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Coral Reef

Coral Reef

A coral reef is a living community built around the accumulated mineralized remains of coral animals, which belong to phylum Cnidaria. The hardened calcium carbonate secretion from coral animals, with mineralized algal cells and other secretions, create nooks and crannies that shelter up to sixty thousand species, including hundreds of types of corals, as well as eels, lobsters, sea slugs, sea horses, sea urchins, turtles, and a huge variety of fishes. A coral reef houses some permanent occupants, and others that come and go. Often life lives within life. For example, snapping shrimp dwell in sponges that occupy crevices in layers of coral.

A living coral animal, called a polyp, is small and soft. Polyps collect atop their preserved ancestors, using their waving tentacles to capture prey that floats by. The sticky calcium carbonate exoskeletons that polyps secrete meld them to each other and to the graveyard below.

The tides deliver nutrients to coral polyps. Algal and dinoflagellate symbionts live inside the corals and actively photosynthesize, providing nutrients to their hosts and contributing the vibrant colors that give coral reefs their rainbow hues. These guests remove wastes from polyps and maintain water pH at a level that stimulates deposition of the exoskeletons. A million such symbionts may occupy a mere 2 cubic inches of coral reef. Were it not for the photosynthesis that the algae and dinoflagellates provide, the coral could not survive.

Extent and Diversity

Corals cover 242,000 square kilometers (232,000 square miles) of ocean. Most reefs lie between 25 degrees north and south latitude, with more isolated growths in cooler waters farther from the equator. Most species require clear, warm water of about 20 degrees Celsius (68 degrees Fahrenheit). Less massive corals are found in colder waters that hug continents, including the fjords in Norway and along vertical banks of the coasts of England, New Zealand, Japan, and the western United States.

Biologists distinguish types of coral reefs by shape and organization. An atoll is a ring-shaped coral colony that encloses a lagoon, whereas a fringing reef forms next to shores where there isn't much rain, such as on one side of a tropical island. Barrier reefs surround islands or run alongside shorelines, enclosing lagoons. The Australian Great Barrier Reef is 1,303 square kilometers (1,250 square miles) long.

Threats to Coral Reefs

Many coral reefs are threatened, either by nature, human activity, or both. Winds destroy the delicate substructure of reefs, which have been damaged both by the large-scale, long-lasting winds of El Niño and more localized but dramatic hurricanes. When stressed by climatic extremes, polyps disgorge their dinoflagellate symbionts, bleaching the coral. In addition, many corals in recent years have fallen victim to bacterial and viral infections. A dozen different viruses, for example, have decimated populations of elkhorn and staghorn corals in the Caribbean.

Building near shores threatens corals. Nitrogen and phosphorus fertilizer and soil in runoff from construction upsets the species balance of photosynthesizing symbionts.

Snorkelers are warned not to sample the coral, which many people erroneously think are plants or nonliving. In some areas, people catch fish by infiltrating living coral with explosives or cyanide, which often kills the coral and humans along with the fish. With all of these insults, ecologists estimate that by the middle of the twenty-first century, up to two-thirds of coral reefs may be gone.

see also Algae; Biome; Bony Fish; Cnidarian; Ocean Ecosystems: Soft Bottoms; Porifera; Symbiosis

Ricki Lewis

Bibliography

Bryant, Dirk, Lauretta Burke, John McManus, and Mark Spalding. Reefs at Risk: A Map-Based Indicator of Threats to the World's Coral Reefs. Washington, DC: World Resources Institute, 1998.

Crossette, Barbara. "World's Imperiled Shores and Coral Reefs to Get Millions in Aid." The New York Times (15 March 2001): F1.

Humann, Paul, and Ned Deloach, eds. The Reef Set. Lancaster, CA: New World Publications, 1994.

Steene, Roger. Coral Seas. Willowdale, Ontario: Firefly Books, 1998.

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coral reefs

coral reefs, limestone formations produced by living organisms, found in shallow, tropical marine waters. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone). The accumulation of skeletal material, broken and piled up by wave action, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although corals are found both in temperate and tropical waters, reefs are formed only in a zone extending at most from 30°N to 30°S of the equator; the reef-forming corals do not grow at depths of over 100 ft (30 m) or where the water temperature falls below 72°F (22°C). Corals are not the only, and in some cases not even the major, reef-forming organisms. Calcium carbonate is also deposited by coralline algae, the protozoan foraminiferans, some mollusks, echinoderms, and tube-building annelid worms. However, any reef formed by a biological community is usually called a coral reef.

Geologically, coral reefs are classified into three main types. Fringing reefs are coral platforms that are more or less continuous with the shore and exposed at low tide. Barrier reefs are separated from the shore by a wide, deep lagoon or surround a lagoon that has a central island. An atoll is a reef surrounding a lagoon that has no central island, with passages through the reef to the sea. It is generally believed that fringing reefs formed as a result of upward and outward growth of corals that became established on rocks near shore; there is disagreement about the nature of barrier reef and atoll formation. Charles Darwin postulated a progression from fringing reef to barrier reef to atoll, as a result of a slow, steady sinking of the seafloor that creates a lagoon and a simultaneous upward and outward growth of coral. Where entire volcanic islands sink, only the reef remains above water, forming an atoll. Not all scientists accept Darwin's proposal, but most current theories involve subsidence of the seafloor, although changes of the ocean level may also be involved.

Sediments accumulate on the lagoon side of atolls and support vegetation; in time the entire lagoon may fill, creating an island. Many such atolls and islands, common in the Pacific and Indian oceans, are inhabited. The Great Barrier Reef of NE Australia is the largest known complex of coral reefs. It is 10 to 90 mi (16–145 km) wide and about 1250 mi (2010 km) long, and is separated from the shore by a lagoon 10 to 150 mi (16–240 km) wide.

Reefs are under numerous environmental pressures, including damage from increased coastal development, water pollution, tourism, runoff containing agricultural chemicals, abrasion by ships' hulls and anchors, and smothering by upstream sedimentation. Coral reefs are sometimes destroyed in fishing when poison or dynamite are used to catch fish and by the harvesting of coral for use in jewelry. During the 1990s, many previously unknown diseases began attacking coral reefs worldwide, causing rapidly spreading damage.

See A. Emery, The Coral Reef (1981); J. A. Fagerstrom, The Evolution of Reef Communities (1987).

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coral reef

coral reef A massive, wave-resistant structure, built largely by coral (see CORALS), and consisting of skeletal and chemically precipitated material. Coral reefs extend over an area of more than 175 × 106 km2 in tropical and subtropical seas, being best developed where the mean annual temperature is 23–5°C; they do not develop significantly at less than 18°C. Surface illumination is important and reefs do not grow in regions of high sedimentation, their skeletal formation depending on the activity of symbiotic algae and zooxanthellae.

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coral reef

coral reef A massive, wave-resistant structure, built largely by coral, and consisting of skeletal and chemically precipitated material. Coral reefs extend over an area of more than 175 × 106 km2 in tropical and subtropical seas, being best developed where the mean annual temperature is 23–25°C; they do not develop significantly at less than 18°C. Surface illumination is important and reefs do not grow in regions of high sedimentation, their skeletal formation depending on the activity of symbiotic algae and zooxanthellae.

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coral reef

coral reef Rock formation found in shallow tropical seas. Such reefs are formed from the calcium carbonate secreted by living coral organisms as protection against predators and wave action. The way in which the coral, and therefore the reef, grows is strongly influenced by the prevailing currents and the temperature of the surrounding sea water.

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