Coral Reef.

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Coral Reef.
I

INTRODUCTION

Coral Reef, coastal and oceanic ecosystem renowned for its beautiful life forms and for providing one of the most biologically diverse habitats on Earth. A reef is a ridge
or outcrop of rock in the sea that comes close to the surface. A coral reef is a reef that has been built largely or entirely by corals, tiny animals that live together in
colonies. Over hundreds or thousands of years the limestone skeletons of coral build up, with new corals growing on the skeletons of the dead ones. This physical
structure, with its living surface of corals and other organisms, is a coral reef.
The corals that build reefs are found only in warm tropical waters where sea temperatures rarely fall below 18°C (64°F). They thrive only in clear saltwater where bright
sunlight can penetrate. This is because corals cannot exist without the symbiotic algae called zooxanthellae that live in coral tissues and require sunlight for
photosynthesis.
Many reefs are found around islands, or quite far from land at the edge of continental shelves. Some of the most extensive areas of coral reefs are found in the Pacific
Ocean, around the islands of Southeast Asia and off the coast of Australia. Large areas of coral reef are also found in the Indian Ocean and around the Red Sea and
Arabian Sea. Many coral reefs also exist in the Caribbean Sea.
Because their skeletons are made of limestone, corals leave clear and abundant fossils. Some of the earliest forms of life--mound- or pillar-shaped fossilized rocks
known as stromatolites--resemble corals. The oldest stromatolites date back 3.5 billion years. They were laid down by some of the simplest organisms on Earth--bluegreen algae (also known as cyanobacteria). About 560 million years ago, limestone reefs built by bacteria, algae, and sponges first appeared. The first complex corals,
known as tabulate corals, are recorded at about 500 million years ago.
The corals we know today first appeared about 220 million years ago, before the age of the dinosaurs. The first true coral reefs began to appear about 205 million to
210 million years ago. Since this time there have been several periods of major reef-building, but also periods when corals declined and did not build reefs.
In all, shallow coral reefs occupy only about 284,000 sq km (110,000 sq mi), or less than one-tenth of 1 percent (0.1 percent) of the world's oceans. Yet in this tiny
area, coral reefs house a quarter of all marine fish species. Reefs are highly productive resources for human beings. Hundreds of millions of people live within easy
reach of coral reefs and rely on them to provide food. Coral reefs also help protect human settlements from large waves during storms.

II

CORAL REEF FORMATION

The most important organisms for the development of coral reefs are, of course, corals. Corals are tiny animals with a simple tubular body known as a polyp. They have
a mouth on top, surrounded by a ring of tentacles. Only some corals build reefs. These are the stony corals, or scleractinians, which live in colonies. Thousands of polyps
live and grow together, laying down a shared skeleton of limestone (calcium carbonate). Over the years such skeletons may become large structures, with a living outer
layer of coral animals.
Despite their importance, corals are not the only creatures involved in the building of coral reefs. Other animals such as mollusks build skeletons from limestone. A
number of important groups of marine plants known as algae also lay down limestone. These include a group of pink or red coralline algae that encrust bare rock and
often act like cement on a reef.
Corals will generally only settle and begin to grow on a hard surface. Over time corals will grow upon corals, and the physical structure of the sea bottom begins to
change. Some corals may be broken up by storms, or by the actions of animals that eat corals or bore holes into them. The broken rubble and sand that result from
these activities form a critical part in the building of coral reefs. Over time all this material is compacted together around the standing skeletons of the corals and
coralline algae. A coral reef begins to grow. Coral reefs grow upward at rates of 1 to 20 cm (0.4 to 7.8 in) per year.

