United States Academic Pentathlon
SCIENCE
RESOURCE GUIDE
AN INTRODUCTION TO MARINE BIOLOGY
SKT Education - China, CH
®
2021–2022
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Table of Contents
SECTION I: THE OCEAN
PLANET . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Introduction . . . . . . . . . . . . . . . . . . . . . . . . .5
An Introduction to Marine Biology . . . . . . 5
What Is Marine Biology? . . . . . . . . . . . . . . . . . 5
The History of Marine Biology and
Oceanography . . . . . . . . . . . . . . . . . . . . . . . . . 6
Modern Marine Biology and Oceanography . 8
How Do We Study Marine Life? . . . . . . . . .8
The Scientific Method . . . . . . . . . . . . . . . . . . . 8
The Geography and Geology of the
Ocean . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Ocean Basin Geography . . . . . . . . . . . . . . . . 10
The Formation of the Earth and the Ocean . . 10
The Ocean Floor and Plate Tectonics . . . . . .11
The Formation of the Basins . . . . . . . . . . . . .14
Marine Provinces . . . . . . . . . . . . . . . . . . . . . 14
Wave Generation . . . . . . . . . . . . . . . . . . . . . .28
Tsunamis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Tides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Cause of Tides . . . . . . . . . . . . . . . . . . . . . . . . 29
Tidal Patterns . . . . . . . . . . . . . . . . . . . . . . . . 30
Tidal Patterns and Marine Organisms . . . . . 30
Energy from Tides . . . . . . . . . . . . . . . . . . . . .30
Section I Summary . . . . . . . . . . . . . . . . . . 31
SECTION II: MARINE LIFE . . . . . . 32
Introduction . . . . . . . . . . . . . . . . . . . . . . . .32
The Origins of Life . . . . . . . . . . . . . . . . . . 32
Defining Marine Life . . . . . . . . . . . . . . . . 32
The Building Blocks of Life . . . . . . . . . . . 32
Photosynthesis and Chemosynthesis . . . . .33
The Fuel of Life . . . . . . . . . . . . . . . . . . . . . . . 33
Biogeochemical Cycles . . . . . . . . . . . . . . .34
Continental Margins . . . . . . . . . . . . . . . . . . . . 14
The Deep-Sea Floor . . . . . . . . . . . . . . . . . . . . 15
The Carbon Cycle . . . . . . . . . . . . . . . . . . . . . 35
The Nitrogen Cycle . . . . . . . . . . . . . . . . . . . . 35
The Phosphorous Cycle . . . . . . . . . . . . . . . . .36
Biological Provinces . . . . . . . . . . . . . . . . . . . 19
Classifying Marine Life . . . . . . . . . . . . . . 37
Water and Seawater . . . . . . . . . . . . . . . . . . 21
The Chemical and Physical Properties of
Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Seawater . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Dissolved Gases in Seawater . . . . . . . . . . . . 24
Interactions of the Ocean and the
Atmosphere . . . . . . . . . . . . . . . . . . . . . . . .24
Atmospheric Circulation . . . . . . . . . . . . . . . . 25
Oceanic Circulation . . . . . . . . . . . . . . . . . . . 26
Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Wave Properties . . . . . . . . . . . . . . . . . . . . . . 27
The Tree of Life . . . . . . . . . . . . . . . . . . . . . . . 37
The Microbial World . . . . . . . . . . . . . . . . .38
Viruses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Archaea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Unicellular Algae . . . . . . . . . . . . . . . . . . . . . .39
Protozoans . . . . . . . . . . . . . . . . . . . . . . . . . . .40
Plankton . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Seaweeds and Plants . . . . . . . . . . . . . . . . .41
Seaweeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Flowering Plants . . . . . . . . . . . . . . . . . . . . . .43
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INTRODUCTION . . . . . . . . . . . . . . . . . .4
Invertebrates: Animals without a
Backbone . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Sponges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Gelatinous Animals . . . . . . . . . . . . . . . . . . . . 44
Worms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Molluscs . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46
Arthropods . . . . . . . . . . . . . . . . . . . . . . . . . . .47
Echinoderms . . . . . . . . . . . . . . . . . . . . . . . . . 48
Tunicates and Cephalochordates . . . . . . . . . 49
Vertebrates . . . . . . . . . . . . . . . . . . . . . . . . 49
Jawless Fishes . . . . . . . . . . . . . . . . . . . . . . . . 50
Cartilaginous Fishes . . . . . . . . . . . . . . . . . . .50
Bony Fishes . . . . . . . . . . . . . . . . . . . . . . . . . .51
Marine Reptiles . . . . . . . . . . . . . . . . . . . . . . .51
Marine Birds . . . . . . . . . . . . . . . . . . . . . . . . . 52
Marine Mammals . . . . . . . . . . . . . . . . . . . . . .52
Section II Summary . . . . . . . . . . . . . . . . . 54
SECTION III: MARINE
ECOSYSTEMS . . . . . . . . . . . . . . . . . . . 55
Introduction . . . . . . . . . . . . . . . . . . . . . . . .55
What Is Marine Ecology? . . . . . . . . . . . . .55
Environmental Factors Limiting
Organismal Distribution . . . . . . . . . . . . . . 56
Ecological Principles . . . . . . . . . . . . . . . . .57
Habitats . . . . . . . . . . . . . . . . . . . . . . . . . . .59
The Intertidal Zone . . . . . . . . . . . . . . . . . . . . 59
Seaweed Communities . . . . . . . . . . . . . . . . . .60
Estuaries and Salt Marshes . . . . . . . . . . . . . .61
Coral Reefs . . . . . . . . . . . . . . . . . . . . . . . . . . 62
The Open Ocean . . . . . . . . . . . . . . . . . . . . . . 63
The Deep Sea . . . . . . . . . . . . . . . . . . . . . . . . 64
Feeding and Food Webs . . . . . . . . . . . . . . 65
Section III Summary . . . . . . . . . . . . . . . . .67
SECTION IV: HUMANS AND THE
OCEAN . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Introduction . . . . . . . . . . . . . . . . . . . . . . . .68
Resources from the Ocean . . . . . . . . . . . . 68
Living Resources . . . . . . . . . . . . . . . . . . . . . .68
Nonliving Resources . . . . . . . . . . . . . . . . . . . 69
Anthropogenic Impacts . . . . . . . . . . . . . . .70
Marine Pollution . . . . . . . . . . . . . . . . . . . . . . 70
Eutrophication . . . . . . . . . . . . . . . . . . . . . . . . 72
Habitat Modification . . . . . . . . . . . . . . . . . . .73
Overfishing . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Introduced Species . . . . . . . . . . . . . . . . . . . . 74
Climate Change Impacts on the Oceans . . . . 75
Conservation and Protection . . . . . . . . . . .76
Marine Protected Areas . . . . . . . . . . . . . . . . 76
Habitat Restoration . . . . . . . . . . . . . . . . . . . . 77
Section IV Summary . . . . . . . . . . . . . . . . 77
CONCLUSION . . . . . . . . . . . . . . . . . . . .79
NOTES . . . . . . . . . . . . . . . . . . . . . . . . . . 80
BIBLIOGRAPHY . . . . . . . . . . . . . . . . .82
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Salt Marsh Plants . . . . . . . . . . . . . . . . . . . . . 43
Mangroves . . . . . . . . . . . . . . . . . . . . . . . . . . .43
Introduction
The science-fiction writer and undersea explorer
Arthur C. Clarke said, “How inappropriate to call this
planet Earth when it is quite clearly Ocean.” Our planet
is an ocean world, with the ocean covering more than
70 percent of the surface of the Earth. Many people
are inherently interested in the ocean and what lies
below. Marine Biology is the study of the ocean and
the marine organisms that live within it. Although it
is often stated that we know more about the surface
of the moon than we do about the deep sea, the ocean
provides vast resources, aids technological advances,
and contributes massively to the basic needs of human
beings. The ocean regulates climate, produces more
than half the world’s oxygen, and provides income for
millions of people. We derive food, energy, medicines,
and more from the oceans. Learning about the ocean
is vital to understanding the challenges facing our
planet, including climate change, pollution, and loss of
biodiversity.
physical, and geological processes of the ocean.
In the first section of this guide, we will discuss our
ocean planet, providing an introduction to and an
overview of oceanography and marine biology. The
topics covered in this first section are the foundational
topics of marine biology. We will discuss why and
how we study marine science as well as the chemical,
When you reach the end of this guide, you will have a
better understanding of the processes that govern our
ocean planet and the life that it holds. Learning about
the ocean and the marine life that inhabits it is critical
as human health is connected to the health of all life on
our planet.
Section three will cover marine ecology, the
relationships among species, and how marine
organisms interact with their environment. We will
also cover different types of marine ecosystems and
discuss their importance.
Finally, in section four, we will survey the ways in
which humans and the ocean interact. Humans have
dramatically altered the physical ocean and marine
life. In this section, we will also explore the ways
in which humans conserve resources and protect a
valuable food supply.
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In section two, we will explore the diversity of marine
life in the ocean. First, we will define what it means to
be alive and discuss the processes that maintain marine
life. Then, we will explore the diversity of living
organisms in the ocean.
Section I
The Ocean Planet
Earth is a planet defined by its oceans and the life
within them. In this section, we will introduce marine
biology, describe why we study marine life, and
cover the major physical and chemical processes of
the ocean. After reading this section, you will have a
better understanding of marine biology, how the ocean
works, and how the land and sea are connected.
The ocean is a place of wonder and beauty and provides
a habitat for countless life forms. Most people have
some kind of natural interest in the oceans and the life
they hold. The ocean and marine biology are important
for many practical reasons. First, it is likely that life
on Earth originated in the ocean. Further, humans use
many resources from the sea in their everyday lives.
The ocean provides us with food, medicines, and natural
resources. The ocean offers abundant recreation and
supports tourism across the globe. Seafood (fish and
shellfish) is a staple in the diets of many people who
live near coasts. Many medical advances have resulted
from studying marine organisms. For example, the
pharmaceutical drug Remdesivir, which has been used
to treat COVID-19, was derived from sea sponges. The
ocean provides many natural resources (e.g., oil, gas,
salt, sand, gravel) that humans use on a daily basis.
Lastly, plant-like organisms in the ocean provide more
than half of the oxygen we breathe and help to regulate
our climate. Thus, marine life represents an important
source of wellbeing for human life, and humans, in turn,
have had a tremendous impact on the ocean.
AN INTRODUCTION TO
MARINE BIOLOGY
What is Marine Biology?
Marine biology is the scientific study of biological
processes and organisms in an ocean setting. Marine
biology is made up of many different disciplines,
approaches, and viewpoints. Typically, marine biologists
FIGURE 1−1
An ocean planet.1
focus on studying living things or components of
living things in the ocean. Oceanography, a field of
study closely related to marine biology, is the scientific
study of the oceans that integrates biology, chemistry,
engineering, geology, and physics. Oceanographers tend
to study chemical and physical processes in the sea.
Our planet truly is an ocean world. An ocean is a vast
body of saline water that occupies the depressions on the
surface of the Earth. The following statements are some
basic statistics about our ocean planet. Seventy-one
percent of Earth’s surface is ocean. The average depth of
the ocean is more than 3,000 meters deep. The average
temperature of the ocean is cold, close to 4°C. The
ocean affects and moderates daily temperatures around
the world and influences global weather patterns. The
ocean provides food and is the primary source of animal
protein for approximately half the world’s population.
In fact, nearly half of the population on our planet lives
within 150 miles of a coastline. Close to one-third of
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INTRODUCTION
FIGURE 1−3
The Greek philosopher and early marine biologist Aristotle.2
the petroleum and natural gas that we use is harvested
from the ocean floor. Moreover, the ocean is a primary
shipping and communications route. The ocean has
been utilized by humans for thousands of years.
The History of Marine Biology and
Oceanography
The history of marine biology and oceanography really
starts with the history of voyaging. As people gained
skills in seamanship and navigation, knowledge of
the ocean expanded. The history of ocean exploration
began with early Melanesians nearly 100,000 years
ago, although early exploration was confined primarily
to coastal areas. Later, ancient Pacific Islanders, who
were talented voyagers, began exploring vast reaches
of oceans in the Polynesian Triangle starting at least
as early as 1500 BCE. These ancient Pacific Islanders
were knowledgeable about marine life and had detailed
information about wind, waves, and navigation patterns
that had been passed down through oral traditions.
At approximately the same time, ocean exploration
Captain James Cook, the first scientific oceanographer to
make skillful measurements.3
was also underway in the northern hemisphere.
Phoenicians were among the earliest Western ocean
explorers, and they developed nautical charts and thus
the written record of marine biology. The Phoenicians
sailed around the Mediterranean Sea, Rea Sea, and
Black Sea as well as the eastern Atlantic Ocean
and the Indian Ocean. Ancient Greeks were also
quite knowledgeable about marine life. Aristotle is
sometimes considered the first marine biologist, as he
described many ocean life forms and recognized that
gills were the breathing apparatus of fish.
During the early Middle Ages, the formal study of
marine life waned in Europe. During this time, the
Vikings continued to explore the northern Atlantic and
were skilled voyagers who learned about the ocean.
During the Renaissance, Europeans again began to
investigate the world around them. Ferdinand Magellan
set sail on the first expedition to circumnavigate
the globe. The advancement of scientific voyaging
continued with an English sea captain, James Cook,
who was a skillful navigator, cartographer, writer,
artist, diplomat, sailor, and scientist. Beginning
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FIGURE 1−2
FIGURE 1−4
in 1768, while onboard the HMS Endeavor,
Captain James Cook was the first to take scientific
measurements using a chronometer, a timepiece used
for determining longitude, and he prepared marine
charts or maps. Cook was the first to circumnavigate
the world at high latitudes, and he mapped the Great
Barrier Reef and made many notes on the natural
history of marine environments.
By the 1800s, it was common for vessels to have a
naturalist onboard to study the organisms that were
encountered. One of the more famous naturalists
was Charles Darwin, who sailed around the world
on the HMS Beagle for five years. Despite being
terribly seasick for the majority of the voyage, Darwin
made detailed observations about the natural world.
These observations led to his theory of evolution by
natural selection. In addition, Darwin made many
contributions to marine biology, taking detailed natural
history notes, mapping coastlines, and explaining the
formation of atolls, rings of coral reef that form around
subsided volcanoes.
The United States also contributed to marine biology
in the 1800s with the United States Exploring
Expedition of 1838–42, often called the “Wilkes
Expedition.” This expedition was composed of a fleet
of vessels, and its achievements were impressive. The
FIGURE 1−5
Charles Darwin, famous naturalist and marine biologist.5
Wilkes Expedition crossed the Atlantic, Pacific, Indian,
and Southern Oceans, confirmed Antarctica was a
continent, and described more than two thousand
marine and terrestrial species. This expedition was
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The path of Captain James Cook’s third voyage around the globe.4
and remotely operated vehicles (ROVs) and improved
computing power offer emerging opportunities to study
marine systems. Ocean observing systems and satellites
are crucial in gathering information and data about
marine systems.
FIGURE 1−6
National Science Foundation research vessels Laurence M.
Gould and Nathaniel B. Palmer.6
the first international expedition sponsored by the U.S.
government, and it helped to lay the foundation for the
Smithsonian Institution and other scientific research
that was funded by the government.
One of the more notable marine expeditions was
led by Charles Wyville Thompson aboard the HMS
Challenger from 1872 to 1876. Funded by the British
government, the Challenger expedition sailed around
the world, systematically collecting data as well
as samples of seawater, specimens, and sediment
(particles of organic and inorganic material that loosely
accumulate on the seafloor). The contributions of the
Challenger expedition were enormous and brought
forth more information about the ocean than had been
recorded from previous expeditions. Furthermore, this
expedition set new standards for marine research and
laid the foundation for modern marine science.
Modern Marine Biology and
Oceanography
Modern-day marine biology and oceanography includes
marine laboratories, vessels dedicated to ocean research,
submersibles, underwater marine laboratories, ocean
observing systems, and the development of technologies
such as sonar (sound navigation ranging) and scuba
(self-contained underwater breathing apparatus). Each
of these technologies and developments has led to
incredible advancements in marine biology. Marine
laboratories and research vessels are often equipped
with the most advanced and cutting-edge technology
available, making them critical for marine research and
education. New technologies such as remote sensing
We study marine life by employing the scientific
method. Science is a systematic process of asking
questions about our observations of the world by
gathering information and studying data. Scientists,
including marine biologists, employ the scientific
method to answer questions about the ocean. The
scientific method is used by scientists across the world
and trusted by the public because it works. Events
in the natural world do not just happen, nor do we
explain them in a haphazard way. There is a scientific
explanation for how and why events occur the way in
which they do. The goal of science is to discover facts
about the natural world and to help explain the world
around us. Science is everchanging as scientists are
continually learning and making new discoveries.
The Scientific Method
Scientists use a set of procedures called the scientific
method to learn about the natural world. Most
scientists agree on the basic steps of the scientific
method. Scientific conclusions are drawn based
on observations that describe the natural world.
Observation is the currency of scientific methods.
Observations arise from exploration and description,
two vitally important components of marine biology.
Scientists make observations about the natural
world, and then seek to explain and predict these
observations, using two forms of thinking: induction
and deduction. When using induction, a scientist has
no goal or predetermined outcome and is, in theory,
objective. As such, a scientist uses the observations
to reach a conclusion. For example, a marine biologist
might examine a shark, a goby, and a lanternfish
and find that they all have gills used for breathing.
Then, the scientist might draw the conclusion that
because sharks, gobies, and lanternfishes all have
gills, all fish have gills used for breathing. Scientists
must exercise caution when using induction because
general conclusions are made based upon specific
observations. In the example above, a scientist might
have concluded that all animals in the ocean have gills,
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HOW DO WE STUDY MARINE
LIFE?
FIGURE 1−7
FIGURE 1−8
which is one of the great strengths of the scientific
method. Hypotheses are constructed in a way that
allows scientists to critically evaluate them. For
example, the hypothesis that sea turtles have gills is
easy to test. Additionally, this would also disprove the
hypothesis that all marine animals have gills.
An overview of the process of the scientific method.7
but in reality, not all animals in the ocean have gills—
some marine animals have lungs.
A scientist must also use deduction to predict what
the specific consequences would be if the statement
made above is true. With deduction, a scientist might
conclude that sea turtles are marine animals, and if
all marine animals have gills, then sea turtles must
have gills. Of course, we know that this is not true.
Induction and deduction help scientists to make
statements about the natural world that might be true.
These statements are called hypotheses. A scientific
hypothesis is a proposed explanation of what has
been observed by a scientist that might be true and
is testable. Scientists test hypotheses over and over,
Often, hypotheses are more complicated, but they
nevertheless must still be testable. A testable hypothesis
is one that can be tested and potentially proved false.
In contrast, no scientific hypothesis can be absolutely
proven true. There are no absolute truths in science.
After a hypothesis has been rigorously tested, it is
conditionally accepted as true, given the available
evidence. Rather than proving hypotheses, scientists
accept hypotheses, knowing that at least for now, given
what we know, the hypothesis fits the observations. We
place more confidence in a hypothesis that has been
tested again and again and has stood up to rigorous
testing.
Hypothesis testing requires careful planning and
experimental design. Sometimes scientists are not able
to test hypotheses in nature and must create conditions
to manipulate nature by performing an experiment.
Using experiments, scientists create artificial situations
to test hypotheses. In an experiment, there are
variables, or factors that might affect the observations.
In all experiments, scientists use controlled variables
(often also called simply “controls” or “constants”),
which are not changed in the course of the experiment.
By using controls, scientists can be more confident that
the observed outcomes (dependent variables) are in
fact due to the independent variables being tested.
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A hawksbill sea turtle.8
Results arrived at via the scientific method are not
wholly infallible. Scientists are humans and thus are
prone to the same shortcomings common to all humans.
Sometimes scientists make mistakes, and sometimes
scientists are wrong. Fortunately, errors in science are
often corrected because hypotheses are tested by many
people. Science, when properly conducted, is not based
on values, feelings, or beliefs—science is factual.
THE GEOGRAPHY AND
GEOLOGY OF THE OCEAN
Geological processes have shaped and sculpted the
continents and ocean basins.
Ocean Basin Geography
The oceans cover 71 percent of the earth, but the
oceans are not distributed equally with respect to
the equator. Specifically, the northern hemisphere is
approximately 61 percent ocean, whereas the Southern
Hemisphere is about 80 percent ocean.
Conventionally, oceans are partitioned into four large
basins. The largest and deepest of the ocean basins is
the Pacific Ocean—it is almost as large as all the other
oceans combined. The Pacific Ocean was named by
Ferdinand Magellan in 1520. The Atlantic Ocean is the
second largest ocean basin but is approximately half of
the size of the Pacific Ocean. The Atlantic Ocean was
named after Atlas, a titan of Greek mythology. The third
largest basin is the Indian Ocean, which is smaller
than the Atlantic Ocean, but similar in depth. The
Indian Ocean is primarily in the southern hemisphere
and is named for its proximity to India. The fourth
major ocean basin is the Arctic Ocean, which is about
7 percent of the size of the Pacific Ocean. The Arctic
Ocean is the shallowest ocean, has a layer of sea ice,
and is named for its location in the Arctic region.
The Southern Ocean is not considered an ocean basin
but is a body of water that flows continuously around
the continent of Antarctica. Unlike an ocean basin,
which is defined by continents, the Southern Ocean
is defined by the meeting of currents near Antarctica
called the Antarctic Convergence. Thus, the Southern
Ocean is made up of parts of the Pacific, Atlantic, and
Indian Oceans that lie south of 50 degrees latitude.
The Formation of the Earth and the
Ocean
The Earth is thought to have originated 4.5 billion
years ago from dust. The dust particles collided with
each other, joining to form larger particles and clumps
when colliding with rocks. Eventually, this colliding
process formed the Earth. After Earth was formed,
layers started to separate according to density. Density
is the mass of a substance of a given volume. When two
substances are mixed, like oil and vinegar, the denser
material (vinegar) tends to sink, and the less dense
material (oil) tends to float. Protoearth, or first Earth,
was likely homogenous, but then heat and gravitational
pressure caused Earth to partially melt. The densest
material, mostly iron, flowed toward the center of
the planet, while the lighter materials migrated to the
surface to form the crust. The internal layered structure
of the Earth indicates the early beginnings of our planet.
The core, or innermost layer of the Earth is mostly
made of iron, with a solid inner core and a liquid
outer core. The pressure in the core is immense, and
the temperature is estimated to be over 4,000 degrees
Celsius.
The middle layer of the Earth is known as the mantle.
The mantle is the thickest layer, consists mostly of
magnesium and iron silicate, and flows like a liquid.
The crust, or outermost layer of the Earth, is thin and
floats on top of the mantle. The crust can be divided
into continental and oceanic crust, and these two types
of crust vary greatly. Continental crust is relatively
light, but thick and is made of silicate materials such
as granite. Oceanic crust is thin and dark in color,
but heavy and consists of a mineral called basalt.
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In summary, scientists use the scientific method
to answer questions about the natural world. The
scientific method starts with an observation and then
a question is asked. Next, a hypothesis is formed,
predictions are made, and the predictions are tested.
Then, the results are used to make a new hypothesis.
When a hypothesis has been tested again and again
and passed every possible test, it can be referred to
as a scientific theory. While in common use the
word theory may describe an untested and often
questionable idea, in science a theory is an idea that is
supported by overwhelming evidence and represents
a comprehensive explanation of how the world works.
Thus, a scientific theory is a hypothesis that has been
extensively tested and is generally accepted as true.
However, as with any hypothesis, it is still subject to
rejection if new evidence is discovered.
The major ocean basins on Earth.9
Oceanic crust is also high in magnesium and iron.
Both continental and oceanic crust float on the mantle,
but because oceanic crust is denser than continental
crust, it does not float as high. Thus, the continents lie
well above sea level, and the ocean crust lies below
sea level, flooded with water. Continental crust is old,
with rocks dating back to 3.8 billion years ago, while
oceanic crust is only about 200 million years old.
FIGURE 1−10
After the stratification of the Earth, the atmosphere
and the oceans eventually began to form. Oceans
formed about 3.8 billion years ago from volcanic gases
that added water vapor, carbon dioxide, nitrogen, and
other gases to the atmosphere. These trapped gases
caused rain, which some scientists estimate continued
for 10,000 years. Comets bombarding the Earth with
ice also likely delivered some of Earth’s surface water.
