Ocean acidification stunts molluscan growth at CO₂ seeps

Ocean acidification can severely affect bivalve molluscs, especially their shell calcification. Assessing the fate of this vulnerable group in a rapidly acidifying ocean is therefore a pressing challenge. Volcanic CO₂ seeps are natural analogues of future ocean conditions that offer unique insights into the scope of marine bivalves to cope with acidification. Here, we used a 2-month reciprocal transplantation of the coastal mussel Septifer bilocularis collected from reference and elevated pCO₂ habitats to explore how they calcify and grow at CO₂ seeps on the Pacific coast of Japan. We found significant decreases in condition index (an indication of tissue energy reserves) and shell growth of mussels living under elevated pCO₂ conditions. These negative responses in their physiological performance under acidified conditions were closely associated with changes in their food sources (shown by changes to the soft tissue δ¹³C and δ¹⁵N ratios) and changes in their calcifying fluid carbonate chemistry (based on shell carbonate isotopic and elemental signatures). The reduced shell growth rate during the transplantation experiment was further supported by shell δ¹³C records along their incremental growth layers, as well as their smaller shell size despite being of comparable ontogenetic ages (5-7 years old, based on shell δ¹⁸O records). Taken together, these findings demonstrate how ocean acidification at CO₂ seeps affects mussel growth and reveal that lowered shell growth helps them survive stressful conditions.