III

TYPES OF CORAL REEFS

Scientists have identified many types of coral reefs. They are known as patch reefs, fringing reefs, barrier reefs, bank reefs, and atolls.
Patch reefs occur along a continental shelf where mound-shaped hillocks on the sea floor are close enough to the surface to allow corals to settle and grow.
Fringing reefs occur along a rocky coastline where corals or coral remains extend outward from the shore and form an outermost line or ridge that runs parallel to the
shore. After many centuries, the reef may grow up to the sea surface. As marine creatures, the corals cannot grow above the surface. The shallowest part of the reef
becomes quite level and is called the reef flat. Few corals can survive there. In fringing reefs the reef flat extends out all the way from the shore. Where the reef flat
reaches the deeper water offshore, an area known as the reef crest forms where the waves break. From here the structure of the reef drops away into deeper water
down the reef slope. In this area conditions remain good for coral growth, and the reef continues to grow both upward and outward.
In certain conditions coral reefs develop far from the continents, around small islands, or toward the edge of the continental shelf. Here the reefs do not join up to the
mainland, but grow upwards on all sides. Large, elongated structures far offshore are called barrier reefs. They are separated from the land by a lagoon. When barrier
reefs grow up in remote ocean areas, they are termed bank reefs.
An unusual type of reef is the coral atoll. This is a large, shallow bank, with a depression in the middle, typically ring-shaped. The British naturalist Charles Darwin
correctly theorized how coral atolls are formed. Coral atolls begin as fringing reefs around active oceanic volcanoes. When the volcanic eruptions cease, an island
remains. Over long geological periods the island begins to sink. The fringing reef continues to grow as the island sinks. Soon what was a fringing reef around the shore
becomes a barrier reef separated from the shrinking island by a deeper lagoon. After thousands of years, the volcanic island sinks completely below sea level, but the
corals continue to grow, forming a circular coral reef, an atoll.
Scientists have now drilled considerable depths into coral atolls. In the Marshall Islands their drilling went through 1,400 m (4,600 ft) of limestone before reaching the
volcanic rock below. Although it has not been growing continuously, the deepest limestone on this coral atoll was deposited by corals more than 50 million years ago.

IV

CONDITIONS FOR CORAL GROWTH

Corals in general are found in all seas and oceans, even in deep oceans and cold waters. Reef-building corals, by contrast, require quite specific conditions in order to
thrive. None are found in areas where the water temperature drops much below 18°C (64°F) for more than a few days. Likewise, although some species in the Arabian
Sea regularly encounter temperatures of 36°C (96.8°F), more normally corals are adapted to thrive in a much smaller temperature range.
Apart from temperature, corals also require clear waters. This is partly because they need sunlight to support the algae that live within their tissues. They are also very

sensitive to particles of mud or sediment settling on them, which means that corals rarely grow close to rivers or other sources of sediment. In the sea, light is filtered
out by depth, so reef-building corals can only grow in relatively shallow water. Even in the clearest oceans few reef-building corals grow below a depth of 80 to 100 m
(260 to 328 ft).
Although corals need nutrients, they cannot thrive in areas where there are large amounts of nutrients. Typically, microscopic organisms in the plankton, or seaweeds
(macroalgae), grow to excess in nutrient-rich water and smother the corals, blocking out the light.
Waves can often be seen crashing onto coral reefs. Being supported by limestone skeletons, corals can survive in very tough conditions. However, in the strongest
storms many corals will be smashed to pieces. In the short term this can be damaging, but many of the broken pieces will continue to grow. New corals will arrive from
other areas, so the reef can recover, often within a few years.

V

PLANT AND ANIMAL LIFE IN CORAL REEFS

The complex shapes of the coral reef provide a surface for many plants and animals to settle and grow. The great mass of holes and contours provide shelter,
protection, or even places for predatory fish to ambush other fish. The total number of species observed in coral reefs throughout the world is about 100,000, but
scientists believe about 900,000 other species may be awaiting discovery. The variety and numbers of species still baffle scientists. One study of a dead coral colony
yielded 8,265 individual animals burrowed within the coral rock. These included about 220 different species.

