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Coral Reefs
What are Coral Reefs?
 The 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."
Corals themselves are tiny animals which
belong to the group cnidaria (the "c" is silent). Other cnidarians
include hydras, jellyfish, and sea
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.
Virtually 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.
Because 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.
Coral 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.
Coral 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 species.
 Boulder
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.
 Elkhorn
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.
 Pillar
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
growing spires.
 In
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.
 Sponges
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.
 Corals
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
world.
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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