Large gryphaeid oysters as habitats for numerous sclerobionts: a case study from the northern Red Sea

The shell of a living specimen of the Indo-Pacific gryphaeid giant oyster Hyotissa hyotis was colonized by numerous encrusting, boring, nestling and baffling taxa which show characteristic distribution patterns. On the upper valve, sponge-induced bioerosion predominates. On the lower valve intergrowth of chamid bivalves and thick encrusting associations-consisting mostly of squamariacean and corallinacean red algae, acervulinid foraminifera, and scleractinian corals-provides numerous microhabitats for nestling arcid and mytilid bivalves as well as for encrusting bryozoans and serpulids. Such differences between exposed and cryptic surfaces are typical for many marine hard substrata and result from the long-term stable position of the oyster on the seafloor. The cryptic habitats support a species assemblage of crustose algae and foraminifera that, on exposed surfaces, would occur in much deeper water.