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different species have been identified. Some of these are the important reef builders, which are long-lived and form solid structures, whereas others are colonizers, able to live on the reef as long as they receive some support and protection from other corals. Some corals are delicate and grow only in sheltered places, whereas others prefer exposed sites. Corals appear to grow best on the edge of the reef, particularly on the sea- ward side, where they receive some exposure to wave action; this keeps them supplied with food in the form of plankton and plenty of oxygen for respiration. Waste products are also carried away easily. The following examples are but a cross-sec- tion of the many disparate life forms found on coral reefs. Other reef creatures (discussed later) include some sharks, groupers, and other large fish.

rare and specialized, but all are built on the same basic plan. Corals must live in shallow seas because their tissues contain millions of microscopic plants that require light to produce their food by pho- tosynthesis. Even the clearest tropical water absorbs some of the light that passes through it so that below a depth of about 165 feet (50 meters), there is not enough light energy to be useful to plants. Corals cannot tolerate expo- sure to the air, so the upper limit of their colo- nization is the lowest point to which the tides fall—the extreme low-water mark of spring tides. No coral polyp can survive in a free-float- ing state because it must be anchored to build its chalky skeleton. These three restrictions limit the corals to warm, clear, relatively shal- low waters with an abundance of firm rocks. Each reef contains many species of coral, and in some particularly rich reefs, more than 200

Reef sharks find most of

their food close to the reef and spend much of their time cruising along near the bottom or along

submerged cliff edges.

The tentacles of the orange

cup coral polyps show the many stinging cells used to immobilize prey. This particular specimen was photographed at night on a shipwreck in the British Virgin Islands.

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