Pearl formation in the Japanese pearl oyster ( Pinctada fucata ) by CaCO 3 polymorphs: Pearl quality‐specific biomineralization processes and their similarity to shell regeneration

Abstract

Pearl oyster shell consists of two layers: a calcite prismatic layer (outer layer) and an aragonite nacreous layer (inner layer). Calcite and aragonite are CaCO3 polymorphs, and their formations are controlled by shell-forming tissue called mantle. Pearl sacs originating in the mantle form cultured pearls. Therefore, it has been widely accepted that pearl and shell are produced by the same process. However, this idea has been called into question by some recent mineralogical studies indicating microstructural and crystal-polymorphic diversity in pearls. The pearl biomineralization process is still not fully understood in detail. Thus, in this study, we focused on the diversity of CaCO3 polymorphism of non-nacreous structures (NNSs) underlying the nacreous layer in pearl and regenerated shell, to reveal the biomineralization process of the Japanese pearl oyster (Pinctada fucata). Using Meigen's stain and scanning electron microscope-energy dispersive X-ray (SEM-EDX), NNSs polymorphs in valuable and valueless pearls, in addition to regenerated shell, were compared. Aragonite was exclusively observed in the NNSs of valuable pearls, whereas calcite was dominant in those of valueless pearls. The same analysis of NNSs of regenerated shells was carried out. As in valueless pearls, almost all regenerated shell NNSs consisted of calcite, but one NNS was composed of aragonite. Accordingly, it seems that pearls are formed by the same biomineralization process as shell regeneration rather than shell formation.