Lesson 3
What Lives in the Ocean?
Fast Fact
Going to School This photo shows part of an aquatic ecosystem—a school of fish. Almost 80
percent of all fish species school, or swim in groups, at some point during their life cycles.
Some schools can be small, with only 10 to 20 members. Other schools can have millions
of members and cover an area as large as a small city! In the Investigate, you will make
and observe an aquatic ecosystem.
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Investigate
Ecosystem in a Jar
Materials
 glass jar with lid
 gravel
 500-mL beaker
 tap water
 pond water
 pond snails
 aquatic plants (duckweed or elodea)
 duct tape
 masking tape
 marker
Procedure
1. Label a clean glass jar with your name and the date. Cover the bottom of the jar with gravel
or small rocks. Half-fill the jar with room- temperature tap water.
2. Measure 200 mL of pond water. Add this water to the jar. Put some aquatic plants into the
jar. Use the gravel at the bottom of the jar to anchor the plants.
3. Add 1 or 2 pond snails. Add more room- temperature tap water to the jar until the water
level is within 2.5 cm of the top.
4. Put the lid on the jar, and seal the lid closed with duct tape.
5. Place the jar in a bright area, but do not put it in direct sunlight. Observe the jar for several
days.
Draw Conclusions
1. Describe some observations you made of your mini-ecosystem.
2. How do you think temperature, light, and moisture affected the organisms in your
ecosystem?
3. Inquiry Skill Scientists use models to infer things about how organisms interact with their
environment. What can you infer about aquatic ecosystems by examining your model of a
freshwater ecosystem?
Investigate Further
Many aquatic organisms are microscopic. Predict what organisms may be living in the
pond water. Put a drop of pond water on a microscope slide to test your predictions.
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Reading in Science
VOCABULARY
intertidal zone p. 228
near-shore zone p. 230
open-ocean zone p. 230
coral reefs p. 232
SCIENCE CONCEPTS
 what kinds of ecosystems can be found in the ocean
 what environmental conditions help define each ocean ecosystem
READING FOCUS SKILL
COMPARE AND CONTRAST Look for the ways different ocean ecosystems are alike and
different.
Intertidal Zones
Earth's oceans contain the world's largest animals as well as countless microscopic organisms.
With all of this biological diversity, it should be no surprise that the oceans have many
different types of ecosystems. Recall that an ecosystem is a community of organisms and
their nonliving environment. Each of the ecosystems in the ocean exists in a major ocean
zone. Each ocean zone is a layer of the ocean that has unique types of plant and animal
communities.
Ocean zones are determined by the depth of the water. As depth increases, there is less light.
The shallowest and brightest ocean zone is the intertidal zone. The intertidal zone is the
area of the ocean between the levels of high tide and low tide. The environment of the
intertidal zone is always changing. At low tide, organisms in this zone may have to find
shelter from the hot sun. As the tide comes in, they must survive the
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rough water of incoming waves. They may spend hours in an undersea world until the tide
goes out again.
Intertidal organisms handle the changes in their surroundings in different ways. Many of them
bury themselves in the mud during low tide. This keeps them moist and protected. Crabs
hide under rocks and in other sheltered, moist spots when the tide goes out.
Mussels and barnacles close their shells tightly during low tide, trapping water inside. During
high tide, they open up to feed on plankton (PLANGK•tuhn) and other organic matter.
Plankton is made up of microscopic organisms that live near the ocean's surface. A single
gallon of seawater may contain more than a million of these organisms! Many use sunlight
to produce food by photosynthesis. Because many marine organisms feed on plankton,
plankton is the base of most marine food chains. This means that most marine organisms,
just like land organisms, acquire their energy directly or indirectly from sunlight.
COMPARE AND CONTRAST How does the environment of the intertidal zone differ
during high tide and low tide?
Math in Science
Interpret Data
The amount of sunlight determines which organisms can live at which depth in the ocean.