A

Plant Life in Coral Reefs

Plants lie at the base of almost all food chains. In the sea, simple plants, known as algae, are the most important. One microscopic alga lives within the corals
themselves. Other microscopic algae are found free-floating in the plankton. More complex algal plants include seaweeds, which are an important diet for many fish.
Coralline algae are a type of red algae that lay down skeletons of calcium carbonate and work with corals in building reefs and cementing the rubble together.
Two other groups of plants are often associated with coral reefs. Mangroves are trees and shrubs that grow in the intertidal zone between the land and the sea, while
seagrasses are the only true plants that live completely submerged in the sea. Both mangroves and seagrasses are particularly abundant and diverse in tropical waters.
Many reef animals move between these ecosystems during their lives. The mangroves and seagrasses are also important for filtering and holding sediments and thus
keeping the water over the coral reefs clearer.

B

Animal Life in Coral Reefs

Sponges are common on coral reefs. The sponge is one of the simplest and most ancient forms of animal life. Some sponges encrust rocks, but many others have
simple skeletons. Giant barrel sponges on coral reefs can reach 2 m (6.5 ft) in height.
Corals are part of a group, or class, known as anthozoans. Apart from the reef-building corals there are many other forms, including soft corals, which do not have a
rigid skeleton. Related to the anthozoans are jellyfish, which sometimes drift over coral reefs.
Largely hidden from sight, a multitude of worms are found on coral reefs. Many of these burrow into the sand or even into the rock and the coral skeletons. They
include bristle worms, ribbon worms, and flatworms. Feather duster and Christmas tree worms are bristle worms that protrude a fan-shaped mouthpiece from their
holes to filter the water for passing food.
The crustaceans are well-known, including shrimps, lobsters, and crabs. One special group on coral reefs is the mantis shrimps or stomatopods. These voracious
predators have fantastic eyesight and powerful limbs. Some are capable of spearing passing fish, while others have clublike appendages that can smash open mollusk
shells.
Mollusks are abundant on coral reefs. They include a huge range of types of snails (gastropods), such as conchs and cowries with large or beautiful shells, but others,
such as nudibranchs, have no shell at all. Bivalves have shells in two matching halves and include the world's largest shell, the giant clam, reaching to over 130 cm (50
in) in length and weighing over 230 kg (500 lb).
The most unlikely mollusks are the cephalopods, which include the octopus, cuttlefish, and squid. These free-swimming creatures are highly intelligent hunters, with
good eyesight and powerful tentacles to catch their prey.
Another conspicuous group on coral reefs is the echinoderms. These include starfish and sea urchins, but also the lesser-known feather stars, brittle stars, and sea
cucumbers. On coral reefs, sea urchins graze on algae and help provide new spaces for corals to settle and grow.
About 4,000 species of fish are regularly seen on coral reefs. Many are highly specialized to survive in this extremely competitive environment. Among the predators are
groupers, snappers, and grunts. The largest of these, the groupers, include the jewfish in the Caribbean and the giant grouper found in the Pacific and Indian oceans.
Both reach to more than 2 m (6.5 ft) in length.
Butterfly fish are small and brightly colored; some feed on corals. Other colorful species include the angelfish and damselfish. Among the largest groups are the wrasses
and the closely related parrot fish. Parrot fish have powerful teeth that have fused together to form a beaklike mouth. With it they can break off chunks of rock or coral
in order to digest algae and other creatures living on the rock surface. Surgeonfish are another group that grazes on algae, often traveling in large groups. They get
their name from a sharp blade that lies folded away inside their bodies near the base of their tails. Sharks are regular visitors to the reef, and some make their home
there. Other visitors include jacks or trevallies and barracudas, as well as rays.
Marine turtles are regular visitors to coral reefs, and there are sea snakes in parts of the Indian and Pacific oceans. Dolphins, too, occasionally come in to feed in coral
reef areas.
In diverse ecosystems, the competition for space, food, and shelter can be intense, driving many species to specialize in their diets or their modes of life. Other species
develop complex partnerships. One of the best known is that of the clownfish and the giant sea anemone. Normally, sea anemones eat small fish. However, the
clownfish has developed a technique to match the chemical signature of the anemone so that it does not try to sting them. They move about freely in the stinging
tentacles, receiving protection. The anemone benefits, too, as the clownfish chase off potential predators, and the anemone also feeds on the scraps dropped by feeding
clownfish.
Coral reefs appear to be isolated oases. However, life continuously moves between them. Almost every animal on the reef produces eggs or larvae that drift in the
water. These join a host of other plants and animals that spend their entire lives drifting in the ocean. These floating, drifting organisms are known as the plankton. The
animals, and the young eggs and larvae, are called zooplankton, whereas the microscopic plants are known as phytoplankton. Currents in the sea move the creatures of
the plankton about, and young corals and fish are carried to new places, where they settle and grow into adults.

