Insights into architecture, growth dynamics, and biomineralization from pulsed Sr-labelled Katelysia rhytiphora shells (Mollusca, Bivalvia)

The intertidal bivalve Katelysia rhytiphora, endemic to south Australia and Tasmania, is used here for pulsed Sr-labelling experiments in aquaculture experiments to visualize shell growth at the micro- to nanoscale. The ventral margin area of the outer shell layer composed of (i) an outermost outer shell layer (oOSL) with compound composite prismatic architecture with three hierarchical orders of prisms and (ii) an innermost outer shell layer (iOSL) with crossed-acicular architecture consisting of intersecting lamellae bundles. All structural orders in both layers are enveloped by an organic sheath and the smallest mineralized units are nano-granules. Electron backscatter diffraction reveals a strong preferred orientation of the aragonite c axes perpendicular to the growth layers, while the a and b axes are scattered within a plane normal to the local growth direction and >46 % twin grain boundaries are detected. The Young's modulus shows a girdle-like maximum of elastically stiffer orientations for the shell following the inner shell surface.

For 6 d, the bivalves were subjected twice to seawater with an increased Sr concentration of 18× mean ocean water by dissolving 144 µg g⁻¹ Sr (159.88 Sr∕Ca mmol ∕ mol) in seawater. The pulse labelling intervals in the shell are 17× (oOSL) and 12× (iOSL) enriched in Sr relative to the Sr-spiked seawater. All architectural units in the shell are transected by the Sr label, demonstrating shell growth to progress homogeneously instead of forming one individual architectural unit after the other. Distribution coefficients, D_{Sr ∕ Ca}, for labelled and unlabelled shells are similar to shell proportions formed in the wild (0.12 to 0.15). All D_{Sr ∕ Ca} values are lower than values for equilibrium partitioning of Sr in synthetic aragonite.