The amino acid composition of the organic matrix of some recent and fossil shells of some west coast species of Mytilus

Abstract

The organic matrix from each structural unit of Mytilus californianus has a distinctive amino acid composition. Systematic variations in composition occur within the outer prismatic layer and the periostracum. In small shells there are profound changes in the periostracum composition as the animal grows during the first year or so. There is a relationship between the amount of organic material present in the original shell unit and the relative amounts of the basic and acidic amino acids in the organic matrix. The matrix from calcitic shell structures has a larger net excess of acidic residues than does the matrix from aragonitic structures. Temperature changes appear to be reflected in compositional changes in the nacreous aragonite layer with increasing temperature favoring a higher number of net acidic amino acid residues. Thus, as temperature increases, the differences in matrix composition between the nacreous aragonite layer and the outer prismatic calcite layer become progressively smaller. Decreasing salinity seems to have a similar effect to increasing temperature. From a study of a series of radiocarbon-dated shell materials, it is found that the composition of the organic matrix undergoes a significant amount of change within a few thousand years under a rather dry environment. The oxidation products of certain amino acids are present in progressively larger amounts in older materials, suggesting oxidation as one of the possible mechanisms involved in fossilization.