Bivalve shell structure and organic matrix

More than 10 morphological types of shell structure such as the simple prismatic, nacreous, foliated, composite prismatic, crossed lamellar structures and the others are discriminated in bivalve shells. The combination of these morphological types in each taxon is mainly divided into three groups: the nacreous, foliated and crossed lamellar groups [I. Kobayashi, Earth Science 73(1964) 1; I. Kobayashi, Sci. Rep. Niigata Univ., Ser. E 2(1971) 27.]. It should be emphasized that there is correlation between phylogenetic trends of bivalves and the groupings.Organic matrices contained in bivalve shells showed great diversity in their molecular weights and amino acid compositions. Samata [T. Samata, The Veliger, 33(1990) 191.] demonstrated that the amino acid composition of organic matrix differed markedly in relation to the shell structures. The difference was most remarkable among three types of shell structures in particular, (1) the nacreous and prismatic, (2) the foliated and (3) the crossed lamellar groups, in which the total amount and degree of heterogeneity of organic matrix components, as well as the composition depended on the shell structure. The secretion of organic matrix is fundamentally controlled by genes involved in shell formation and it can be pointed out that shell structures might reflect the gene structures. Therefore, recognition of relationship between the morphological types seen in shell structures and primary structures of organic matrices (genes) can open a speculation for the further study of molluscan evolution.