The Ocean Floor and Plate Tectonics
Geologic change is responsible for the formation the
ocean basins and the drifting apart of continents,
resulting in the form that we see today. The face
of the planet is ever changing and shifting. Alfred
Wegener (1880–1930), a German geophysicist, first
proposed a detailed hypothesis of continental drift in
Layers of the Earth.10
1912. Wegener formed his hypothesis on the basis of
evidence that Sir Francis Bacon made note of in the
1600s. Sir Francis Bacon observed that the continents
on opposite sides of the Atlantic Ocean fit together
like puzzle pieces. Wegener then suggested that all
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FIGURE 1−9
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FIGURE 1−11
Continental drift from Pangaea to the present.11
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Tectonic plate boundary types: divergent plate boundaries, convergent plate boundaries, and transform plate boundaries.12
of the continents had once been joined together in a
single supercontinent he called Pangaea. Wegener’s
hypothesis was not widely accepted and was largely
ridiculed because he did not have a mechanism to
explain how the continents moved. It was not until
the 1950s and 1960s that scientists were able to gather
the evidence that allowed for the development of
the theory of plate tectonics. This scientific theory
states that the Earth’s outer layer is made up of
moving plates, and it helps to explain the formation of
mountains, volcanoes, and earthquakes.
Tectonic plates are rigid plates that can consist of
oceanic crust, continental crust, or both. Tectonic
plates float on the mantle, and their movement is
driven by convection currents from the mantle.
Convection currents are movements of mantle that are
driven by the downward movement of cooler, denser
material and the upward movement of hotter, less
dense material. This movement causes the tectonic
plates to push together and pull apart, creating several
oceanographic features.
Where plate boundaries meet, plate interaction
occurs. Plate boundaries can be divided into three
types. Divergent plate boundaries occur where two
plates are moving apart from each other, causing
splitting and rifting to occur. Convergent plate
boundaries occur when two plates move toward each
other, causing buckling and shortening of the plates.
Transform plate boundaries occur when two plates
slide past each other, causing shearing.
After the development of sonar during World War II,
surveys of the deep-sea floor resulted in the discovery
of the mid-ocean ridge. The mid-ocean ridge is a
continuous chain of underwater volcanoes that encircles
the globe like seams on a baseball. Mid-ocean ridges
form at the edges of divergent plate boundaries where
two underwater plates are pulling away from each
other. Here, volcanic activity creates new seafloor
through a process called seafloor spreading. Plates
spread at approximately 2 to 18 cm per year, varying
from place to place. At the ridge where the plates have
recently pulled apart, the crust is new and has not yet
accumulated sediment. As the crust is pushed away
from the ridge, it ages, and sediment builds up as it
“rains” down from above. The farther you move from
the ridge, the older and thicker the sediment becomes.
Divergence on one part of the Earth must be offset
by convergence somewhere else. If this did not occur,
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FIGURE 1−12
Convergence can also occur when an oceanic plate
interacts with another oceanic plate. Usually, one of the
oceanic plates is older and thus cooler and denser than
the other. The older, heavier plate is pulled downward by
gravity and slips below the lighter plate, forming a deep
trench, and often resulting in island arcs, such as the
Aleutian Islands. Finally, convergent plate boundaries
can occur where a continental plate collides with
another continental plate. In this case, both continental
plates are similar in density, which results in the plates
being compressed together, folded, buckled—because
the force is too great—and ultimately uplifted to form
mountains. The Himalayan Mountains were formed
when India collided with the rest of Asia.
At transform plate boundaries, plates shear past one
another, rather than creating or destroying crust. When
two plates slide past each other horizontally, there is a
tremendous amount of friction. Additionally, because
this plate movement is occurring on a sphere, rather
than on a flat surface, the movement of the plate often
ends abruptly when connecting to a divergent or
convergent plate boundary, hence the name transform
plate boundary. Earthquakes are common at transform
plate boundaries. The San Andreas Fault in California
is a junction between the Pacific and North American
plates and is the largest and most well-known
transform plate boundary.
The Formation of the Basins
When the continents were all joined as one, known
as Pangaea, the supercontinent was surrounded by a
single enormous ocean called Panthalassa. The ocean
basins that we know today formed as the supercontinent
began to pull apart and separate into distinct continents.
Approximately 180 million years ago, Pangaea
separated into two large continents: Laurasia and
Gondwana. Laurasia was composed of what is now
North America and Eurasia, while Gondwana was made
up of what is now South America, Africa, Antarctica,
and India. Soon after, a rift appeared between North
America and the combined continents of South America
and Africa, marking the beginning of the Mid-Atlantic
Ridge. This rift formation was the start of the North
Atlantic Ocean.
Around the same time, another rift occurred in
Gondwana, marking the birth of the Indian Ocean.
The South Atlantic Ocean began when a rift occurred
between South America and Africa. This rift
eventually joined with the Mid-Atlantic Ridge to form
a single mid-ocean ridge. The Pacific Ocean, formerly
Panthalassa, continued to shrink as the continents
continued to shift. The continents are still adrift,
headed for collision, only to pull apart again.
Marine Provinces
Ocean basins and the seafloor are not flat, vast bathtublike basins. Instead, as a result of plate tectonics,
the seafloor is varied in topography. Bathymetry is
the discovery and scientific study of the ocean floor
contours. Scientists study bathymetry using echo
sounders, which are multibeam sonar systems that
bounce sound off of the seafloor to measure ocean
depth.
In addition, satellites are used to help measure
variations in sea surface elevation, and when echo
sounding and satellites are combined, a clear picture
of the topography of the seafloor emerges. Much of
the topography on Earth exists below the ocean. The
average depth of the ocean is approximately 3,000
meters, while the average elevation of the continents
is ~840 meters. The seafloor is divided into two main
regions: continental margins and the deep-sea floor.
Continental Margins
Continental margins are the edges of the continents
that are submerged by the ocean. They are the
boundaries between the continental and oceanic crust.
Continental margins are classified as either passive
margins or active margins. Passive margins are
continental margins facing the edges of diverging
plates. These margins experience very little volcanic
and earthquake activity and surround the Atlantic
Ocean basin. Active margins are continental margins
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the Earth would get bigger and bigger, having to grow
to accommodate the continuous seafloor spreading.
Convergent plate boundaries are sites of violent
geological activity where plates are pushing together. At
convergent plate boundaries, oceanic crust is destroyed
by a process called subduction, where one plate slides
beneath another. Convergent plate boundaries can occur
where an oceanic plate and a continental plate collide.
In this case, the lighter continental plate rides up over
the heavy oceanic plate, causing the oceanic plate to
be subducted along a deep trench. Here, volcanoes
and earthquakes are numerous, often forming coastal
mountain ranges like the Andes Mountains.
Multibeam bathymetry of Pao Pao Seamount (right) and an unnamed guyot (left).13
near the edges of converging plates; they are common
in the Pacific Ocean basin where earthquakes and
volcanic activity are frequent.
Continental margins consist of a shallow, flat
continental shelf, a steep continental slope, and
a continental rise, a gently sloping thick apron of
sediment that blends the margin into the deep ocean
basin. The continental shelf is the shallow, submerged
extension of the granite continental crust, which gently
slopes to the edge of the shelf. Though the continental
shelf is only about 8 percent of the Earth’s ocean area,
it is biologically the richest part of the ocean, where the
majority of sea life can be found and where the bulk of
fishery harvests occur. The shelf is studded with hills,
canyons, depressions, and sedimentary rocks, as well
as mineral and oil deposits. In fact, a large portion of
hydrocarbon extraction takes place on the continental
shelf. The shelf slopes gently downward, varying in
width depending on the location. For example, the
Atlantic Ocean shelf extends over 350 km, while off
the Arctic coast of Siberia the shelf extends for more
than 750 km.
The continental shelf ends abruptly at the shelf break,
where the steepness of the slope greatly increases.
Generally, the shelf break is about 150 meters deep,
though it is deeper in Antarctica and Greenland
because ice weighs down the continental crust. The
continental slope is the transition between the shelf
and the deep ocean floor. The slope is the truest edge
of the continent and is steeper along active margins
than at passive margins. Submarine canyons cut into
the continental shelf and slope, terminating on the
deep-sea floor as a fan-shaped wedge of sediment.
Submarine canyons are formed by turbidity currents,
where turbulence mixes sediments into water above
the slope. The continental rise is the base of the
continental slope that is covered by an apron of
accumulated sediment. The sediment has slowly
descended, like sand moving down a slide, to the ocean
floor and has become trapped by turbidity currents.
The Deep-Sea Floor
More than half of the ocean is deep-sea floor, with
most of the habitat ranging in depth from 3,000 to
5,000 meters. Much of the deep-sea floor is relatively
flat and is blanketed with thick sediments. However,
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FIGURE 1−13
FIGURE 1−14
Continental margin consisting of continental shelf, continental slope, and continental rise.14
Abyssal plains cover approximately one-third of
the planet’s surface and extend from the edge of the
continental rise, averaging between 4,500 meters and
6,000 meters deep. Abyssal plains are covered with
thick blankets of sediment that have settled down from
above for millions of years. Most of the abyssal plains
occur in the Atlantic and Indian Ocean basins, with
a few occurring in the Pacific Ocean. Abyssal plains
are studded with volcanic peaks, with some extending
above sea level to form islands.
Often, volcanic peaks do not break the surface and
are called seamounts. Sometimes volcanoes below
the surface have a flattened top as a result of wave
action and subsidence and are called guyots. Any
volcanic features on the abyssal plains that are less
than 1,000 meters tall are called abyssal hills. Abyssal
hills are one of the most common features on Earth
and are mostly created from the stretching of crust
during seafloor spreading at mid-ocean ridges. Over
time, with sediment buildup, many abyssal hills
have become buried, especially in the Atlantic and
Indian Ocean basins. In the Pacific Ocean basin, the
abundance of trenches helps to trap sediment, so the
rate of sedimentation is lower. As a result, abyssal hills
are more prevalent and visible in the Pacific Ocean
basin because they are not buried by sediment.
Ocean trenches are narrow, steep-sided, arc-shaped
depressions in the ocean floor. Ocean trenches occur
at convergent plate boundaries where two plates have
collided, and one plate has been subducted. When
a continental plate and an oceanic plate collide, the
landward side of the trench can rise as a volcanic arc
that can sometimes produce islands or mountains.
Trenches are curved because the plate movement is
occurring on a sphere. The Aleutian Islands are a
volcanic arc and are referred to as an island arc. A
volcanic mountain range along the edge of a continent,
such as the Andes Mountains, is referred to as a
continental arc. Trenches are the deepest places in the
ocean. The deepest trench known on Earth, called the
Mariana Trench, is 11,034 meters deep and is located in
the Pacific Ocean basin. Trenches contain the densest,
coldest water in the ocean. Trenches are primarily found
in the Pacific Ocean, where the majority of convergent
plate boundaries are located—only a few trenches are
found in the Atlantic and Indian Oceans.
The mid-ocean ridge—a continuous mountain range
that is found in all of the ocean basins—is a major
feature of the deep-sea floor. The mid-ocean ridge
stretches for more than 40,000 miles across the planet.
As previously discussed, mid-ocean ridges arise at
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the deep-sea floor is rimmed with trenches and studded
with submarine channels, ocean ridges, abyssal hills,
plateaus, rises, and other features.
FIGURE 1−15
Sea Surface
200 m
Continental Shelf
Volcanic Island
Continental Slope
2,000 to 3,000 m
Continental Rise
Abyssal Plain
4,000 to 6,000 m
Oceanic Trench
10,000 m
Ocean basin topography.15
Oceanic crust
Lithosphere
Volca
nic
arc
Tr
e
nc
h
FIGURE 1−16
Continental crust
Lithosphere
Asthenosphere
Active margin diagram with formation of a continental arc.16
divergent plate boundaries where seafloor spreading
is occurring. In a few places on Earth, the mid-ocean
ridge breaks the surface, forming islands such as
Iceland and the Azores. In the North Atlantic, the
mid-ocean ridge is called the Mid-Atlantic Ridge
and is taller than all of the mountain ranges found on
the continental crust. Earthquakes and volcanoes are
common along the mid-ocean ridge.
Hydrothermal vents are among the most notable
geological features of the mid-ocean ridge.
Hydrothermal vents (hydro = water, thermo =
heat) are deep-water hot springs on active ridges.
Hydrothermal vents are created when water seeps
through the cracks along the ridge, gets heated from
being near the magma chamber, and then forces its
way back toward the surface of the ocean floor through
a vent. The temperature of the water when it is forced
back up through a vent determines the appearance
of the vent. For example, warm-water vents have
water temperatures below 30 degrees Celsius and are
generally clear in color. White smokers have water
temperatures running from 30 degrees Celsius to 350
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Submarine Ridge
FIGURE 1−17
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Region
Image of the Mariana Trench in the Pacific Ocean basin.17
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World map of the mid-ocean ridge system.18
degrees Celsius and look white because of the presence
of light-colored compounds such as barium sulfide.
Black smokers emit water that is black because of the
presence of dark-colored metal sulfides such as iron,
nickel, copper, and zinc and have water temperatures
above 350 degrees Celsius. Despite their seemingly
unlivable temperatures, hydrothermal vents are host
to a variety of organisms. They will be discussed in
further detail in the ecosystem section of this guide.
FIGURE 1−19
Biological Provinces
Marine biologists categorize marine habitats according
to where they are located in marine systems. Marine
biologists generally group organisms based upon the
organisms’ lifestyles. Organisms that live on the bottom
are classified as living in the benthic environment and
are referred to as the benthos. Organisms that live in
the benthic environment can either move around or
live attached in one place (sessile). Sea stars or crabs
are examples of benthic organisms that move around,
while sponges or corals would be examples of benthic
organisms that are sessile.
A hydrothermal vent chimney.19
Organisms that live up in the water column, away from
the benthic zone, occupy the pelagic environment.
Pelagic organisms are further divided according to
how they can locomote in the water column. Plankton
are organisms that are at the mercy of ocean currents
and are carried from place to place. The word plankton
is derived from the Greek word for “drifters.” Plankton
largely drift from place to place in the ocean, although
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FIGURE 1−18
Oceanic divisions of the marine environment based on distance from land, water depth, and whether organisms
are benthic or pelagic.20
there is some evidence that some plankton can direct
their movement. Plankton can either be phytoplankton,
which are microscopic marine algae, or zooplankton,
which are tiny drifting animals. Zooplankton include
tiny protozoans and copepods as well as juvenile
forms of many larger marine animals such as fish and
crustaceans.
benthic zone because they spend the majority of their
time on the bottom, rather than swimming in the water
column. Many marine animals, including the egg and
larval stages of many fish and benthic invertebrates,
will live the early part of their life as plankton in
the pelagic zone and then transform to dramatically
different lifestyles as adults.
Animals that swim well enough to oppose currents are
called nekton and are mostly vertebrates, primarily
marine mammals and fish. However, squids are a great
example of invertebrate nekton. Some creatures blur
the lines between these human-made classifications
for zones. For instance, sting rays are considered to be
part of the nekton because they can swim and oppose
currents, but they are also considered to be part of the
The benthic and pelagic zone can be further divided
based on the characteristics of each zone. The benthic
zone is divided based on depth and the continental
shelf. The first zone of the benthic zone on the
continental shelf is called the intertidal zone, which is
the boundary between the land and the sea, between
the tides. This area is exposed when the tide is out
but is underwater at high tide. The zone below the
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FIGURE 1−22
The pelagic environment is also divided with reference
to the continental shelf. The neritic zone is the pelagic
environment over the continental shelf to the shelf
break. The oceanic zone is the water beyond the shelf
break. The pelagic environment is also divided into
zones based on depth and light. The shallowest zone,
called the epipelagic zone, extends to ~200 meters and
has plenty of light for photosynthesis to occur. Plankton
thrive in the epipelagic zone because there is enough
sunlight to provide plenty of food. Below the epipelagic
zone is the mesopelagic zone. The mesopelagic zone
is the deeper water beyond the continental shelf. The
mesopelagic zone extends to ~700 meters deep. This
zone is referred to as the twilight zone because there is
enough light to see by, but not enough light to support
photosynthesis. The deepest parts of the ocean are the
bathypelagic, the abyssopelagic, and the hadopelagic
zones. The bathypelagic zone is considered the midnight
zone and extends from 1,000 to 4,000 meters. The
abyssopelagic zone is considered the lower midnight
zone and extends from 4,000 meters to wherever it
meets the seafloor. Lastly, the hadopelagic zone is
deep and dark and is the water in the trenches. The
bathypelagic, abyssopelagic, and hadopelagic make up
the deep-sea environment.
WATER AND SEAWATER
Water is all around us and is so common that we
often take it for granted. Water is a unique substance
with few properties shared by other substances.
Furthermore, the chemical properties of water are
essential for life, with water being the primary
component of all living organisms. Water on our planet
makes life possible. In fact, when we look for life
elsewhere, we look for water.
The Chemical and Physical Properties of
Water
Water molecules are a compound of two hydrogen
atoms and one oxygen atom. A compound is a substance
that contains two or more different elements in fixed
proportion. Atoms are the building blocks that make up
elements. A molecule is a group of two or more atoms
of different elements held together by mutually shared
electrons. Atoms are composed of subatomic particles
FIGURE 1−21
A water molecule is comprised of two hydrogen atoms and
one oxygen atom.21
called protons, neutrons, and electrons. Protons and
neutrons are bound together in the nucleus by strong
forces. Protons have a positive electrical charge, while
neutrons have no electrical charge. Electrons are found
surrounding the nucleus and have a negative charge.
Thus, the electrical attraction between the positively
charged protons and the negatively charged electrons
holds the electrons in shells around the nucleus.
Water is a group of three atoms held together by
chemical bonds. When atoms come together to form
a molecule, they share or trade electrons to establish
bonds. The chemical formula for water is H2O, which
signifies that a water molecule is composed of two
hydrogen atoms and one oxygen atom. A hydrogen
atom has one proton and one electron, and an oxygen
atom has eight protons and eight electrons. The
chemical bonds in water are covalent bonds, which are
formed by shared pairs of electrons. Covalent bonds
are relatively strong bonds, and thus a lot of energy is
needed to break them.
The atoms of a water molecule are not bonded in a
straight line, but rather they have a unique geometry
which comes from the covalent bonds. The two
hydrogens bound to the oxygen are separated by an
angle of about 105 degrees because the oxygen atom
pulls the electrons more strongly than the hydrogen
atom. This angle gives a slight overall negative charge
to the end of the molecule that contains the oxygen
atom and a slight overall positive charge to the end of
the molecule that contains the hydrogen atoms. These
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intertidal zone, is called the subtidal zone. Beyond the
continental shelf, the benthic zone is divided into the
bathyal, abyssal, and hadal zones. These three zones
are considered part of the deep-sea floor.
FIGURE 1−23
Hydrogen bonds between water molecules.22
A water strider uses water surface tension.23
differences in charge across the molecule give it an
electrical polarity, making water molecules dipolar.
Batteries or bar magnets are common dipolar objects
in our everyday lives. Somewhat like two magnets that
are joined negative to positive, two water molecules
can be weakly bound together at the dipolar ends via a
hydrogen bond.
When water cools, the molecules move more slowly
and are packed more closely together, and the volume
of water decreases. The water is also denser because
the volume has decreased, but the mass has not
changed. Thus, colder seawater is denser than warmer
seawater. Water freezes when the molecules move so
slowly that the hydrogen bonds take over. When this
occurs, the bond angles between the hydrogen and
the oxygen widen from 105 degrees to 109 degrees,
forming a three-dimensional hexagonal crystal lattice
structure. This results in a 9 percent expansion when
water freezes. Ice is less dense than liquid water
because the same mass of water occupies more volume
as ice than as liquid water. Thus, ice floats in liquid
water. In the Arctic and Antarctica, a floating layer
of ice serves as an important insulator for the marine
organisms in the water below. If ice did not float, much
of the ocean volume would remain frozen.
Hydrogen bonds between water molecules are much
weaker than covalent bonds. Thus, the bonds between
different water molecules are weak, while the bonds
within the water molecule are strong. Although the
hydrogen bonds are weaker than covalent bonds, they
still give water some unique properties. In particular,
hydrogen bonds between water molecules allow water
molecules to cluster together and exhibit cohesion,
which means that water molecules have the ability to
stick to each other. Cohesion creates water surface
tension, allowing insects such as water striders to
“walk on water.” Mercury is the only substance that
has a higher surface tension than water. Hydrogen
bonds also give water the property of adhesion, or the
tendency of water molecules to stick to other surfaces,
like dew on spiderwebs or blades of grass.
All substances on Earth can exist in three different
states: solid, liquid, or gas. The only substance on
Earth that naturally occurs in all three different states
is water. When water is a liquid, hydrogen bonds hold
most of the molecules together. When water is heated,
and the molecules move fast enough to break free from
hydrogen bonds, they escape and turn into a gas. This
process is called evaporation.
FIGURE 1−24
Arctic Sea ice.24
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FIGURE 1−22
When enough heat is added to a liquid, it converts to a
gas. The temperature at which boiling occurs is known
as the boiling point. The latent heat of vaporization is
the amount of energy required to break the hydrogen
bonds between water molecules causing evaporation.
Water has the highest latent heat of vaporization of
any substance known. Approximately 434,000 cubic
meter of water evaporates from the surface of the
ocean each year.
The amount of heat needed to raise the temperature of 1
gram of a substance by 1°C is called the speci ic heat
capacity. Water has a high heat capacity and therefore
resists changing temperature when heat is added or
removed. The high heat capacity of water comes from
the strength and number of hydrogen bonds. The high
heat capacity of water allows it to help regulate the rate
of the temperature change of the air. Specifically, the
ocean has a significant impact on temperature and
climate on Earth. In particular, areas close to the ocean
experience mild changes in temperature, while places
far from the ocean often see temperature extremes.
Think about the climate of Seattle compared to that
of Minneapolis. While these two places are similar in
latitude, Seattle’s climate is far milder in winter and
summer compared to Minneapolis’ climate. The unique
thermal properties of water allow the ocean to act as a
temperature buffer and help to moderate climate.
Seawater
Water is a powerful solvent and easily dissolves salts.
In fact, water can dissolve more things than any other
substance, and thus it is referred to as the universal
solvent. Salts are composed of particles with opposite
FIGURE 1−25
Table salt, sodium chloride.25
electrical charges. The electrically charged particles
can be either single atoms or groups of atoms called
ions. Table salt (NaCl) consists of a positively charged
sodium ion (Na+) and a negatively charged chloride
ion (Cl–). These oppositely charged ions are attracted
to each other and are joined in ionic bonds. If no
water is present, these ions bond together to form salt
crystals. When water is present, the ions pull apart, or
dissociate, and the salt dissolves.
Seawater derives its characteristics from the physical
and chemical properties of water and the materials
dissolved in it. Some salts and minerals in seawater
come from runoff from land masses, and others are
released by hydrothermal vents from the interior of
the Earth. Seawater is 96.5 percent pure water and
3.5 percent dissolved solids and gases. Most of the
dissolved materials, or solutes, in seawater are made up
of only six ions: chloride, sodium, sulfate, magnesium,
calcium, and potassium. In fact, sodium and chloride
make up about 85 percent of the dissolved materials,
which is why seawater tastes salty. More than eighty
other chemical elements have been identified in
seawater, some of them in minuscule amounts, but they
are still often crucial for life.
Salinity is the total quantity of dissolved inorganic
solids in water. Salinity is usually expressed as the
number of grams of salt left behind when 1,000 grams
of water are evaporated. If 35 grams of salt are left
behind after evaporation, then the water had a salinity
of 35 parts per thousand. In modern marine biology
and oceanography, salinity is determined by using
electronic instruments rather than by evaporating a
water sample. The ions in seawater are good electrical
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For water to change states, heat or energy must be
added or removed. For example, if you add heat to
ice cubes, this will cause the ice to melt, whereas
removing heat from the water will cause it to form
ice. The temperature at which melting occurs is called
the melting point. The temperature at which freezing
occurs is called the freezing point. The melting point
and freezing point for pure water occur at 0°C. The
amount of heat required to melt a substance is called
its latent heat of melting. Water has a higher latent heat
of melting than any other common substance. As heat
is added, ice begins to melt, and heat is absorbed, but
the temperature does not change until all of the ice has
melted. This is why ice works so well to keep a drink
cold. The added heat goes into melting the ice, not
raising the temperature.
more readily dissolve in cold water compared to warm
water, and thus dissolved gas concentrations are higher
in polar water than in tropical water.
Dissolved solids in seawater give water many special
properties. Specifically, heat capacity decreases with
increasing salinity. Moreover, as salinity increases,
the freezing point of water decreases. The salts act
as an “antifreeze,” lowering the freezing point. The
same principle applies when salt is applied to keep
roads from getting icy in the winter. Additionally, as
salinity increases, evaporation slows. Water is removed
from the ocean through evaporation or by freezing.
When seawater evaporates or freezes, the solutes
are left behind, and the salinity increases. Icebergs
formed from seawater are not salted because the salts
were excluded in the freezing process. Freshwater is
added to the ocean through precipitation—both rain
and snow—and through runoff as well as through the
melting of glaciers and ice sheets.
