Increased Relative Calcification, Shell Dissolution and Maintained Larval Growth in Mussel (Mytilus edulis) Larvae Exposed to Acidified Under-Saturated Seawater

Ocean acidification (OA) is known to affect bivalve early life-stages. It is often assumed that aragonite saturation state (Ω_a) is the main driver of the biological response. However saturation state of different CaCO₃ forms is not the main driver of most physiological processes and pH/pCO₂ are playing an overarching role (e.g. acid-base regulation). The aim of our study was to investigate the effects of a wide range of seawater pH on different physiological parameters (e. g. calcification; growth) of blue mussel (Mytilus edulis) developing larvae in order to identify a physiological tipping point beyond which they are no longer capable of carrying out those functions necessary to their survival and recruitment into the adult population.

Our results confirmed that increasing seawater acidity and decreasing saturation state increases larval mortality rate and the percentage of abnormally developing larvae. No larvae reared at pH_T ≈ 7.1 were able to reach the D-shell veliger stage and their development appeared to be arrested at the trochophore stage. However – despite morphological shell abnormalities - larvae were capable of reaching the D-shell stage when reared at pH_T ≈ 7.35 and normally D-shaped larvae were observed in all the remaining treatments (pH ≈ 7.6, 7.85 and 8.1) including in under-saturated seawater with Ω_a as low as 0.75 ± 0.03 (mean ± SE). Growth rate of these larvae was not affected by lower pH_T despite potential increased energy costs associated with compensatory calcification in response to increased shell dissolution. Overall, our results suggest a shift in energy allocation toward growth in larvae exposed to ocean acidification.