Trematode prevalence and abundance in hosts are known to be affected by biotic drivers as well as by abiotic drivers. In this study, we used the unique salinity gradient found in the south-western Baltic Sea to: (i) investigate patterns of trematode infections in the first intermediate host, the periwinkle Littorina littorea and in the downstream host, the mussel Mytilus edulis, along a regional salinity gradient (from 13 to 22) and (ii) evaluate the effects of first intermediate host (periwinkle) density, host size and salinity on trematode infections in mussels. Two species dominated the trematode community, Renicola roscovita and Himasthla elongata. Salinity, mussel size and density of infected periwinkles were significantly correlated with R. roscovita, and salinity and density correlated with H. elongata abundance. These results suggest that salinity, first intermediate host density and host size play an important role in determining infection levels in mussels, with salinity being the main major driver. Under expected global change scenarios, the predicted freshening of the Baltic Sea might lead to reduced trematode transmission, which may be further enhanced by a potential decrease in periwinkle density and mussel size.

Improved understanding of the occurrence and spatio-temporal distribution of bivalve larvae holds significant benefits for ecological studies, shellfisheries management and aquaculture. Morphological methods for identification have proved difficult to develop because of the small size of these larvae and similarities in their shape and colour. Molecular methods based on DNA extraction can confirm the presence of a species in a plankton sample, but without sample sorting and individual larval analysis, provide no estimate of larval abundance and are incapable of providing an estimate of larval growth rate. Fluorescence in situ hybridization (FISH) using species-specific DNA probes has the potential to resolve these issues. However, utilization of this technique is constrained by the strong autofluorescence, common in marine larvae. Here we evaluate the effect of eight different fixatives on the autofluorescence intensity of bivalve larvae using fluorescein isothiocyanate (FITC) and Cy3 filters. In addition, fifteen autofluorescence reduction treatments were evaluated and their compatibility with FISH assessed. Relative to fresh larvae, chemically fixed larvae had significantly higher autofluorescence in both filter sets. Larvae preserved by freezing at –80°C exhibited no significant increase in autofluorescence over a 3-year period. Autofluorescence levels were generally lower with the FITC filter set than the Cy3 filter set. For archived larvae preserved in modified saline ethanol and exhibiting fixative-induced autofluorescence, the autofluorescence intensity could be reduced to 20–30% with saturated Sudan Black B and to 30–40% with Chemicon™. Both of these autofluorescence reduction treatments were compatible with subsequent FISH protocols using a FITC-labelled probe.