coral

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. Some coral is valued for decoration or jewellery, for example, Mediterranean red coral Corallum rubrum.

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 about 1,600 km/1,000 mi long, has a total area of 20,000 sq km/7,700 sq mi, and adds 50 million tonnes of calcium to the reef each year. The world's reefs cover an estimated 620,000 sq km/240,000 sq mi.

Coral reefs provide a habitat for a diversity of living organisms. In 1997 some 93,000 species were identified. One third of the world's marine fishes live in reefs. The world's first global survey of coral reefs, carried out in 1997, found around 95% of reefs had experienced some damage from overfishing, pollution, dynamiting, poisoning, and the dragging of ships' anchors. A 1998 research showed that nearly two-thirds of the world's coral reefs were at risk, including 80% of the reefs in the Philippines and Indonesia, 66% of those in the Caribbean, and over 50% of those in the Indian Ocean, the Red Sea, and the Gulf of Arabia.

Diseases
Since the 1990s, coral reefs have been destroyed by previously unknown diseases. The white plague attacked 17 species of coral in the Florida Keys, USA, in 1995. The rapid wasting disease, discovered in 1997, affects coral reefs from Mexico to Trinidad. In the Caribbean, the fungus Aspergillus attacks sea fans, a soft coral. It was estimated in 1997 that around 90% of the coral around the Galapagos islands had been destroyed as a result of ‘bleaching’, a whitening of coral reefs which occurs when the coloured algae evacuate the coral. This happens either because the corals produce toxins that are harmful to the algae or because they do not produce sufficient nutrients. Without the algae, the coral crumbles and dies away. Bleaching is widespread all over the Caribbean and the Indo-Pacific. A report published in November 1998 by Reef Check, an international organization, claimed that coral reefs are dying at a record rate throughout the world, and reefs which had existed for hundreds of years had suddenly died in 1998. Worldwide warming has caused bleaching of coral reefs in 50 or more countries in that year.