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Reef sponges create an environment that is exploited by a variety of other creatures. Small crabs and shrimps and even fish hide in the tubes and cavities. Crinoids perch on upright sponges to filter plankton from the current. And sea cucumbers and other detritus feeders graze on the organic material that collects on the sponge's surfaces.

Corals and their relatives

Corals, soft corals, sea anemones, gorgonians, hydroids, jellyfish and the other members of the phylum Cnidaria (formerly Coelenterata) cause a great deal of confusion for the diver trying to identify the teeming mass of branched and tentacled life he sees attached to the reef. Taxonomists identify these animals by their stinging cells, nematocysts, and simple coelenteron, from the Greek koilos, "hollow," and enteron, "gut." All have the form of a polyp at some stage in their lives. Other than these shared characteristics, the form of these animals varies widely.

Aristotle considered them an intermediate form between plants and animals, and they were first placed by taxonomists in a group called Zoophyta, "animal-plants." Only in 1723 were corals properly identified as animals, and Jean Andre Peyssonel, the naturalist who proposed this to the French Academy of Sciences, was laughed at and quit science in disgrace.