Approximately half of the dissolved gas in seawater is
nitrogen. The upper layers of the ocean are saturated
with nitrogen, and nitrogen is required by living
organisms to build proteins and to make important
biochemicals. Interestingly, organisms cannot use
nitrogen directly from the water. First, nitrogen must
be “fixed” or bound to oxygen or hydrogen before it
can be biologically used by other organisms. Nitrogen
fixing is often done by bacteria or archaea.
Ocean surface salinity varies depending on the latitude.
At higher latitudes, like in the Antarctic, salinity is
lower because of lower solar radiation and limited
evaporation and because of abundant precipitation and
runoff from melting ice sheets. At latitudes near the
Tropics of Cancer and Capricorn, evaporation rates are
high, with little precipitation, and thus salinity increases.
At the equator, evaporation rates are high because of
high temperatures, but precipitation is also high. Thus,
water is added and removed at similar rates in the
tropics, and so salinity is maintained.
Salinity and thus density also vary based on depth in
the ocean because temperature and density create a
stratified, or layered, ocean. The saltier the water, the
denser the water. Specifically, cold, salty water is denser
than warmer, less salty water. As such, denser saltwater
sinks. Density increases with increasing salinity,
increasing pressure, and decreasing temperature.
This layering affects the movement of water and the
distribution of marine organisms.
Dissolved Gases in Seawater
In addition to solutes, gases are also dissolved in
seawater. Plants and animals in the ocean require
dissolved gases to survive. Nitrogen, oxygen,
and carbon dioxide are important gases dissolved
in seawater, and their solubility in water differs
compared to their solubility in air. Solubility refers to
a substance’s ability to be dissolved. In general, gases
Another essential dissolved gas in seawater is oxygen,
which is dissolved at approximately six parts per
million. Oxygen is a byproduct of photosynthesis,
and therefore dissolved oxygen is abundant near the
surface as there is plenty of light for photosynthesis
by marine plant-like organisms. The concentration
of oxygen decreases below the photic zone because
photosynthesis is absent. However, below the photic
zone animals and bacteria are still utilizing cellular
respiration, which consumes oxygen.
The ocean acts as a carbon reservoir, with carbon
dioxide being highly soluble in water because it reacts
chemically when it dissolves. When carbon dioxide
combines chemically with water, it forms carbonic
acid. (This chemical reaction and the resulting ocean
acidification will be discussed later in this guide in the
section covering human impacts.) At the surface of
the ocean, carbon dioxide is quickly used for marine
photosynthesis, but below the sunlit layer, carbon
dioxide builds up because of cellular respiration. The
amount of carbon dioxide increases with depth because
the solubility of carbon dioxide increases as pressure
increases and as temperature decreases.
INTERACTIONS OF THE OCEAN
AND THE ATMOSPHERE
The atmosphere above the earth is a thin layer of
gases, water vapor, and airborne particles. Water and
gases are freely exchanged between the atmosphere
and the ocean. The gases that enter the atmosphere
from the ocean have effects on climate, with important
implications for global warming (which will be
discussed in greater detail later in this guide). Thus,
the atmosphere and the ocean are interconnected and
act together as part of a system.
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conductors because of their electrical charges. Thus,
marine biologists can use the conductivity of seawater
to infer its salinity.
Layers of the atmosphere.26
Atmospheric Circulation
The atmosphere is composed of a series of layers. The
most turbulent layer and the layer closest to Earth is the
troposphere, which contains most of the atmosphere’s
mass. The troposphere is where the weather is produced
and occurs. The next layer up is the stratosphere, which
is calmer and less turbulent than the troposphere.
Airline pilots and passengers generally appreciate the
calm stratosphere for air travel. Above the stratosphere
is the mesosphere, which is cold and thin. Beyond
this is the thermosphere, a layer where temperature
increases with altitude due to the sun’s energy. The
lower atmosphere is a fairly homogeneous mixture of
gases made up of nitrogen, oxygen, and water vapor.
Water vapor makes up about 4 percent of the volume of
the atmosphere. The density of the air is influenced by
temperature and water content.
The circulation of the atmosphere is powered by
sunlight. About half of the incoming energy from
the sun is absorbed by land and water. This energy
is converted into heat and then is transferred into the
atmosphere. The Earth has a heat “budget,” which is
the balance of heat input and heat outflow. A stable
heat budget should be in thermal equilibrium, where
incoming heat equals the outflow of heat.
While the circulation of the atmosphere is powered by
solar heating, the amount of solar energy received on
Earth varies with latitude. Lower latitudes, like those
near the equator, receive a large amount of solar energy.
Higher latitudes, like those near the poles, receive less
solar energy, and much of the energy received at the
poles is reflected by ice. While the tropics receive far
more input of heat than the poles, heat is transferred
from the equator to polar regions by winds and ocean
currents.
The solar heating of the Earth also varies with the
season. In fact, seasons are caused by variations in the
amount of incoming solar energy. The Earth is tilted
23.5 degrees on its axis. Thus, the northern hemisphere
leans toward the sun in June and receives more heat,
while in December, the northern hemisphere leans
away from the sun and receives less heat with shorter
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FIGURE 1−26
drawing scientific conclusions.
FIGURE 1−27
Oceanic Circulation
The direction of air circulation in the northern and southern
hemispheres.27
days. Seasonal changes in the angle of the sun and the
length of daylight have a large influence on the climate
of the Earth.
The uneven heating of the Earth is the ultimate driver
of atmospheric circulation. Atmospheric circulation is
specifically driven by convection currents, which result
from differential heating. Warm air from the equator
rises and begins to move toward the poles, but because
the temperature nearer to the poles is much lower, the
air cools, and its density increases. However, because
the earth is spinning on its axis, air currents do not
travel in a direct path toward the poles. The spinning
of the Earth causes bodies of air to be deflected or
turned. This phenomenon is called the Coriolis effect.
Air turns to the right in the northern hemisphere and to
the left in the southern hemisphere. The Coriolis effect,
in combination with the uneven heating of the Earth,
drives atmospheric circulation.
The circulation of the atmosphere drives the circulation
of the ocean, and because of the unusual thermal
properties of water, the ocean significantly influences
global weather and climate patterns on Earth. Weather
is the condition of the atmosphere at a given place
and time, whereas climate is the long-term average of
weather. It is important to understand and distinguish
the differences between weather and climate when
Surface currents form gyres, or currents that flow
continuously around the periphery of each ocean
basin. The five gyres of the world are the North Pacific
Gyre, the South Pacific Gyre, the North Atlantic Gyre,
the South Atlantic Gyre, and the Indian Ocean Gyre.
Generally, each gyre is composed of four currents that
flow progressively into one another. The four main
currents that compose gyres are equatorial currents,
western boundary currents, northern or southern
boundary currents, and eastern boundary currents.
The centers of ocean gyres are calm because there is
minimal wind and thus little current flow. As a result,
there is very little life in the middle of ocean basins,
though floating material such as seaweed and debris
tends to collect in the center.
Oceanic currents are critical because they distribute
tropical heat worldwide by transporting warm water to
high latitudes. As the water moves to higher latitudes,
heat is transferred to the air, which cools and then
moves back to low latitudes, where it absorbs heat
again. Thus, ocean surface currents directly influence
the climate of the nearby land masses.
In addition to horizontal water movement, there is
also vertical circulation of the ocean. Wind-driven
horizontal movement of water can cause the vertical
movement of deep water to replace surface water
that has been pushed away by wind. Upwelling is the
upward movement of water that brings deep, cold,
nutrient-rich water toward the surface. Oppositely,
downwelling is the downward movement of water that
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Ocean currents move large masses of water from one
place to another. Ocean currents are either driven by
wind from atmospheric circulation or by differences
in density. Surface currents are ocean water that
flows horizontally in the uppermost 400 meters of the
surface of the ocean. Surface currents are primarily
driven by wind friction, where the wind tugs on the
surface of the ocean, causing a mass flow of water. As
such, water piles up in the direction the wind blows,
and water pressure is higher on the piled-up side of
the water. Gravity then pulls the water down the slope
from the piled-up side, and the Coriolis effect causes
the water currents to flow to the right of the wind
direction in the northern hemisphere and to the left of
the wind direction in the southern hemisphere.
A map of global ocean gyres.28
supplies the deeper ocean with dissolved gases such
as oxygen. For instance, when surface water sinks, it
displaces the water below it and mixes with deeper
water and then sinks to a depth determined by the
density of the water. This form of water circulation is
called thermohaline circulation.
This movement of surface water to deeper water and
vice versa is known as the great ocean conveyor, which
is a large-scale, horizontal, and vertical movement of
ocean water. Thermohaline circulation occurs on a
global scale, straddling the hemispheres, with tropical
water being transported to the poles. Thermohaline
circulation distributes dissolved gases and solids,
mixes nutrients, and transports larvae. Without
the great ocean conveyor, driven by thermohaline
circulation, life on our planet would cease to exist.
WAVES
Waves are the undulations of the surface of the water
that result from energy transfer. Waves are important
in terms of recreational activities such as swimming,
sailing, and surfing, and waves are also important in
building and maintaining beaches. Waves are also
responsible for the erosion of coastlines and beaches.
Large waves called tsunamis result from a large
displacement of water and can cause major damage.
Wave Properties
Waves are disturbances caused by the movement
of energy from a source through a medium. In the
ocean, the surface of the water oscillates up and
down, forming waves. The highest point of elevation
of a wave above the average water level is called a
wave crest. The lowest part of the wave, or the valley
between wave crests below the average water level, is
called a wave trough. The horizontal distance between
two adjacent crests or troughs is called the wavelength.
The wave height is the vertical distance between the
wave crest and the adjacent trough. The wave period
is the time it takes a wave to move a distance of one
wavelength, and wave frequency is the number of wave
crests or troughs that pass a fixed point per second.
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FIGURE 1−28
FIGURE 1−29
Waves can be classified by the disturbing force, such as
wind, that creates them; by the restoring force, such as
gravity, that tries to flatten them; or by their wavelength.
Disturbing force waves are created by a force that
disturbs the medium. For example, wind, storm surge,
sudden changes in atmospheric pressure, landslides,
earthquakes, and volcanic eruptions are all disturbing
forces that can generate waves. Disturbing force waves
can either be free waves or forced waves. Free waves are
waves that are formed and propagate across the surface
of the ocean without further inclusion from the force.
Tsunami waves are free waves and will be discussed
later in this section. Forced waves are waves that are
maintained by the disturbing force. Tides, discussed
in the next section, are forced waves that come from
the gravitational forces from the Earth, moon, and sun,
combined with the Earth’s rotation.
Restoring force waves are classified by the dominant
force that returns the water surface to flatness. Often
the restoring force, such as gravity, overcompensates
and causes oscillation to occur. Specifically, gravity
waves pull the crests downward, but inertia causes the
crests to overshoot and become troughs.
Waves can also be classified by their wavelength. The
wavelength of a wave can indicate a type of wave
based on a typical length. For example, tides are waves
that are caused by the gravitational attraction and
rotation of the Earth, are restored by gravity, and have
a wavelength that is half of the Earth’s circumference.
Wave Generation
Waves form as soon as the wind begins to blow. Wave
size depends on how fast and long the wind blows, as
well as the fetch, or the span of open water over which
the wind blows. Storm winds can generate seas, or
waves with peaked crests and flat troughs. When the
waves move out of the storm area, they start to settle
into swells, with smoothly rounded crests and troughs.
As the waves approach a shoreline and move into
shallower water, the waves get higher and shorter and
closer together. Ultimately, the waves become unstable
and they break, creating surf. The energy from the wave
is now expended on the beach as the wave breaks.
Tsunamis
Tsunamis, also known as “harbor waves,” are waves
that are produced by water displacement resulting from
earthquakes, landslides, volcanoes, asteroid impacts, or
icebergs falling from glaciers. Each of these phenomena
can cause a large displacement of water, launching the
water forward and turning it into tsunami waves. Most
tsunami waves occur in the Pacific Basin near the Ring
of Fire, a geologically active area where volcanoes
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Global ocean thermohaline circulation.29
FIGURE 1−30
and earthquakes are common. Tsunami waves have
longer wavelengths and are faster than ordinary waves.
Tsunami waves can move faster than an airplane.
be navigated during a low tide. Tides can be used as an
energy source, and tides are important for many marine
species that live in the intertidal zone.
Out in the open ocean, tsunami waves are small in
height, but the wave height increases as they get closer
to shore. When a tsunami wave approaches the shore,
a vacuum effect is created, and coastal water is sucked
out, exposing the benthic habitat. This retreating water
is an important warning sign of the tsunami that will
soon reach the shoreline as a literal wall of water, as
tsunamis sometimes reach heights greater than thirty
meters, causing great destruction as they crash ashore.
The best defense against a tsunami is an early warning
system to allow people in coastal areas enough time to
get to higher ground.
Causes of Tides
TIDES
Water in coastal areas moves in and out as a result of
the gravitational pull of celestial bodies on each other.
Tides are the periodic, short-term change in the height
of the surface of the ocean at a particular place. Tides
are caused by the gravitational pull of the moon and the
sun on the Earth. Tides are the longest waves on Earth,
with a wavelength that is half of the circumference of
the Earth. Tides are important to note because many
shipping harbors or channels are not deep enough to
Gravity in our solar system tends to pull the Earth and
moon toward each other, but centrifugal inertia keeps
them apart, and the moon is thus in a stable orbit around
the Earth. The moon’s gravity also attracts and pulls
the ocean toward the moon because it is a fluid body.
Additionally, because of the motion from the rotation
of the Earth around the center of mass of the Earthmoon system, another bulge of water is created on the
side of Earth opposite the moon. Thus, combined there
are two bulges. These two bulges form the lunar tides.
Lunar tides are tides caused by the gravitational pull
and inertial interaction of the moon and the Earth. A
complete rotation occurs in twenty-four hours and fifty
minutes. In a twenty-four-hour solar day, the moon
moves eastward about 12.2 degrees. Thus, the moon
must rotate another 12.2 degrees to make a complete
orbit around the Earth, taking another fifty minutes.
Similar to the moon, the sun’s gravity also attracts the
Earth’s mass. Because the sun is 27 million times more
massive than the moon, and 387 times as far from the
Earth as the moon, the sun’s influence on tides is about
half that of the moon. Solar tides are caused by the
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Diagram of wave anatomy.30
some locations, high tides and low tides each occur
twice a day. In others, each occur once a day. Tidal
patterns are heavily influenced by basin shape.
FIGURE 1−31
Tidal range, the high-water to low-water height
difference, is small in lakes, moderate in enclosed seas,
and largest at the edges of ocean basins, especially
in inlets or bays. The Bay of Fundy on Canada’s east
coast experiences some of the most remarkable tidal
ranges on Earth. Here, the tidal range can be over
eleven meters. The smallest oceanic tidal ranges on
Earth are seen in the Mediterranean and Caribbean
Seas, where tidal ranges are less than a meter.
Lunar and solar tides.31
gravitational pull and inertial interaction of the sun
and the Earth. Thus, when solar tides and lunar tides
are combined, we see the cumulative tidal patterns
exhibited on Earth.
Tidal Patterns
High tides are the bulges of water pulled on each side of
the earth. These bulges are the crests of the planet-sized
waves. Low tides are the troughs of the planet-sized
waves, or the area between the bulges. The Earth is
rotating beneath the bulges, and these bulges of water
tend to stay aligned with the moon as the Earth spins.
When the Earth, the moon, and the sun are all in a
straight line, the lunar and solar tides or bulges of
water are additive and are called spring tides. Spring
tides have higher highs and lower lows, occurring at
two-week intervals corresponding to new and full
moons. When the Earth, the sun, and the moon form
a right angle, the solar tide or bulge diminishes the
lunar tide, and such tides are called neap tides. The
difference between the high and low tides of neap tides
is less than the difference between the highs and lows
of spring tides. Neap tides occur at two-week intervals
corresponding to first- and third-quarter moons.
Tidal patterns on Earth are influenced by several
physical factors, including land masses and basin shape.
Land masses can obstruct tidal crests, causing the tide
to be diverted and slowing the movement of the water.
As a result, the obstruction from land masses can cause
differences in the arrival timing of the tidal crests. In
From high tide to low tide, the shoreline periodically
is underwater and periodically exposed to the air, and
thus species that live along the shoreline must cope with
alternating periods of being under water and exposed
to the air. Despite this being a seemingly tough habitat,
many marine organisms live in the intertidal zone, or
the zone where the sea level changes from the high
tide line to the low tide line. Sessile organisms, such
as barnacles, anemones, or seaweed, are cemented
in place and are submerged and emerged as the tide
comes in and out. In the intertidal zone, a pattern of
clear zonation is often seen in which organisms are
distributed based upon their tolerance for being out of
the water. Additionally, some organisms migrate with
the tides. For instance, small crustaceans and clams
such as Emertia spp. and Donax spp. migrate up and
down the beach so they stay in the surf zone as the tide
moves. Other organisms make burrows at the high tide
line or use high tides for spawning events.
Energy from Tides
Tides are constant on our planet and will occur as long
as the sun, moon, and Earth have mass and so will
have gravitational attraction to each other. Given their
constancy, the tides are a possible source of energy
that is clean and renewable. Some tidal energy can be
captured by large electricity-generating turbines that
can be placed in the ocean where tidal currents move
rapidly. Tides are only strong enough in a few places on
our planet for such purposes. In these places, turbines
and dams can be placed across bays or estuaries to
capture tidal energy to generate electricity. A few issues
do arise when attempts are made to generate electricity
from tidal energy. First, tides do not generate electricity
during the period of slack water when the tidal currents
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Tidal Patterns and Marine Organisms
SECTION I SUMMARY
6 Marine biology is the scientific study of
biological processes and organisms in an ocean
setting.
6 Water covers 71 percent of the surface of the
Earth, making our planet a water planet.
6 Oceanography is a field of study closely related
to marine biology and is the scientific study of
the oceans. Oceanography integrates biology,
chemistry, engineering, geology, and physics.
6 The early exploration of the oceans was
achieved through scientific voyaging and
discovery.
6 Today, we study marine biology using the
scientific method, with testable hypotheses, in
combination with marine laboratories, vessels
dedicated to ocean research, submersibles,
underwater marine laboratories, ocean
observing systems, and cutting-edge technology.
6 Geological processes have shaped and sculpted
the continents and ocean basins. There are four
large ocean basins on Earth.
6 The Earth formed about 4.5 billion years ago.
The oceans formed about 3.8 billion years ago
from volcanic outgassing and water delivery
from comets.
6 The ocean habitat is composed of continental
crust submerged at the edges of continents, and
the edge of the continental crust joins the deepsea crust at the edge of the deep sea.
6 Oceanic divisions of the marine environment
are based on distance from land, water depth,
and whether organisms are benthic or pelagic.
6 Water is essential for life and is the primary
component of all living organisms. The unique
properties of water make life on our planet
possible. Seawater is a mixture of water, salts,
dissolved gases, and other particulates.
6 The gases that enter the atmosphere from the
ocean have effects on climate, with important
implications for global warming. The
atmosphere and the ocean are interconnected
and act together as a system.
6 Oceans circulate through both surface and
deep-water currents, moving water around our
planet on a global scale.
6 Waves are important in terms of recreational
activities such as swimming, sailing, and
surfing and are also important in building
and maintaining beaches. Waves are also
responsible for the erosion of coastlines and
beaches and can even cause major damage
when they are tsunami waves.
6 Tides are the periodic, short-term changes in
the height of the surface of the ocean
at a particular place. Tides are caused by
the gravitational pull of the moon and the
sun on the Earth. Tides influence organism
distribution in the ocean.
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are low. Second, placing a tidal dam across an estuary or
bay can significantly alter the ecology of that ecosystem.
Often places that are suitable for capturing tidal energy
are also areas of biodiversity.
Section II
Marine Life
Life on Earth evolved around 3.5 billion years ago. All
life on Earth is related and shares a common ancestor.
Life on earth evolves or changes over time in response
to the physical and chemical environment. Charles
Darwin and Alfred Russel Wallace are credited with
first proposing the idea of evolution and the processes
by which organisms adapt to their environment.
Evolution is fueled by genetic mutations, which are
heritable changes (meaning they are passed on to
offspring) in the genes of organisms. Most of these
genetic mutations are unfavorable, but some of them
randomly result in a variant that is better suited for
survival and reproduction. Thus, evolution occurs in
response to the physical and chemical environment.
Over many generations, populations of organisms
respond to environmental pressures by accumulating
changes. These changes could be in shape, size, color,
biochemistry, behavior, or any other aspect of an
organism.
THE ORIGINS OF LIFE
The Earth formed approximately 4.5 billion years ago.
By at least 3.5 billion years ago, life emerged on our
planet. Scientists think that protocells developed into
single-celled bacteria-like organisms. Fossil rocks
dating back billions of years help provide evidence for
the timing of the emergence of life. However, where
life originated on Earth is still subject to debate, though
many scientists think deep-sea hydrothermal vents may
be where life on Earth originated. If life started here,
the early single-celled organisms likely obtained energy
by absorbing hydrogen sulfide molecules from the
hydrothermal vents.
DEFINING MARINE LIFE
Though generally people know the difference between
living and nonliving entities, defining life can be
challenging. Living organisms have the ability to
capture, store, and transmit energy. Living things
also have the ability to reproduce. Living things in
the marine environment are called marine organisms.
Marine organisms range from microscopic organisms,
like bacteria and unicellular algae, to the largest
animal on Earth, the blue whale.
All life on Earth is composed of approximately twentythree of the 107 known chemical elements. Moreover,
99 percent of the mass of living organisms is made up of
the following four chemical elements: carbon, hydrogen,
oxygen, and nitrogen. These chemical elements can
combine to create four types of macromolecules:
carbohydrates, lipids, proteins, and nucleic acids.
THE BUILDING BLOCKS OF
LIFE
The basic unit of life is a cell. Besides water, most of the
chemicals in a cell are organic compounds that contain
carbon, hydrogen, and oxygen. Carbon, hydrogen, and
oxygen are high-energy molecules. An organism’s
ability to use energy comes from its ability to break
down these compounds and use the energy that is
released.
Most organic compounds can be grouped into the four
main categories of macromolecules: carbohydrates,
lipids, proteins, and nucleic acids. Carbohydrates
are mostly made of carbon, hydrogen, and oxygen
and include simple sugars as well as more complex
molecules. One of the simplest carbohydrates, glucose,
is an important energy source for many living things.
Complex carbohydrates are formed by the combination
of simple sugars into chains. Starches and other complex
carbohydrates consist of much longer chains, which
may contain other components besides simple sugars.
Carbohydrates play a vital role in the basic functions of
metabolic processes in organisms and serve as energy
stores. Additionally, some carbohydrates provide
structure and protection for marine organisms.
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INTRODUCTION
Glucose.32
Proteins are the second major group of organic
macromolecules. Proteins are composed of chains of
smaller subunits called amino acids. Like carbohydrates,
amino acids are made up of carbon, hydrogen, and
oxygen, but amino acids also contain a substantial
amount of nitrogen. Proteins have a wide variety of
functions and purposes in organisms. For example,
muscles are made primarily of protein. Enzymes that
are used to speed up, or catalyze, reactions are also
proteins. Furthermore, some hormones, which serve
as chemical messengers to send signals to different
parts of the body of an organism, are proteins. A
protein’s function is determined by the shape it takes
as long amino acid chains fold into compact, functional
proteins.
Lipids, or fats, oils, and waxes, are the third major
group of organic molecules. Lipids are used for
energy storage and to repel water. Many marine birds
and mammals have a coating of oil that helps keep
their feathers and fur dry. Additionally, some marine
organisms that are exposed out of the water at low tide
have a coating of wax to help keep them from drying
out. Lipids also provide insulation from the cold, often
as a layer of blubber, and aid in buoyancy in many
marine mammals.
Like DNA, RNA is a nucleic acid chain that consists
of four types of nitrogenous bases. However, instead
of thymine, RNA has uracil in addition to adenine,
cytosine, and guanine. RNA molecules most commonly
act as intermediaries in converting information from
DNA to proteins. In some microbes and viruses,
RNA itself stores the genetic information. Other RNA
molecules help to regulate gene expression, regulate
embryonic development, and can also trigger immune
responses and help defend against viral infections.
PHOTOSYNTHESIS AND
CHEMOSYNTHESIS
The Fuel of Life
The organic molecules and compounds that make up
living things utilize energy and also generate energy.
Living organisms have to capture, store, and transmit
energy to live. Energy can be made by photosynthesis
or by chemosynthesis. Photosynthesis is the process
by which organisms convert sunlight energy into food
energy. Organisms that are capable of conducting
photosynthesis and producing their own food are called
autotrophs or “self-feeders.” In the ocean, seaweeds,
unicellular algae, and bacteria are the most common
and important autotrophs, and these organisms are
also called primary producers. Primary producers
synthesize organic molecules from inorganic
substances through photosynthesis or chemosynthesis.
