Marine invertebrate of the class Anthozoa in the phylum Cnidaria, which also includes sea anemones and jellyfish. It has a skeleton of lime (calcium carbonate) extracted from the surrounding water. Corals exist in warm seas, at moderate depths with sufficient light. Corals form large colonies known as coral reefs, which cover less than 0.25% of the total area of marine environments but provide a home to 25% of the world's known marine life.
Corals live in a symbiotic relationship with microscopic algae (zooxanthellae), which are incorporated into the soft tissue. The algae obtain carbon dioxide from the coral polyps, and the polyps receive nutrients from the algae. Corals also have a relationship to the fish that rest or take refuge within their branches, and which excrete nutrients that make the corals grow faster.
The majority of corals form large colonies, although there are species that live singly. Their accumulated skeletons make up large coral reefs and atolls. The Great Barrier Reef, to the northeast of Australia, is the world's largest coral reef system and largest living structure. Coral reefs provide a habitat for a diversity of living organisms. A third of the world's marine fishes live at least part of their lives in reefs.
Fringing reefs are so called because they form close to the shores of continents or islands, with the living animals mainly occupying the outer edges of the reef. Barrier reefs are separated from the shore by a saltwater lagoon, which may be as much as 30 km/20 mi wide; there are usually navigable passes through the barrier into the lagoon. Atolls resemble a ring surrounding a lagoon and are usually formed by the gradual subsidence of an extinct volcano, with the coral growing up from where the edge of the island once lay.
As living structures, the health of coral reefs is affected by changes to their enivronment caused by climate change, human activities, and disease.
The biggest threat to the world's coral reefs is climate change. Coral bleaching occurs when the environmental conditions of the coral reef, especially water temperature, change sufficiently to cause the zooxanthellae to leave, which in turn causes the coral to die. Changes in salinity, pH, or water level can also kill coral.
Human activities that threaten coral include the development of coastal towns, which produce water pollution in the form of chemical and sewage discharges. Excess nutrients, resulting from agricultural activities, can promote the production of algae, reducing the levels of sunlight reaching the coral. Physical damage to coral reefs is a common consequence of the building of harbours and dredging of shipping canals. Destructive fishing practices, both direct (such as cyanide poisoning and dynamiting) and indirect (such as overfishing) adversely affect the health of the coral reef ecosystem. Poorly managed tourism, especially the removal of coral as souvenirs and pollution from tourist transports, can also cause harm.
At least thirty diseases are known to affect coral. These can be caused by fungi, algae, worms, and bacteria. The commonest diseases known to affect the Great Barrier Reef are brown band, black band, and white syndrome.
Careful management, as with the Great Barrier Reef, keeps coral healthy and leads to improved fish yields and increased revenues from tourism.
Corals and Coral Reefs
coral reef in the Red Sea
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