Molluscan shells, mainly composed of calcium carbonate, also contain organic components such as proteins and polysaccharides.
Shell organic matrices construct frameworks of shell structures and regulate crystallization processes during shell formation. To date, a number of shell matrix proteins SMPs have been identified, and their functions in shell formation have been studied. However, previous studies focused only on SMPs extracted from adult shells, secreted after metamorphosis. Using proteomic analyses combined with genomic and transcriptomic analyses, we have identified 31 SMPs from larval shells of the pearl oyster, Pinctada fucata , and from the Pacific oyster, Crassostrea gigas.
Larval SMPs are almost entirely different from those of adults in both species. RNA-seq data also confirm that gene expression profiles for larval and adult shell formation are nearly completely different.
Therefore, bivalves have two repertoires of SMP genes to construct larval and adult shells. Despite considerable differences in larval and adult SMPs, some functional domains are shared by both SMP repertoires. These conserved domains are thought to play crucial roles in shell formation.
Furthermore, a comprehensive survey of animal genomes revealed that the CA and VWA—CB domain-containing protein families expanded in molluscs after their separation from other Lophotrochozoan linages such as the Brachiopoda. After gene expansion, some family members were co-opted for molluscan SMPs that may have triggered to develop mineralized shells from ancestral, nonmineralized chitinous exoskeletons. In living organisms, biomineralized tissues serve multiple functions: tissue support, storage of mineral ions, protecting the soft body from predators and from environmental factors such as UV radiation Lowenstam ; Simkiss and Wilbur As in other metazoan lineages, acquisition of diverse mineralized exoskeletons is one of the reasons for the rapid establishment of shell-bearing molluscs at the dawn of Cambrian times Kawasaki et al.
