Each polyp, which may be as small as a tiny seed or as large as a lily pad, secretes a hard calcium substance that forms a cup in which the polyp lives. Within polyp tissues live colonies of algae that process polyp wastes before they are excreted, helping the polyp to retain valuable nutrients. Most corals are colonial, creating masses of cups fused together. The bulk of any reef colony is composed of the abandoned calcium cups built by previous colonies. Only the upper layer is covered by living corals, which form a thin, ever-changing skin over their dead predecessors.
Coral reefs are complex, tightly bonded ecosystems.
Removal or disturbance of one key component of a reef can trigger a nasty chain reaction. When fishermen took too many parrot fish and sea urchins from reefs around the South Pacific's Cook Islands in the mid-1980s, algal growth soon smothered the coral, killing the polyps. Parrot fish and urchins help to keep the reefs clean by grazing extensively on excess algae. No other marine creature or ecosystem rivals the productive capacities of coral reefs. Southeast Asian reefs alone can support an estimated 5 to 15 times the number of fish found in the North Atlantic.