Oceans 11 – Dalesandro
Assignment 8 – Coral Reefs
Instructions: Read the attached article and answer the questions below.
Questions:
1) For how long have corals existed on Earth?
2) How many kinds of corals are there?
3) List the three layers of a coral’s body.
4) What are the mouth, tentacles, and stinging cells used for?
5) How does a coral make a reef?
6) Where in the world do reefs exist?
7) What kinds of conditions to corals need to live and grow?
8) a) Who was the first famous scientist to study corals?
b) What did he say about them?
9) Using the internet, identify three major coral reefs, their locations,
and their sizes. Find a picture of each reef and include it with your
write-up. [2 pts.]
10) Write a short (5 sentence) response to this assignment. What are
your thoughts on what you have read and what you have learned?
List one question you have about coral reefs that was not answered
in the article. [5 pts.]
Value: 15 pts.
Due date: One week from today (Tues, Nov. 10th).
Corals and Coral Reefs
Appearing in fossils from more than 400 million years ago, corals are an
extremely ancient type of animal. Coral reefs are unique and complex systems,
rivaling rainforests in longevity of their ecological communities. Well-developed
and well-protected reefs reflect thousands of years of history.
Corals and their Kind
There are over 6,000 known species of corals (scientific name anthozoans).
Stony corals make up the largest group of corals, and are the group primarily
responsible for laying the foundations of, and building up, coral reefs. For the
most part, corals are colonial organisms, which means they are composed of
hundreds or of thousands of individuals, called polyps.
Corals have a very basic body plan. Each coral polyp consists of three basic
tissue layers: 1) an outer epidermis,
2) an inner layer called the gastrovascular cavity which acts as
a space for digestion, and,
3) a layer called the mesoglea in between.
All coral polyps have a cavity called the
mouth, wheich takes in food and expels
waste. A second feature all corals possess is
a circle of tentacles, extensions of the body
wall that surround the mouth. Tentacles
help the coral to capture and eat plankton
food, and clear away debris from the mouth.
The tentacles are also the animal’s only
means of defense against predators.
Found throughout the tentacles are
nematocysts (stinging cells). Because
nematocytes are capable of delivering
powerful, often lethal toxins, they are
essential to capturing prey and defending
the polyp.
From Polyp to Reef
Massive reef structures are formed when
each stony coral polyp secretes a skeleton of
calcium carbonate. Most stony corals
have very small polyps, averaging 1 to 3 mm in diameter, but entire colonies can
grow very large and weigh several tons. Although all corals secrete CaCO3, not
all are reef builders. Some corals are solitary and have single polyps that can
grow as large as 25 cm in diameter. Other coral species are cannot produce
sufficient quantities of calcium carbonate to form a reef.
Structure of a typical coral polyp.
The skeletons of stony corals are secreted by the lower portion of the polyp. This
process produces a cup, called the calyx, in
which the polyp sits. A polyp will lift off its
base and secrete a new floor to its cup. This
allows the coral to grow. Most polyps only
extend themselves out of their protective
calyx when they feed on plankton at night.
Colonies of reef-building (hermatypic)
corals exhibit a wide range of shapes.
While the growth patterns of stony coral
colonies vary, a colony’s geographic
This large healthy elkhorn coral (Acropora sp.)
location, environmental factors (e.g., wave
exhibits a branching pattern.
action, temperature, light exposure), and
the density of surrounding corals may affect
and/or alter the shape of the colony as it grows.
In addition to affecting the shape of a colony’s growth, environmental factors
influence the rates at which various species of corals grow. One of the most
significant factors is sunlight. On sunny days, corals can grow twice as fast as on
cloudy days. Experiments have shown that rates of growth slow significantly
when corals are kept in shade or
darkness.
As they become larger, corals
place a heavier load on their
relatively small bases, rendering
the colony increasingly unstable.
Under these circumstances, the
branches are prone to snapping
off during strong wave action.
Where Reefs Exist
Major coral reef sites are seen as red dots on this world map. Most of the
reefs, with a few exceptions are found in tropical and semitropical
waters, between 30° north and 30° south latitudes.
Reef-building corals are restricted
in their geographic distribution. This is because they require a narrow and
consistent band of climate conditions to grow The formation of reefs only occur
where the temperature does not fall below 18°C for extended periods of time.
This temperature restriction does not, however, apply to all corals. In Japan,
about half of all corals grow where the sea temperature regularly falls to 14°C,
even 11°C! Most corals require very saline (salty) water. The water must also
be clear to permit light to penetrate. These requirements explain why most
corals live in the epipelagic zone (sunlit zone), approximately 200 metres deep
or less.
The number of species of corals on a reef declines rapidly in deeper water. High
levels of sediments (cloudy water) can smother corals, clogging their mouths and
choking them to death. Suspended sediments can also serve to decrease the
depth to which light can penetrate. In colder regions, murkier waters, or at
depths below 200 m, corals may still exist but their lifespans are greatly
reduced.
In light of such strict environmental restrictions, reefs generally are confined to
tropical and subtropical waters.
Another factor that seems to affect the diversity of reef-building corals is the
ocean in which they are located. At least 500 reef-building species are known to
exist in the waters of the Indian and Pacific oceans. In comparison, the Atlantic
Ocean contains only 62 known species. The fossil record shows that many
species once found across the Atlantic, Pacific and Indian Oceans gradually went
extinct in the Atlantic, where the affects of ice ages had strong impacts on the
Caribbean area wherein most of the Atlantic reefs reside.
The Structure of Coral Reefs
Coral reefs begin to form when free-swimming coral larvae (“baby corals”) attach
to the submerged edges of islands or continents. As the corals grow and expand,
they begin to form a reef.
In the 1830s, Charles Darwin became the first well-known scientist to study coral
reefs. He soon suggested a sequence of coral reef formation.
Profile of coral reef with typical reef “zones” (based on Darwin’s work).
Darwin claimed that ocean bottom geography, water depth, wave and current
strength, light intensity, temperature, and level of suspended sediments all acted
to create the appearance of coral reefs we see today.