The Epifauna of a California Siltstone Reef

Between 300 and 400 described species of macroscopic animals (including 22 fishes) were encountered on 1,385 m 2 of a silt—stone, submarine hogback and adjacent mixed bottom located in the shallow sublittoral (9.5 to 18 m depth) about 500 m off the coast of Corona del Mar, California. The components of the epifauna are not distributed uniformly over the reef. Both the numbers of species and individuals per unit area of reef surface vary inversely with depth. Although more species achieve maximum population densities at the top of the reef than elsewhere, some species are most abundant on the reef's mid—section, on the base, or on the rock—sediment bottom. Delineations among these major groups are sufficiently marked to warrant recognition of 3 vertical epifaunal zones on the reef itself and a 4th on the surrounding bottom within a meter of the reef base. Although changing conditions associated with the highly irregular rocky surface create a high degree of patchiness in the epifauna, within any of the zones some aggregations of species are sufficiently uniform and widespread to permit recognition of communities. In general the predominant species in the upper 2 zones are sessile suspension feeders, which form thick incrustations that form biotopes for large numbers of small sessile and motile species. In the lower zones, on the other hand, the predominant species tend to be motile scavengers and deposit feedes, and the incrustations that are present are thin and formed by single species. The degree of water movement on the reef, which results primarily from the propagation of surface waves, also varies inversely with depth. It is suggested that this pattern of water movement determines the distribution of the major units of the epifauna through its effects upon the availability of food of different types at particular points on the reef. Suspended organic matter is not only more abundant on the upper levels of the reef, but its rate of delivery to crustforming organisms is enhanced by turbulence. Deposition of organic materials removed from the upper part of the reef is facilitated by the sharp reduction in degree of water movement along the lower sides and bottom of the reef.