Nicole Höher

• The Baltic Sea is one of the largest brackish water areas in the world. The maintenance of its functioning strongly depends on habitat-forming key species, such as Mytilus spp. The health of these key species is dependent on effective immune responses that provide protection against infectious agents (bacteria, viruses, fungi and parasites) and tumours. Environmental contaminants have been shown to impair immune defences, leading to increased disease susceptibility. The present study, being part of the BEAST project, aimed to test the suitability of selected immunotoxic effects of contaminants on the bioindicator Mytilus spp., with particular focus on the confounding factor salinity. Even though, organisms have adapted to low ambient salinities in the Baltic Sea, permanent osmoregulation is energetically costly and affects stress susceptibility. Until now, the synergistic effects of salinity and pollution on the immune system of invertebrate species have not yet been explored. Studies conducted addressed the impact of the model contaminant copper on immune responses and disease susceptibility of Mytilus spp from differing ambient salinities, the effect of contaminant mixtures and ambient salinity in the natural environment in individuals indigenous to the region of concern and in transplanted organisms. Salinity slightly affected immune responses, in particular phagocytic activity and bacterial clearance, in Mytilus exposed to copper in the laboratory study. However, results from both field surveys showed contradictory results. In individuals indigenous to the locations of interest it was shown that contaminants had a greater impact on cellular immune responses than salinity. In conclusion, I recommend the implementation of immunological parameters, accompanied with assessments of higher biological levels, in monitoring surveys of environments with low salinities and brackish water conditions. In particular, the functional endpoint “phagocytic activity” and the morphometric endpoint “cytoplasmic vacuolation” are promising tools for the assessment of immunotoxicity. Nevertheless, further research on the impact of salinity with or without additional anthropogenic stressors are needed to elucidate the underlying mechanisms of the biological effects that are being assessed.