Do you think more organisms live closer to the surface or closer to the bottom of the
ocean? Explain.
---see chart pg.228
Sunlight Zone
0m to 200m
Twilight Zone
200m to 1000m
Midnight Zone
1000m to 4000m
Abyssal Zone
4000m to 6000m
Hadal Zone
6000m to 11,000m
---see pictures pg.228
Dolphins and other marine mammals must live close to the ocean's surface so they can
come up for air to breathe.
This squid lives deep beneath the surface of the ocean.
---see pictures pg.229
The intertidal zone is an important feeding ground for birds and other animals.
When the tide goes out, tide pools still hold water. Tide pools are home to many different
organisms.
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Near-Shore and Open-Ocean Zones
Moving seaward from the intertidal zone, you enter the near-shore zone. The near- shore zone
includes most of the ocean over the continental shelf, where the water gets no deeper than
about 200 m (650 ft). The near- shore zone is relatively shallow and receives a great deal
of sunlight. It is a much more stable ecosystem than the intertidal zone. This is why it is
teeming with life. Many different kinds of marine organisms inhabit the near-shore zone.
These include fish, jellyfish, krill (a small shrimp- like animal), seaweed, shrimp, and
plankton. The plentiful supply of fish attracts sea birds. It also attracts larger marine
animals. Dolphins, porpoises, sharks, and whales often feed on fish and krill in the nearshore zone.
Some types of whales, even the huge baleen whales, feed almost entirely on krill, small fish,
other small crustaceans, and plankton! Baleen whales eat by sucking seawater in through
their mouths. Comb-like structures in their mouths capture the food.
Farther out to sea, past the continental shelf, you enter the open-ocean zone. The open- ocean
zone includes most of the water over the continental slope and abyssal plain. Most of the
animals in the open-ocean zone live near the surface. Dolphins, krill, seals, swordfish,
tuna, whales, and plankton live in the open-ocean zone.
Because of the depth of the open-ocean zone, food is limited to water near the surface. In this
zone, many organisms are active swimmers. They must be able to swim long distances to
obtain food.
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Food becomes even scarcer as the ocean becomes deeper. At about 1000 m (3200 ft) below the
surface of the ocean, there is little or no light. The extreme darkness means that no
producer organisms that rely on sunlight can live there. Those that can be found have sunk
down from the surface. Life is also difficult at these depths because of enormous pressure
from the water above. At this depth, most organisms with lungs would be crushed!
However, sperm whales can dive to 3000 m (9600 ft), where they feed on giant squid. This
deep region of the ocean, which makes up about 90 percent of all the oceans, is like the
barest of deserts on land. Much of this zone is cold, deep, and dark.
COMPARE AND CONTRAST What are some differences between the near-shore zone
and the open-ocean zone?
Insta-Lab
Jellyfish Movement
Many jellyfish are unable to propel themselves effectively from place to place. How do
jellyfish move around? Set an aluminum baking pan on a table that is protected with
newspaper. Fill the pan half-full with water. Sprinkle a handful of confetti over the water
to represent a group of jellyfish. Gently move the pan to make small waves. Observe how
the confetti moves. What can you hypothesize about how jellyfish move around?
---see pictures pgs.230 & 231
Near-Shore and Open-Ocean Zones
The largest animal alive on Earth today, the blue whale, can eat about 3500 kg (7700 lb)
of krill a day!
Squid live in both the near-shore zone and the open-ocean zone. They eat fish, shrimp,
and other squid.
The organisms found in a particular ocean zone depend on the depth and on the amount
of light the zone receives.
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Coral Reefs
Small animals called corals, which live in shallow, sunlit, near-shore ocean waters, form
coral reefs, some of the largest structures on Earth built by living organisms. Despite their
large size, coral reefs are very delicate and easily damaged.
