Behavioural and eco-physiological responses of the mussel Mytilus galloprovincialis to acidification and distinct feeding regimes

The carbon dioxide taken up by the ocean is increasing as levels of atmospheric carbon dioxide increase, thus lowering the ocean pH and altering the carbonate system. In this laboratory study, we evaluated the physiological responses of juvenile mussels Mytilus galloprovincialis from Galician waters (NW Iberian Peninsula) exposed to control (500 μatm) and elevated (800 or 1200 μatm) seawater pCO₂ conditions under 2 different feeding regimes (optimal and sub-optimal). Shell properties such as compressive strength and composition (organic matter and aragonite:calcite ratio) were negatively affected by high seawater pCO₂, regardless of food availability. This result suggests that water chemistry is a main driver for shell development. Under the optimal feeding regime, mussel feeding rates increased in response to elevated pCO₂, presumably as a strategy to maintain a high strength of attachment. In contrast, mussels on the suboptimal diet showed weak attachment and narrow valve opening at the highest pCO₂ condition. Thus, our results suggest that with optimal food availability, mussels were resilient to water acidification with respect to feeding activity, valve opening and attachment strength. Under a suboptimal diet, however, the ability of mussels to respond to acidification was compromised. These results highlight complex ecophysiological interactions for calcifying organisms subjected to climate change.