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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 µ g 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, DSr ∕ Ca , for labelled and unlabelled shells are similar to shell proportions formed in the wild (0.12 to 0.15). All DSr ∕ Ca values are lower than values for equilibrium partitioning of Sr in synthetic aragonite.

We use pulsed Sr-labelling experiments to visualize growth of aragonitic Katelysia rhytiphora (Mollusca, Bivalvia) shells. The outer compound composite prismatic structure is organized into three orders of prisms, and the inner crossed acicular structure consists of intersecting lamellae. Electron Backscatter Diffraction (EBSD) reveals substantial twinning of the aragonite crystals (> 46 %) and an overall reduced and strategically oriented anisotropy of the Young’s modulus in the whole shell compared to that of monolithic aragonite. All structural orders in both layers are enveloped by an organic sheath and the smallest mineralized units are nanogranules. Total organic contents are 2.2 (outer) and 1.4 wt. % (inner layer) and are, thus, intermediate between those of nacreous and crossed-lamellar shells. Prisms in the outer structure can be correlated to yearly, daily and sub-daily growth rates. Average daily growth rates at the ventral margin for the outer structure are 17 % higher than for the inner crossed acicular structure. The calcification front runs evenly across all structures and architectural orders independently of the current growth rate. Sharply defined transitions from labelled to unlabelled areas in the shell indicate that physiological processes driving calcification have no lag. This suggests that the extrapallial fluid cannot be very voluminous. Narrow increments of varying Sr content within labelled shell, despite constant Sr concentrations in seawater, suggest cyclic metabolic activity during calcification. Micro-Raman spectroscopy maps validate a low impact of high Sr-conditions on the aragonite crystal structure. Identical Sr-enrichment factors for labelled and ambient conditions support models of ion transport via a passive selective pathway to the mantle epithelium followed by calcification via amorphous calcium carbonate.