Coral reefs are one of the oldest ecosystems on the planet. They provide a habitat for more than 4,000 marine animals and are often referred to as the "rainforests of the oceans." PHOTOGRAPH REPRODUCED BY PERMISSION OFAP/WIDE WORLD PHOTOS.

Coral polyps are the most important reef-building organism. The species that secrete calcium carbonate are known as hard coral polyps or, simply, stony corals. In the western Pacific Ocean, more than three hundred species of stony corals exist. Coral polyps are invertebrates (pronounced in-VER-tuh-brets), or animals without backbones. They are related to anemones and jellyfish. In fact, a coral polyp looks similar to an anemone: it is a jellylike sac attached at one end to its skeleton. The open end, the mouth, is fringed with six stinging tentacles (or a multiple of six), which the polyp extends at night to feed. The size of coral polyps varies greatly, depending on the species. They may be as small as 0.04 inch (0.1 centimeter) in diameter or as large as 8 inches (20 centimeters) in diameter.

Worldwide, coral reefs cover an estimated 110,000 square miles (284,900 square kilometers). Many stony corals grow best in clear, salty water with a temperature between 70°F and 85°F (21°C and 29°C). Because of this, most reefs are found in the Pacific Ocean, the Caribbean Sea, the Red Sea, the Arabian Sea, and the Indian Ocean between the tropic of Cancer and the tropic of Capricorn, two parallel lines of latitude lying one-quarter of the way from the equator to the North and South Poles, respectively. Reefs are also found around Florida and southern Japan because warm-water currents flow into these areas from the Tropics.

The shape of a coral reef depends on the species of coral building it. Different types of coral produce different shapes. Some are pointed and hard; others are round and soft. They may be robust or delicate. Various shapes in reefs include fingerlike branches, flat branches, knobs or wedges, boulders or balls, mushroom caps, and tablelike structures.