Effects of CO2-induced seawater acidification on the health of Mytilus edulis

The impact of CO2-acidified seawater (pH 7.8, 7.6, or 6.5, control = pH 8) on the health of Mytilus edulis was investigated during a 60 d mesocosm experiment. Mussel health was determined using the neutral red retention (NRR) assay for lysosomal membrane stability and from histopathological analysis of reproductive, digestive and respiratory tissues. Seawater acidification was shown to significantly reduce mussel health as measured by the NRR assay, and it is suggested that this impact is due to elevated levels of calcium ions (Ca2+) in the haemolymph, generated by the dissolution of the mussels' calcium carbonate shells. No impact on tissue structures was observed, and it is concluded that M. edulis possess strong physiological mechanisms by which they are able to protect body tissues against short-term exposure to highly acidified seawater. However, these mechanisms come at an energetic cost, which can result in reduced growth during long-term exposures. Consequently, the predicted long-term changes to seawater chemistry associated with ocean acidification are likely to have a more significant effect on the health and survival of M. edulis populations than the short-lived effects envisaged from CO, leakage from sub-seabed storage. Ocean acidification could reduce the general health status of this commercially and ecologically important marine species.