Coral reefs constitute a varied, complex structure that serves as a habitat for many other marine animals, including echinoderms, mollusks, crustaceans, and especially fish.
At first glance coral reefs seem to be almost entirely lacking in plant life. The reef-building colonial corals, however, supply much of the photosynthetic basis for this rich community. The cells of hermatypic corals are packed with microscopic single-celled algae known as zooxanthellae. The association between coral and zooxanthellae is mutualistic-- both benefit. The coral shelters the algae, and metabolic by- products of the algae are a major source of nutrients for the coral. As an additional source of food, corals capture plankton.
The coral-algal association accounts for the strange, plantlike structure of many reef-building corals. Corals spread out and compete for light just as terrestrial plants do. Overtopping by rapidly growing corals shades and kills slower growing colonies. Some corals are able to damage and kill their neighbors by a digestive process should they come in contact. The plantlike nature of reef-building corals is the reason that reefs are found only in shallow waters. A few corals, lacking algae, form small colonies in deep water.
In response to environmental stress, some reef corals suffer from a condition known as "bleaching," in which they expel the algae, become weak, and sometimes die. Particularly widespread in the Caribbean region, this condition has damaged reef corals in all parts of the world.
Among the organisms occupying coral reefs, echinoderms (see ECHINODERM) play an important role. The crown-of-thorns starfish is a major coral predator in the Indo-Pacific area, able to devastate large areas of living coral. Starfish population explosions, however, occur sporadically and are short-lived, which suggests that the crown-of-thorns is a natural part of the fluctuating buildup and breakdown of portions of coral reefs. This is not the case in other oceans, however. One objection to proposals for a sea- level canal through Central America is that it might unleash this voracious predator on Caribbean corals, which have never evolved any defenses.
Another important echinoderm is the spiny sea urchin, Diadema. This grazing urchin feeds on algae growing on dead portions of the reef and upon the sea grass that often surrounds shallow reefs. Diadema is such an efficient grazer that it creates a cleared "halo" around the reef margin.
The giant clam, Tridacna, is the most spectacular mollusk associated with coral reefs (in the Indo-Pacific area only). Like many corals, its tissues also contain zooxanthellae. Many other species of small bivalves, as well as many snails, are found among coral reefs. One snail, Charonia, is a major predator of the crown-of-thorns starfish. Poisonous (even to humans) cone snails are common reef inhabitants, feeding on fish and small invertebrates.
Many Crustacea roam the surface of coral reefs. Large, brightly colored painted shrimp live in pairs and feed upon the crown-of -thorns starfish. Another unusual crustacean is the gall crab. The female crab settles in a notch of a branching coral, and the coral grows around and permanently encases her. The vividly colored mantis shrimp occupies cavities in dead coral, fighting to obtain and defend its dwelling from other shrimp.
Among the most spectacular and visible life on the reefs are the numerous and beautiful fish. Some, such as the large PARROT FISH, feed on the living coral. DAMSELFISH defend territories around the reef from larger fish as well as from invertebrates and one another; otherwise, sea urchins and other fish would soon eliminate their algal food supply.
Perhaps the most fascinating reef fish are the cleaner fish. These small fish remove external parasites from the skin, gills, and mouth of most other reef fishes. The cleaner fish maintain stations at which other fish collect and wait their turn to be cleaned. The cleaner fish are rarely eaten by other fish. In some locations a blenny mimics the shape and coloration of the local cleaner species, darting up to unsuspecting victims and nipping the fins or skin.
Among other interesting fish species living on the reef are the poisonous stonefish and lionfish, moray eels that dart out from crevices in the reef to attack their prey, and fishes that change from female to male as they age.
Coral is a common name for several species in the phylum Coelenterata (or Cnidaria), a major group of invertebrate animals. Other COELENTERATES include hydras, jellyfish, and sea anemones. The latter, along with most corals, are members of the class Anthozoa. All coelenterates are characterized by their radically symmetrical (as opposed to bilaterally symmetrical) bodies, and an internal space for digestion, called a gastrovascular cavity, whose only opening is surrounded by tentacles to aid in food capture.
Corals are most often recognized by their stony skeletons, a calcium carbonate crystalline framework secreted by the epidermis of individual coral organisms. The skeleton provides a substratum to support the living organism, or polyp, and serves as a protective enclosure into which a polyp might retreat when threatened. Most corals are colonial, although a few are solitary. Individual polyps, usually under 3 mm in diameter (0.12 in), can live together in enormous structures known as CORAL REEFS.
Coral reefs in the tropics provide limestone materials for road-building. Other types of coral have been used as ornaments. Much of the coral valued as jewelry grows in the Mediterranean Sea and in the Sea of Japan.