A comparison of living and dead molluscs on coral reef associated hard substrata in the northern Red Sea — implications for the fossil record

Abstract

Fidelity of death assemblages to live shelly faunas is one of the major palaeontological questions, but quantitative data are scarce and most case studies on this topic have been performed in non-reef sediments. Therefore we studied different types of subtidal reef-associated hard substrata (reef flats, reef slopes, coral carpets, coral patches, rock grounds), each with different coral associations, in order to determine the agreement of assemblages of living and dead shell-bearing molluscs. A total area of 340.5 m 2 was investigated and 2846 individuals were counted at 68 sample localities ranging from shallow subtidal to 40 m water depth. Most taxa found dead in the study area were also found live and vice versa; differences in this respect can be related to quantitatively unimportant taxa. However, strong differences exist in the proportion of living and dead fauna, dominant taxa, and molluscan distribution patterns. The ratio of live to dead molluscs is high. Living molluscs are strongly dominated by taxa with distinct relations to corals, mainly Pedum, Coralliophila and Tridacna, and the encrusting gastropod Dendropoma. Five distinct groups of living molluscs can be differentiated and related to specific hard substrata, which are characterized by distinct molluscan life habits. In contrast, the death assemblages are always strongly dominated by encrusting bivalves, mainly Chamoidea and Spondylidae, and cerithiid gastropods in varying dominances. Correspondingly there is no significant correlation of the total abundance of living and dead molluscs and their overall similarity is only 6%. Similarity between living and dead faunas is above 50% at only 12 of the 68 sample locations, and at 17 sample locations significant correlations of living and dead molluscs were recognized. These correlations are mainly based on Chamoidea, which dominate both the living and dead fauna, on rock grounds. Therefore rock grounds are the only bottom type with consistent correlations and similarities of living and dead molluscs. The observed bias is due to the close relationship of molluscan life habits and post-mortem history of shells. Molluscs that live permanently attached to or within living corals (mostly bivalves and encrusting Dendropoma) can easily be overgrown after death by the large amounts of living substrata available. Rapid transport of dead shells into surrounding sediments or into crevices within corals is typical of gastropods that feed on corals. Molluscs that colonize dead surfaces preferentially accumulate on rock grounds. The ecologic information that can be derived from the shells depends on the different post-mortem histories of the faunas.