The shell of Ostreoidea is specific among that of bivalves, as it comprises many crystal microstructures and crystal textures. This stems from the fact that into the basic shell structure, consisting of columnar, foliated, granular calcite and myostracal aragonite, voids are incorporated: the pores of the vesicular segments and the blades/laths of the chalk lenses. Furthermore, cementation to uneven-surfaced substrate requires the formation of corrugated shell morphologies and adjustments of crystal organization for the generation of convex/concave shell surfaces.
We investigated the shell of Magallana gigas, Ostrea stentina, Ostrea edulis (Ostreidae) and Hyotissa hyotis, Hyotissa mcgintyi, Neopycnodonte cochlear (Gryphaeidae) with high-resolution, low-kV, electron backscatter diffraction (EBSD) measurements and scanning electron microscopy (FE-SEM) imaging. From a crystallographic perspective we, (i) characterized the sub-micrometer crystal assembly of ostreoid microstructures and textures, (ii) investigated crystal organization at the changeover from one microstructure into the other, and (iii) examined how curved crystal surfaces are generated at inner shell surface and within the shell, in and at aggregations of folia. We show that Ostreoidea are capable to secrete, within the same shell, single crystalline, graded and polycrystalline calcite, the latter with almost random orientation. We demonstrate that Ostreoidea myostracal aragonite is twinned, while Ostreoidea calcite is not twinned, neither the calcite within the different microstructures, nor the calcite at the changeover from one microstructure into the other. We highlight the very specific microstructure of the foliated shell and demonstrate the strongly regulated gradedness of both, the c- and a*-axes orientation of the foliated calcite crystallites.