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Nucleic acids are the fourth major group of organic
molecules. Nucleic acids are important for storing
and transmitting the genetic information of life.
Nucleic acids are made up of smaller subunits
called nucleotides. Nucleotides consist of a simple
sugar joined to a phosphate group and to molecules
called nitrogenous bases. Nucleic acid chains can be
thousands of nucleotides long, but these chains are
only made up of four types of nitrogenous bases:
adenine, cytosine, thymine, and guanine. The sequence
of the different nucleotides determines the order of
amino acids to make specific proteins. Nucleic acids
can be either DNA (deoxyribonucleic acid) or RNA
(ribonucleic acid). DNA is used in most organisms
to supply the instructions for the construction and
maintenance of the body. DNA is passed from one
generation to the next through gametes or sex cells
(sperm and egg). The entirety of an organism’s genetic
information is called its genome.
FIGURE 2−1
FIGURE 2−2
FIGURE 2−3
light
oxygen
carbon
dioxide
carbohydrates
water
Plants, algae, cyanobacteria, phytoplankton, and
some other organisms conduct photosynthesis by
using light energy from the sun to synthesize the
carbohydrate glucose from carbon dioxide and water.
First, photosynthetic organisms absorb sunlight,
or solar energy, with photosynthetic pigments. The
most common photosynthetic pigment on Earth
is chlorophyll, which often gives photosynthetic
organisms a green color. After solar energy is captured
by chlorophyll and other pigments, it is converted
into chemical energy in the form of ATP (adenosine
triphosphate). ATP is a common high-energy molecule
that combines with carbon dioxide (CO2) and water
(H2O) to make glucose. Glucose is then used as an
energy source to synthesize other organic compounds.
A byproduct of photosynthesis is oxygen gas (O2).
The majority of the oxygen on Earth is produced from
photosynthetic organisms.
Chemosynthesis is the production of food from energy
released by inorganic molecules in the environment.
Chemosynthesis uses an inorganic molecule, such
as hydrogen sulfide or methane, to produce glucose.
Bacteria are common chemosynthesizers. For example,
bacteria at hydrothermal vents use hydrogen sulfide in
combination with carbon dioxide and water to make
glucose.
Many organisms are not capable of producing their
The cycle of cellular respiration and photosynthesis.34
own food; thus, they must obtain energy from organic
matter produced by autotrophs. These organisms are
called heterotrophs. Both heterotrophs and autotrophs
perform cellular respiration to utilize the energy
originally obtained by photosynthesis. In the process of
cellular respiration, sugars can be broken down using
oxygen—a process called aerobic respiration— while
carbon dioxide and water are byproducts.
When the glucose is broken down, and oxygen is
consumed, energy is released. Energy is stored in the
form of ATP. Some organisms use a different form of
respiration, called anaerobic respiration, which does
not use oxygen. Anaerobic respiration is not as efficient
as aerobic respiration. Many organisms can switch to
anaerobic respiration when oxygen has been depleted
from their muscles and blood. Some organisms, like
bacteria, live in oxygen-depleted environments, like
the stomachs of fish or oxygen-poor sediments, and
exclusively use anaerobic respiration.
BIOGEOCHEMICAL CYCLES
There are three major biogeochemical cycles on Earth.
Biogeochemical cycles move elements such as carbon,
nitrogen, and phosphorus between living organisms
and nonliving matter. These materials can be used over
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The process of photosynthesis.33
The carbon cycle.35
and over in a repeating cycle. Each of these materials—
carbon, nitrogen, and phosphorus—comes from the
atmosphere, the interior of the Earth, or the weathering
of rocks. They are then converted into other elemental
forms and incorporated into the bodies of organisms.
Digestion, respiration, and decomposition break the
materials down again, and they are released into the
environment for re-uptake by other organisms.
The Carbon Cycle
The element carbon is constantly cycling between living
organisms and the atmosphere. In the carbon cycle,
carbon exists in its inorganic state in the atmosphere
as carbon dioxide. Carbon dioxide is highly soluble
in water and is readily dissolved in the ocean. In the
ocean, inorganic carbon is converted into organic
compounds through the process of photosynthesis.
Cellular respiration by producers, consumers, and
decomposers breaks down organic compounds, making
them carbon dioxide again. This carbon dioxide is once
again available to primary producers. Unfortunately,
humans are altering the natural carbon cycle by adding
carbon dioxide to the atmosphere. The clearing of
forests and the consumption of fossil fuels contributes
to increased carbon dioxide in the atmosphere, causing
global warming and ocean acidification, which will be
discussed in more detail later in this guide.
The Nitrogen Cycle
The nitrogen cycle uses free nitrogen that is present
in the atmosphere. However, most organisms cannot
use free nitrogen (N2). In order for algae, plants, and
animals to get nitrogen, the nitrogen must be “fixed.”
Cyanobacteria, other bacteria, and archaea are capable
of performing nitrogen fixation whereby nitrogen
is converted into a usable, biologically active form.
Nitrogen is fixed with hydrogen or oxygen in the form of
ammonia (NH4+), nitrate (NO3–), or nitrite (NO2–). Then
nitrogen is recycled as animals consume and excrete
ammonium and urea. As with the carbon cycle, human
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FIGURE 2−4
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FIGURE 2−5
The marine nitrogen cycle.36
activities are interrupting the nitrogen cycle. Through
the use of agricultural fertilizers, human and animal
waste, and the cultivation of nitrogen-fixing crops,
human beings are producing and releasing into the
environment more fixed nitrogen than occurs naturally.
This results in eutrophication, or the excessive richness
of nutrients, in nearshore habitats. Eutrophication will
be discussed further in Section IV of this guide.
The Phosphorus Cycle
In the phosphorus cycle, the majority of phosphorus
enters the ocean from rivers in the form of phosphate
(PO4 –3), though a small amount comes from the
atmosphere. In the ocean, primary producers
incorporate phosphate into organic matter, which
is then passed up the food chain or contributes to
detritus or dissolved organic matter. Phosphorus is
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The phosphorus cycle on Earth.37
also deposited in ocean sediments from biogenous
sediments (sediments from once-living organisms) or
through chemical precipitation. Like the nitrogen cycle,
the phosphorus cycle is also interrupted by human
activities, including the use of fertilizers and other
substances that cause eutrophication.
CLASSIFYING MARINE LIFE
Humans have long classified life on Earth into
various groupings. We group similar organisms
together to facilitate the study and discussion of
these organisms. The naming and grouping of related
organisms is called taxonomy, which is the study
of biological classification. The eighteenth-century
Swedish naturalist Carolus Linnaeus (also known
as Carl Linnaeus) developed a system of classifying
organisms, which is still widely used today, and thus
Linnaeus is known as the father of taxonomy.
Organisms are identified by two names that make
up the binomial species name. The two-name
system introduced by Linnaeus is called binomial
nomenclature. Scientific names are most often Latin
or Greek, and they are used by biologists because
common names are not very precise. The same
common name may be applied to several different
species or used differently in different places across
the world. For example, the common name “dolphin”
could be used to describe the charismatic relative of
whales or a game fish, also known as mahi-mahi.
Scientific names are used worldwide and span different
languages. Each organism has a genus name and a
species epithet, or species name, that together make up
its scientific name. A genus is a group of closely related
species. For example, the largest animal on Earth,
the blue whale, has the scientific name Balaenoptera
musculus. Other whales, such as minke whales and fin
whales, are also in the genus Balaenoptera, but each
has a unique species name.
The Tree of Life
Organisms on Earth are currently classified into three
major domains: Bacteria, Archaea, and Eukarya.
Bacteria and Archaea are prokaryotes, or single-celled
organisms without nuclei or organelles. Eukaryotes
can be either unicellular or multicellular, but their cells
have nuclei and organelles. Fungi, protists, animals,
and plants are all eukaryotic organisms. While bacteria
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FIGURE 2−6
FIGURE 2−7
FIGURE 2−8
and archaea may superficially look similar, they are as
genetically different from each other as they are from
all other eukaryotic organisms.
THE MICROBIAL WORLD
Microorganisms are mostly invisible to the naked eye,
and yet they are the most abundant forms of marine
life. Microorganisms can be found in every marine
habitat, from shallow intertidal zones to the deepest
parts of the ocean. All three of the domains of life
include microorganisms. Life on Earth evolved from
microorganisms, and microorganisms continue to play
critical roles in marine ecosystems. Microorganisms
are important primary producers, meaning they are
organisms that make organic matter from carbon
dioxide by photosynthesis or chemosynthesis.
Viruses
Viruses are not included in the tree of life because
biologists are not in agreement that viruses should
be considered living organisms, nor do they agree on
where to place viruses on the tree of life. Viruses span
the living and the nonliving. The basic structural unit
of viruses is called a virion, not a cell. Containing
only a short chain of nucleic acid, which can be DNA
or RNA, viruses have only a few genes, which are
protected by an outer protein coat. Viruses can only
reproduce and develop when they have a living host
cell to infect. Viruses are extremely small, and most
can only be seen with powerful microscopes. Although
Unicellular bacteria from a microbial mat in Guerrero
Negro, Baja California, Mexico.39
viruses are best known for causing disease, they play
a major role in marine food webs through the cycling
of nutrients. Viruses infect and destroy bacteria and
unicellular algae in the ocean, thereby releasing
organic molecules, nutrients, and cellular debris that
can be taken up by other organisms.
Bacteria
Bacteria are prokaryotic organisms that are structurally
simple and are incredibly abundant in marine
ecosystems. Bacterial cells can be many shapes,
including spheres, spirals, rods, and rings. The cell
walls of bacteria are rigid and strong because of their
unique chemistry. A bacterium’s cell wall is made
of peptidoglycan, which contains muramic acid and
chains of amino sugars. This gives bacterial cell walls a
slippery or slimy covering, which adds an extra layer of
protection. Bacteria play many different roles in marine
ecosystems, including that of being a photosynthetic
primary producer. Bacteria are found everywhere in the
ocean, on almost all surfaces, in the sediment, within
other organisms, and in the water column.
Decay bacteria are common in detritus and break
down waste products as well as dead organic matter
and release nutrients back into the environment. These
bacteria are essential for life because they recycle
essential nutrients in marine food webs making up
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The domains of life.38
the microbial loop. In addition, these bacteria serve as
a food source for many bottom-dwelling organisms.
Other marine bacteria are involved in degrading oil
and other toxic pollutants, such as plastics, that find
their way into the ocean. Some bacteria can cause
diseases in marine animals and even humans.
FIGURE 2−9
Archaea
Unicellular Algae
Algae are an incredibly diverse group of simple marine
and freshwater photosynthetic organisms that range
in size from microscopic to giant seaweeds. Algae
are eukaryotic organisms, and thus their cells contain
a nucleus and other membrane-bound organelles.
Unicellular algae often behave more like animals
than plants. For example, some unicellular algae
swim by using their flagella, and while algae conduct
photosynthesis, others also consume food particles like
animals do. These unicellular algae are called protists.
Diatoms are a diverse group of unicellular algae that
often aggregate together into chains or stars. The cell
of a diatom is enclosed by a cell wall made largely
of silica (SiO2), a glass-like material. This glass-like
shell is called a frustule and is made up of two tightly
fitting valves. The frustule can be quite ornate, with
many perforations and spines or ribs that allow light
to pass through, so the chloroplasts can harvest light
energy for photosynthesis. Diatoms are incredibly
efficient photosynthesizers. In fact, more than half the
energy from the sun that is harvested by diatoms can
be converted into glucose. Diatoms also store excess
Marine diatoms from McMurdo Sound, Antarctica.40
energy as fatty acids and oils, which helps with flotation
and also makes diatoms a great potential source for
alternative biofuels. Additionally, diatoms are a major
contributor to the oxygen produced on Earth.
Dinoflagellates are another common type of
photosynthetic unicellular algae found in marine
ecosystems. Mostly planktonic, dinoflagellates
possess two flagella, one wrapped around the midline
of the cell and one trailing the cell like a tail. These
flagella direct the movement of the cell, with the
tail driving the organism forward, and the midline
flagella allowing the cell to rotate in the water. Most
dinoflagellates have a cell wall made of cellulose
with armored plates. While dinoflagellates carry out
photosynthesis, they are also capable of ingesting food
particles. Dinoflagellates are important planktonic
primary producers in warm waters.
FIGURE 2−10
Dinoflagellates.41
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Like bacteria, archaea are also relatively simple,
unicellular organisms. Archaea cells are small and can
be spherical, spiral, or rod-shaped. Although archaea
were originally thought to be bacteria, there is genetic
evidence that demonstrates they are more closely
related to eukaryotes than to bacteria. Archaea were
first discovered in extreme environments, like hot
sulfur springs, saline (salty) lakes, and highly acidic
or alkaline environments. In the ocean, archaea can
be found in very deep water, surviving incredible
pressure from the weight of the ocean, and they can
also be found living at incredibly high temperatures
near hydrothermal vents. More recently, genetic tools
have demonstrated that archaea are common in many
marine environments, including in the water column
and in the sediments. Some also live symbiotically in
sponges, fish, and other invertebrates.
is the only characteristic that unites protozoans as a
group. The classification of protozoans is still debated,
though they are generally considered to be protist-like
unicellular algae. Protozoans have incredible diversity
in structure, function, and lifestyle. Protozoans live
everywhere in marine environments, including inside
other organisms and even in the guts of fish. While
some protozoans contain chlorophyll and conduct
photosynthesis, they are considered heterotrophs, as
they also ingest food from external sources.
Symbiodinium, commonly called “zooxanthellae.”42
When dinoflagellate growth is unchecked and rapid, a
bloom can occur. Some dinoflagellates produce toxic
substances during a bloom, which can be poisonous to
humans and marine organisms. Other dinoflagellates
are capable of bioluminescence, or the production
and emission of light from a living organism.
Bioluminescence is a chemical reaction that occurs
when an organism contains luciferin, a molecule that
produces light when it reacts with oxygen. The light
emitted is a blue-green light that can be seen by the
human eye.
Zooxanthellae, another notable group of dinoflagellates,
live in close association with other animals. Animals
that contain zooxanthellae range from sponges and
corals to giant clams. Reef-building corals may be
the most significant hosts of zooxanthellae. This close
association is called a symbiotic relationship. The
zooxanthellae/host relationship is beneficial to both
partners. Dinoflagellates are sheltered in the animal
host while the host in turn receives benefits from the
algae. In corals, the dinoflagellate Symbiodinium fixes
carbon via photosynthesis and releases organic matter
to the coral to help in the formation and growth of coral
skeletons.
Protozoans
Protozoans are animal-like unicellular eukaryotic
organisms that are structurally simple. Protozoans
are made up of several groups of organisms that are
of unrelated origins. In actuality, having a single cell
Ciliates are also protozoans and have many hair-like
cilia that are used for feeding and getting around.
Ciliates are common on seaweeds and in sediments.
Some ciliates can be found living in the stomachs and
intestines of fishes and invertebrates while others live
in the gills of clams.
PLANKTON
Plankton are pelagic organisms that live suspended in
seawater. Plankton can either drift or swim weakly, but
their movement is primarily driven by ocean currents.
Plankton are less able to move laterally through the
ocean, but they can move vertically (up and down)
in the water column. Plankton range in size from
microscopic to almost eight meters in length. Plankton
are collected from marine environments using
plankton nets, conical nets made of mesh nylon. The
nets are pulled through the water, and the organisms
get flushed to the pointed end, called the cod end,
where they then can be collected for analysis.
Plankton are divided into two major categories:
phytoplankton and zooplankton. Phytoplankton are
autotrophic plankton that generate glucose through the
process of photosynthesis. Phytoplankton are the base
of the food web and are crucial in oxygen production
on Earth. Cyanobacteria, diatoms, and dinoflagellates
are important phytoplankton.
Zooplankton are heterotrophic plankton, or planktonic
organisms that eat primary producers. Zooplankton
are primary consumers and graze on cyanobacteria,
diatoms, and dinoflagellates. Diversity of zooplankton
is high, with representatives from nearly every major
animal group. Microflagellates, microcilliates, and
copepods are abundant holoplankton. Meroplankton
are temporary visitors to the plankton and will later
adopt a benthic or nektonic lifestyle. Crab, urchin, and
fish larvae are great examples of meroplankton.
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FIGURE 2−11
FIGURE 2−12
FIGURE 2−13
Mixed zooplankton sample.43
One of the ocean’s most notable zooplankters is krill,
an abundant, pelagic, shrimp-like crustacean. Krill
graze on diatoms in productive waters; thus, they
are commonly found where significant upwelling
currents occur. Krill are considered keystone species
in Antarctic ecosystems. Keystone species are species
that are fundamental for keeping an ecosystem intact
such that when they are removed, the ecosystem
changes dramatically. Krill are eaten by sea birds,
fishes, whales, and penguins. While krill are at the
mercy of currents for horizontal movement in the
ocean, they migrate vertically each day to feed,
sinking back down when their stomachs are full.
Gelatinous animals are also common zooplankton.
Animals such as jellies or jellyfish, salps, and
ctenophores are drifters in the pelagic zone.
SEAWEEDS AND PLANTS
Seaweeds and plants are multicellular autotrophs,
producing their own food through photosynthesis. In
coastal areas, seaweeds and plants play an important
role and provide many ecosystem services.
Seaweeds
Seaweeds are multicellular algae commonly found on
rocky shores and in other shallow, nearshore marine
environments. Sometimes seaweeds are referred to as
macrophytes or macroalgae. Large seaweeds are rare
in warm, nutrient-poor tropical waters but are common
in cold, nutrient-rich temperate waters. Seaweeds
are eukaryotic organisms that lack the specialized
structures and reproductive mechanisms found in most
land plants. Specifically, unlike most plants, seaweeds
are nonvascular, meaning they do not need conductive
vessels to transport oxygen, water, and nutrients. In
fact, seaweeds lack true leaves, stems, and roots, and
although they have visually similar anatomical parts,
these are not analogous to the structures found in
plants. The body of a seaweed is called the thallus. The
leaf-like portions of the thallus are known as blades.
Blades have a large surface area and are the main sites
of photosynthesis. Blades differ from leaves in that they
lack veins, and the upper and lower surfaces of blades
are identical. Seaweeds have gas-filled bladders called
pneumatocysts to help keep the blades close to the
surface to maximize their ability to absorb sunlight.
Seaweeds are divided into three different types—
green, brown, and red—according to the color
pigments in their tissues. In nature, distinguishing
seaweeds by color is sometimes challenging because
the proportion of their pigments can vary. Green
algae, also known as Chlorophyta, are less common in
marine habitats compared to freshwater and terrestrial
habitats. In fact, only about 10 percent of green algae
are marine. The pigments and food reserve storage
of green algae are the same as land plants. Thus, it
is thought that land plants evolved from green algae.
Green algae are typically colored bright green. Sea
lettuce (Ulva sp.) and Dead Man’s Fingers (Codium sp.)
are common types of marine green algae.
Brown algae, also known as Phaeophyta, are often
the color brown, though some can appear more olive
green due to the difference in yellow-brown pigments.
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Antarctic krill.44
FIGURE 2−14
FIGURE 2−15
Giant kelp.46
Assorted green algae.45
Almost all species of brown algae are marine. Brown
algae are common on temperate and polar rocky
coasts. Kelps are the most complex and largest of all
brown algae and are commonly found in temperate and
subpolar latitudes. Kelps provide immense ecosystem
resources, offering food and shelter to many marine
organisms. Many individual kelp often grow together,
forming incredibly rich and productive kelp forests.
Kelps are true giants compared to other seaweeds. Giant
kelp (Macrocystis sp.) is the largest benthic organism on
Earth. Nutrient-rich waters fuel kelp growth, allowing
kelp to grow quickly. The three-dimensional structure
of kelp forests provides a great habitat for many
other marine organisms. Animals like fish, worms,
crustaceans, snails, and echinoderms are common in
kelp forests. When kelp forests are removed by urchin
outbreaks or other disturbances, the diversity of other
organisms in the area is greatly reduced.
Red algae, also known as Rhodophyta, are incredibly
speciose in the ocean and inhabit shallow marine
environments. In fact, there are more marine species
of red algae than of marine green and brown algae
combined. Red algae have red pigments called
phycobilins, which mask chlorophyll, causing the algae
to appear red. Some species of red algae are harvested
for food and various other economic goods. Coralline
Seaweeds are of great economic importance—they
are harvested for food as well as for their vitamins,
minerals, fiber, and antioxidants. Seaweeds are
also harvested for their gelatinous chemicals called
phycocolloids. Algin is one phycocolloid of particular
importance that is used as a stabilizer and emulsifier
in many consumer products, such as ice cream, cream
cheese, icing, shampoo, shaving cream, plastic, paint,
paper, cosmetics, and so on. Seaweeds can also be used
as fertilizers, added to feed for animals, and can even
be used for dressing wounds.
FIGURE 2−16
Crustose coralline red algae.47
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red algae are red algae that deposit calcium carbonate
within their cell walls. Coralline algae are of particular
ecological importance in the formation and development
of coral reefs, helping to cement these living structures.
FIGURE 2−17
FIGURE 2−18
Fully submerged seagrasses.48
Flowering Plants
Seagrasses are the only true marine flowering
plants. Seagrasses live fully submerged in seawater
most of the time. Seagrasses superficially look like
grasses, but they are not actually grasses. Seagrasses
have horizontal stems, called rhizomes, that grow
beneath the sediment to help anchor them in marine
habitats. Seagrasses have flowers, though they are
small and inconspicuous. The pollen of seagrasses
is carried by water currents, fertilizing tiny seeds in
the flowers. Once fertilized, seeds are transported by
water currents or by grazers that feed on seaweeds.
Seagrasses are limited in depth distribution due
to their need for light to carry out photosynthesis.
Seagrasses form thick beds, which serve as a habitat
and food for many marine organisms. Animals such as
snails, worms, shrimp, sea stars, juvenile fishes, and
conch are common in seagrass beds.
Salt Marsh Plants
Cordgrasses, which are true grasses, are not considered
marine species but rather are land plants that tolerate
salt. Because of this salt-tolerance, cordgrasses
are considered halophytes (Greek for “salt plant”).
Cordgrasses do not tolerate total submergence under
seawater. If they were fully submerged for long periods,
Partially submerged cordgrass.49
they would not survive. However, when the tide comes
in, cordgrasses are partially submerged in salt marshes.
Salt marshes are a coastal ecosystem in the intertidal
zone that is regularly flooded by the tides. Cordgrasses
are common in soft-bottom coastal areas, and they
provide a habitat and breeding grounds for many
marine organisms and protect the land from erosion.
Other important halophytes in salt marshes include
Pickleweed (Salicornia) and Rushes (Juncus), which are
found higher in the marshes.
Mangroves
Mangroves are trees and shrubs that are adapted to
live along tropical and subtropical shores, but like
cordgrasses, they do not tolerate total submergence.
Mangroves are land plants that tolerate salt. Mangrove
forests are common in muddy or sandy coastlines
that are protected from wave action in subtropical and
tropical zones. Mangroves use their roots to trap and
hold sediment. Mangroves are well-adapted to salt,
high water loss, soft sediments, and poor oxygen. Many
species of marine organisms live in and among the roots
and branches of mangroves. Many fish and invertebrates
take shelter, reproduce, and live in mangrove forests.
Large sponges grow on the roots, while many other
invertebrates, such as worms, shrimp, and crabs, live
among the roots. Land animals such as birds, snakes,
lizards, and bats use mangrove tree branches for shelter.
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Angiosperms, or flowering plants, are dominant on
land but are less abundant in the ocean. Angiosperms
are vascular plants that use conductive vessels to
transport oxygen, water, and nutrients. Flowering
plants have true leaves, stems, and roots. Only a few
angiosperms have re-colonized the ocean and live in
shallow coastal waters.
FIGURE 2−19
FIGURE 2−20
Red mangrove in Florida.50
INVERTEBRATES: ANIMALS
WITHOUT A BACKBONE
The majority of multicellular species on Earth are
animals, and of these, more than 97 percent lack a
backbone. Animals without a backbone, or row of bones
called vertebrae, are called invertebrates. Animals
with a backbone are called vertebrates. All major
groups of invertebrates have representatives in marine
environments. Moreover, some invertebrate groups, like
echinoderms, are found only in marine habitats.
Sponges
Sponges, which belong to the phylum Porifera, are
filter-feeding animals made up of aggregations of
specialized cells. Sponges lack true tissues and organs.
All sponges are sessile, meaning they are fixed in one
place. Sponges feed by pumping water through their
bodies to strain food particles from water. Almost all
sponges are marine, with a few species being found
in freshwater habitats. Sponges are incredibly diverse,
with a wide variety of shapes, sizes, and colors. They
can be found in both deep and shallow marine habitats
from the poles to the tropics.
