Fig. 1: Nacre growth visualised by Sr pulse-chase labelling.

a Photo of the inner shell surface of a M. galloprovincialis shell showing nacre constituting most of the inner shell surface inside the white dotted line and prismatic calcite outside the line. b Schematic cross-section of a shell. The shell grows in length by extending the outer calcite layer along the ventral margin (black arrow). The inner nacreous shell layer extends by thickening the inner shell surface (two white arrows). c SEM-BSE image of a polished shell cross-section (specimen M2S2R) showing Sr-labelled nacre layers as three light grey bands tracing parallel to the inner shell edge, while shell portions grown in normal seawater composition are shown in darker grey. In this specific example, the nacre layer grew 25 µm over the experimental period of 46 days marked by the start of the innermost Sr label. Scale bars are 1 mm (a) and 5 µm (c).