Factors influencing the distribution of trematode larvae in blue mussels Mytilus edulis in the North Atlantic and Arctic Oceans

Blue mussels, Mytilus edulis, serve as second intermediate hosts for several trematode species, but little is known about the mechanisms underlying parasite infections in mussels. To reveal these mechanisms, the prevalence and intensity of trematode larvae (metacercariae in species of Gymnophallus, Himasthla and Renicola) were examined in blue mussels at 19 intertidal sites from the North Atlantic (Ireland, Iceland, Norway) to the Arctic Ocean (north-western Russia). Mussel samples were taken in 2005–2008. The impact of a number of environmental (maximal sea surface temperature, SSTmax) and biological factors (mussel age and density and the abundance of avian final hosts) on trematode infection was examined. Infection levels correlated with the interaction between local bird abundance and mussel age and density. They increased with mussel age but their dependence on mussel density and bird abundance varied among parasite genera. Prevalence and intensity increased with SSTmax for Renicola spp., but no relationship was found for Gymnophallus and Himasthla spp. The ambiguous effect of SSTmax is likely explained by the broad range of optimal temperatures for the normal functioning of trematode larvae (cercariae) infective for mussels and by the high dependence of the level of mussel infection on a combination of local ecological factors. High infection levels were observed even in localities with a low SSTmax. No mussels were infected in the most north-eastern population, probably due to extreme conditions in the Arctic intertidal preventing trematode transmission. Future warming of the Arctic may accelerate trematode transmission in this system.