Where corals live

Corals live in all the oceans of the world. Many types grow in colonies that continue to enlarge year after year. Other types are solitary, meaning that they live alone. Several different types of corals can together form enormous colonies. These colonies are called coral reefs, coral islands, and coral atolls. The largest reefs are found in the warmer portions of the Pacific and Indian oceans. However, coral reefs are also found in the Caribbean and in the Gulf of Mexico as far north as southern Florida.

Coral reefs are composed of different types of corals. The reefs create underwater habitats that are essential for many species of marine organisms. Certain kinds of fish spend their life among the corals, using them as a refuge from predators. The largest coral reef in the world, the Great Barrier Reef off the coast of Australia, is more than 1,250 miles (2,000 kilometers) long. More than 350 species of coral have been found in the Great Barrier Reef.

Physical characteristics

Coral reefs have different shapes and patterns because of the growth pattern of the millions of tiny individual animals that make up a colony. Reefs can look like beautiful underwater gardens. Some corals resemble the tendrils of plants, while some may look like leaves. The brain coral is nearly spherical and has grooves that make it look like a human brain. Corals with flat, fan-shaped structures that spread out from a narrower base are called sea fans. The organ-pipe corals, which are found in the tropical oceans of the Indo-Pacific, are typically long rigid tubes.

The corals' skeletons, external or internal, have a stonelike, horny, or leathery texture. The body of a coral animal consists of a polyp—a hollow, cylindrical structure attached at its lower end to some surface. The largest of the solitary polyps grows to a diameter of about 10 inches (25 centimeters). The polyps of the corals that form reefs range from 0.04 to 1.2 inches (0.1 to 3 centimeters) in diameter.

Life cycle

Corals reproduce in two different ways. In one process, fertilized eggs develop into tiny, swimming organisms called planulae. Planulae eventually settle on the bottom of the ocean, on a rock or on another coral, and develop into polyps. Each polyp builds a limestone skeleton attached to the surface on which the polyp has landed. After the coral establishes itself, the upper part of the body becomes dome-shaped and develops a stomach and mouth.

Reproduction also can occur by budding. A bud develops the body of a polyp and remains attached there. A colony develops by the constant addition and growth of new buds. As new polyps develop, the older, underlying polyps die, leaving behind their skeletons. In some kinds of coral the buds may break away to become separate individuals. Vast coral colonies are built by budding, with the animals connected by their extensive skeletal network.

Food habits

Ecological issues

By the 1990s coral reefs off the coasts of more than 20 countries were being destroyed for a number of reasons. Rising water temperature is said to cause bleaching, which the corals cannot withstand. Oil spills and chemical pollution also are a major threat to corals. Long-term damage occurs when stands of coral are broken by freighters or boat anchors. Overfishing has caused the loss of coral-protecting sea urchins. This has left the coral vulnerable to the crown of thorns, a kind of coral-eating starfish.