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).

The Columbia Encyclopedia, 6th ed. Copyright© 2014, The Columbia University Press.

Selected full-text books and articles on this topic

FREE! Methods of Study in Natural History
L. Agassiz.
Ticknor and Fields, 1863
Librarian’s tip: Chap. XI "Formation of Coral Reefs" and Chap. XII "Age of Coral Reefs Showing Permanence of Species"
Zoogeography of the Sea
Sven Ekman; Elizabeth Palmer.
Sidgwick & Jackson, 1953
Beneath Tropic Seas: A Record of Diving among the Coral Reefs of Haiti
William Beebe.
G.P. Putnam's Sons, 1928
East African Ecosystems and Their Conservation
T. R. McClanahan; T. P. Young.
Oxford University Press, 1996
Librarian’s tip: Chap. 4 "Coral Reefs and Nearshore Fisheries"
Principles of Paleoclimatology
Thomas M. Cronin.
Columbia University Press, 1999
Librarian’s tip: "Coral Reefs, Sea Level, and Ice Volume" begins on p. 151
Study of the Earth
J. F. White.
Prentice-Hall, 1962
Librarian’s tip: "Questions of the Coral Reefs" begins on p. 121
Preserving Paradise
Zurick, David N.
The Geographical Review, Vol. 85, No. 2, April 1995
Peer-reviewed publications on Questia are publications containing articles which were subject to evaluation for accuracy and substance by professional peers of the article's author(s).
Life-Saving Products Form Coral Reefs: Coral Reefs Are Storehouses of Genetic Resources with Vast Medicinal Potential, but They Must Be Properly Managed. (Perspectives)
Bruckner, Andrew W.
Issues in Science and Technology, Vol. 18, No. 3, Spring 2002
Ocean Security, Sustainable Development and Peace
Jacques, Peter.
International Journal of Humanities and Peace, Vol. 18, No. 1, Annual 2002
Emerging Diseases Threaten Conservation
Epstein, Paul R.; Chivian, Eric; Frith, Kathleen.
Environmental Health Perspectives, Vol. 111, No. 10, August 2003
Peer-reviewed publications on Questia are publications containing articles which were subject to evaluation for accuracy and substance by professional peers of the article's author(s).
America's Coral Reefs: Awash with Problems; Government Must Acknowledge the Magnitude of the Crisis and Fully Engage the Scientific and Conservation Communities in Efforts to Solve It
Agardy, Tundi.
Issues in Science and Technology, Vol. 20, No. 2, Winter 2004
Indonesia's Coral Reefs on the Line
Ryan, John C.
World Watch, Vol. 14, No. 3, May 2001
Monitoring Coral Reefs from Space
Soliday, Brian.
The World and I, Vol. 16, No. 9, September 2001
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