
-Photograph by Liza Schillo
Coral Reefs
are among the most unique biological
communities on Earth. Incredibly diverse, very productive
and at the same time extremely fragile, they are found in warm,
clear shallow waters of tropical oceans worldwide. Often referred
to as the "rainforests of the sea" because of their incredible
diversity, coral reefs are home to over 4,000 species of fish, 700
species of coral and thousands upon thousands of other marine
plants and animals.
Corals are a part of the group of small aquatic animals called
"Cnidarians." Cnidarians include the corals, sea anemones, hydroids, and
jellyfish. Some are motile (free-swimming), like the jellyfish and
hydroids, and others are sessile (attached), like the corals and sea
anemones. Corals are a very diverse group of Cnidarians. They are made
up of many tiny organisms living together in a colony, and each
individual organism is called a "polyp." Corals in the class Anthozoa
are the chief reef builders. They do so by producing calcium carbonate
from their tiny coral polyps which once secreted build up the reefs into
communal structures slowly over time. This type of coral reef is almost
entirely confined to warm, shallow waters, and it is their limestone
skeletons which are critical to coral reef formation. While these
corals are the chief architects of reef structure, they are not the only
organisms at work. Coralline algae cement various corals together, and
various mollusks contribute their hard skeletons as well. A coral colony
may consist of thousands of polyps which are predominately carnivorous,
feeding on smaller particles floating in the warm ocean waters and which
have a symbiotic relationship with unicellular algae called
zooxanthellae. Corals reproduce both sexually and asexually and an
entire colony many meters across can start out as a single individual
polyp.
Important for many
reasons, coral reefs are also home to numerous organisms which are
important sources for drugs and medicines for mankind It is entirely
possible that a cure for cancer still awaits discovery on the incredibly
diverse coral reefs of the world. In addition, many island
cultures depend on the coral reefs for food, and income from tourists
attracted to the stunning beauty of the coral reefs is growing yearly.
From a purely biological
standpoint, coral reefs are extremely important to the ecosystem of the
oceans at large. Islands of diversity ,they function also as nurseries,
and feeding and breeding grounds to countless species. They also protect
shorelines from erosion, and provide recreational opportunities and
sources of income for millions of people who rely on them. Although they
cover less than one percent of the ocean floor, they are home to a
quarter of the known marine plant and animal species.
Because of their narrow
range of tolerance for environmental conditions, coral reefs are
especially sensitive to damage from natural and human causes. Biologists
have seen the future, and their message regarding coral reefs could not
be clearer. -all over the world coral reefs are dead or dying
because of human activity. While they are the most spectacular of
underwater environments, coral reefs are also the most fragile. They
have evolved over millions of years to cope with natural forces such as
hurricanes, floods, ocean currents and diseases, but they are no match
for man's destructive patterns and activities. Coastal development, over
fishing, coral mining, sewage, fertilizer and chemical pollution,
sedimentation, ocean warming and the use of cyanide and dynamite on the
reefs as fishing practices are all taking a major toll, and they all can
be traced by to man's activities.

Corals
are of two types: perforate and imperforate. Perforate
corals have porous skeletons with connections between the polyps through
the skeleton while imperforate corals have solid skeletons. Coral reefs
consist of many diverse species of corals which in turn are made up of
tiny organisms called polyps. A polyp is a small marine
invertebrate (spineless) animal. Some coral polyps are as small as the
head of a pin and others can be as large as a foot in diameter, but the
majority of them are small. A polyps' body has radial symmetry, which
means that if a line were drawn any way through the middle of the body,
it would produce two identical halves. The body structure is simple – it
is shaped like a tube. The tube is closed on one end where the polyp
attaches to its growing surface. The other end of the tube is the mouth,
which is used for both taking in food and excreting wastes. Tentacles
surround the mouth for gathering food, and are usually found in
multiples of six or eight. The body tissues of polyps are extremely
simple, with only two cell layers – the outer epidermis, which has
direct contact with the water, and the inner gastrodermis. The space
between the two cell layers is filled with a jelly-like substance called
"mesoglea.
Marine Reef: The Anatomy of Coral

Zooxanthellae
are small unicellular yellow-brown dinoflagellate algae which live
symbiotically in the gastrodermis of reef building corals, and it
is the nutrients supplied by the zooxanthellae from their photosynthetic
processes that allow the corals to grow and reproduce quickly enough to
produce reef structures. Reciprocally, coral provides the zooxanthellae
with protection and access to sunlight. Limited to waters less
than 100 meters deep because of their need for light, zooxanthellae are
a key energy providers for coral, helping their hosts meet carbon and
light energy needs. They also give the corals their distinctive
colorations.
Hermatypic Coral -The Role of the Zooxanthellae
The University of York Department of Biology: Coral Symbioses With
Zooxanthellae

Corals
obtain their food in a number of different ways. The reef building
corals rely mainly on the symbiotic zooxanthellae to provide them with
nutrient by-products from photosynthesis, while other corals
capture zooplankton for food using one of two methods -nematocyst
adhesion or mucus entrapment.
Most corals feed at night because that is when the zooplankton travel
most and thus become available for capture by the corals. During the
days t he corals keep their tentacle largely retracted to avoid
predation, protect themselves from ultraviolet light and to avoid
sharing their zooxanthellae.
About.com: What Do Corals Eat?
NOAA's Aquarius Report: Effects of Water Flow and Prey Behavior on Coral
Feeding

