Coral reef

Coral reefs represent some of the oldest and most complex communities of plants and animals on Earth. About 200–400 million years old, they cover about 231,660 mi² (600,000 km²) worldwide. (The most popular reefs range from 5,000–10,000 years old.) The primary structure of a coral reef is a calcareous skeleton formed by marine invertebrate organisms known as cnidarians, which are relatives of sea anemones. Corals are found in most of the oceans of the world, in deep as well as shallow seas and temperate as well as tropical waters. But corals are most abundant and diverse in relatively shallow tropical waters, where they have adapted to the constant temperatures provided by these waters. The reef-forming corals, or hermatypic corals, have their highest diversity in the Indian and Pacific Oceans, where over 700 species are found. By contrast, the Atlantic Ocean provides the habitat for less than 40 species. Other physical constraints needed for the success of these invertebrate communities are clear water, a firm substrate, high salinity , and sunlight. Clear water and sunlight are required for the symbiotic unicellular plants that live in the surface tissues of the coral polyps. This intimate plant-animal association benefits both participants. Corals obtain oxygen directly from the plants and avoid having to excrete nitrogenous and phosphate waste products because these are absorbed directly as nutrients by the plants. Respiration by the coral additionally provides carbon dioxide to these plants to be used in the photosynthetic process.

The skeletons of hermatypic coral play a major role in the formation of coral reefs, but contributions to reef structure, in the form of calcium carbonate, come from a variety of other oceanic species. Among these are red algae, green algae, foraminifers, mollusk shells, sea urchins, and the exoskeletons of many other reef-dwelling invertebrates. This limestone infrastructure provides the stability needed, not only to support and protect the delicate tissues of the coral polyps themselves, but also to withstand the constant wave action generated in the shallow, near-shore waters of the marine ecosystem .

There are essentially three types of coral reefs. These categories are fringing reefs, barrier reefs, and atolls. Fringing reefs form borders along the shoreline. Some of the reefs found in the Hawaiian Islands are fringing reefs. Barrier reefs also parallel the shoreline but are found further offshore and are separated from the coast by a lagoon . The best example of this type of reef is the Great Barrier Reef off the coast of Australia . Because the coral colonies form an interwoven network of organisms from one end of the reef to the other, this is the largest individual biological feature on earth. The Great Barrier Reef borders about 1,250 mi (2011 km) of Australia's northeast coast. The second largest continuous barrier reef is located in the Caribbean Sea off the coast of Belize, east of the Yucatan Peninsula. The third type of reef, the atoll, is typically a ring-shaped reef, from which several small, low islands may project above the surface of the ocean. The ring structure is present because it represents the remains of a fringing reef that formed around an oceanic volcano . As the volcano eroded or collapsed, the outwardly-growing reef is ultimately all that remains as a circle of coral. Possibly the most infamous atoll is the Bikini Atoll , which was the site of the United States' hydrogen bomb tests during the 1940s and 1950s.

Besides the physical structure of the coral and the reef itself, the most significant thing about these structures is the tremendous diversity of marine life that exists in, on, and around coral reefs. These highly productive marine ecosystems may contain over 3,000 species of fish, shellfish, and other invertebrates. About 33% of all of the fishes of the world live and depend on coral reefs. This tremendous diversity provides for a huge commercial fishery in countries such as the Philippines and Indonesia. With the advent and availability of SCUBA gear to the general public in this half of this century, the diversity of life exhibited on coral reefs has been a great lure for tourists to these ecosystems throughout the world.

Even with their calcium carbonate skeleton and exquisite beauty, coral reefs are being degraded and destroyed daily, not only by natural events such as constant wave action and storm surges, but, more importantly, by the actions of man. Of the 109 countries that have coral reef formations within their territorial waters, 90 are losing them because of man-induced environmental degradation . Most is the result of physical abuse or pollution which alters the narrow range of physical and chemical parameters necessary for the coral, or their plant symbionts, to remain viable and thrive. Today, 10% of the world's coral reefs are completely degraded, 30% have reached a critical stage. Scientists have determined that if degradation at this rate continues, 70% of all coral reefs could be gone in 40 years. Reefs can be salvaged however. In 1995, participants from 44 countries representing governments, nongovernmental organizations, international development agencies, and the private sector gathered to launch the International Coral Reef Initiative. In 1997, some 1,400 participants declared the year as the International Year of the Reef, a period they hoped would heighten awareness and further reef salvage activities worldwide.

These reefs, most of which are between 5,000 and 10,000 years old, and some of which have been building on the same site for over a million years, are being degraded and destroyed by a vast array of water pollutants. Silt , which washes into the sea from erosion of clearcut forests miles inland, cloud the water or smother the coral, thus prohibiting the photosynthetic process from taking place. Oil spills and other toxic or hazardous chemicals that find their way into the marine ecosystem through man's actions are killing off the coral and/or the organisms associated with the reefs. Mining of coral for building materials takes a massive toll on these communities. Removal of coral to supply the ever increasing demand within the aquarium trade is destroying the reefs as well. The tremendous interest in and appeal of marine aquaria has added another problem to this dilemma. In the race to provide the aquarium market with a great variety of beautiful, brilliantly-colored, and often quite rare, marine fishes, unscrupulous collectors, who are selling their catches illegally merely for the short term monetary gain, spray the coral heads with poison solutions (including cyanide) to stun the fishes, causing them to abandon the reefs. This efficient means of collecting reef fishes leaves the coral head enveloped in a cloud of poison, which ultimately kills that entire section of the reef.

An unusual phenomenon has developed within the past decade with regard to coral reefs and pollution. In the Florida Keys in the early 1980s divers began reporting that the coral, sea whips, sea fans, and sponges of the reefs, around which they had been swimming, had turned white. They also reported that the waters felt unusually warm. The same phenomenon occurred in the Virgin Islands in the late 1980s. As much as 50% of the reef was dying due to this bleaching effect. Scientists are still studying these occurrences; however, many feel that it is a manifestation of global warming, and that a mere change in the water temperature around the coral reefs of 2–3°C is inducing the bleaching and death of the coral.

Tourism and recreation are inadvertently degrading coral reefs throughout the world as well. Coral is being destroyed by the propellers of recreational boats as well as divers who unintentionally step on coral heads, thus breaking them to pieces, and degrading the very structure of the ecosystem they came to see. Many of the reefs undergoing this degradation are sections that have been set aside for protection. Even with almost a quarter of a million square miles of coral reefs in the world, and about 300 protected regions in 65 countries, ever increasing levels of near-shore pollution, coupled with other acts of man, may be destroying these extremely complex communities of marine organisms at a rate faster than we can control.

[Eugene C. Beckham ]

RESOURCES

BOOKS

Brown, B., and J. Ogdon. "Coral Bleaching." Scientific American 268 (January 1993): 64-70.

Derr, M. "Raiders of the Reef." Audubon 94 (February 1992): 48-56.

Falkner, D. This Living Reef. New York: The New York Times Book Co., 1974.

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 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.