Another relationship affects all life on coral reefs. Microscopic algae known as zooxanthellae are held within the tissues of the coral itself. There may be millions in any
single coral polyp, where they cannot be eaten by grazing animals. The corals benefit considerably from their guests in what is known in biology as a symbiotic
relationship. Like all plants, the algae use the Sun's energy to convert basic raw materials into sugars that are used as food. These algae produce enough food both for
themselves and the corals, so that reef-building coral animals rarely have to capture food for themselves.
However, coral animals also feed at night on zooplankton, which they capture with their tentacles. Coral animals prey on zooplankton not so much for the calories as for
scarce nutrients, especially phosphorus. Through digestion, coral animals release these nutrients to the algae. Coral and algae then apparently cycle these nutrients
between them, reducing nutrient loss to the water.

VI

THREATS TO CORAL REEFS

Despite their considerable importance to humans, many coral reefs around the world are in decline. Some scientists estimate that 58 percent of the world's coral reefs
are at risk from human damage. The most threatened reefs are those of Southeast Asia, where some 82 percent of reefs are at risk. About 61 percent of the Caribbean
reefs are also under considerable threat. In many cases these reefs are not simply threatened by one thing, but by a combination of natural and human-caused
stresses.

A

Natural Threats to Coral Reefs

Reefs have always been subject to natural threats. Storms and hurricanes regularly sweep across some tropical areas, bringing massive waves capable of smashing
corals even at depths of 10 m (33 ft) or more. Heavy rainfall can also harm corals by bringing fresh water and sediments onto the reefs.
Certain creatures eat coral. This is usually a natural process on a coral reef, but one creature, the crown-of-thorns starfish, sometimes reaches plague proportions and
can destroy all living coral on a reef in a few weeks. These plagues may be natural, and reefs can certainly recover. However, many scientists believe that overfishing of
the natural predators of these starfish may be worsening the problem. Starfish populations may also be growing because they benefit from increased fresh water and
sediments.
Corals appear to be susceptible to a number of diseases. Huge declines in certain types of coral, particularly in the Caribbean, have been associated with particular coral
diseases, notably white-band and black-band diseases. Very little is known about most coral disease, but fungi and bacteria may be responsible.

