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Corals and coral reefs are most abundant and best developed offshore of the Florida Keys. These reefs, nearly 100,000 years old, form a tract almost 240 km (150 mi) long and about 6 km (4 mi) wide sloping to the edge of the Florida Straits (fig. 17). The reef tract slope is not uniform but consists of a series of banks and channels parallel to the keys. Reefs of two types are present in the reef tract: (1) patch reefs which grow in the back reef zone and (2) outer reefs which form the seaward edge of the reef tract platform. The present day corals are the same species which formed the Key Largo Limestone and built the upper Florida Keys. The best examples of living reefs exist off Key Largo where tidal channels are few and where the offshore water is relatively free of suspended sediment (Hoffmeister, 1974).

In the back reef area behind the outer reef, the water is calmer and silt accumulates. Small patch reefs are scattered here among sand banks and grass beds. Almost all the life forms of the outer reef occur on the patch reefs, but the dominance of animal species differs, as does the growing shape of some corals. Sea fans and feathers seem more common here than on the outer reef, and there is a higher percentage of grass-feeding fishes. These fishes appear to utilize the patch reefs as a daytime resting place and then move onto nearby grass beds to feed at night. Patch reefs often have a halo of white sand around their perimeter caused largely by the browsing of the black-spined sea urchins on the adjacent grasses (Ogden and others, 1973).

The greatest variety of corals and coral reef animals lives on the outer reefs. That environment has the stablest temperature and salinity and the clearest water. Most reef-building corals require clear water for photosynthetic algae living in their soft tissues. The corals in turn benefit from oxygen and nutrients produced by the algae. The Florida Current, which moves parallel to the reef tract, provides a rich source of plankton, a food source for many fishes and invertebrates of the outer reefs. The fish population on the outer reefs is one of the most varied in the world, containing more than 500 recorded species (Starck, 1968). In addition, there are more than 20 species of reef-building corals including the branching staghorn corals, large specimens of the star coral, and several species of brain coral. These corals provide a haven for fish, crustaceans, mollusks, worms, and sea urchins. Also, the dead coral limestone offers attachment surface to a multitude of marine algae and invertebrates. Nearly 1,400 species of marine plants and animals were recorded for a small area of the Florida reef tract (Voss and others, 1969). These coral reefs are perhaps the most diverse and colorful marine habitats within the continental United States.

FIGURE 17. (left) The Florida reef tract. [larger image]	(right) Staghorn coral on a south Florida reef. [larger image]

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