What is a coral reef?
Coral reefs are massive limestone structures, made
from the skeletons of millions of marine animals called coral polyps. As
polyps die, new polyps grow on top of and next to the dead ones. Many
layers of skeletons from the dead polyps form the limestone structure of
a coral reef. Other types of animals and plants also contribute to the
reef structure, such as algae, sponges and mollusks. However, the
primary architects are coral polyps.
Coral reefs are ecosystems which are like bustling
cities, with buildings made of coral and thousands of marine inhabitants
coming and going, interacting with one another, carrying out their
business. In this sense, coral reefs are cities under the sea.
Coral reefs provide shelter for nearly one quarter of
all known marine species. Over the last 350 million years, reefs have
evolved into one of the largest and most complex ecosystems on the
planet. The reefs are home to over 4,000 species of fish, 700 species of
coral, and thousands of other forms of plant and animal life. The reefs
are home to over 4,000 species of fish, 700 species of coral, and
thousands of other forms of plant and animal life.
What is coral?
Coral reefs are made of hundreds of different species
of coral. Coral is generally classified as either "hard"
coral (reef building coral) or "soft" coral.
Although people often mistake coral for a rock or
plant, coral is actually a colony of tiny animals called coral polyps.
Thousands of these polyps can live as neighbors on one coral branch or
mound. Polyps grouped together in large numbers are referred to as coral
colonies. Each coral "tree" or "mound" is one
colony of coral polyps. When someone refers to coral, they are
referring to these branches or mounds.
Polyps are invertebrates, spineless animals, which
makes them cousins of anemones and jellyfish. Some polyps are as small
as a pinhead and other polyps are much larger, sometimes over a foot in
diameter, and can live individually on the sea floor.
A polyp has a sack-like body with one opening, or
mouth, encircled by stinging tentacles. Polyps of hard coral species,
called hermatypic species, make calcium carbonate from surrounding
seawater to build a cup-shaped skeleton of limestone. This hard
structure protects the delicate body of the polyp from predators and
under water currents.
During the day, the polyp keeps its tentacles folded
inside of its skeleton, but at night it extends them out to feed. Coral
polyps eat tiny floating animals known as plankton, which they catch in
their tentacles. They also house tiny algae, called zooxanthellae
(pronounced zo-zan-THEL-ee), which provide the coral polyps with food
Soft corals are ahermatypic, or non-reef building.
These corals do not have stony skeletons, but instead grow in colonies
with a wood-like core for support and a fleshy rind to protect the
polyps. Many soft coral colonies look like underwater branches or ferns
attached to a rock by a single holdfast, with branches swaying in the
wind. Although soft corals do not help to build the rocky structure of
the reef, they play an important role in reef ecosystems as hiding
places for fish and filters for organic material. In addition, many
species of soft coral, such as sea fans, form delicate and intricate
shapes whose beauty attracts the admiration of divers everywhere!
How does coral get its color?
Zooxanthellae, are microscopic, single-celled algae
that live and grow within the tissue of hard coral polyps. Several
million zooxanthellae live in just one square inch of coral. Coral
polyps and their skeletons are actually clear or white and it is pigment
in the zooxanthellae that gives coral more than 90% of its colorful
Zooxanthellae and coral have a symbiotic
relationship. The two organisms live together and depend upon each other
for survival, making them "biological partners". The algae
provide the polyp with food and process the polyp's wastes to retain
important nutrients. Zooxanthellae also help to produce calcium
carbonate for the polyp’s skeleton. In return, coral polyps build a
safe and protected home for the algae.
Up to 98% of a hard coral's nutritional needs can be
met by the surplus food produced by zooxanthellae. The tiny algae use
energy from sunlight to make food through photosynthesis, converting
carbon dioxide into oxygen and carbohydrates. This process requires a
lot of sunlight, which is why coral colonies must live in clear, shallow
water. Hard corals are extremely reliant on their zooxanthellae and
cannot survive long without them.
What other reef animals are symbiotic?
