What are Coral Reefs?
mention of coral reefs generally brings to
mind warm climates, colorful fishes and clear waters. However,
the reef itself is actually a component of a larger ecosystem.
The coral community is really a system that includes a collection
of biological communities, representing one of the most diverse
ecosystems in the world. For this reason, coral reefs
often are referred to as the "rainforests of the oceans."
themselves are tiny animals which belong to the group cnidaria
(the "c" is silent). Other cnidarians include hydras, jellyfish,
anemones. Corals are sessile animals, meaning they are
not mobile but stay fixed in one place. They feed by reaching
out with tentacles to catch prey such as small fish and planktonic
animals. Corals live in colonies consisting of many individuals,
each of which is called polyp. They secrete a hard calcium
carbonate skeleton, which serves as a uniform base or substrate
for the colony. The skeleton also provides protection, as
the polyps can contract into the structure if predators approach.
It is these hard skeletal structures that build up coral reefs
over time. The calcium carbonate is secreted at the base of
the polyps, so the living coral colony occurs at the surface
of the skeletal structure, completely covering it. Calcium
carbonate is continuously deposited by the living colony,
adding to the size of the structure. Growth of these structures
varies greatly, depending on the species of coral and environmental
conditions-- ranging from 0.3 to 10 centimeters per year.
Different species of coral build structures of various sizes
and shapes ("brain corals," "fan corals," etc.), creating
amazing diversity and complexity in the coral reef
ecosystem. Various coral species tend to be segregated
into characteristic zones on a reef, separated out by competition
with other species and by environmental conditions.
all reef-dwelling corals have a symbiotic
(mutually beneficial) relationship with algae called zooxanthellae.
The plant-like algae live inside the coral polyps and perform
photosynthesis, producing food which is shared with the coral.
In exchange the coral provides the algae with protection and
access to light, which is necessary for photosynthesis. The
zooxanthellae also lend their color to their coral symbionts.
Coral bleaching occurs when corals lose their zooxanthellae,
exposing the white calcium carbonate skeletons of the coral
colony. There are a number of stresses or environmental changes
that may cause bleaching including disease, excess shade,
increased levels of ultraviolet radiation, sedimentation,
pollution, salinity changes, and increased temperatures.
the zooxanthellae depend on light for photosynthesis, reef
building corals are found in shallow, clear water where light
can penetrate down to the coral polyps. Reef building coral
communities also require tropical or sub-tropical temperatures,
and exist globally in a band 30 degrees north to 30 degrees
south of the equator. Reefs are generally classified in three
types. Fringing reefs, the most common type, project seaward
directly from the shores of islands or continents. Barrier
reefs are platforms separated from the adjacent land
by a bay or lagoon. The longest barrier reefs occur off the
coasts of Australia and Belize. Atolls rest on the tops of
submerged volcanos. They are usually circular
or oval with a central lagoon. Parts of the atoll may emerge
as islands. Over 300 atolls are found in the south Pacific.
reefs provide habitats for a large variety of organisms. These
organisms rely on corals as a source of food and shelter.
Besides the corals themselves and their symbiotic algae, other
creatures that call coral reefs home include various
sponges; molluscs such as sea slugs, nudibranchs, oysters,
and clams; crustaceans like crabs and shrimp; many
kinds of sea worms; echinoderms like star fish and sea urchins;
other cnidarians such as jellyfish and sea anemones;
various types of fungi; sea turtles; and
many species of fish.
reefs, and their associated systems of mangroves and seagrasses,
are the world's most biologically diverse marine ecosystems.
Reef building corals contain tiny cells of symbiotic algae
that convert sunlight and nutrients into fuel for coral growth
and production. Other types of corals that do not require
warm water or sunlight are found in deep water, providing
important habitats for commercial, recreational and other
and massive corals, like this boulder star coral,
are the "builders" of the reef. A coral head is a colony of
small animals called polyps. These corals provide the reef
foundations that is home to millions of reef species. Over
4,000 species of reef fish have been described so far.
coral is a branching coral. Branching corals grow
in the shallow areas of the reef crest and
serve to break up the wave action as it comes onto the reef.
The branches of elkhorn coral resemble an elk's rack
of antlers, thus its name.
coral forms numerous heavy cylindrical spires that
grow upward from an encrusting base. Unlike other hard corals,
which feed at night, pillar coral extends its polyps to feed
during the day. Fallen pillers may give rise to new upward
addition to the hard corals, there are a variety of soft corals
like this common sea fan. The calcium carbonate
skeleton of soft corals is located within their bodies, allowing
them to move with the wave action. Over 800 species of corals
have been described to date.
like this orange elephant ear sponge are water filters for
the reef. They filter up to 30,000 times their body volume
every day. Researchers are discovering unique chemical compounds
in sponges and other reef species that may
have important medicinal properties and other uses.
are large colonies of small animals called polyps.
These polyps reside within a cup-like calcium carbonate skeleton.
They have a central opening surrounded by tentacles which
can be extended to feed on phytoplankton in the water column.
Why Care about Coral Reefs?
Healthy coral reefs are some of the most biologically
diverse and economically valuable ecosystems on earth, providing
food, jobs, recreational opportunities, coastal protection
and other important services to billions of people world-wide.
Unfortunately, many of the world's coral reefs (including
the associated seagrass beds and mangrove habitats)
have been damaged or destroyed due to increasing human impacts,
climate change, and other factors.
According to the Status of Coral Reefs
of the World: 2004, 70% of the worlds' coral reefs are threatened
or destroyed, 20% of those are damaged beyond repair, and
within the Caribbean alone, many coral reefs have lost 80%
of coral species. The decline and loss of coral reef
ecosystems has significant social, economic, and ecological
impacts on people and communities in the U.S. and around the
Benefits of Coral Reefs
Coral reefs provide habitat for
one-third of all marine fish species, build tropical islands,
protect coasts from waves and storms, contain an array of
potential pharmaceuticals, and support tourism and fishing
industries worth billions of dollars. Coral reefs
are also fundamental to the fabric of local communities, providing
a source of food, materials and traditional activities.
Threats of Coral Reefs
Critical information is still lacking about
the causes of coral decline but evidence suggests a variety
of human forces, including population increases, shoreline
development, land-based sources of pollution, increased sediments
in the water, damage by tourists and divers, groundings, poor
water quality from runoff and sewage treatment, and over-fishing.
In addition, increased stress from global warming and sea
level rise act separately and in combination with natural
factors (hurricanes and disease) to degrade reefs.
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