Environmental drivers of host-parasite interaction in the Baltic and Adriatic Sea under a global change perspective

In coastal areas undergoing fast changes, such as the Adriatic and the Baltic Sea, investigating on the possible effects of environmental drivers on host-parasite interaction becomes more urgent. The aim of this study was to provide an inventory of parasites infecting key species of molluscs of the Adriatic and Baltic Sea. The species investigated were Mytilus galloprovincialis in the Adriatic Sea, and Littorina littorea and Mytilus edulis in the Baltic Sea. Parasites prevalence, richness, abundance were evaluated in two large scale field samplings along the northern coast of the Adriatic Sea and the south-western coast of the Baltic Sea. Parasites prevalence and abundance were then related to biotic and abiotic environmental drivers, such as salinity, eutrophication, host density and host length. Then, the effects of salinity and temperature on trematode emergence from the first intermediate host, survival and transmission to the second intermediate host were experimentally investigated. Results of the Adriatic sampling showed Parvatrema timondavidi as the dominant species infecting mussel all over the northen coast, followed by Eugymnanthea inquilina, more widespread in the north-eastern side. Parasites distribution was mainly driven by eutrophication and salinity. Results of the Baltic sampling showed the trematodes Renicola roscovita and Himasthla elongata as the dominant parasite species. Statistical analysis showed salinity as the main factor driving trematodes distribution, followed by upstream host density and mussel size. Through the laboratory experiment, salinity was confirmed as the most important driver, displaying significant positive effects in cercarial emergence and transmission to the second intermediate host to infection. Under a global change prospective, results of this study reveal a potential decrease in parasitism with freshening in the Baltic Sea. In the Adriatic Sea, the potential decrease of eutrophication will might be detrimental for parasites as trematodes and beneficial for the intermediate host as mussels. The effects of decreasing eutrophication might be, however, buffered by increasing salinity