Coral, is a limestone formation formed in the sea by millions of tiny animals. Coral formations may look like branching trees, large domes, small irregular crusts, or tiny organ pipes. The living coral-forming animals color the formation in beautiful shades of tan, orange, yellow, purple, and green. When the animals die, they leave limestone "skeletons" that form the foundations of barriers and ridges called coral reefs. Coral reefs look like lovely sea gardens because of the many colorful sea animals that live among the corals. These animals include fishes, starfish, and sea anemones.
Sometimes coral masses build up until they rise above the water to form coral islands. The grinding, battering sea helps to build coral islands. It breaks up the coral growths and piles them up. Other creatures, such as calcifying algae, cement the pieces together and a rigid structure is formed. Often, soil lodges on the coral and vegetation begins to grow. Many Pacific islands were formed this way.
Coral reefs are found mostly in warm, shallow, and tropical seas, because the reef-forming corals cannot live in water colder than 65 °F . Reefs abound throughout the South Pacific, in the East Indies and the Indian Ocean to Sri Lanka, and around Madagascar on the southeastern African coast. They also form along the tropical eastern coast of Brazil, through the West Indies, along the Florida coast, and at Bermuda. There are three types of coral reefs: (1) fringing reefs; (2) barrier reefs; and (3) atolls.
Fringing reefs are submerged platforms of living coral animals that extend from the shore into the sea. Barrier reefs follow the shoreline, but are separated from it by water. They form a barrier between the water near the shore and the open sea. A barrier reef may consist of a long series of reefs separated by channels of open water. Such reefs usually surround volcanic islands of the South Pacific. The Great Barrier Reef of Australia, about 1,250 miles long, is the largest coral reef in the world. An atoll is a ring-shaped coral island in the open sea. It forms when coral builds up on a submerged mudbank or on the rim of the crater of a sunken volcano. The atoll surrounds a body of water called a lagoon. One or more channels connnect the lagoon to the open sea. Many coral islands of the South Pacific Ocean are atolls.
Coral reefs do not develop on the east coast of North America north of Florida and Bermuda. But small patches of coral grow as far north as New England. Certain kinds of coral grow as far north as the Arctic Circle. How coral is formed. The animals that form coral belong to the same animal group as the hydras, jellyfish, and sea anemones. Most individual coral animals, called polyps, are less than 1 inch in diameter, but a small percentage measure as much as 1 foot. A coral polyp has a cylinder-shaped body. At one end is a mouth surrounded by tiny tentacles. The other end attaches to the limestone skeletons of dead polyps. Most coral polyps live together in colonies. The stony corals attach themselves to each other with a flat sheet of tissue that connects to the middle of each body. Half of the coral polyp extends above the sheet and half below.
Coral polyps build their limestone skeletons by taking calcium out of the seawater. Then they deposit calcium carbonate (limestone) around the lower half of the body. As new polyps grow, the limestone formation becomes larger and larger. Coral polyps feed mainly on tiny swimming animals, such as the larvae of many kinds of shellfish. Reef corals cannot live without special, single-celled algae that live in the polyp's own tissue. The polyps use food manufactured and released by the algae. The algae also help the coral animals secrete their limestone skeletons. Coral reefs grow only in water with enough light for photosynthesis to occur in the algae. Coral polyps reproduce either from eggs or by budding. Small, knoblike growths called buds appear on the body of an adult polyp, or on the connecting sheet, from time to time. These buds grow larger, separate from the parent, and begin to deposit their own limestone in the colony. Budding helps the colony increase its size. New colonies of coral polyps form when the adult polyps of an old colony produce eggs. The eggs grow into tiny forms that swim away. Then the developing animals settle to the sea bottom and begin to form new colonies by budding.
Various marine animals eat living coral-forming animals. The loss of coral to such animals is usually balanced by the development of new coral colonies and the growth of old ones. But beginning in the 1960's, large numbers of crown-of-thorns starfish destroyed stony coral colonies on many reefs of the southwest Pacific Ocean. Scientists are trying to determine what has caused this starfish to become so numerous. Precious and gorgonian corals. Other kinds of coral, in addition to stony corals, are found in the world's oceans. These corals are also colonies of polyps, but the skeletons they form are internal rather than external.
Precious coral is a species valued for jewelry. It has a hard core that can be polished. Polishing brings out beautiful red, rose, or pink colors. Craftworkers carve it into beads and other ornaments. Precious coral grows in bushlike formations in the Mediterranean Sea and Sea of Japan. Gorgonian corals have internal skeletons of a flexible, horny substance. These corals look like bushes, fans, or whips. They may be soft yellow, rose, purple, brown, tan, or black. In clear West Indian waters, gorgonian corals look like sea gardens as they wave on the reefs.