Mussel larvae modify calcifying fluid carbonate chemistry to promote calcification

Understanding mollusk calcification sensitivity to ocean acidification (OA) requires a better knowledge of calcification mechanisms. Especially in rapidly calcifying larval stages, mechanisms of shell formation are largely unexplored-yet these are the most vulnerable life stages. Here we find rapid generation of crystalline shell material in mussel larvae. We find no evidence for intracellular CaCO₃ formation, indicating that mineral formation could be constrained to the calcifying space beneath the shell. Using microelectrodes we show that larvae can increase pH and [CO₃^2-] beneath the growing shell, leading to a ~1.5-fold elevation in calcium carbonate saturation state (Ω_arag). Larvae exposed to OA exhibit a drop in pH, [CO₃^2-] and Ω_arag at the site of calcification, which correlates with decreased shell growth, and, eventually, shell dissolution. Our findings help explain why bivalve larvae can form shells under moderate acidification scenarios and provide a direct link between ocean carbonate chemistry and larval calcification rate.