Variability in the crystallographic texture of bivalve nacre

Abstract

The crystallographic texture of nacre in eleven bivalve species belonging to the superorders Opponobranchia, Pteriomorpha, and Paleoheterodonta, was studied. Our analysis confirmed a uniform orientation of the crystallographic c-axes of aragonite platelets in all the species studied, but a variable arrangement of a-and b-axes. New data suggest that the rate of alignment of a-and b-axes from girdle-like (unordered nacre) to a single crystal-like texture (ordered nacre) differs considerably among the analyzed species. Numerical evaluation of the data obtained from species with ordered nacre also revealed systematic differences in texture strength among the main crystallographic axes of aragonite platelets. The texture strength of a-axes (direction parallel to growth lines) is always weaker than that of b-and c-axes. Observed differences in shape of the growth lines on the surface of nacreous platelets, in twinning percentage as well as in rate of alignment of a-and b-axes suggest variations in nacre growth process in individual phylogenetic lineages of the Class Bivalvia. These facts may imply not only evolutionary changes in microstructure and crystallographic texture of the nacre but also in molecular mechanisms driving its production during the long molluscan evolution.