Parasitism and epibiosis in native and non-native gammarids in freshwater in Ireland

The introduced amphipod crustaceans Gammarus pulex and G. tigrinus are disp lacing the native G. duebeni celticus in a number of freshwater sites in No rthern Ireland. We investigated parasite and epibiont infection in populati ons in the River Lagan and Lough Neagh where both native and invading speci es occur. Prevalence of the four parasites and epibionts observed was highe r in the native G. d, celticus than in the invading amphipods at both field sites. In Lough Neagh, G. d. celticus individuals suffered higher burdens of the rotifer Embata parasitica and the protozoan Epistylis in comparison with the invading species. These patterns may reflect host specificity by t he parasites or may result from different susceptibilities of the native an d invading host species. We consider the influence of parasitism on host in vasions and resulting species distributions.