Sponges mostly reproduce asexually when branches or
buds break off and grow into new, identical sponges.
Sponges can also reproduce sexually when specialized
cells develop into gametes, or sperm and egg cells.
Most animals have gonads that produce gametes, but
sponges lack tissues and organs. Most sponges are
hermaphrodites, producing sperm and egg from the
same individual. Sponges release sperm into the water
by broadcast spawning. Eggs are often retained inside
the sponge body, and fertilization takes place via the
filtration of water containing sperm.
Sponges grow into a variety of asymmetrical shapes and
may be branching, tubular, round, or volcano shaped.
Some sponges are encrusting, growing over the surface
of rocks or coral, while others are boring sponges, which
bore thin channels into shells or corals. Also, of note
are glass sponges, such as the Venus’s flower basket
sponge (Euplectella), a deep-sea sponge that has a glass
skeleton made of fused spicules. Marine sponges are of
commercial importance as some are harvested as bath
sponges. These sponges are different from synthetic
sponges, as they are the remaining spongin fibers of
once-living animals from the ocean.
Gelatinous Animals
Cnidarians (phylum Cnidaria) and Ctenophores (phylum
Ctenophora) are common multicellular, heterotrophic,
gelatinous animals in marine ecosystems. Cnidarians
include jellyfishes or jellies, sea anemones, and
corals. Cnidarians have true tissues and exhibit radial
symmetry, with body parts arranged around a central
axis, like a pie. Although they have no head, Cnidarians
have a mouth on one end, which is surrounded by
tentacles. All cnidarians are predatory organisms that
capture prey items with specialized stinging structures
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Caribbean sponges.51
FIGURE 2−22
Representatives of Cnidaria.52
called nematocysts, which are found within specialized
cells in the tentacles. There are two basic body forms
in cnidarians. The polyp form is sac-like with the
mouth and tentacles pointed upward, as in the sessile,
adult stage of anemones. The medusa is a jellyfish-like,
upside-down polyp that is adapted for swimming. In
a medusa, the mouth is oriented toward the substrate.
Some cnidarians, like corals and sea anemones, are only
polyps; some have both a polyp and medusa stage; and
others are only medusae.
Marine flatworm.53
in such a way that the left and right halves are mirror
images. There are many different groups of worms that
live in marine environments. We will briefly discuss a
few of the major and most common groups of worms
here. Flatworms (phylum Platyhelminthes) are called
flatworms due to their flattened body shape. These are
some of the simplest animals with tissues and organs
as well as a central nervous system, where information
is stored and processed. Additionally, flatworms
have an aggregation of nerve cells in the head, which
functions like a simple brain.
With over 10,000 species, cnidarians are grouped into
major types. Hydrozoans are feathery or bushy colonies
of tiny polyps. Siphonophores are a specific group
of hydrozoans that form drifting colonies of polyps.
Often siphonophores have specialized polyps that are
focused for feeding, defense, or reproduction. The
most common siphonophore is the Portuguese manof-war (Physalia physalis), which can cause a painful
burning sting in humans. Scyphozoans are jellies or
jellyfish that are common in all oceans and swim with
rhythmic contractions of their bell, or rounded medusa.
Cubozoans, or box jellyfish, have only four tentacles
or four groups of tentacles, one on each corner of the
medusa. The sea wasp (Chironex fleckeri) and other box
jellies are some of the most dangerous marine animals.
Anthozoans, like corals and sea anemones, are solitary
or colonial polyps that lack a medusa stage. Coral reefs
will be discussed further in Section III of this guide.
Ribbon worms (phylum Nemertea) are worms that
display several features that indicate more complex
organization. In ribbon worms, we see a complete
digestive system as well as a circulatory system in
which blood transports oxygen and nutrients to tissues.
Ribbon worms also have a distinct structure called a
proboscis, which is a long fleshy tube that is used to
entangle and consume prey, such as other worms and
crustaceans.
Worms
Segmented worms, or annelids (phylum Annelida),
are a large group of worms, with the most well-known
member of this group being terrestrial earthworms.
Worms are the simplest animals that exhibit bilateral
symmetry, which is the arrangement of body parts
Nematodes (phylum Nematoda), also called
roundworms, are incredibly numerous in marine
habitats. Roundworms are common in sediments as
well as in the tissues of other organisms. As their
common name implies, roundworms have slender,
cylindrical bodies that are pointed at both ends.
Nematodes that inhabit sediments eat mostly bacteria
and organic matter.
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FIGURE 2−21
FIGURE 2−23
FIGURE 2−24
Hydrothermal vent tube worms.54
Peanut worms, also called sipunculans (phylum
Sipunclua), are soft-bodied worms that burrow in
muddy bottoms, rocks, and corals or hide in empty
shells. All peanut worms are marine, and they live
mostly in shallow waters. The head region contains
a mouth and set of branching tentacles that can be
pulled into the remaining portion of the body. When
contracted into the body, the worm is compacted and
looks like a large peanut.
Molluscs
Clams, snails, and octopuses are members of phylum
Mollusca. There are more species of molluscs (also
written “mollusks”) in the ocean than of any other
animal group. Molluscs exhibit incredible diversity
and are found in a variety of marine habitats ranging
from intertidal communities to deep-sea hydrothermal
vents. Many molluscs are consumed by humans across
the globe as a source of protein. In general, molluscs
have a soft body enclosed in a calcium carbonate shell.
Mollusc bodies are covered by a mantle, or thin layer
of tissue that is responsible for secreting the shell. For
locomotion, molluscs have a muscular foot. A radula
is a structure unique to molluscs that is formed by a
Marine snails.55
ribbon of small teeth on the ventral surface used for
feeding. All molluscs have this basic body plan, but
it is often incredibly modified in different species.
For example, the shell is absent or is a rudiment in
nudibranchs, octopuses, and squids. Additionally, in
some molluscs, the radula is modified or absent.
Gastropods—a group that includes snails, limpets,
abalones, and nudibranchs—are the most common
and varied group of molluscs. A gastropod is typically
a coiled mass of vital organs enclosed by a shell that
sits on the dorsal surface. Most gastropods feed with
a radula by scraping algae from rock surfaces. Other
gastropods are deposit feeders, feeding on mud from
the bottom, or are predators, feeding on clams, oysters,
worms, and small fish. Nudibranchs, or sea slugs, are
gastropods in which the shell is absent. Nudibranchs
are beautiful marine organisms, with brightly colored
branches of the gut or exposed gills. Because they lack
a shell, most nudibranchs are chemically defended by
toxins that they produce to avoid predation.
Clams, mussels, and oysters are bivalves and have a
laterally compressed body that is enclosed in a shell
with two valves. The umbo, or hinge, near the hump
of the valves connects each shell. Adductor muscles
inside the bivalve are used for closing the valves.
Clams use their muscular foot to burrow in the sand
or mud. Mussels attach to rocks using strong byssal
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A key feature of annelids is that their bodies are
composed of a series of similar compartments or
segments. Polychaetes are the most common marine
annelids. Each polychaete segment has a pair of
parapodia, or flattened extensions that contain stiff
bristles called setae, which are used for locomotion.
Polychaetes can be benthic, crawling on rocks and
burrowing in the sediment, as well as sessile, in tubes
made of mucus, protein, seaweed, cemented mud, and
sand grains. At hydrothermal vents, tube-dwelling
polychaetes are common. Other polychaetes swim up
in the water column, using their setae like oars.
FIGURE 2−25
FIGURE 2−26
Clam valves.56
Cephalopods, commonly known as octopuses, squids,
cuttlefishes, and nautili (singular: nautilus), are
specialized predators with an incredible capacity for
locomotion. They are agile swimmers with a complex
nervous system and large eyes. In cephalopods, the foot
is modified into arms and tentacles, which are used for
capturing prey items. Cephalopods swim by forcing
water out of a siphon, a muscular tube formed from
the foot, which projects under the head. The siphon is
incredibly flexible, allowing cephalopods to move in
any direction using jet propulsion. Octopuses have eight
arms and lack a shell. They are efficient hunters and
use their beak-like jaws to bite prey. Like many other
cephalopods, octopuses have an ink sac that can be used
to secrete a cloud of dark fluid to distract predators.
Squids are well adapted for swimming and have eight
arms and two tentacles. The largest invertebrate on
Earth is the deep-sea colossal squid (Mesonychoteuthis
hamiltoni), which can grow up to ten meters and weigh
as much as 700 kg. Cuttlefishes have eight arms and
two tentacles and a flattened body, with a fin running
along the sides. Cuttlefishes have an internal calcified
shell, called a cuttlebone, which is used for buoyancy.
Often cuttlebones are sold as a source of calcium for
pet birds. Nautiluses have a coiled shell that contains
a series of gas-filled chambers that aid in buoyancy, as
well as numerous short tentacles used to capture prey.
Arthropods
The largest phylum of animals on Earth is Arthropoda.
The majority of arthropods are insects; however,
Copepod.57
arthropods are also dominant in marine habitats.
The most diverse group of arthropods in marine
habitats are crustaceans, a group that includes crabs,
lobsters, shrimp, barnacles, and many other smallshelled animals. Horseshoe crabs and pycnogonids
are marine arthropods that are not crustaceans. The
body of an arthropod is segmented and bilaterally
symmetrical, with jointed appendages. Arthropods have
an exoskeleton—a tough nonliving external skeleton.
Exoskeletons provide support and protection but limit
growth because the animal must molt or shed the
exoskeleton to grow.
There are several groups of small crustaceans in
marine environments that are incredibly important
to food webs and ecosystems. Copepods are small
crustaceans that are numerous and important in
the plankton. Copepods swim in the plankton with
elongated antennae. Copepods eat phytoplankton
and are themselves consumed by many other marine
organisms. Amphipods are another group of small
crustaceans with a curved body that is laterally
flattened. Some amphipods can be found intertidal
zones, while others live in the benthic zone, burrow in
the skin of whales, or live in the plankton.
Krill, or euphausiids, are planktonic, shrimp-like
crustaceans that filter feed on diatoms. Incredibly
common in temperate and polar waters, krill are a
critically important food source for many whales,
penguins, seals, and other animals. Barnacles are small
filter-feeding crustaceans that live attached to surfaces
such as whales, docks, boats, and piers. As such, they
are considered biofouling organisms.
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threads. Oysters cement their shell to a hard surface
and filter feed to collect food. The largest of the
bivalves is the Giant clam (Tridacna sp.), which can
grow to be more than one meter in length.
FIGURE 2−27
FIGURE 2−28
Decapods are a large group of crustaceans that have ten
legs. This group includes shrimp, lobsters, and crabs.
Many of these species are important economically as
food. Shrimps and lobsters have laterally compressed
bodies with elongated abdomens or tails. Shrimps are
found mostly in benthic habitats from shallow to deep
waters. Shrimps and lobsters are scavengers, feeding
on detritus from the bottom. Crabs have a small
abdomen, which is tucked under the main body called
the cephalothorax. In females, the abdomen is used for
carrying eggs. Crabs are common along sandy beaches
and rocky shores. Land crabs are crustaceans that are
modified to live on land, only returning to the sea to
release their eggs.
Echinoderms
Echinoderms (phylum Echinodermata) are spinyskinned animals and include sea stars, sea urchins, sea
cucumbers, feather stars, and sea lilies. Echinoderms
are secondarily radially symmetrical, which means
that their planktonic larvae are bilaterally symmetrical,
and only adults develop radial symmetry. More
specifically, adult echinoderms have five-point radial
symmetry, giving them a star-like appearance. Most
echinoderms have an interior skeleton made of calcium
carbonate. Echinoderms are exclusively marine and
contain a system of water-filled canals that are used
for locomotion as well as for food, waste, and oxygen
transport. Echinoderms are capable of regeneration,
meaning they can regrow lost or damaged body parts.
Some echinoderms can regenerate lost arms, while in
some species a lost arm can grow into an entire new
individual.
Decapod crab.59
Echinoderms are members of benthic communities
from the poles to the tropics, from shallow to deeper
waters. Sea stars, sometimes called starfishes,
mostly have five arms that radiate from the central
disk. Despite living a benthic lifestyle, sea stars are
incredibly mobile, using thousands of tube feet for
locomotion. Brittle stars also are shaped like a star,
with most species having five, flexible arms attached
to the central disk. The arms of brittle stars move with
snake-like motion to move around benthic habitats.
Brittle stars are often tucked away, hiding under rocks
and corals or buried in mud or sand.
Sea urchins have moveable spines that are jointed to
sockets in the rounded shell. Sea urchins are major
grazers on seaweeds and seagrasses. A specialized
structure called an Aristotle’s lantern is found in the
mouth of sea urchins and is used to graze on algae.
Not all sea urchins have rounded shells. Instead, some
have a flattened shape and are called heart urchins
or sand dollars—these animals are adapted to living
buried in sand or mud. Sea cucumbers have a worm-like
appearance and lack spines. The sea cucumber lies on
one side and either deposit feeds or suspension feeds
by filtering food from the water. Sea cucumbers are
also able to expel their own internal organs as a means
to scare or distract predators. Crinoids, which include
feather stars and sea lilies, are suspension feeders that
usually live in deeper waters. Sea lilies are sessile and
attached in one place, while feather stars are known for
graceful swimming.
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Laterally flattened amphipod.58
FIGURE 2−34
FIGURE 2−30
Sea squirt.61
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FIGURE 2−31
Echinoderm diversity.60
Tunicates and Cephalochordates
The phylum Chordata is divided into three major
groups: tunicates, cephalochordates, and vertibrates.
Tunicates and cephalochordates lack a backbone, while
members of the third group, known as vertebrates, have
a backbone. All chordates, including humans, have four
major characteristics that they all share at some point
in their lives. Specifically, they have a single hollow
nerve cord that runs along the dorsal length of the body;
pharyngeal gill slits—small openings along the anterior
part of the gut; a flexible rod called a notochord, which
is used for support and lies between the nerve cord and
the gut; and a post-anal tail (a tail that extends beyond
the anus).
Tunicates (subphylum Urochordata), the first
subphylum of invertebrate chordates, are all marine.
The most common tunicates are sea squirts, which
have a sac-like body and are often attached to a hard
surface. Sea squirts are filter feeders and use an
incurrent siphon to bring water and food particles in
and use an excurrent siphon to expel water. Pelagic
tunicates called salps have transparent barrel-shaped
bodies that float in the water column. Some colonial
Lancelet.62
salps can reach several meters in length.
Cephalochrodates (subphylum Cephalochordata), or
lancelates, are an invertebrate chordate group with
only a few known species. The body of a lancelet is
elongated and compressed like a fish. Lancelets live
on soft bottoms and are filter feeders, using their gill
slits to capture food particles. These organisms are
considered the invertebrate that is most closely related
to vertebrate animals.
VERTEBRATES
Vertebrates have a vertebral column or spine, which
we call a backbone. The vertebrae protect the nerve
cord, or spinal cord, which terminates in a brain that
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FIGURE 2−32
FIGURE 2−33
A hagfish.63
Fishes were the first vertebrates on Earth, appearing
about 500 million years ago, and likely evolved from
an invertebrate chordate relative similar to lancelets.
Fishes are diverse and incredibly abundant in the ocean.
In fact, fishes are the most successful and abundant
vertebrates on Earth. Fishes range from being tiny
in size to weighing more than forty-five tons and are
an important source of food in marine ecosystems as
well as for humans. Three major groups of fishes are
generally recognized: jawless fishes, cartilaginous
fishes, and bony fishes.
Jawless Fishes
The most ancestral fishes currently alive on Earth are
the jawless fishes (Agnatha). Jawless fishes feed by
suction with a round muscular mouth because they
lack jaws. The bodies of jawless fishes are cylindrical
and elongated and lack paired fins and scales.
Hagfishes and lampreys are the only extant members
of jawless fishes. Hagfishes are slimy, deep-water
Cartilaginous fishes.64
fishes that feed mostly on dead or dying prey items.
Lampreys are primarily freshwater fishes that breed in
rivers and lakes but move to the ocean as adults. They
are parasitic, attaching to other fishes or invertebrates.
Cartilaginous Fishes
Sharks, skates, rays, and ratfishes are cartilaginous
fishes (Chondrichthyes). Cartilaginous fishes have a
skeleton made of cartilage, a tissue more flexible than
bone. They have movable jaws with well-developed
teeth, paired fins, and rough scales called placoid
scales. Sharks are mainly fast-swimming predators
with a fusiform, or torpedo-shaped, body. Their tail, or
caudal fin, is well developed and makes them powerful
swimmers. Sharks also often have two dorsal fins on
their dorsal (upper) surface. Their paired pectoral fins
are broad and wide and are used for steering. Sharks
have five to seven gill slits behind their head on each
side of their body. Sharks vary in size from the tiny
spined pygmy sharks (Squaliolus laticaudus) to the
giant whale shark (Rhincodon typus), the largest fish
in the sea. Although they are found throughout the
oceans from shallow to deeper waters, most sharks are
concentrated in the tropical seas.
Rays and skates are dorsoventrally flattened
cartilaginous fishes that live mostly in the benthic zone.
Rays and skates have five pairs of gill slits on the ventral
(underside) of their bodies. Their pectoral fins are flat,
expanded, and function like wings. Stingrays and their
relatives have a whip-like tail that has one or more
stinging spines used for defense. Torpedo rays have
special organs that produce electricity on each side of
the head to stun prey items and defend against predators.
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is protected by a skull. Vertebrates originated in the
ocean approximately 500 million years ago, and their
ancestral forms were fish-like. Approximately 350
million years ago, vertebrates from the ocean came onto
land. The earliest land vertebrates came from a lineage
called tetrapods, meaning “four-footed.” Tetrapods
evolved from a group of marine fishes that had lungs.
Amphibians, or land vertebrates that need water or
moist environments to survive, were the first land
tetrapods. Reptiles, or air-breathing vertebrates that have
scales or bony plates as protective covering, evolved
from amphibians. Birds and mammals evolved from
different groups of extinct reptiles. More recently, many
reptiles, birds, and mammals, returned to the ocean.
FIGURE 2−34
FIGURE 2−35
Examples of bony fish.65
Bony Fishes
The majority of fishes are bony fishes (Osteichthyes)
and have a skeleton made at least partially of bone.
Bony fish live in marine habitats, from tide pools to the
abyss, and are an extremely diverse group, consisting
of the largest class of extant vertebrates on Earth. They
have highly modified fins and show great diversity in
body shape, size, color, feeding habits, and reproductive
patterns. Bony fish use coloration and body shape for
a locomotive advantage or for camouflage. The jaws
of bony fishes are protrusible, meaning they project
outward from the mouth. Many bony fishes have a swim
bladder, which is a gas-filled balloon-like sac about the
stomach that aids in buoyancy. Bony fish are a major
protein source for humans across the globe.
Marine Reptiles
Marine reptiles include lizards, snakes, turtles, and
crocodiles that live in the ocean. Marine reptiles first
appeared 300 million years ago and thrived in the
ocean in the Age of Reptiles. Now, only a few reptiles
still roam the sea. Marine reptiles have skin covered in
scales and salt glands to concentrate and excrete excess
salts. The majority of marine reptiles must still return
to land to lay eggs.
Sea turtles are an ancient group of reptiles with a body
Marine reptiles.66
enclosed in a shell, or carapace, that is fused to the
backbone. Sea turtles cannot retract their heads into
their shells and have legs that are modified into flippers.
Sea turtles eat mainly seaweeds, seagrasses, jellyfish,
and other invertebrates. There are only seven species
of sea turtles currently extant, and they primarily live
in warm ocean waters. Due to human interference and
overharvesting for eggs, meat, and leather, all sea turtle
species are listed as either threatened or endangered
by the United States Endangered Species Act of 1973.
The largest sea turtle is the Leatherback (Dermochelys
coriacea), and it is also the widest ranging in the oceans.
Sea snakes are found in the tropical Indian and Pacific
Oceans. Sea snakes are laterally flattened with a
paddle-shaped tail. They are closely related to land
cobras, making them incredibly venomous. Though
their bites would be fatal to humans, they are rarely
aggressive, with a mouth that is too small to get a good
bite. Many sea snakes give birth to live young.
Other marine reptiles include the marine iguana and the
saltwater crocodile. The Marine iguana (Amblyrhynchus
cristatus) lives in the Galapagos Islands off the Pacific
coast of South America. Marine iguanas spend their
days basking in the sun, warming up after a cold dive
in the Pacific for seaweed. The saltwater crocodile
(Crocodylus porosus) inhabitants swamps and estuaries
in the Indian Ocean, Australia, and some of the western
Pacific islands. Salties, as saltwater crocodiles are
commonly called, are among the most aggressive of all
marine animals and are known to attack humans.
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Some rays, like eagle and manta rays, filter feed up in
the water column and return to the benthic habitat when
not feeding. Skates are similar to rays but lack the whiplike tail and stinging spines. Skates lay egg cases, often
referred to as mermaids’ purses, whereas rays give birth
to live young. Ratfishes, or chimaeras, are deep-water,
cartilaginous fishes. They have one pair of gill slits
covered by a flap of skin, and some species have a long,
rat-like tail. Ratfishes feed on crustaceans and molluscs.
FIGURE 2−36
FIGURE 2−37
King penguins.67
Birds, in general, evolved from fast-running dinosaurs
approximately 160 million years ago. Only a small
portion of birds on Earth are considered seabirds.
Seabirds are birds that spend a significant portion
of their lives at sea and feed mainly on marine
organisms. The majority of seabirds live in the southern
hemisphere. Many seabirds, such as albatrosses, have
webbed feet, salt glands in their head to help excrete
excess salts, and breed in large colonies. Most seabirds
are predators of fishes, squids, and bottom invertebrates.
Probably the most well-known and charismatic seabirds
are penguins. Penguins are flightless birds with
modified wings for swimming. They are incredibly
powerful swimmers with streamlined bodies. They are
adapted for cold temperatures, equipped with a layer
of fat under their skin, and have dense, waterproof
feathers. Penguins live only in the southern hemisphere,
with the majority of species restricted to the Antarctic.
Many penguins feed on krill and make great migrations
between feeding and nesting grounds.
Tubenoses are seabirds that have distinctive tube-like
nostrils and heavy beaks with a curved tip. Tubenoses
are skilled flyers, catching their prey items at the
surface of the water. The largest flying bird on Earth
is the Wandering albatross (Diomedea exulans),
a tubenose that has a wingspan up to 3.4 meters.
Tubenoses make incredibly long-distance migrations
from breeding grounds in the Antarctic to feeding
grounds in the Arctic.
Several other seabirds, such as pelicans, cormorants,
frigatebirds, and terns, are adapted for life and feeding
in the water. Shorebirds are included among the
seabirds, but they do not have webbed feet and thus
do not swim very much. They live mostly near coastal
waters, utilizing the resources on the shoreline, bays,
and inland waters.
Marine Mammals
Mammals evolved from now-extinct reptiles
approximately 200 million years ago. About 65 million
years ago, mammals underwent a rapid diversification
when dinosaurs were no longer dominant. All
mammals, with only a few exceptions, produce live
offspring instead of laying eggs. Newborn mammals
are fed by the mother’s mammary glands, and the
mother invests a lot of time and energy into parental
care. At least five different groups of land mammals
have independently and successfully invaded the
ocean. The ocean is the only uniting feature of the
different groups of marine mammals.
Sea lions, seals, and walruses are marine mammal
carnivores in the suborder Pinnipedia and have
paddle-shaped flippers. Pinnipeds evolved from a
terrestrial carnivore ancestor they share with dogs,
cats, and bears. The majority of pinnipeds live in cold
water and have a thick layer of blubber for insulation.
Pinnipeds must leave the ocean for mating and for
raising their young. Sea lions (including fur seals) are
most obviously distinguished from true seals by their
ear pinnae. Sea lions and fur seals have external ear
pinnae while seals do not. In addition, sea lions and fur
seals can rotate their rear flippers forward, allowing
them to run with all four limbs on land. Contrastingly,
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Pinnipeds.68
Marine Birds
FIGURE 2−39
Polar bear.69
seals have to inch along on their bellies to move on
land. The walrus is a large pinniped with a pair of
distinctive tusks that lives in the Arctic. Both male and
female walruses have tusks.
Sea otters (Enhydra lutris) are also a member of the
carnivores and are the smallest marine mammal.
Instead of a layer of blubber, sea otters have a thick
layer of fur that traps air to help insulate them from
the cold. In the eighteenth and nineteenth centuries,
humans hunted sea otters to near extinction for
their fur coats. Polar bears (Ursus maritimus) are
semiaquatic marine mammals that spend a good
portion of their lives on sea ice. Feeding primarily on
seals, polar bears stalk and capture prey. Polar bears
are under threat due to decreasing sea ice in the Arctic
as a result of global warming.