B

Human Threats to Coral Reefs

The great bulk of human threats to coral reefs falls into four categories: pollution, sedimentation, overfishing, and global warming. Corals are susceptible to various
forms of pollution, but probably the most common is simply the flow of nutrients from untreated sewage and from agricultural land, especially where there is heavy use
of pesticides or fertilizer. These nutrients cause large increases in algae, both in the plankton and in algae growing on the seafloor, which then smother the corals or
block out the light.
The amount of sediments reaching the sea has increased rapidly over the past hundred years or more. This can be linked to the loss of forests and the development of
new agricultural techniques that lay the land open to rain and erosion. The sediments are swept into the oceans through rivers and streams and, like the algae, can
smother and kill the coral.
Fishing has been going on around coral reefs for thousands of years. Problems arise when too many people try to catch fish from the reefs, or when, often out of
desperation, they turn to destructive measures to capture fish. The rapid growth of human populations, combined with the development of increasingly efficient fishing
methods, means that reefs in many areas are overfished. The reefs have fewer and smaller fish than in the past, a disaster both for the reef ecosystem and for the
fishers. In a few places, notably in Southeast Asia, some fishers use explosives to capture fish. These explosives kill all fish within a certain area, as well as destroying
the corals nearby. It takes many years for these damaged areas to recover.
Even reefs remote from humans have been affected by fishing as certain target species, such as large fish, giant clams, and sharks, have become so valuable that
fishing vessels are willing to travel considerable distances to capture the remaining individuals. Today it is rare to find a reef with its full complement of these
magnificent creatures.
At the end of the 20th century scientists realized that a new threat to the world's coral reefs existed. Corals are extremely sensitive to changes in temperature. If
temperatures are increased even by just one or two degrees above the normal maximum for a few days, corals exhibit a stress response called coral bleaching. The
relationship between the corals and the zooxanthellae breaks down, and the corals lose their color and become white. These bleached corals are still alive, but they are
weakened. If conditions continue for too long, they die.
Scientists have already seen dramatic increases in the numbers of extreme coral bleaching events since the 1970s and have linked these to a warming of the oceans
related to climate change caused by human activity. By burning fossil fuels, human activity has released large amounts of the greenhouse gas carbon dioxide into the
atmosphere. This heat-trapping gas helps warm the lower atmosphere, Earth's surface, and ocean water. Future predictions are that oceans will continue to warm.
Some scientists have predicted that most coral reefs will be regularly affected by devastating coral bleaching events by 2050, and many will be lost.
In the early 21st century, scientists drew attention to another repercussion of global warming that could have devastating effects on coral and coral reefs. British
scientists with the Royal Society reported in 2005 that the release of more than 25 billion metric tons of carbon dioxide into the air each year is turning the oceans
gradually more acidic. This dramatic change in ocean chemistry worldwide is reducing the availability of the carbonates that corals need to produce limestone skeletons
and may also be weakening their existing skeletons. Reduced growth rates and greater vulnerability to erosion are predicted.

C

Coral Reef Protection

Despite these threats, coral reefs can be protected. The results of this protection are beneficial for people as well as for the reefs. In places where there is chronic
overfishing, better, more sustainable management practices can increase the total fish numbers, allowing for more fish to reproduce and raising the catches of the
fishers. Sewage treatment and better management of land areas can help reduce pollution and sedimentation, allowing reefs to recover from degraded states.
The most important tools for coral reef conservation are education and the establishment of protected areas. Closing off even relatively small areas of coral reefs to
fishing can have dramatic results. Large numbers of fish build up in these areas and spill over to the surrounding reefs, where fishers may catch them. These same
areas become valuable destinations for tourists as they offer opportunities to see the reefs. They are also likely to be important areas to support the recovery of reefs
when there is destruction of reef areas through natural impacts or coral bleaching.

As of 2005 more than 660 coral reefs were being protected, including two of the world's largest protected areas--the Great Barrier Reef off Australia and the coral reefs
of the northwest Hawaiian Islands. The 660 protected reefs represent about 19 percent of the world's coral reefs. However, the majority of protected areas only receive
partial protection. Probably less than 5 percent of the world's coral reefs are sufficiently protected at the present time. Some scientists estimate that strict protection for
about 30 percent of the world's reefs would be ideal.
Some reefs appear to be more resilient to coral bleaching than others. The protection of these reefs from other harmful human impacts will be critical, not only for their
own survival, but also because these same reefs may provide new corals to repopulate damaged reefs. But in the long run, designating coral reefs as protected zones
will not spare any reefs from the global effects of climate change. Only drastic reductions in greenhouse gas emissions, particularly carbon dioxide emissions, can
protect coral reefs from the dangers of global warming.

Contributed By:
Mark D. Spalding
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