The corals that make up reefs use minerals dissolved in ocean water to form hard outer
skeletons. The living corals attach themselves
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to the skeletons of dead corals. Very slowly, a reef forms. Many kinds of algae and some
kinds of plants live on reefs. The reefs provide underwater surfaces that sunlight can reach
easily. They also provide shelter for many kinds of animals, as well as a large variety of
producers. Producers in coral-reef food webs include seaweed and plankton that live within
the corals.
COMPARE AND CONTRAST Contrast a coral reef with the deep ocean.
---see pictures pgs. 232 & 233
Coral reefs take up a little less than I percent of the ocean floor, but about 25 percent of
different marine species live in or around coral reefs! Sea urchins, corals, sponges,
surgeonfish, ant` many other organisms are found around coral reefs.
Coral Reef Habitat
Most corals are polyps that form groups. A polyp (PAHL•ip) has a body that is shaped
like a cylinder. Its mouth, which is surrounded by stinging tentacles, is located at the
end of its body.
This type of coral is not like the others. It is called an octocoral and can often be found in
and near reefs.
Many of the animals, including the coral animals themselves, rely on plankton for their
food source. Some types of fish eat the corals. Parrotfish eat the algae on the corals.
They often bite chunks off the reef and grind up the coral skeletons with teeth in their
throats.
These are soft corals. These animals are like corals, but they do not form a hard skeleton.
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Deep-Ocean Vents
Until recently, scientists hypothesized that all of Earth's ecosystems used sunlight as their
energy source. However, researchers have discovered a new kind of ecosystem in the deep
oceans. These ecosystems, which lie more than 1000 m (3200 ft) beneath the surface, seem
to get energy from chemical reactions.
On some parts of the ocean floor in the abyssal zone, volcanic vents spew water heated to
around 350°C (662°F). The water contains dissolved sulfur- and iron-based chemicals.
Certain bacteria use these chemicals, rather than sunlight, to produce and store energy.
These bacteria are the primary producers in deep-ocean vent ecosystems. All other
members of the ecosystem depend on the energy the bacteria store. In fact, some animals
in these ecosystems have no stomachs or mouths.
Instead, they live with these bacteria in their bodies. They take nutrients from the bacteria
directly into their blood. Many species living near deep-ocean vents were unknown before
these ecosystems were discovered.
COMPARE AND CONTRAST How do deep-ocean vent ecosystems differ from all other
known ecosystems?
---see pictures
Deep-ocean vent ecosystems have been found in the Atlantic, Pacific, and Indian Oceans.
These meter-long (39-in.-long) tube worms are among the many unusual organisms that
live near deep-ocean vents.
A certain kind of bacterium is the primary producer in deep- ocean vent ecosystems.
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1. COMPARE AND CONTRAST Draw and complete the graphic organizer to tell how
intertidal zones, near-shore zones, and deep-ocean vents are alike and different.
---see organizer
2. SUMMARIZE Draw a cross section of the ocean floor that shows all of the ocean zones.
Include labels that describe each zone.
3. DRAW CONCLUSIONS What would most likely happen to the ocean ecosystems
discussed if the species that make up plankton suddenly became extinct?
4. VOCABULARY Use the lesson vocabulary to explain the various ecosystems found in the
oceans.
Test Prep
5. Critical Thinking Why might coral reefs be considered to be the "tropical rain forests" of
the ocean?
6. Which of the following organisms would most likely be found in the open-ocean zone?
A. bacteria
B. barnacles
C. corals
D. whales
Writing
Narrative Writing
Suppose you have built an exploratory submarine that can take you from the surface of the
ocean all the way to the bottom of the abyssal plain. Write a story about a trip you make in
the sub. Describe what you see.
Math
Calculate Pressure
Air pressure at sea level is about 14.7 pounds per square inch (psi). Near the bottom of the
Mariana Trench, the water pressure is 16,883 psi. About how many times as great as the
pressure at sea level is this?
Art
Coral Reefs
Coral reefs are some of the most colorful places on Earth. Find some pictures of coral reefs.
Use them as inspiration to paint a picture or make a sculpture of some coral reef
organisms.
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