Cetaceans, or whales, dolphins, and porpoises, are
fully aquatic marine mammals. Cetaceans evolved
from ungulates, four-legged land animals. With
streamlined bodies, cetaceans are excellent swimmers.
Like all mammals, they breathe air and nurse their
young. Instead of terminal nostrils in a nose, cetaceans
have a modified opening on the top of their head called
a blowhole. Cetaceans have front flippers, but their
rear limbs fail to develop as they were lost through
evolutionary processes. The body of a cetacean ends in
a muscular tail called a fluke. Cetaceans have blubber
for insulation and buoyancy.
Diversity of Cetacea.70
Cetaceans are divided into two main groups: baleen
whales and toothed whales. Baleen whales have rows
of flexible plates called baleen that are used for filter
feeding. Baleen whales are the largest animals on earth.
In fact, the blue whale (Balaenoptera musculus) is the
largest animal to ever live on Earth. Toothed whales are
cetaceans with teeth and are adapted for a diet of fish,
squid, and other prey items. The largest toothed whale
is the sperm whale (Physeter macrocephalus). Both
baleen whales and many toothed whales were hunted
by humans, dating back to prehistoric times. Whaling
continued into the 1900s, by which time populations
of many whales were severely depleted. Commercial
whaling still occurs in some places today despite an
international ban on the practice.
Manatees and dugongs, also known as sirenians, are
relatives of elephants. Both manatees and dugongs
have front flippers and a paddle-shaped horizontal
tail. Sirenians are gentle herbivores and usually live in
groups. Sirenians were exploited by humans for their
meat, skin, and blubber, and as a result, populations
have severely declined. In general, most marine
mammals are long-lived with low reproductive rates.
Given their low reproductive potential, marine mammal
populations have been seriously depleted by hunting,
and many of these populations still have not recovered.
However, some marine mammals, like humpback
whales and Steller sea lions, are among the few species
that have undergone recovery in the modern era.
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FIGURE 2−38
are three major biogeochemical cycles on Earth
that help cycle elements from living organisms
to nonliving matter.
FIGURE 2−40
6 Taxonomy is the study of biological
classification. Organisms on Earth are currently
classified into three major domains: bacteria,
archaea, and eukarya. Bacteria and archaea are
prokaryotes, or single-celled organisms without
nuclei or organelles. Eukaryotes can be either
unicellular or multicellular, but their cells have
nuclei and organelles. Fungi, protists, animals,
and plants are all eukaryotic organisms.
6 Viruses are not included in the tree of life
because biologists are not in agreement that
viruses are alive, and they do not agree on
where to place them on the tree of life. Viruses
span the living and the nonliving.
SECTION II SUMMARY
6 The planet is 4.5 billion years old, and life
evolved around 3.5 billion years ago. Life
almost certainly originated in the sea.
6 Organisms are made of carbon, hydrogen,
oxygen, and nitrogen that combine to make
carbohydrates, lipids, proteins, and nucleic
acids.
6 Living organisms must capture, store, and
transmit energy to live. Energy can be stored
via photosynthesis or by chemosynthesis.
Photosynthesis is a process by which
organisms convert sunlight energy into food
energy. Chemosynthesis is the production
of food from energy released by inorganic
molecules in the environment
6 Autotrophs are organisms that can nourish
themselves from inorganic sources using
photosynthesis or chemosynthesis. Heterotrophs
are organisms that are not capable of producing
their own food and must obtain energy from
organic matter produced by autotrophs.
6 The carbon, nitrogen, and phosphorus cycles
6 Archaea cells are small and can be spherical,
spiral, or rod-shaped. Although they were
originally thought to be bacteria, there is
genetic evidence that archaea are more closely
related to eukaryotes than to bacteria.
6 Algae are an incredibly diverse group of
simple marine and freshwater photosynthetic
organisms that range in size from microscopic
to giant seaweeds.
6 Plankton are pelagic organisms that live
suspended in seawater, going where the ocean
goes. Plankton can be animal-like or plant-like.
6 Seaweeds are multicellular algae commonly
found on rocky shores and in other marine
environments. Seagrasses and mangroves are
considered true marine flowering plants that
tolerate salt.
6 Most multicellular species on Earth are
animals. More than 97 percent of these animals
lack a backbone. Animals without a backbone,
or row of bones called vertebrae, are called
invertebrates. Animals with a backbone are
called vertebrates.
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Diversity of Sirenia.71
6 Bacteria are prokaryotic organisms that are
structurally simple but are incredibly abundant
in marine ecosystems.
Section III
Marine Ecosystems
The scientific study of the relationships of living
organisms with one another and with their natural
environment is called ecology. Organisms live within
an ecosystem—a biological community of interacting
organisms and their physical environment. Organisms
interact with each other in a multitude of ways,
and these interactions have profound effects on the
organisms themselves and on their broader ecosystem.
Specifically, organisms consume each other, they fight
for space and other resources, they provide shelter or
habitat for other organisms, and they even live together
in symbiotic relationships. Each living entity in an
ecosystem is linked to other living and nonliving
things. This section of the resource guide provides an
introduction to marine ecology and ecosystems.
WHAT IS MARINE ECOLOGY?
Each habitat on Earth is unique and distinct, and these
features help to determine the types of organisms
that live in these habitats. The nonliving components
of an environment are called abiotic factors. Abiotic
factors in marine environments can include water,
temperature, acidity, pressure, and so on. The biotic
features of an ecosystem are the living components, or
the organisms. In an ecosystem, a group of individuals
of one species that lives together is called a population.
All the populations of different species that live together
in the same place are called a community. Organisms
in a community are dependent upon each other. An
ecological niche describes both the range of conditions
in which an organism can survive and the role each
species occupies within an ecosystem (in relationship to
food, resources, competitors, and predators).
To some degree, each individual organism has the
ability to adapt or acclimate to the environment by
changing characteristics such as behavior, metabolism,
etc. This type of individual flexibility in behavior or
metabolism is called plasticity. For example, coral
FIGURE 3−1
A coral reef ecosystem.72
animals can adapt to different light levels by changing
their growth form. Corals that grow at greater depths
with less sunlight often take on a plate-like shape
to maximize their ability to capture sunlight for
photosynthesis.
Whereas plasticity refers to an individual organism’s
ability to change in response to its environment,
evolution by natural selection is the process by which a
population changes through time in response to biotic
and abiotic conditions. Individuals in a population have
differences, or variation, in their traits. For example, a
population of intertidal snails may differ in shell color.
These differences are heritable, meaning that they are
encoded in the organism’s DNA and can be passed on
to offspring. The best adapted individuals pass on their
adaptive traits to their offspring. Specifically, if an
adaptation results in the survival of traits that are best
suited to the abiotic and biotic environment and the
individuals are more successful and survive, then the
adaptation is passed on to the next generation.
To return to our example, if lighter snails stay cooler
and are better able to survive being exposed out of
the water, then they will produce more offspring than
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INTRODUCTION
darker snails. Over time, the population will have a
higher proportion of lighter snails. This process is
known as natural selection, where the best-adapted
individuals are better suited to the environment in each
successive generation. The process of traits within a
population changing through time is called evolution.
It is important to understand that individuals do not
evolve; rather populations evolve as a result of natural
selection acting upon individuals.
FIGURE 3−2
ENVIRONMENTAL FACTORS
LIMITING ORGANISMAL
DISTRIBUTION
Light is a major physical environmental factor that
limits the distribution of photosynthetic as well as
non-photosynthetic organisms in the ocean. Light is
limited by both water depth and the number and the
characteristics of particles in the water. The more
particles that are in the water, the less the light is able
to penetrate to greater depths. The ocean is divided
into zones based on light presence. The photic zone
is the sunlight zone where light can penetrate. The
photic zone is divided into the euphotic zone and the
disphotic zone. The euphotic zone is the zone with
enough sunlight for photosynthesis to occur. In the
disphotic zone, some light is present, but there is not
enough light for photosynthesis to produce net glucose.
Thus, photosynthesizing organisms do not typically
A marine iguana.73
occur in the disphotic zone. The aphotic zone is the
zone of permanent darkness, where no light reaches.
Another major physical environmental factor in the
ocean is temperature. Temperature varies greatly
in the ocean according to depth and latitude. The
ocean surface is generally warmer at the equator
and gets colder toward the poles. The deep ocean is
typically uniformly cold. Temperature affects chemical
reactions in organisms in terms of their metabolic rate.
Specifically, metabolic rate increases with an increase in
temperature. Ectothermic organisms are those animals
whose internal metabolism is of negligible importance
to controlling their body temperature. These “coldblooded” organisms instead warm their bodies from the
environment to increase their metabolism. Most fish and
marine reptiles, such as the marine iguana, are examples
of ectothermic marine organisms.
Organisms that use internal metabolism to maintain
temperature are referred to as endothermic. These
“warm-blooded” organisms typically have stable and
high body temperatures. Most marine mammals,
such as humpback whales, birds, and a few fish, are
endothermic organisms.
Nutrients and dissolved gases are two environmental
factors that help to shape organismal distribution.
Nutrients are compounds that are required by
organisms for the production of organic matter. The
most commonly cycled nutrients are nitrogen and
phosphorus. The distribution of nutrients varies greatly
across marine habitats. Coastal areas and areas with
deep water upwelling are typically high in nutrients
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The distribution of marine organisms in the ocean
is determined by many environmental factors,
both physical and biological. Physical factors are
environmental factors such as light, dissolved gasses,
temperature, salinity, acid-base balance, hydrostatic
pressure, and nutrients. Biological factors are
environmental factors such as feeding relationships,
competition for space, metabolic wastes, and defense
of territory. Each of these major categories of
environmental factors can contribute to shaping the
community and the distribution of marine organisms.
When a physical or biological factor controls the
distribution of an organism or limits its ability to
occupy a particular habitat, it is called a limiting
factor. Limiting factors are environmental factors
that can be harmful if they are present in quantities
that are too great or too small. For example, corals
are distributed only in parts of the ocean where water
temperatures are between 18 and 30 degrees C. Thus,
temperature is a limiting factor for coral distribution.
FIGURE 3−6
A humpback whale.74
and are usually high in primary production. Tropical
waters near the equator are usually nutrient poor. The
majority of living biomass in the ocean is focused near
the continental shelves where nutrients are plentiful.
Dissolved gases, such as oxygen and carbon dioxide,
are required for life and impact organismal distribution.
Concentrations of oxygen and carbon dioxide in the
ocean vary with temperature. Oxygen does not readily
dissolve in seawater. There is far more oxygen in the
atmosphere than there is in the ocean, but there are
still sufficient concentrations for life to persist in the
oceans. Carbon dioxide readily dissolves in the ocean,
with the ocean holding far more carbon dioxide than the
atmosphere.
A streamlined penguin.75
dive down just a few feet in water, you can feel the
pressure change in your ear drums. The pressure of
water increases the equivalent of 1 atmosphere for every
ten meters of depth in the ocean. Marine animals must
cope with increasing pressure and the consequences
of changing pressures. Some marine organisms cope
by eliminating large, compressible air pockets. For
example, sperm whales dive to great depths to capture
squid by collapsing their rib cages and lungs to help
them avoid getting decompression sickness.
ECOLOGICAL PRINCIPLES
Marine organisms also capitalize on the transparency
of water. For instance, some marine organisms have
large eyes to help them see well underwater and to aid
in prey capture. Additionally, many marine organisms
have transparent bodies to help them elude predators
and stalk prey without being seen. Other marine
organisms have silver-sided bodies that function like
mirrors, reflecting light and allowing them to blend in
with their surroundings.
In an ecosystem, individuals of different species
frequently interact with each other, and biological
factors often contribute to organismal diversity and
distribution. Organisms may compete for resources,
especially when resources are short in supply or
scarce. Competition can occur with members of the
same species as well as with members of different
species. Whereas interspecific competition is
competition between species, intraspecific competition
is competition among members of the same species.
On coral reefs, interspecific competition for space
is typically intense and controls the distribution of
sessile species. Corals compete for space by growing
over each other, with the fastest, most aggressive, and
chemically defended species taking over. Competition
can be reduced through niche partitioning, where each
organism has a specific role in terms of what it eats,
where it lives, how it reproduces, and how it behaves,
with little overlap with other species.
Pressure is an environmental factor in the ocean that
increases with depth from the weight of the water and
can influence marine organismal distribution. If you
Sometimes species use each other as a food resource
rather than competing for resources. These interactions
are known as trophic interactions. Predation is the act
A unifying environmental factor for all marine
organisms is water. Life in water has driven adaptation
in several organisms toward a streamlined or
flattened body shape that is tapered to the back end. A
streamlined body shape allows marine organisms to
glide through the water with minimal resistance from
friction.
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FIGURE 3−3
FIGURE 3−5
FIGURE 3−6
A pair of humpback whales feeding.76
of one organism eating another. The predator is the one
that does the eating, and the prey item is the one that is
eaten. Predation includes animals that eat other animals,
known as carnivores, as well as organisms that eat
plants or algae—such animals are called herbivores.
Brightly colored nudibranch.77
FIGURE 3−7
Many organisms develop defense mechanisms such
as effective fleeing strategies, camouflage, protective
structures such as spines or shells, or poisonous
chemicals to help them avoid predators. For example,
many sea slugs have developed a defense of poisonous
chemicals to protect themselves from predators
because they lack a shell.
Organisms also interact through symbiotic relationships.
A symbiosis is a relationship in which two or more
organisms live together in close association. In a
symbiotic relationship, the host is the larger partner,
and the smaller partner is the symbiont. Typically,
symbiotic relationships are divided into three major
types according to how the organisms interact. In a
commensal symbiosis, one species gains shelter, food,
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In many ecosystems, predators often have an impact
on species in their community besides their prey
species, a process known as an indirect interaction.
Often, indirect interactions can cause a flow of impacts
through the ecosystem. A trophic cascade occurs when
a predator affects not only the abundance of its prey,
but also the abundance of its prey’s resources. For
example, when predatory sea otters are present, they
control the abundance of herbivorous urchins, thus
allowing kelp (the urchin resource) to flourish. If sea
otters are removed, the kelp forest can be destroyed
by unchecked urchins. A trophic cascade can thereby
affect the entire ecosystem.
Barnacles on a sea turtle.78
or some other benefit without really affecting the other
species. Barnacles that live on the surface of another
organism such as a whale or turtle are good examples of
commensal relationships. The barnacle gains food and
a place to live, while the whale or turtle appears to be
neither harmed nor helped by the barnacle.
In a parasitic relationship, the symbiont benefits at the
expense of the host. Parasites in marine ecosystems
are quite common. For example, marine isopods are
commonly-seen parasites in the gills and mouths of
fishes. In a mutualism, both partners benefit from
the relationship. The benefit is not always equal, but
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FIGURE 3−9
A moray eel being cleaned by a wrasse.79
HABITATS
Across the globe, marine habitats can be divided into
distinct communities based on their characteristics.
The major marine habitats will be presented here.
While there are marine organisms in every habitat in
the ocean, the majority of life is concentrated around
the continental shelves where productivity is highest
and nutrients are found in the greatest abundance.
The Intertidal Zone
The intertidal zone, or the space between high and low
tide, is one of the best-known marine habitats. Because
humans can literally walk to this habitat during low
tide, it is one of the best studied habitats in the ocean.
Intertidal habitats are regularly exposed to air, which is
called emersion, and regularly submerged underwater,
or experience immersion. There are two main types of
intertidal habitats that are divided based on the bottom
type, or substrate. Rocky-shore intertidal habitats
have hard, rocky bottoms with abundant structures to
which sessile organisms can attach. In contrast, softbottom or sandy intertidal habitats are lacking in hard
Rocky-shore intertidal.80
structure and are composed of loose sediment.
The intertidal is a particularly challenging habitat for
marine organisms. Here, organisms are challenged by
desiccation, or the tendency to dry out when exposed to
air. When the tide goes out and organisms are emersed,
or exposed to the air, they have to prevent drying out.
Intertidal organisms cope with desiccation in one of
two ways. They can employ either a “run-and-hide”
strategy, or they can “clam-up.” For the run-and-hide
strategy, organisms simply find somewhere that remains
submerged until the tide comes back. Crabs and snails
are common intertidal organisms that use the run-andhide strategy. Tide pools, or depressions in the rocks
that hold seawater when the tide goes out, are refugia
habitats for organisms that are using the run-and-hide
strategy.
Organisms that employ the “clam-up” strategy are
sessile organisms that have a protective covering, like a
shell, to help hold in water. Barnacles and mussels are
intertidal organisms that use the “clam-up” strategy,
while limpets clamp down tightly to the rocks to keep
from losing water. Organisms in the intertidal also have
to cope with wave shock, wide shifts in temperature and
salinity, and restricted food when the tides are out.
In rocky shore intertidal habitats, clear vertical zonation
is exhibited. Specifically, physical and biological factors
cause distinct and vertically separated bands of species.
Species in the upper intertidal are usually limited in
their distribution by physical factors, like heat and
desiccation, while species in the lower intertidal are
typically limited by biological factors like predation and
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both partners gain from the relationship. A mutualism
can be either a facultative symbiosis or an obligate
symbiosis. In a facultative symbiosis, both partners
can survive without each other if they have to. In
an obligate symbiosis, one or both of the partners
depends upon the other for survival. Cleaning
associations in which small cleaner fishes and shrimps
have mutualistic relationships with larger fishes are
examples of facultative symbiosis. Lichens, an obligate
relationship between fungus and algae, cannot live
without both partners. Corals and their photosynthetic
partners, called zooxanthellae, are obligate symbionts.
FIGURE 3−10
FIGURE 3−12
Soft-bottom intertidal.81
A kelp forest community.83
A tide pool.82
competition. In the upper intertidal, organisms are rarely
submerged and are adapted to withstand exposure to air.
Here, organisms are splashed by the highest waves and
by ocean spray. Lichens and mats of cyanobacteria that
can withstand this harsh environment are common in
the upper intertidal. The middle intertidal is regularly
submerged and exposed to air as the tides move in
and out. Barnacles, mussels, and algae (seaweeds) are
common in the middle intertidal. The lower intertidal is
immersed for most of the time, which allows predators
like sea stars and snails to feed. The lower intertidal
consists mostly of algae and is dominated by predators.
In soft-bottom intertidal habitats, organisms live on or
buried in the sediment. Clams, worms, and crustaceans
are common in this habitat, often burrowing in
the sediment. Organisms that live burrowed in the
sediment are commonly suspension feeders, filtering
food from the surrounding water. Many of the
organisms that live in soft-bottom intertidal habitats
also move up and down the intertidal zone as the tide
comes in and out. Zonation is less pronounced in softbottom intertidal habitats than in rocky shore intertidal
habitats, but some zonation does exist. The upper
beach is mainly dominated by beach hoppers and crabs
while worms and clams are common throughout the
rest of the soft-bottom intertidal.
Seaweed Communities
Seaweed communities exist in shallow, clear waters
where light and nutrient conditions are optimal. Thus,
seaweed communities are common in temperate and
polar regions and are highly productive ecosystems.
Kelp forests, such as those formed by the giant kelp,
Macrocystis pyrifera, off the Pacific Coast of North
America, are among the most well-known, fastest
growing, and largest seaweed communities. Kelp forests
provide shelter and habitat for many marine species
such as echinoderms, molluscs, and fishes. Typically,
seaweeds are attached to the bottom, though a few
species, such as Sargassum, float in ocean currents.
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FIGURE 3−11
FIGURE 3−13
FIGURE 3−14
A fjord in New Zealand.85
An estuary.84
Estuaries and Salt Marshes
The space where freshwater from rivers enters the sea is
called an estuary. Estuaries are typically semi-enclosed
with some level of mixing of freshwater and saltwater.
Estuaries are also commonly known as lagoons or bays.
Estuarine habitats are very productive and serve as
nurseries for many marine species. These habitats are
divided into four major groups based upon their origins.
Drowned river valleys or coastal plain estuaries
formed when sea level rise occurred at the end of the
last ice age and inundated lowlands and river mouths.
They are the most common type of estuary on Earth.
The Chesapeake Bay and Delaware River mouth are
two major examples. Drowned river valleys are also
extensive along the northern coast of the Gulf of
Mexico.
Bar-built estuaries form from the accumulation of
sediment that builds up along the coast forming
sandbars or barrier islands. Sandbars and barrier
islands act as a wall between the freshwater from the
rivers and the ocean. Bar-built estuaries are common
Tectonic estuaries formed when land sank or subsided
after crust movement. San Francisco Bay in California
is a tectonic estuary. The fourth and final type of
estuary are fjords, which were formed when retreating
glaciers cut deep valleys along coastlines. Fjords are
common in southeastern Alaska, British Columbia,
Norway, southern Chile, and New Zealand.
Salinity fluctuates drastically in estuaries as tides move
in and out of coastal areas. Because saltwater is denser
than freshwater, the saltwater flows along the bottom,
with the freshwater floating on top. This forms what is
known as a salt wedge in some estuaries. The salt wedge
moves in and out with the tide, and often organisms
move along with the salt wedge to avoid dramatic
fluctuations in salinity. Euryhaline species are those that
can tolerate a wide range of salinity, while stenohaline
species are those that can only tolerate a narrow range
of salinity. Brackish water species are those that find
intermediate salinity to be optimal.
Estuaries are home to a wide variety and abundance
of organisms. Many fishes, crabs, molluscs, and
polychaetes live in estuaries. Mudflats and saltmarshes
are common estuarine communities and are formed
when the bottoms of estuaries become exposed at
low tide. As this is an intertidal habitat, organisms
here face the same challenges faced by the intertidal
organisms that were discussed earlier. Seaweeds
and unicellular algae like diatoms are common on
mudflats. Bacteria and archaea are incredibly abundant
here as well. Dominant animals on mudflats are those
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in the United States along the Gulf of Mexico and in
North Carolina.
FIGURE 3−16
A salt marsh.86
that live buried in the sediment and are known as
infauna. Ghost shrimp, clams, snails, and worms are
common infauna of mudflats.
Salt marshes typically border estuaries and are
dominated by extensive grassy areas. Though most
often associated with estuaries, salt marshes can
also develop along sheltered coastlines. Salt marsh
communities are dominated by hardy cord grasses
and other halophyte (salt tolerant) species. Some
burrowing animals and fishes inhabit salt marshes as
do nematodes, small crustaceans, predatory snails,
and the larvae of land insects. Salt marshes are
exceptionally good at sequestering and storing carbon,
which is an ecosystem service that is particularly
important to help slow climate change.
Estuaries are high in primary production due to major
nutrient influx and rapid cycling from tides and rivers.
Energy from primary production flows to consumers,
supporting a diverse and abundant community of
organisms. Estuaries also serve as nurseries for many
marine species, including many that are food species
for humans. Human intrusion in estuaries has been
catastrophic for biodiversity in these ecosystems, and
many coastal areas have been destroyed by human
disturbance. For example, estuaries are often dredged
to make harbors and marinas for large vessels, while
others are filled in for urban development.
Estuaries are also faced with eutrophication, or the
input of excess nutrients from humans via agricultural
runoff and waste. While increased nutrients may sound
positive, an overabundance of nitrogen, phosphorus,
A coral reef ecosystem.87
and other nutrients can be disastrous for coastal
marine ecosystems. Excess nutrients allow primary
producers, for which nutrients are normally a limiting
factor, to grow out of control. When phytoplankton
or algae grow unchecked, they can prevent light from
reaching photosynthetic organisms. Further, when
these organisms die and decompose, oxygen levels can
become severely depleted, causing marine dead zones,
where most marine life either dies or leaves the area.
Coral Reefs
Coral reefs are ecosystems that are considered the
rainforest of the sea due to the incredible diversity of
their inhabitants. While coral reefs cover less than 1
percent of the area of the ocean, they are home to fully
25 percent of marine species. Coral reefs are large
geological structures made from the calcium carbonate
(CaCO3) skeleton of marine cnidarian animals called
corals. Individual coral animals are small anemonelike polyps, but they form colonies of hundreds
or thousands of individuals. Reef-building corals
contain symbiotic algal partners called zooxanthellae.
Zooxanthellae are photosynthetic and provide corals
with energy in the form of glucose. In return, corals
provide zooxanthellae with a protected environment
and the compounds needed to photosynthesize. Coral
reefs are distributed in tropical oceans because they
require shallow, warm, clear, nutrient-poor water to
grow. Climate change and ocean acidification are
having great impacts on coral reef ecosystems and will
be discussed further in Section IV of this guide.
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FIGURE 3−15
FIGURE 3−19
FIGURE 3−20
Many reefs on Earth fall into one of three classic types
of coral reefs, which were first categorized by Charles
Darwin. The first major type of coral reef is called
a fringing reef. Fringing reefs are the simplest, most
common type of coral reef and develop close to shore.
