Seawater acidification affects mortality, growth and shell integrity in juvenile bivalves

Anthropogenic emissions of carbon dioxide are leading to changes in the carbonate chemistry of seawater and decreases in pH. Ocean acidification may negatively affect the ability of marine organisms to produce calcareous structures, at the same time influencing physiological responses and growth. The aim of the study was the evaluation of the effects of reduced pH on survival and growth of two juvenile bivalves, Mytilus galloprovincialis and Chamelea gallina. A long-term experiment (6 months) was carried out at the Hydrobiological Station of Chioggia (Northern Adriatic, Italy). An experimental outdoor flow-through plant was set up to test two pH levels (natural seawater pH as control, 7.4 pH as treatment). Monthly, bivalve mortality was checked, and shell length and fresh weight were measured. After three and six months from the beginning of the experiment, shell damage and thickness were also evaluated. Mortality was low throughout the experiment for both mussels and clams, but a significant increase in treated animals was observed at the end of the sixth month. Mortality was sensibly higher in clams than in mussels. No significant difference in growth was found between control and treated mussels. Conversely, a significant decrease in weight and length of treated clams was observed. In controls of both species no shell damage was recorded. In treated bivalves, damage proceeded with different mode and extent, being maximum in mussels after three months exposure and progressively increasing in clams till to the end of the experiment. In a global change scenario, decreasing growth performance in bivalve natural populations may be expected, followed by reduction of presence/abundance and increased risk of extinction. Between the two species, C. gallina demonstrated to suffer more severely from seawater acidification, thus posing further concerns about maintenance of clam beds, mainly in the Adriatic sea.