Molluscan assemblages on coral reefs and associated hard substrata in the northern Red Sea

Information on spatial variability and distribution patterns of organisms i n coral reef environments is necessary to evaluate the increasing anthropog enic disturbance of marine environments (Richmond 1993; Wilkinson 1993; Day ton 1994). Therefore different types of subtidal, reef-associated hard subs trata (reef flats, reef slopes, coral carpets, coral patches, rock grounds) , each with different coral associations, were investigated to determine th e distribution pattern of molluscs and their life habits (feeding strategie s and substrate relations). The molluscs were strongly dominated by taxa wi th distinct relations to corals, and five assemblages were differentiated. The Dendropoma maxima assemblage on reef fiats is a discrete entity, strong ly dominated by this encrusting and suspension-feeding gastropod. All other assemblages are arranged along a substrate gradient of changing coral asso ciations and potential molluscan habitats. The Coralliophila neritoidea Bar batia foliata assemblage depends on the presence of Porites and shows a dom inance of gastropods feeding on corals and of bivalves associated with livi ng corals. The Chamoidea-Cerithium spp. assemblage on rock grounds is stron gly dominated by encrusting bivalves. The Drupella cormus-Pteriidae assembl age occurs on Millepora-Acropora reef slopes and is strongly dominated by b ivalves associated with living corals. The Barbatia setigera-Ctenoides annu lata assemblage includes a broad variety of taxa, molluscan life habits and bottom types, but occurs mainly on faviid carpets and is transitional amon g the other three assemblages. A predicted degradation of coral coverage to rock bottoms due to increasing eutrophication and physical damage in the s tudy area (Riegl and Piller 2000) will result in a loss of coral-associated molluscs in favor of bivalve crevice dwellers in dead coral heads and of e ncrusters on dead hard substrata.