They grow in a band along the shore, and because
they are close to the coastline, they are particularly
vulnerable to runoff and human disturbance. One of
the longest reefs in the world is a fringing reef found in
the Red Sea.
The second major category of coral reefs is barrier reefs,
which are similar to fringing reefs but are typically
farther from shore. Most barrier reefs are separated from
the shore by a lagoon or stretch of calm water between
the coast and the reef. Seagrass beds and inhabitants
such as fish, inverts, and turtles are common in shallow
parts of lagoons. Often on barrier reefs, waves and
currents cause sediment to build up and form sand cays
or keys. The most well-known and largest barrier reef is
the Great Barrier Reef, which is actually a complex of
reefs that runs along the northeastern coast of Australia.
Atolls, or a ring of reef that surrounds an island,
comprise the third category of reefs. The majority of
atolls occur in the Indo-West Pacific, where active plate
boundaries and subsequent volcanoes are common.
Atolls can develop far from land when a volcano
rises from the depths. Atolls form when volcanoes or
seamounts begin to subside. Corals first develop as a
fringing reef around the newly formed island and then
grow to form a barrier reef. Ultimately, the volcano
completely subsides, and what remains is a ring of reef
with a lagoon in the middle.
An anemonefish in an anemone.89
Coral reef communities are incredibly diverse and
complex. Competition on coral reefs is fierce because
space and light are at a premium. Many coral colonies
battle for space by digesting or stinging their neighbor.
Coral reefs are home to a wide range of organisms
such as unicellular algae, fishes, sponges, and a
variety of invertebrates. Additionally, coral reefs are
host to many symbiotic relations between different
species. For example, anemonefishes have mutualistic
relationships with anemones. Typically, anemones
are capable of stinging and killing fish; however,
the anemonefish has a protective mucus coating that
prevents it from being stung. The anemonefish benefits
with a place to live and brood its eggs. In return,
the anemone benefits because the fish cleans away
parasites, provides nutrients in the form of waste, and
chases away anemone predators.
The Open Ocean
The vast open ocean is often referred to as the pelagic
zone. In the open ocean, many organisms spend
their life suspended in water. Food webs in the open
ocean are supported by photosynthesis from primary
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An atoll reef.88
FIGURE 3−21
FIGURE 3−22
Countershading in a bluefin tuna.91
producers, which serve as a major food source for
consumers. Both phytoplankton and zooplankton are
common inhabitants of the open ocean. Other common
inhabitants include small crustaceans, such as krill and
copepods. These small crustaceans serve as major food
sources for fishes, seabirds, and whales. Gelatinous,
transparent animals called salps are also common in
the open ocean. Salps feed on phytoplankton through
the process of water filtration.
Additionally, pteropods, a group of small, grazing
molluscs, are also common in the open ocean, capturing
phytoplankton for food. Pteropods serve as a major food
source for many marine fish. Arrow worms and jellyfish
are also common in the open ocean, drifting with the
currents. Nektonic organisms such as fishes, marine
mammals, turtles, and squids also inhabit the open
ocean.
Organisms that live in the open ocean have mechanisms
to aid in buoyancy. Many open ocean organisms store
lipids or fats to help them float. Other organisms,
such as bony fish, have gas-filled swim bladders to
aid in buoyancy. Organisms in the open ocean also
use mechanisms to help conceal themselves to avoid
predation. For example, many open-ocean animals, such
as salps and jellies, are transparent to avoid detection
by predators. Other open-ocean animals, like fish, use
countershading, where the dorsal or back surface is
dark, and the belly is white or silver. When a predator
looks down at an organism that has countershading,
the dark back of the fish blends in against the dark
background below. When looking up, predators see the
bright light from above and the white belly of the fish
also blends in.
The Deep Sea
The deep sea is dark and cold and lacking in
photosynthesis. Without photosynthesis, many
organisms in the deep sea depend on food that sinks
down from surface layers. As a result, much of the deep
sea is sparsely populated with organisms. Organisms
that do occupy this zone are often unusual, alien-like
creatures, reminiscent of those from science fiction
movies.
The deep sea is constantly dark and cold, and salinity
remains relatively uniform. Due to the weight of the
water from above, the deep sea is highly pressurized.
Most animals of the deep-sea are drab gray, off-white,
or black. A few deep-sea fishes and shrimps are bright
red, which is a mechanism of camouflage because red
light does not penetrate deep water, so the organisms
appear black. Animals in the deep sea commonly use
bioluminescence for prey attraction, communication,
and courtship displays. Because of bioluminescence,
some animals of the deep sea do have small, functional
eyes, though many are blind.
As a result of scarce food in the deep sea, organisms
here have adapted by growing slowly with low
metabolic rates, reproducing late in life, and adopting
a sluggish and sedentary lifestyle. Most deep-sea
fishes have huge mouths with fearsome teeth, so
they consume prey that are larger than them. In
addition to large mouths, many have expandable
stomachs to accommodate the large prey once it has
been swallowed. Anglerfishes, well-known deep-sea
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Salps.90
FIGURE 3−21
FIGURE 3−22
Hydrothermal vent fauna.93
A drawing of a deep-sea anglerfish.92
FIGURE 3−23
Deep-sea microbes also play an important role in
helping to produce food for other deep-sea inhabitants.
Specialized deep-sea habitats such as hydrothermal
vents, cold seeps, and whale falls are ecosystems
that teem with life and are dependent on primary
production from chemosynthetic microbes. With no
sunlight for photosynthesis, the primary producers in
hydrothermal vent communities are chemosynthetic
bacteria and archaea. These chemosynthetic organisms
capitalize on hydrogen sulfide from the vents,
converting it to organic matter. Some organisms, such
as tube worms, even have symbiotic relationships with
chemosynthetic bacteria inside their guts that pass
organic matter to the worm.
Hydrothermal vents were first discovered along the
mid-ocean ridge by the submersible Alvin in 1977.
The scientists who discovered the vents found a rich
and incredibly productive community of organisms.
Worms, clams, shrimps, crabs, and fishes were found
flourishing in these habitats. Hydrothermal vents have
since been discovered near hydrothermally active areas
all around the world.
Whale falls are created when whale bodies sink to
the bottom. The whale carcass provides a sudden
but important food source for many other deep-sea
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inhabitants, have a modified dorsal fin that acts as a
pole, dangling bioluminescent bait to lure prey.
A whale fall community.94
organisms. As the whale carcass decomposes, different
marine organisms feed on and colonize the dead body.
Hagfish, sharks, and other scavenging organisms are
the first on the scene to remove whale tissue. After
approximately six months on the seafloor, only a
skeleton is left. At this time, worms called Osedax
mucofloris, or bone-eating worms, burrow into the
skeleton of the whale, consuming oil from the bones.
FEEDING AND FOOD WEBS
The living and nonliving parts of an ecosystem interact
in complex ways to sustain life. All living things use
energy to make and maintain the organic molecules
necessary for life. Energy is the currency of an
ecosystem, and it flows from autotrophs to heterotrophs.
Autotrophs, or the primary producers, typically gain
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FIGURE 3−24
Arctic food web.95
energy from the sun, use the energy to make their
own food, and then they themselves serve as food
for heterotrophs. When a heterotroph consumes an
autotroph, the organic material and the energy stored
in the autotroph is passed from the autotroph to the
heterotroph. That is, energy and chemical substances
flow from the nonliving components to the living
components of the ecosystem. The living components
then transfer the energy from producers to consumers
and on to other consumers. This flow of energy through
the ecosystem is called a food chain.
Each step in the food chain is called a trophic level.
In reality, many ecosystems have several primary
producers and consumers that eat more than one
food type (omnivores), and thus the food chain is
actually interconnected like a web and is termed a
food web. The first trophic level in a food chain or
web includes the primary producers, like diatoms and
other phytoplankton. Consumers that feed directly on
the primary producers are called primary consumers.
Secondary consumers, such as small crustaceans or
fish, predate on the primary consumers, while tertiary
consumers, larger fish, turtles, and birds, predate on
secondary consumers, and so on. At the very top of the
food web are the top predators such as sharks or orcas.
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SECTION III SUMMARY
6 The scientific study of how organisms
interact with each other and with the natural
environment is called ecology. An ecosystem
is a biological community of interacting
organisms and their physical environment.
6 The nonliving components of an environment
are called abiotic. Abiotic factors in marine
environments can include water temperature,
acidity, pressure, and so on. The biotic features
of an ecosystem are the living components, or
the organisms.
6 The distribution of marine organisms in the
ocean is determined by limiting environmental
factors. Environmental factors can be physical
or biological.
6 In an ecosystem, different species interact
with each other, and often biological factors
contribute to organismal diversity and
distribution. Organisms compete for resources,
predate upon each other for food, or can have
symbiotic relationships.
6 Across the globe, marine habitats can be
divided into distinct communities based on
their characteristics.
6 The intertidal zone, the space between high
and low tide, is one of the best-studied marine
habitats.
6 Seaweed communities exist in shallow waters
that are highly productive ecosystems where
light and nutrient conditions are optimal.
6 The space where freshwater from rivers enters
the sea is called an estuary. Estuaries are
areas that are semi-enclosed where freshwater
and saltwater mix. These habitats are very
productive and serve as nurseries for many
marine species.
6 Coral reefs are large geological structures that
are built by organisms and are made of calcium
carbonate (CaCO3).
6 The vast open ocean is often referred to as
the pelagic zone. In the open ocean, many
organisms spend their life suspended in water.
6 The deep sea is full of alien-like creatures,
reminiscent of those from science fiction
movies. The deep sea is dark, cold, and lacking
in photosynthesis. The deep sea makes up
the majority of marine habitats. Without
photosynthesis, many organisms in the deepsea depend on food that sinks down from
surface layers. As a result, much of the deepsea is sparsely populated with organisms.
6 All living things use energy to make and
maintain the organic molecules necessary
for life. Energy flows from autotrophs
to heterotrophs. Autotrophs, or primary
producers, typically gain energy from the sun,
using the energy to make their own food, and
they, in turn, serve as food for heterotrophs.
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Energy is lost at each level because some of the
consumed matter is excreted and not utilized, and also
because the consumers expend energy in the activities
they undertake. Indeed, ecological efficiency across
trophic levels is only about 10 percent. Bacteria, fungi,
and other decomposers assist in the breakdown of
organic matter into its original components, cycling it
back to the primary producers.
Section IV
Humans and the Ocean
Humans depend upon the ocean for many of the
resources and materials we use daily. The human
population on Earth is growing exponentially, with
more than 7.8 billion people currently living on
our planet. Humans have radically altered Earth’s
ecosystems, including the world’s oceans. The
anthropogenic impacts on the ocean, or the effects
of human activities, include climate change, loss
of biodiversity and habitat, pollution, oil spills, and
overfishing, among others. Humans have modified
nearly every habitat on Earth, in some cases degrading
habitat and water quality, endangering many species,
and creating health hazards, such as microplastics.
FIGURE 4−1
RESOURCES FROM THE
OCEAN
Humans extract and rely on many resources from the
ocean, using both living and nonliving resources from
the sea. Unfortunately, we have used and misused
marine resources and habitats in a manner that has
caused their degradation.
Living Resources
The ocean is the largest factory of organic matter on
the planet. Food is the primary ocean living resource
humans use. Harvesting biological resources from
the ocean provides food as well as jobs for billions of
people across the world. A wide variety of organisms
are harvested from the ocean for human consumption,
ranging from sea cucumbers and worms to fishes,
molluscs, and crustaceans. Fish that are harvested from
the ocean for food are referred to as finfish and make
up the majority of the seafood catch. Molluscs and
crustaceans that are harvested for human consumption
are together referred to as shellfish.
Seafood is a major source of protein for humans
A fisheries vessel.96
around the globe, especially in coastal areas.
Specifically, the world produces over 150 million
tons of seafood each year. The harvesting of seafood
provides employment and profit. Sadly, the majority of
the fish harvested for food are exhausted, with fisheries
being overexploited or harvested at a rate higher
than the population can recover. Overfishing will be
discussed more toward the end of this section.
Fisheries resources are primarily harvested from
the continental shelf, as this is where the majority
of biomass is concentrated in the ocean. Harvests
from this area are collected from both the pelagic and
benthic zones using nets and trawls. Clupeoid fishes,
like sardines, herring, and anchovies, make up a major
portion of the fisheries harvested from the ocean.
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INTRODUCTION
FIGURE 4−2
FIGURE 4−4
Clupeoid Fish.97
FIGURE 4−3
An oil rig platform.99
Nonliving Resources
A mangrove ecosystem.98
Typically, clupeoid fishes congregate over the
continental shelf and are collected using large purse
seine nets. These fish are often ground into fish flour,
which is used as a protein and dietary supplement for
humans, or fish meal, which is used to feed poultry and
livestock. Clupeoides are also pressed to make fish oil,
which is rich in omega-3 fatty acids and is used as a
human dietary supplement.
Benthic fishes such as cod and haddock are caught using
trawls or nets that are dragged along the bottom. Due to
heavy fishing pressure, Atlantic cod populations were
decimated in the latter part of the twentieth century,
which resulted in a closure of the fisheries in eastern
Canada and the eastern U.S. Neither the fisheries nor the
region has fully recovered from this collapse.
In addition to fisheries harvested for food, the marine
environment provides living resources for commerce
and recreation. For example, mangroves are harvested
for timber, and some medicines are obtained from
marine organisms. The ocean also provides a way
The primary non-living resources from the ocean
are oil, gas, and minerals. These are non-renewable
resources, or resources that are not replenished at the
rate at which they are used. Oil and gas are extracted
from the sea floor. There are oil rigs on continental
shelves of every continent except for Antarctica.
Ocean engineering has overcome incredible obstacles,
including powerful currents, waves, and weather,
to extract oil and gas from deeper and deeper parts
of the ocean. Exploratory drilling to find more oil
reserves occurs from drill ships or platforms that can
be anchored in place on the seafloor. If oil and gas
are found, then a platform is typically constructed to
extract the resource.
Minerals are also mined from the ocean. The main
mineral obtained from seawater is table salt, or sodium
chloride (NaCl). Salt is mined by evaporating seawater.
When water evaporates, the salts are left behind. Other
resources are also mined from the ocean. In particular,
sand and gravel are mined for the construction and
glass industries. Additionally, we may soon begin
mining metals, such as manganese, nickel, copper,
and cobalt, from the deep ocean basins. This deep-sea
mining could present dangers to deep ocean life, and
ocean ecosystems could be affected by plumes from
this mining.
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to relax and spend vacation time. Humans travel to
coastal areas to partake in recreational activities such
as diving, snorkeling, fishing, and surfing. Ocean
tourism is currently valued globally at $390 billion.
FIGURE 4−5
A desalination plant.100
Oil spill effects on marine life.101
Freshwater is another nonliving resource that humans
can obtain from the ocean. Freshwater is made via
desalination plants, where seawater is converted
into freshwater. Desalination plants are particularly
common in coastal areas that lack a sufficient amount of
freshwater, such as desert regions. While desalination
can provide freshwater, it requires a large amount
of energy and creates significant carbon emissions,
and it is therefore expensive to employ and may have
indirect consequences on the oceans. When renewable
energy sources can be paired with desalination, they
make this industry more economically practical and
environmentally responsible.
as well as for raw materials for plastics, rubber, and
fertilizers. Petroleum is one of the most widespread
pollutants in the ocean, entering from both runoff,
chronic leakage, and major oil spills. Though spills are
greatly impactful when they occur, the majority of the
petroleum in the ocean comes from runoff associated
with human activity. Petroleum pollution from landbased sources enters the ocean from runoff from city
streets, from waste dumped down drains, or from trash
in landfills. Petroleum in the ocean also comes from
marine transportation and fuel from small boats and jet
skis. Oil spills at sea can originate from tanker spills,
pipeline spills, or from accidents that occur during the
exploration and extraction of oil from the seafloor. Oil
can have a toxic effect on marine organisms, causing
interference with reproduction, development, growth,
and behavior.
ANTHROPOGENIC IMPACTS
The overall health of ocean ecosystems is declining as
a result of human impacts. This section will explore
the numerous ways in which humans have altered the
ocean and its communities.
Marine Pollution
Human activities are the source of the majority of
pollutants that make their way to the ocean. Marine
pollution is the introduction of a substance or energy
that results in a change in the quality of the ocean.
Marine pollution can affect the physical, chemical, or
biological environment. A pollutant causes damage by
directly or indirectly interfering with the mechanical
or biochemical process of an organism. The response
of the organism depends upon the quantity, toxicity,
and persistence of the pollutant.
Crude oil or petroleum is refined by humans for fuel
In 2010, a large, catastrophic oil spill known as the
Deepwater Horizon spill occurred in the Gulf of
Mexico when a drilling platform exploded. A variety
of organisms, ranging from seabirds and mammals
to sea turtles, were immediately impacted by the
spill. The spill also negatively affected fisheries in the
region, causing an estimated $8.7 billion in economic
impact and a total loss of 22,000 jobs. Containing
and cleaning up an oil spill is incredibly challenging.
Attempts are made to contain the spill with booms and
skim the oil from the surface with boats. Other times,
chemical dispersants are added to break the surface oil
into small droplets to allow the oil to disperse in the
water. Dispersants do not cause the oil to disappear,
but rather cause it to sink. The use of dispersants is not
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FIGURE 4−6
FIGURE 4−7
FIGURE 4−9
The Deepwater Horizon oil spill.102
Marine debris washed up on the beach.104
FIGURE 4−8
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FIGURE 4−10
Microplastics.105
Biomagnification.103
ideal because they are incredibly toxic to marine life,
and in some cases, dispersants are as harmful, if not
more harmful, as the oil itself.
In addition to oils, other toxic substances, such
as pesticides and heavy metals, are also common
marine pollutants. Pesticides and heavy metals are
termed persistent toxins because they remain in the
environment for years or even decades. Pesticides are
used to kill insects and herbicides are used for weed
control on land, but they make their way to the ocean
by rivers and runoff. While pesticides and herbicides
have been helpful to humans, they are known to be
harmful to marine life. In particular, pesticides in the
ocean are absorbed by unicellular algae. Pesticides
and other persistent toxins often dissolve in fats and
are not excreted; thus, they remain in the organisms
for life. At each level of the food chain, these toxins
become more concentrated. This process is known as
biomagnification. Concentrations of toxins are highest
in organisms at the top of the food chain, such as birds
and mammals.
Biomagnification also occurs with heavy metals such
as mercury and lead. An excess of heavy metals can
be toxic to organisms. Mercury, for example, is used in
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FIGURE 4−11
Garbage patches in the Pacific Ocean basin.106
Solid waste in the ocean is mostly composed of plastics.
Plastic is an essential part of our everyday lives;
however, single-use plastics often end up in landfills or
are disposed of improperly. Marine debris is a persistent
pollution problem that is found throughout the ocean.
Marine debris originates from humans and can range
from tiny microplastics—plastic pollution smaller than
5mm—to abandoned fishing gear. Lost, abandoned, and
discarded fishing nets, pots, and other gear continues to
trap target and non-target fish and marine mammals, a
process called “ghost fishing.”
Many marine species are impacted by marine debris
when it is ingested or when they become entangled.
Marine debris tends to collect in the center of ocean
basins from rotating currents, which push the debris
into the middle of the basins. Areas in the ocean where
this marine debris collects from the circulating ocean
currents are termed garbage patches. Unfortunately,
garbage patches exist in every major ocean basin on
Earth.
FIGURE 4−12
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the production of plastics and enters the ocean through
runoff. High mercury levels have been found in large
fishes such as tuna and swordfish, which can have
levels too high for human consumption. At the very
top of the food chain, orcas have shown extremely high
levels of heavy metal accumulation. The exposure is
made worse when changing ocean conditions deprive
them of adequate prey, and they must metabolize fats,
where heavy metals are stored. As you see, human
impacts can act in concert to affect marine life.
Eutrophication.107
Eutrophication
When the marine environment has been enriched with
excessive amounts of nutrients, eutrophication can
result. Eutrophication is the overgrowth of algae as a
result of increased nutrients in the ecosystem. Excess
nutrients (particularly nitrates and phosphates) come
from fertilizers used in agriculture, from runoff, and
from sewage. Though nutrients are needed for primary
production in the ocean, humans have input excess
nutrients into the ecosystems, causing overgrowth,
primarily in coastal areas. Eutrophication can cause
damage to coastal environments such as seagrass beds
and coral reefs. Harmful algal blooms and dead zones
can result from eutrophication. Harmful algal blooms
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occur when algae grow out of control and produce a
toxin that can have a harmful effect on shellfish, birds,
marine mammals, and even people. Some illnesses
that result from harmful algal blooms can be fatal for
humans and marine organisms. Dead zones, which are
commonly seen with eutrophication, occur from an
overgrowth of decay bacteria that results from algal
overgrowth. Over 405 dead zones have been identified
worldwide. In dead zones, the bacteria use up all the
oxygen, creating anoxic conditions where the water
is lacking in oxygen. As a result, the oxygen-depleted
area is void of life because the living organisms either
leave if they can swim, or die if they are not mobile.
Habitat Modification
Fisheries resources are renewable resources, meaning
the resources can replace themselves when harvested
sustainably. However, many resources from the ocean
are overexploited, and overfishing results. Overfishing
occurs when humans take fish from the ocean at a rate
too high for the fished species to replace itself. As a
result, catches dwindle, and the fish that are caught
are smaller and smaller. To harvest a fish species in a
sustainable way, it is of utmost importance to consider
the species population growth rate. Enough individuals
must be left behind that are capable of reproduction
in order to replenish the population. To be harvested
in a sustainable way, the number of fish caught must
be less than the number of fish that are added through
reproduction in the same time period. Many populations
of fishes on Earth are overfished or fully exploited,
meaning they are fully depleted. The cod fishery is a
great example of an overfished, fully exploited fishery.
Importantly, different species have different levels of
resilience to fishing pressure. Clupeoids grow rapidly and
reproduce early, so they are able to replenish themselves
even with heavy fishing pressure. In contrast, large, slowgrowing fishes such as tunas, sharks, and swordfish are
much more easily overfished. Thus, overfishing is one of
the biggest threats to the mostly large marine fishes that
are on the endangered species list.
Fisheries resources need to be harvested in a way that
does not put them at risk for overfishing. The first
step in properly managing fisheries resources is to
estimate the sustainable number of individuals of a
species that can be harvested. However, this can be very
difficult. For example, fisheries biologists need detailed
information about each species in terms of how fast they
grow and reproduce, how long they live, and what they
eat. All this information is difficult to measure for many
species. Additionally, other factors such as destruction
of habitat, pollution, and climate change can affect the
reproductive capacity of a fished species and complicate
their sustainable management.
Bycatch, or species caught unintentionally while fishing
for other species, is also a contributing problem to
overfishing. Many types of fishing gear catch non-target
species, and often the non-target species are discarded.
Shrimp nets catch more bycatch by biomass than shrimp
due to the small mesh size of the net. Bycatch such as
turtles, seabirds, and mammals are also caught in nets
and often die as a result of drowning.
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Sea turtle bycatch.108
Humans have modified and destroyed many marine
habitats through dredging, landfilling, development,
fishing, and explosives. Most habitat destruction occurs
along the coasts, close to where humans live. Bays and
estuarine habitats are often the most impacted. These
coastal habitats often serve as nurseries for many ocean
species. In coastal areas, increased human populations
are placing increased pressure on marine habitats.
Trawling—dragging nets along the bottom to collect fish
and shrimp—can also cause major damage to intertidal
habitats. The trawl nets scour the seafloor, leaving
scars on the bottom. Additionally, trawling causes the
resuspension of sediment and toxic chemicals, killing
suspension feeders. Seabed mining is an up-and-coming
threat that will have impacts on marine habitats as
humans extract resources from the seafloor. Perhaps the
most extreme form of habitat modification was the use
of remote atoll reefs in the Pacific Ocean between 1946
and 1962 for nuclear testing.
Overfishing
FIGURE 4−13
FIGURE 4−14
FIGURE 4−15
A fish farm in Greece.109
In addition to food, other species are grown for cultured
pearls and for the aquarium trade. Farming at sea is not
without environmental impacts, and there are several
challenges to sustainable mariculture. For instance,
pollution can be a major consequence of mariculture
operations. Fish are concentrated in ponds along the
coast or in floating pens, which releases feces, urine,
and leftover food to the surrounding environment. This
imparts an excess of nutrient input in the ecosystem
(eutrophication) and degrades water quality. Other
impacts of mariculture include introduced chemicals,
such as antibiotics and pesticides, as well as synthetic
pigments used to dye the fish flesh. Escaped mariculture
species can breed with their wild cousins, resulting
in negative genetic effects on the population. Often,
critical coastal habitats, such as mangrove forests and
salt marshes, are cleared for marine farming, causing
damage to important marine ecosystems.
