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Clear tropical seawater is nutrient poor, an aquatic desert. The strange and varied forms of the members of coral reef communities allow each to fill a niche in a complex nutrient cycle, beginning with the fixing of nutrients by the photosynthesis of algae, and working up to the barracuda that snatches an aging fish from the school. The ammonia and feces secreted by the predator are cycled right back into the reef ecosystem.

Over 240 million years, when scleractinian coral reefs first formed, this community has made a remarkable geological impact. The stony coral skeletons become overgrown and compacted into rock, eventually building up a prodigious thickness of limestone. When forced upward by the buckling of the earth's crust, this old reef rock forms islands.

Distribution of Coral Reefs

Reef-building corals require large amounts of sunlight, and thus are only found in the tropics, and even there only in shallow water. The effective limit of coral growth is usually given as 100 meters, although in Indonesia coral usually stops at 60-70 meters. Corals, even stony corals, are found as deep as 6,000 meters, but these grow slowly and do not form the diverse communities of tropical coral reefs.