Role of competition and predation in determining habitat occupancy of Cerithiidae (Gastropoda: Prosobranchia) on the rocky,intertidal, Red Sea coasts of Sinai

Six cerithiid snail species occur on rocky intertidal flats along the Sinai coasts of the Red Sea: Clypeomorus moniliferum, Cerithium caeruleum, C. scabridum, C. columna, Clypeomorus tuberculatum and Cerithium echinatum. The present study, conducted at 22 stations covering almost the whole length of Sinai, covered the 4 yr period from October 1972 to August 1976, and describes the habitat occupancy of 5 of these species (C. echinatum is excluded for lack of data). Several of these species (sometimes all) often occur together, and in such cases are distincly segregated by habitat. However all species considerably overlap in their distribution along the axes of four major interrelated abiotic gradients, thus excluding the possibility that habitat segregation is determined by larval settlement preferences. Other distributional patterns observed at some sites, such as lack of overlap or contact between belts of the various species and the relative abundance of food available to all species, make postlarval competitive interactions unlikely. The existence and the degree of proximity of a coral reef with its associated predatory fishes, influence the cerithiids' distributional patterns. Differences between the cerithiid species in their vulnerability to fish predation, associated with differences between sites in the abundance and the accessibility of predatory fishes, and in the availability of refuges for each cerithiid species, can satisfactorily explain the observed distributional patterns including co-occurrence with habitat segregation. It is proposed that habitat segregation is caused by predation on young stages by generalist fishes which may totally eliminate a certain species at a given site; the same site may provide refuges for recruits of another species, allowing these to survive to an advanced age. In general, the flat's structural complexity is associated with its diversity of refuges from predation, and hence with the number of co-existing species. This mechanism for co-existence and habitat segregation in tropical Cerithiidae may also be instrumental in maintaining the high species diversity of other tropical benthic communities.Paper No. 12 in the series Colonization of the Eastern Mediterrancan by Red Sea species immigrating through the Suez Canal