Corals
exhibit both sexual and asexual reproduction modes, and the coral colony
expands in size by budding, which may be intratentacular (new bud forms
from the oral discs of the old polyp) or extratentacular (new polyp
forms from the base of the old polyp). Fragmentation is a very
common form of asexual reproduction in which broken pieces of corals
that land on a suitable substrate may begin to grow and ultimately
produce a new colony. Also, many coral species spawn, typically with a
24-hour period in which they all release their eggs and sperm and
fertilization takes place in the surrounding waters.
Great Barrier Reef Marine Authority: Reef Snapshots of Coral Spawning
Marine Life Learning Center: Coral Spawning
SeaWorld.Org: Coral Reproduction
UOG Marine Lab: Coral Spawning

Corals
are known to be susceptible to at least four diseases: white band
disease (WBD), black band disease (BBD), bacterial infection, and shut
down reaction. These are all stress related, and adverse environmental
impacts from man may increase a coral's susceptibility to these
diseases. Corals may also be infected by diseases indirectly and those
that infect other reef inhabitants may also attack corals.
Coral diseases generally
occur in response to biological stresses, such as bacteria, fungi and
viruses, and nonbiological stresses, such as increased sea surface
temperatures, ultraviolet radiation and pollutants. One type of stress
may exacerbate the other. Black Band Disease, shown in the photo to the
left, is the only known coral disease that can be successfully treated.
The frequency of coral diseases has increased significantly over the
last 10 years, causing widespread mortality among reef-building corals.
Many scientists believe the increase is related to deteriorating water
quality associated with human-made pollutants and increased sea surface
temperatures.
Diseases of Coral Reefs
Global Coral Reef Alliance: Mysterious New
Diseases Devastate Coral Reefs
Reef Base Diseases
Reef Base Biological Infestations
Threats
to coral reefs include both natural and manmade: These include climate,
coral bleaching, coral fish trade, coral mining, destructive fishing
practices, diseases and biological infestations, eutrophication, exotic
species (Crown of Thorns starfish), habitat loss from development,
military and nuclear testing, overharvesting and physical impacts from
ship groundings and anchor, pipeline and cable damage, pollution,
sedimentation and tourism. In short, it is a long list and growing.
Already 10% of the world's coral reefs are lost and some scientists
predict that without change 70% of all corals reefs on the planet will
be destroyed in the next twenty to forty years.
Link To "Threats To Coral Reefs" Section Of This Website

Coral
reefs now throughout the world now face a serious problem -coral
bleaching, caused primarily by rising sea surface temperatures, a direct
result of global warming. Other environmental stresses including excess
shade, increased levels of ultraviolet radiation, sedimentation,
pollution and salinity changes can also lead to coral bleaching. When
this happens, the relationship between the host coral and the symbiotic
Zooxanthellae yellow-brown
dinoflagellate algae collapses and the plant cells are ejected. The
white calcium carbonate skeleton that is left now becomes visible and
the coral slowly starves in to the absence of its nourishment providing
zooxanthellae.
Coral Reef Ecology
Environmental Protection Agency
International Coral Reef Network
NOAA's Coral Health and Monitoring
Oceans Alive
Odyssey Expeditions
Reef Base
Reef Futures
US Dept of State Report: Coral Bleaching, Coral Mortality, and Global
Climate Change

Coral
reefs are home to a staggering number of fish species, with the total
number estimated to be over 4000. Biological diversity is great on coral
reefs and this principle extends also to the fish that live permanently
on reefs or are only occasional visitors. Of all the creatures living on
coral reefs, fish are by far the most active and most noticeable, and
display an incredible range of sizes, colors, shapes and behaviors. The
high diversity of the fish communities in the coral reefs is produced
directly by their complexity which provide a myriad of different ways
for fishes to feed, live and reproduce.
Coral Reef
Fish.com
Fishbase

Coral
reefs also provide habitats for a wide variety of other organisms that
rely on corals for shelter and food. Sponges, starfish, gastropods, sea
worms, crustaceans, mollusks, jellyfish, sea anemones and turtles are
among the most commonly seen.
Dr. Jungle's Coral Reef Animals Of The World
STARFISH: Coral Reef Animals

There are essentially four types of coral reefs: Fringing Reefs, Barrier
Reefs, Atolls and Bank, or Platform Reefs.
Fringing Reefs:
Fringing
reefs are the most common type of reef and develop from the simple upward growth of a calcium carbonate
platform from a shelf along the coastline. Growth is most rapid and
prolific in the warm, shallow waters and the corals grow quickly to the
surface and produce a shallow platform which is usually around the level
of the lowest tide. Offshore, growth still takes place but it is slower.
Typically here there is a sharply defined edge, the reef crest, beyond
which there is a steeply shelving reef front dropping to the ocean
floor.
Barrier Reefs:
Typically
these are much older structures rising up from a deeper base farther out
from the shoreline, and more than likely have a lagoon associated with
them. Some have their origins as fringing reefs but take on a new stage
of growth when the shoreline they are associated with subsides or is
flooded by rising sea levels. Under these conditions, the barrier reefs
continue to grow on their own upwards but deeper water fills in between
the reef and the shoreline and a lagoon is formed.
Atolls:
Atolls
are really unique structures, typically circular and enclosing a large
lagoon. They are usually found in oceanic locations, far away from the
continental shelf. They initially form as fringing reefs around
isolated, usually volcanic islands. If these island subside, the reefs
continue to grow, first forming a barrier around the sinking island, but
finally forming a single ring of coral once the islands disappears
completely beneath the sea. The depth of these coral reefs can be
extensive. Some found in the Marshall Island are up to 1.4 kilometers in
depth, dating back over 50 million years!
Bank or Platform
Reefs: Bank or platform reefs are simple physical structures formed
a number of ways. As reefs, they have no physical link to a coastline
but do not have the clear shape of a barrier reef or atoll. Larger or
submerged reef structures of this type are referred to also a shoals.
Bermuda Coral Reefs: Coral Formation
SeaWorld: Types of Coral Reefs
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