Microtexture of larval shell of oyster, Crassostrea nippona: A FIB-TEM study

The initial formation and subsequent development of larval shells in marine bivalve, Crassostrea nippona were investigated using the FIB-TEM technique. Fourteen hours after fertilization (the trochophore stage), larvae form an incipient shell of 100–150 nm thick with a columnar contrast. Selected-area electron diffraction analysis showed a single-crystal aragonite pattern with the c-axis perpendicular to the shell surface. Plan-view TEM analysis suggested that the shell contains high density of {110} twins, which are the origin of the columnar contrast in the cross-sectional images. 72 h after fertilization (the veliger stage), the shell grows up to 1.2–1.4 μm thick accompanying an additional granular layer between the preexisting layer and embryo to form a distinctive two-layer structure. The granular layer is also composed of aragonite crystals sharing their c-axes perpendicular to the shell surface, but the crystals are arranged with a flexible rotation around the c-axes and not restricted solely to the {110} twin relation. No evidence to suggest the existence of amorphous calcium carbonate (ACC) was found through the observation. The well-regulated crystallographic properties found in the present sample imply initial shell formation probably via a direct deposition of crystalline aragonite.