One intriguing example of symbiosis on the reef is
between large fish and certain shrimps or small fish called
"cleaners." Cleaners feed on the parasites and damaged tissues
on the skin of larger fish. The cleaner benefits from the food obtained
and the host rids itself of annoying parasites. Certain areas of the
reef function as "cleaning stations." Big fish can often be
seen in these places, lining up to be cleaned.
Orange spotted Gobies and snapping shrimp also have a
fascinating symbiotic relationship. These Gobies are small fish that
hunt by squatting at the mouth of snapping shrimp burrows and
occasionally moving forward to catch passing prey. When danger
approaches, however, the goby quickly swims down into the shrimp’s
protective burrow, which warns the nearly blind shrimp to do the same.
How does coral reproduce?
Coral reproduction varies by species. For instance,
brain coral and star coral polyps produce both sperm and eggs at the
same time. Other species, such as Elkhorn and boulder coral, produce
single-sexed colonies. All of the polyps in one of these colonies
produce only sperm and all of the polyps in another colony produce only
eggs, so at least two colonies of a species are needed for reproduction.
Coral larvae are formed in one of two ways. For some
species, fertilization occurs within the body of a polyp. Sperm is
released into the water by one polyp and then swims into the mouth of
another polyp that has produced eggs. The sperm and egg combine together
and produce a planula larva. When the larva has matured, it is
"spit out" into the water through the mouth of the coral
polyp. Other species of coral reproduce by ejecting large quantities of
eggs and sperm into the surrounding water in a process called coral
spawning. In some areas, mass coral spawning events occur on one night a
year. In certain regions, spawning cycles are so regular that scientists
can predict when this magical night will occur. During spawning,
trillions of eggs and sperm are simultaneously released into the water
in one of the most astonishing acts of synchronicity in the natural
Once in the sea, larvae are attracted to light. They
swim to the surface of the ocean, where they remain for several days or
even weeks. If predators do not eat a larva during this time, it
eventually falls back to the ocean floor. Here, it attaches itself to a
hard surface and develops into a polyp. The recently established polyp
then divides many times, making exact genetic copies of itself, which
remain fused together. As more and more polyps are added, a coral colony
develops. Eventually, the coral colony becomes mature and reproduces
sexually as the cycle of life continues!
Where do corals live?
Tropical coral reefs grow in warm seas, in
temperatures between 70° and 85°F (20° and 30°C). Soft corals
sometimes grow in temperatures higher or lower than this range, but the
growth rates of such colonies are much slower. Most coral grows between
the tropics of Cancer and Capricorn, with the majority in the Caribbean,
Indian Ocean, Red Sea, Arabian Gulf, South Pacific, and Southeast Asia.
Coral colonies are also found in places where warm water currents flow
out of the tropics, such as Florida and southern Japan. Worldwide, coral
reefs are found in over 100 countries and cover an estimated 98,500
square miles (255,000 square kilometers).
Corals grow best in clear, shallow waters, where
sunlight filters through in large quantities. Some species can survive
at depths of up to 300 feet (91 meters), but most reef-building corals
do not grow well below 60 to 90 feet (18 to 27 meters). Salty water is
another thing that coral colonies need. Fresh water can kill corals, so
reefs are not found near river openings or in coastal areas with
excessive run-off. Silt and pollution in the run-off can also harm or
Fringing reefs lie parallel to the shores of islands and
continents. Narrow, shallow lagoons separate them from land.
Barrier reefs also grow parallel to coastlines, but are separated
from the shore by deep, wide lagoons. At their shallowest point,
barrier reefs can reach the water’s surface, forming a
"barrier" to navigation. The Great Barrier Reef in Australia
is the most famous reef of this type.
Atolls are rings of coral that create protected lagoons, often in
the middle of the sea. Atolls usually form when volcanic islands
surrounded by fringing reefs sink into the sea or the sea rises and
covers the land. The reefs, however, continue to grow upward as the
water rises. These reefs form circular or horseshoe shapes with a
protected lagoon inside.
