Post-larval cementation in the Ostreidae and its implications for other cementing bivalvia

Despite the polyphyletic acquisition of the cemented habit in the bivalves, the actual mechanism of post-larval cementation has remained obscure. This study examines the mechanism of attachment in the dissoconchs of members of the Ostreidae. It is demonstrated that a thin periostracal sheet, preformed at the mantle margins, is secreted throughout the cementation process and that the eventual cement is calcareous rather than composed of mucopolysaccharide. The cement structure is reminiscent of ‘inorganic’ diagenetic cavity fill cements. The composition of the cement is remarkably similar to that of the oyster shell, indicating that it is derived from the same extrapallial fluid. A mechanism is proposed to account for these observations, invoking a permeable periostracum which allows the ‘leakage’ of extrapallial fluids. This means of cementation has interesting implications for the evolution of the cemented habit in the bivalves, and in other encrusting taxa as well as for our understanding of biomineralization processes.