Biocrystal assembly patterns, biopolymer distribution and material property relationships in mytilus galloprovincialis, Bivalvia, and haliotis glabra, Gastropoda, shells

Biologically secreted structural materials serve many purposes: protection, defense, locomotion, support, reinforcement. Their specific functions are achieved through the intimate interaction of biopolymer and mineral tissue characteristics: composition, microstructure/fabric, biomaterial properties. Here we highlight and discuss for bivalve and gastropod shells the interrelation between mineral microstructure, texture, organic component fabric and distribution. We investigate these on the micrometer to submicrometer scale. We choose two model organisms living in distinct marine environments constructing their shells with distinct mineral microstructures and biopolymer assemblies. We compare shell structural characteristics for the mollusk H. glabra that forms its purely aragonitic shell of a prismatic and a columnar tablet portion. In contrast, M. galloprovincialis constructs its shell of calcite and aragonite. Calcite fibers comprise outer and sheeted arrangements of aragonite tablets inner shell portions, respectively. Glutaraldehyde fixed/MOPS buffer soluble organic material exists between aragonite prisms, while insoluble organic membranes encase calcite fibers and aragonite tablets. Electron backscatter diffraction demonstrates for the shell layers the marked difference in microstructure and texture. Quasi-static nanoindentation documents that, of the four investigated microstructures, prismatic aragonite forms the hardest and stiffest material. Nacreous aragonite, columnar or sheeted, is less hard and stiff, while the assembly of calcite fibers yields the softest and the most compliant biomaterial. Is toughness considered, the trend is reverted. We show that the toughness of calcite fibers is capable to outperform the tablet arrangements, despite the fact that fibrous calcite gives the weakest microstructure, relative to aragonitic microstructures.