coral reefs Coral reefs are masses of calcareous rock deposited by living organisms (not all of them corals), representatives of which commonly inhabit reef tops a little below the low-tide level of the ocean. At present, the principal hermatypic (or reef-building) organisms are the calcifying rhodophytes (red algae), molluscs, sponges, polychaetes, and cnidarians. The latter include the corals; the most important contributors to the growth of coral reefs are scleractinian corals, together with a few octocorallians and hydrocorallians (Table 1). The Scleractinia are an order of corals, mostly colonial, which have a calcareous external skeleton with radial partitions or septa. The Octocorallia form fan-shaped colonies with interconnecting branches. The Hydrocorallina also have calcareous skeletons.

Reef-building corals are animals containing soft parts called polyps that contain symbiotic algae. One of the major constituents of a polyp is calcium bicarbonate, which is broken down by the coral into calcium carbonate and carbonic acid. The latter is then broken down into carbon dioxide and water. The carbon dioxide is taken by the algae which, through photosynthesis, produce metabolites that feed the polyp. The process of extracting carbonic acid from calcium bicarbonate entails the precipitation of calcium carbonate, a process that is responsible for the construction of coral reefs:

Patrick D. Nunn

Bibliography

Darwin, C. R. (1842) Structure and distribution of coral reefs. Smith, Elder, London.
Guilcher, A. (1988) Coral reef geomorphology. John Wiley and Sons, New York.
Jones, O. A. and Endean, R. (eds) (1973) Biology and geology of coral reefs, Vol. 1. Academic Press, New York.
Nunn, P. D. (1994) Oceanic islands. Blackwell, Oxford.
Nunn, P. D. (1999) Environment change in the Pacific Basin. John Wiley and Sons, Chichester.
Wiens, H. J. (1962) Atoll environment and ecology. Yale University Press, New Haven.

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.

coral reefs mostly develop in shallow tropical seas, though there are the cold-water Darwin mounds. Typically, a coral reef consists of an inner lagoon with a fringing reef at the edge of deeper water. They develop atop sea mounts forming atolls and also along coasts as barrier reefs, with the Great Barrier Reef being by far the largest.

Each reef has a complex structure built from the skeletons of species of hard (hermatypic) corals. Corals are coelenterates, relatives of anemones and jellyfish, that lay down limestone skeletons (calcium carbonate). In their soft tissues live symbiotic microscopic plant cells called zooxanthellae, which provide their hosts with organic food in exchange for protection. Like all green plants these cells need sunlight to grow, so the reef-forming corals are restricted to the shallow sunlit waters and are very sensitive to any increases in turbidity of the water that may cut out the light.

The reefs are important not only for their rich diversity, but also because they protect the coastline from the ravages of tropical storms. Many of the fishes that live over the reefs graze on the coral, scraping off the soft tissues and the limy skeletons. The latter is voided and is the source of the white coralline sands that make tropical beaches so attractive. Traditionally they have been exploited by local fisheries. One favoured, but highly destructive, method of fishing is to drop explosives, but this kills many more fish than are recovered. Another threat comes from the expansion of trade in colourful tropical fish for aquaria. Poisons are used to stun the fish, which also kill many of the other animals on which the ecology of the reefs depend. Yet another threat to coral reefs results from the sudden rises in sea temperatures generated by the El Niños. Recently reefs in the Indian Ocean have been particularly badly affected, one scientist estimating that more than 90% of the corals were killed by the effects of the 1998 El Niño.

The expansion of diving and ecotourism, while giving a measure of protection to some reefs, is exposing the fragile reefs to damage from boats' anchors, spearfishing and careless divers. On remote islands, coralline rocks from the reefs offer the only local source of building material, and their extraction can cause problems. Corals are now suffering from coral bleaching. When the water becomes too hot the zooanthellae are expelled and the coral dies back. This has been happening right across the tropics and may be a sign of global warming.

Some coral reefs are also under attack from population outbursts of the crown of thorns starfish, a species of echinoderm that eats the corals' living tissue, and also from another of today's environmental issues, eutrophication. Potentially an even more serious threat comes from the increasing concentrations of carbon dioxide in the atmosphere, which hampers the coral reefs' ability to lay down their skeletons of calcium carbonate, so they may no longer be able to grow fast enough to keep up with rising sea levels. This is a major concern for people living along tropical coastlines and on coral islands, because the protection afforded to them from the impact of tropical storms by the reefs will be reduced.

M. V. Angel

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.

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

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.

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.

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.