Introduced Species
Introduced species, often referred to as exotic or
naturalized species, are organisms that are not native to
the place or area where they are currently found; while
many introduced species never become common, some
species, termed invasive species, will flourish in new
Lionfish.110
ecosystems, often causing major damage to the invaded
ecosystem. Invasive species are defined as species that
have been accidentally or deliberately transported to the
new area by humans or human activity. Invasive species
can pose threats to native species, often outcompeting
or predating native species, introducing disease, and
causing a reduction in biological diversity. In marine
environments, introduced species are often transported
via the ballast water of tankers and ships. (Large ships
hold freshwater or saltwater in tanks called ballast tanks
to help provide stability and maneuverability during
transport.) Sometimes, introduced species are brought to
an area on purpose by humans as a form of pest control.
Other times, the introduction occurs accidentally
or intentionally by aquarists. For example, lionfish,
native to the Indo-Pacific, were likely introduced in
the Atlantic Ocean from the aquarium trade when they
were released into a new marine habitat.
Lionfish quickly spread up and down the East Coast
of the United States, in the Caribbean, and along the
East Coast of South America. Lionfish are voracious
predators, with venomous spines that help protect them
from potential predators in their nonnative environment.
As a result, lionfish populations have proven challenging
to control and have altered the communities to which
they were introduced.
Climate Change Impacts on the Oceans
Among the greatest emerging threats to the ocean
are climate change and ocean acidification. Climate
change is having multiple negative impacts on the
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Aquaculture is a method of rearing aquatic organisms
that is becoming critical for supporting the seafood
demands of the growing human population. In 2014,
aquaculture supplied fully half of the fish and shellfish
consumed by humans. Mariculture is the farming
or aquaculture of marine organisms. In mariculture,
ocean animals are bred, reared, and harvested in the
marine environment or in a closed system that mimics
the marine environment. The majority of aquacultured
species are grown for human consumption.
The greenhouse effect.111
ocean. Climate change is causing the oceans to
warm and become more acidic, and it is reducing
concentrations of oxygen in the ocean. Both climate
change and ocean acidification are induced by excess
carbon dioxide emissions that result from human
activities. Temperatures around the world are rising,
and weather patterns are shifting. Emissions of carbon
dioxide, as well as methane and nitrous oxide, have
intensified the greenhouse effect, a natural process that
warms the surface of the Earth.
The temperature of the ocean, particularly near the
surface, is increasing. The warming of the ocean causes
the water cycle to speed up, intensifies existing weather
patterns, slows thermohaline circulation, and increases
density stratification. Ocean warming also contributes
to a decline in oxygen in the ocean, as warmer water
cannot retain as high a concentration of dissolved
oxygen as compared to cooler water. Salinity patterns
are also shifting as a result of the increased evaporation
of ocean water in temperate areas, whereas tropical
areas are seeing an influx of freshwater. Additionally,
ice sheets and glaciers are melting, causing an influx of
freshwater into marine habitats and sea level rise. Sea
level rise is the increase in the height of the sea surface.
Sea level rise stems from an increase in the volume of
water due to thermal expansion. Sadly, sea level rise
is expected to increase coastal erosion, disrupt coastal
ecosystems (e.g., submerging mangrove forests and salt
marshes), and displace millions of humans who live in
low-lying coastal regions.
Many marine organisms may be unable to adapt to
a changing ecosystem, especially with the rate of
change that is currently occurring. Therefore, the rate
of extinctions in the ocean is expected to far outstrip
typical rates of extinction. Coral reef organisms
are particularly vulnerable to climate change, as
they respond to increased sea surface temperatures
by expelling their zooxanthellae. The expulsion of
zooxanthellae due to unfavorable environmental
conditions is called coral bleaching. Without their
symbiotic zooxanthellae, the coral tissue and skeleton
left behind appears white, hence the term coral
bleaching. Corals can recover from temporary heat
stress; however, extended and persistent heat stress
causes coral mortality. Coral bleaching events are
becoming more common and more severe. Between
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FIGURE 4−16
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FIGURE 4−18
Bleached coral.112
2014 and 2017, more than 75 percent of coral reefs
experienced heat stress, and more than 30 percent
experienced heat-related mortality. Without massive
reductions in carbon emissions, few coral reefs are
expected to survive the coming decades.
Ocean acidification refers to the decrease in the pH of
the ocean, making it more acidic and reducing available
carbonate. This change in seawater chemistry results
from the absorption of carbon dioxide. Human activities
have been the direct cause of excess carbon dioxide in
the atmosphere, which is then absorbed by the ocean. A
decrease in the pH of the ocean means that organisms
with shells, such as molluscs and foraminiferans, must
use more energy to obtain dissolved calcium carbonate
to build these shells. Over the past couple of centuries,
the pH of the ocean has decreased by 0.1 pH units,
which represents approximately a 30 percent increase
in acidity. Such a rapid change in pH can have drastic
effects on marine life. For example, bivalves and snails
show a decrease in growth rate, especially in the
larval stages. Other animals that deposit a test made
from calcium carbonate, such as echinoderms, show
a decrease in fertilization success and developmental
rates. These direct effects of ocean acidification also
have indirect effects in the food chain, which ultimately
affect humans.
CONSERVATION AND
PROTECTION
Although humans have altered the ocean in many
negative ways, they have made efforts to conserve,
protect, and restore marine ecosystems. The protection
and conservation of the marine environment is of
immediate importance to human health and survival.
Humans depend upon the ocean for resources and
services. Many jobs, foods, and medicines are tied
to the health of the oceans. The key to the successful
conservation of marine habitats is the employment
of science-informed management and well-enforced
regulations. Using ocean resources wisely and
sustainably is essential for their preservation for
future generations. Efforts are underway to reduce
overfishing by addressing illegal and unregulated
fishing that occurs in international waters where no
single country has jurisdiction and to ban destructive
fishing methods. Legislation has been passed to
address pollution, and there have been bans passed
on one-time-use plastics and on the production of
microplastics. Efforts have also been made to protect
and restore marine habitats such as coral reefs
and mangroves. Importantly, the conservation and
protection of the ocean will require human societies to
address carbon emissions and climate change.
Marine Protected Areas
All over the world, governments have established
marine protected areas (MPAs) to aid in the protection
and management of areas of ecological importance.
Marine protected areas limit human activities in a
part of ocean, allowing the communities that inhabit
the area to flourish in the absence of anthropogenic
stressors, like fishing or drilling for oil. In the United
States, MPAs can be sanctuaries, research reserves,
ocean parks, or marine wildlife sanctuaries. MPAs are
often employed to protect ecosystems; preserve cultural
resources, such as archaeological sites; or to help sustain
fisheries resources. Marine protected areas vary in the
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Ocean acidification.113
Habitat Restoration
Habitat restoration is often employed to improve the
degraded quality of the marine environment due to
human impacts. Habitat restoration helps an ecosystem
recover from destruction and pollution. With habitat
restoration, species are typically transplanted or
restocked from healthier areas. Salt marshes and
mangrove forests are two habitats that have been greatly
impacted by habitat destruction. Thus, these areas often
become the focus sites for habitat restoration. Habitat
restoration sometimes involves protecting the habitat
for natural regeneration, while other times replanting or
transplanting dominant species is involved. For example,
the cordgrass Spartina is often transplanted to help
accelerate the recovery of salt marshes. Often habitat
restoration includes the removal of pest or invasive
species. Oyster beds and kelp forests have also been
focus sites for habitat restoration. Habitat restoration
is a good step in helping to restore destructed habitats,
but of course the best course of action is preventing the
degradation of marine habitats.
The improvement of habitats can also include the
building of artificial reefs. Artificial reefs are humanmade structures that are deployed in the ocean to
provide high-relief structure and surfaces for reef
organisms like fishes and corals to thrive. Artificial
reef materials range from concrete blocks to discarded
ships. Although increases in biomass and yield of
fisheries species occur with artificial reefs, there is
some concern that artificial reefs only concentrate
FIGURE 4−19
An example of an artificial reef.114
wildlife in one spot, making it easier to catch.
SECTION IV SUMMARY
6 Humans extract and rely on many living and
nonliving resources from the ocean.
6 Seafood is one of the primary living resources
humans utilize from the ocean. The marine
environment also provides resources for
commerce and recreation. Humans travel to
coastal areas to partake in recreational activities
such as diving, snorkeling, fishing, and surfing.
6 The primary nonliving resources from the
ocean are oil, gas, and minerals. These are
nonrenewable resources, or resources that are
not replenished at the rate at which they are
used. There are important environmental risks
that need to be managed with the use of these
resources.
6 The overall health and functioning of ocean
ecosystems are declining as a result of human
impacts.
6 Marine pollution is the introduction of a
substance or energy that results in a change
in the quality of the ocean. Marine pollution
can affect the physical, chemical, or biological
environment.
6 Oil is one of the most widespread pollutants in
the ocean, entering both from runoff and oil
spills.
6 Pesticides and heavy metals are termed
persistent toxins because they remain in 6
the environment for years or even decades.
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ways that humans can use them. For example, some
MPAs are open for fishing, while others are not; some
areas allow for shell collecting, while others do not.
Each protected area has its own management and set
of regulations. Over time, MPAs allow for increases in
marine species size, abundance, and diversity. These
positive effects depend on the MPA’s age, size, and how
strictly it limits human activities. The most beneficial
MPAs for recovery from overfishing are “no-take”
marine reserves, which have benefits even outside of
their boundaries. There are more than 1,200 marine
protected areas in U.S. waters alone, and there are
MPAs all over the world. For example, the Great Barrier
Reef in Australia is designated as an MPA. Scientists
estimate that around 30 percent of the world’s oceans
should be protected to reach recovery goals; currently,
only about 7.5 percent of the world’s oceans are under
some form of protection.
6 Marine debris is a persistent pollution problem
that is found throughout the ocean and mostly
consists of plastic waste and discarded fishing
gear.
6 Eutrophication is the overgrowth of algae as a
result of increased nutrients in the ecosystem
from agricultural runoff and sewage waste.
6 Humans have modified and destroyed many
marine habitats through dredging, landfilling,
development, fishing, and explosives. Most
habitat destruction occurs along the coasts,
close to where humans live.
6 Fisheries resources are renewable resources,
meaning the resources can replace themselves
when harvested sustainably However, many
fisheries are currently overfished and are not
able to replace themselves. By-catch, or species
caught unintentionally while fishing for other
species, has led to the decline of many nontarget species.
6 Mariculture is the farming or aquaculture of
marine organisms and is vital for supporting
the seafood demands of many nations. In
mariculture, animals are bred, reared, and
harvested in the marine environment or
in a closed system that mimics the marine
environment.
6 Introduced species are organisms that are
not native to the place or area where they are
currently found. Invasive species are a subset
of introduced species that cause major damage
in the new ecosystem and often reach high
abundance.
6 Among the greatest emerging threats to
the ocean are climate change and ocean
acidification. Both climate change and ocean
acidification are induced by excess carbon
dioxide emissions as a result of human
activities. Temperatures around the world
are rising, and weather patterns are shifting.
Emissions of carbon dioxide, as well as
methane and nitrous oxide, have intensified
the greenhouse effect. Ocean acidification is
the decrease in the pH of the ocean, making it
more acidic.
6 The protection and conservation of the marine
environment is of immediate importance to
human health and survival. Marine protected
areas and habitat restoration are two of many
tools used to conserve marine environments.
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Pesticides are used to kill insects and for weed
control on land but make their way to the
ocean via rivers and runoff.
Though the ocean presents challenges to study, humans
have nevertheless learned a vast amount about the ocean
through exploration and experimentation. The marine
environment provides many ecosystem functions and
services to humans as well as material goods and
cultural benefits. Marine biology is simply the study
of life in the ocean, incorporating chemistry, physics,
math, and engineering. Marine biologists play a vital
role in researching the marine environment and in
investigating the effect of human activities on the planet.
Through this resource guide, you have learned that
marine biology is the multifaceted study of living
organisms in the ocean and how they interact with
their surrounding environment. You have discovered
the importance of the ocean and the marine
resources upon which humans depend. You have
learned about the chemical, physical, and geological
processes of the ocean and how these components
influence the biological factors. You now have a
greater understanding of—and hopefully a greater
appreciation for—the organisms that inhabit the
ocean. You have also explored the ways in which
humans have transformed ocean ecosystems, as well
as the efforts that are now being made to conserve and
protect the ocean. It is vital that human populations
ensure that the ocean is protected and its resources
are conserved. As Jacques Yves Cousteau, a modern
pioneer in marine biology, said, “The sea, once it casts
its spell, holds one in its net of wonder forever.” The
ocean, when treated with care, represents both a vital
resource and a place of beauty and wonder.
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Conclusion
1. NASA https://images.nasa.gov
2. https://commons.wikimedia.org/wiki/File:Aristotle_Altemps_Inv8575.
jpg
3. https://commons.wikimedia.org/wiki/File:Captainjamescookportrait.jpg
4. https://en.wikipedia.org/wiki/File:Cook%27sThirdVoyage58.png
5. https://commons.wikimedia.org/wiki/File:Charles-Darwin-31.jpg
6. h ttps://commons.wikimedia.org/wiki/File:NSF_Research_Vessels_
Laurence_M._Gould_and_Nathaniel_B._Palmer_-_showing_their_
relative_size.jpg
7. https://commons.wikimedia.org/wiki/File:Figure_01_01_05.png
8. N
OAA Image Library Photo by Caroline S. Rogers, available through
NOAA.
9. h ttps://commons.wikimedia.org/wiki/File:World_map_ocean_locator-en.
svg
10. https://commons.wikimedia.org/wiki/File:Earth_layers_NASA.png
11. https://commons.wikimedia.org/wiki/File:Pangaea_to_present.gif
12. https://commons.wikimedia.org/wiki/File:Tectonic_plate_boundaries.
png
13. h ttps://oceanexplorer.noaa.gov/image-gallery/welcome.html#cbpi=/
okeanos/explorations/ex1703/dailyupdates/media/mar9.html
14. https://commons.wikimedia.org/wiki/File:Plataforma_Continental.png
15. https://commons.wikimedia.org/wiki/File:Oceanic_basin.svg
16. B
y en:User:Booyabazooka - http://upload.wikimedia.org/wikipedia/
en/b/b7/Oceanic-continental_convergence_Fig21oceancont.svg, Public
Domain, https://commons.wikimedia.org/w/index.php?curid=3260825
17. https://commons.wikimedia.org/wiki/File:Marianatrenchmap.png
18. h ttps://commons.wikimedia.org/wiki/File:World_Distribution_of_MidOceanic_Ridges.gif
19. I mage courtesy of the NOAA Office of Ocean Exploration and Research,
2016 Deepwater Exploration of the Marianas
20. M
arine Divisions By Chris huh (Original text: K. Aainsqatsi) / K.
Aainsqatsi - self-made (Original text: self-made), Public Domain, https://
commons.wikimedia.org/w/index.php?curid=3251432
21. h ttps://commons.wikimedia.org/wiki/File:Water_molecule_3D_withsigns.svg
22. https://commons.wikimedia.org/wiki/File:210_Hydrogen_Bonds_
Between_Water_Molecules-01.jpg
23. https://commons.wikimedia.org/wiki/File:Water_strider_in_a_pond_2.
jpg
24. N
OAA Image courtesy of Caitlin Bailey, GFOE, The Hidden Ocean
2016: Chukchi Borderlands
25. https://commons.wikimedia.org/wiki/File:Sodium_chloride.JPG
26. https://commons.wikimedia.org/wiki/File:Atmosphere_layers.jpg
27. https://commons.wikimedia.org/wiki/File:Coriolis_effect.svg
28. h ttps://en.wikipedia.org/wiki/Ocean_gyre#/media/File:Oceanic_gyres.
png
29. NASA Image https://www.jpl.nasa.gov/images/earth/20100325/
atlantic20100325-full.jpg
30. h ttps://commons.wikimedia.org/wiki/File:Water_wave_diagram.jpg
31. https://commons.wikimedia.org/wiki/File:Tide_schematic.svg
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
https://commons.wikimedia.org/wiki/File:Alpha-D-Glucopyranose.svg
https://commons.wikimedia.org/wiki/File:Photosynthesis_en.svg
https://commons.wikimedia.org/wiki/File:Auto-and_heterotrophs.png
https://commons.wikimedia.org/wiki/File:Carbon-cycle-full.jpg
J apan Agency for Marine-Earth Science and Technology (JAMSTEC),
https://www.jamstec.go.jp/e/about/press_release/20180523/
https://commons.wikimedia.org/wiki/File:Phosphorus_cycle.png
h ttps://commons.wikimedia.org/wiki/File:The_Different_Types_of_
Animals_.jpg
https://commons.wikimedia.org/wiki/File:Cyanobacteria_guerrero_
negro.jpg
h ttps://en.wikipedia.org/wiki/File:Diatoms_through_the_microscope.
jpg
https://commons.wikimedia.org/wiki/File:Dinoflagellates.jpg
https://commons.wikimedia.org/wiki/File:Zooxanthellae.jpg
h ttps://commons.wikimedia.org/wiki/File:Mixed_zooplankton_sample.
jpg
h ttps://commons.wikimedia.org/wiki/File:Antarctic_krill_(Euphausia_
superba).jpg
h ttps://commons.wikimedia.org/wiki/File:Ulvophyceae_composite.jpg
https://www.nps.gov/subjects/oceans/plants-alga-plankton.htm
https://commons.wikimedia.org/wiki/File:Crustose_coralline_algae,_
South_East_Bay,_Three_Kings_Islands_PA121443.JPG
https://commons.wikimedia.org/wiki/File:Seagrass_at_Rapid_Bay_
Jetty_P1262907.JPG
https://commons.wikimedia.org/wiki/File:Snails_eating_fungus_on_
cordgrass.png
https://en.wikipedia.org/wiki/Florida_mangroves#/media/File:Red_
mangrove-everglades_natl_park.jpg
h ttps://commons.wikimedia.org/wiki/File:Sponges_in_Caribbean_Sea,_
Cayman_Islands.jpg
https://commons.wikimedia.org/wiki/File:Cnidaria.png
h ttps://commons.wikimedia.org/wiki/File:Pseudoceros_bifurcus_-_
Blue_Pseudoceros_Flatworm.jpg
https://commons.wikimedia.org/wiki/File:Riftia_tube_worm_colony_
Galapagos_2011.jpg
h ttps://commons.wikimedia.org/wiki/File:Marine_Snail_(16219581550).
jpg
h ttps://commons.wikimedia.org/wiki/File:Clams_on_Sandy_Hook_
beaches_-_panoramio.jpg
https://commons.wikimedia.org/wiki/File:Copepod.jpg
https://commons.wikimedia.org/wiki/File:Leucothoe_incisa_2.jpg
https://commons.wikimedia.org/wiki/File:Liocarcinus_vernalis.jpg
https://commons.wikimedia.org/wiki/File:Echinodermata.png
https://commons.wikimedia.org/wiki/File:Tunicate_komodo.jpg
h ttps://en.wikipedia.org/wiki/Lancelet#/media/File:Branchiostoma_
lanceolatum.jpg
https://commons.wikimedia.org/wiki/File:Hagfish_knot.jpg
https://commons.wikimedia.org/wiki/File:Chondrichthyes.jpg
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Notes
91. https://en.wikipedia.org/wiki/Pelagic_fish#/media/File:Bluefin-big.jpg
92. https://en.wikipedia.org/wiki/Deep_sea_fish#/media/File:Humpback_
anglerfish.png
93. h ttps://commons.wikimedia.org/wiki/File:Fauna_on_hydrothermal_
vents.jpg
94. https://commons.wikimedia.org/wiki/File:Whale_fall.jpg
95. h ttps://commons.wikimedia.org/wiki/File:Arctic_marine_food_web_2.
jpg
96. h ttps://commons.wikimedia.org/wiki/File:Krabbenkutter_Ivonne_
Pellworm_P5242390jm.JPG
97. h ttps://commons.wikimedia.org/wiki/File:Dorosoma_petenense.jpeg
98. h ttps://commons.wikimedia.org/wiki/File:Mangrove_swamp,_Iriomote_
Island,_Okinawa,_Japan.jpg
99. https://commons.wikimedia.org/wiki/File:Oil_platform_in_the_North_
Sea.jpg
100. https://commons.wikimedia.org/wiki/File:Port_Stanvac_Desalination_
Plant_P1000725.jpg
101. https://commons.wikimedia.org/wiki/File:Oiled_Bird_-_Black_Sea_
Oil_Spill_111207.jpg
102. h ttps://commons.wikimedia.org/wiki/File:Deepwater_Horizon_Oil_
Spill_-_Gulf_of_Mexico.jpg
103. https://commons.wikimedia.org/wiki/File:MercuryFoodChain-01.png
104. https://commons.wikimedia.org/wiki/File:Starr-130914-1458-Nama_
sandwicensis-habit_with_marine_debris_and_Kim-North_DesertLaysan_(24857189889).jpg
105. h ttps://commons.wikimedia.org/wiki/File:Microplastics_in_sediments.
jpg
106. https://commons.wikimedia.org/wiki/File:Garbagepatch1.jpg
107. https://commons.wikimedia.org/wiki/File:Algae_bloom._Bolles_
Harbor_(8741970440).jpg
108. h ttps://commons.wikimedia.org/wiki/File:Corpse_of_a_sea_turtle,_
drowned_in_a_fishing_net.jpg
109. https://commons.wikimedia.org/wiki/File:Fish_farm_Amarynthos_
Euboea_Greece.jpg
110. https://commons.wikimedia.org/wiki/File:Red_lionfish_near_Gilli_
Banta_Island.JPG
111. https://www.nps.gov/goga/learn/nature/climate-change-causes.htm
112. https://en.wikipedia.org/wiki/File:Keppelbleaching.jpg
113. https://commons.wikimedia.org/wiki/File:Effect_of_Ocean_
Acidification_on_Calcification.svg
114. https://commons.wikimedia.org/wiki/File:Artificial_Reef_by_
Reefmaker.jpg
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65. https://commons.wikimedia.org/wiki/File:Osteichthyes-examples.png
66. h ttps://commons.wikimedia.org/wiki/File:Modern-marine-reptiles-001.
jpg
67. https://commons.wikimedia.org/wiki/File:Falkland_Islands_
Penguins_42.jpg
68. https://commons.wikimedia.org/wiki/File:Pinniped_collage.jpg
69. https://commons.wikimedia.org/wiki/File:Polar_Bear_-_Alaska_
(cropped).jpg
70. https://commons.wikimedia.org/wiki/File:The_Cetacea.jpg
71. https://commons.wikimedia.org/wiki/File:Sirenia_Diversity.jpg
72. https://commons.wikimedia.org/wiki/File:Maldivesfish2.jpg
73. h ttps://commons.wikimedia.org/wiki/File:Gal%C3%A1pagos_marine_
iguana.jpg
74. https://en.wikipedia.org/wiki/Aquatic_mammal#/media/File:Humpback_
Whale_underwater_shot.jpg
75. h ttps://commons.wikimedia.org/wiki/File:Pygoscelis_papua_Nagasaki_Penguin_Aquarium_-swimming_underwater-8a.jpg
76. h ttps://en.wikipedia.org/wiki/Shoaling_and_schooling#/media/
File:Humpback_lunge_feeding.jpg
77. https://commons.wikimedia.org/wiki/File:Red_Nudibranch.jpg
78. h ttps://commons.wikimedia.org/wiki/File:Chelonibia_testudinaria.jpg
79. https://en.wikipedia.org/wiki/Cleaning_symbiosis#/media/File:Giant_
Moray_Eel_getting_cleaned.jpg
80. h ttps://en.wikipedia.org/wiki/Rocky_shore#/media/File:Ocean_from_
Leblon.jpg
81. h ttps://commons.wikimedia.org/wiki/File:Pensacola_Beach_1957_
White_Sand.jpg
82. https://commons.wikimedia.org/wiki/File:Tide_pools_in_santa_cruz.
jpg
83. h ttps://en.wikipedia.org/wiki/Kelp_forest#/media/File:Rockfish_
around_kelp_Monterey_Bay_Aquarium.jpg
84. https://commons.wikimedia.org/wiki/File:Estuaries.jpg
85. https://commons.wikimedia.org/wiki/File:DoubtfulSound-Fjord.jpg
86. https://en.wikipedia.org/wiki/Salt_marsh#/media/File:Spartina_
alterniflora.jpg
87. https://commons.wikimedia.org/wiki/File:The_Coral_Reef_at_the_
Andaman_Islands.jpg
88. https://en.wikipedia.org/wiki/Coral_reef#/media/File:Maldives_small_
island.jpg
89. https://commons.wikimedia.org/wiki/File:Anemone_purple_
anemonefish.jpg
90. https://commons.wikimedia.org/wiki/File:Salp.jpg
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