Patch reefs are small, isolated reefs that grow from the shallow
bottom of an island platform or continental shelf. These usually occur
between fringing reefs and barrier reefs. Patch reefs vary greatly in
size and rarely ever reach the surface of the water.
What are the different types of coral reefs?
How do corals get their shape?
The shapes and sizes of coral colonies depend on
their species and on the location of the coral. For example, where there
are strong waves, coral colonies tend to grow in robust mounds or
flattened shapes. In more sheltered areas, the same species may grow in
more intricate patterns of delicate branches. The availability of light
and nutrients also plays a major role in determining what shape coral
colonies will assume.
How long does it take for a coral reef to form?
The geological record indicates that the ancestors of
modern coral reef ecosystems were formed at least 350 million years ago.
The coral reefs existing today began growing as early as 50 million
years ago. Most established coral reefs are between 5,000 and 10,000
years old. Although size can be a good indicator of the age of a coral
reef, it is not a certain indicator. Different species of coral grow at
different rates, depending on the water temperature, oxygen level,
amount of turbulence, and availability of food around them. Most massive
boulder corals are the slowest growing species, adding only between 1/4
to 7/8 inch (5 to 25 millimeters) per year to their length. Branching
and Staghorn corals may grow much faster, adding as much as 8 inches (20
centimeters) to their branches each year.
What is killing our coral reefs?
Consider these statistics:
- Approximately 11% of the world's coral reefs have already been
destroyed by human activity and a further 16% have been severely
damaged during the massive climate-related coral bleaching event in
- Reefs in 93 countries have been damaged in the past four
- At the current rate of loss, 40% of the world's coral will be
destroyed by 2010, and another 20% by 2030.
- Several coral reefs in Florida and the Caribbean are in
"critical condition" – in other words, they are likely
to be destroyed if current trends continue.
- The most severe damage to coral has occurred in the Middle
East, Southeast Asia, and the Caribbean (22%-59% of coral cover
In the last few decades, humans have destroyed over
35 million acres of coral reefs. If the present rate of destruction
continues, 40% of the world's coral reefs will be killed in the next 10
years. The loss of healthy coral reefs could mean the elimination of a
primary source of food, income and employment for millions of people
around the world, as well as the extinction of many fascinating and
beautiful marine species.
Coral reefs have always faced damage from natural
causes such as cyclones, pest outbreaks, and disease. Coral is also
constantly being broken by fish and other forms of marine life. Under
normal conditions, reefs are resilient to such damage and recover over
time. Additional human pressures, however, are weakening the reefs,
reducing their ability to regenerate and recover from natural damage.
Humans continue to destroy Coral Reefs through:
- Sewage, Chemical Pollution and Marine Debris
- Coastal Development
- Destructive Fishing Practices
- Coral Mining
- Global Warming and Coral Bleaching
- Carbon Dioxide
- Ozone Depletion
Reefs are wrecked when Humans treat them
What can a Shore Diver or Snorkeler do?
- Dive responsibly: stay neutrally buoyant, make sure your
equipment doesn’t drag on the reef and don't touch coral reefs or
stir up sediment. (Snorkelers too can help coral reefs by not touching
coral or stirring up sediment.)
- Support reef-friendly dive operators.
- Don't take or buy coral souvenirs. Take pictures instead.
- Volunteer for a coral reef clean-up, conservation or research
- Pack out your plastics and batteries when visiting tropical
islands and try to support hotels and resorts that properly treat
their sewage and waste.
- Stay informed about the threats to coral reefs and how to help
stop them by reading coral reef conservation groups’ newsletters,
web sites and brochures. Visit www.coral.org for more
information and to find a list of other coral reef groups.
- Try to drive less and save energy by turning down your heater or
air conditioner and use energy efficient appliances. Sign up for
renewable energy sources if you have that option in your state.
Support environmental groups working to stop global warming.
- Try to reduce, reuse and recycle as much as possible.
- Using nontoxic household cleansers, lawn and garden products help
keep our seas healthy.
- Become a member of CORAL and support our conservation efforts!