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—blue-green 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.

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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.

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