Reef-dwelling molluscs in open framework cavities, Bonaire N.A., and their potential for preservation in a fossil reef

Six hundred and ten specimens comprising 34 species of gastropods and 40 species of bivalves were collected from 60 growth framework reef cavities over the depth range 3 to 61 m from fringing reefs on the leeward side of Bonaire, Netherlands Antilles. The bivalves represent 20 families and the gastropods 24 families, with the highest familial diversity at 30 m and 36 m, respectively. The number of bivalve species in cavities at each depth is consistently almost twice the number of gastropod species, although both groups show very similar trends in the number of species versus depth. Five species assemblage depth zones are recognized, with boundaries between zones at 3 m, 12 m, 21 m, and 46 m depth. The overall potential for preservation of reef coelobiontic (cavity-dwelling) molluscs is high, but the potential for recognition of fossil molluscs preserved in cavities as having been coelobiontic in life is dependant upon their preservation in situ on the cavity walls and roof. Only 3% of the coelobiontic gastropod species are likely to be preserved in situ, compared with 66% of bivalve species. Therefore, although in the fossil state some coelobiontic molluscs may be recognized, the potential for reconstructing the bivalve and gastropod assemblages in their original proportions and relationships is very poor.