Immunomodulating effects of environmentally realistic copper concentrations in Mytilus edulis adapted to naturally low salinities.

The possibility to assess organisms’ health condition and biological effects of chemicals is a fundamental requirement for the achievement of the Good Environmental Status (GES) as defined in the Marine Strategy Framework Directive (EU). In this respect, the role of natural environmental stressors, possibly acting as confounding factors, should be carefully considered, especially at low doses of exposure. Organisms from the Baltic Sea have adapted to the ambient salinity regime, however energetically costly osmoregulating processes may have an impact on the capability to respond to additional stress such as contamination. In the present study, immune responses of Mytilus spp, adapted to salinities of 12 ‰ (LS) and 20 ‰ (MS) were compared after short-term exposure (1, 7 and 13 days) to low copper concentrations (5, 9 and 16 µg/L Cu). In brief, LS mussels accumulated markedly more copper than MS mussels, and exhibited a strong correlation between bacterial clearance and phagocytic activity. The greatest separating factor of LS and MS was the proportion of granulocytes and hyalinocytes while functional parameters (phagocytic activity and bacterial clearance) were merely affected by salinity, but rather by copper exposure. The overall results demonstrated that immune responses may be suitable biomarkers for the assessment of ecosystem health in brackish waters (10 to 20 ‰ S), and that the impact of salinity on physiological processes should be accounted for.