Maintained larval growth in mussel larvae exposed to acidified under-saturated seawater

Ocean acidification (OA) is known to affect bivalve early life-stages. We tested responses of blue mussel larvae to a wide range of pH in order to identify their tolerance threshold. Our results confirmed that decreasing seawater pH and decreasing saturation state increases larval mortality rate and the percentage of abnormally developing larvae. Virtually no larvae reared at average pH(T) 7.16 were able to feed or reach the D-shell stage and their development appeared to be arrested at the trochophore stage. However larvae were capable of reaching the D-shell stage under milder acidification (pH(T) ≈ 7.35, 7.6, 7.85) including in under-saturated seawater with Ω(a) as low as 0.54 ± 0.01 (mean ± s. e. m.), with a tipping point for normal development identified at pH(T) 7.765. Additionally growth rate of normally developing 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 on OA impacts on mussel larvae suggest an average pH(T) of 7.16 is beyond their physiological tolerance threshold and indicate a shift in energy allocation towards growth in some individuals revealing potential OA resilience.