coral
The formation of a coral atoll by the gradual sinking of a volcanic island. The reefs fringing the island build up as the island sinks, eventually producing a ring of coral around the spot where the island sank.
Corals are marine animals related to jellyfishes and sea anemones. Most hard or stony corals live as colonies of polyps that secrete a rigid external skeleton of lime. But there are also those that live as solitary individuals. Corals grow in a wide variety of forms as the types here demonstrate. Sea fans, for example, have an internal skeleton linking the polyps, while the polyps of the brain coral are arranged in rows. The stagshorn coral gets its name from the resemblance of the colony to a male deer's antlers. The names of the plate and pillar corals also reflect their appearance.
A picture of the coral off the coast of Costa Rica. Corals are animals, and not plants as often supposed, which live in tropical seas and frequently form reefs around islands.
A shoal of carp around a coral reef in the Pacific Ocean. This familiar scene is under threat. Surveys during the 1990s proved that reefs were becoming subject to dangerous levels of pollution and physical damage. They are also experiencing new and pernicious diseases, affecting both the coral and the fish, algae, and other creatures that live in frequently symbiotic proximity.
Coral in the Red Sea is under threat. The sea is a major shipping lane and has several popular tourist centres along its shores. Potentially, the pollution emitted could do serious damage to the coral that lines the coasts. This is particularly disturbing, since one theory regarding the origin of the name ‘Red Sea’ suggests the existence of an important symbiotic relationship between algae and the endangered coral.
A coral reef off Tarawa, northern Kiribati, in the Pacific. Pacific coral is renowned for its coloration – and yet the natural colour of coral (primarily composed of calcium carbonate) is matt white. The colours of coral (and ‘coral’ is itself generally counted as a colour that is not white) are caused by mineral impurities in the water absorbed by the coral organisms. Absorption of iron, for example, contributes to shades between pink and brown.
A view of a classic coral atoll in the western Pacific. In effect it represents a frame for something that is no longer there: a volcanic peak that once protruded through the water surface in the middle, but has since eroded away. What is left is a circular island based upon coral that for the most part remains beneath the surface.
Many coral atolls are surrounded by a barrier reef enclosing a lagoon. This is a secondary phase of development, during which the atoll has taken on the mantle of the volcanic peak that once existed in the middle, and the reef is another atoll that is in turn forming around it.
A squirrel fish, hiding among rocks in the reefs of the Seychelles. In recent years there has been growing concern about coral reef degradation, as tropical shallow-water marine environments come increasingly under pressure from human activities. There are approximately 27,000 ha/66,700 acres of protected marine area and reefs in the Seychelles.
An aerial view of part of Chumbe Island, Zanzibar. Chumbe Island Coral Park is a private nature reserve that includes a coral reef sanctuary and a forest reserve. The reef has been protected since 1992.
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.
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