MOLLUSCS IN A COASTAL ECOSYSTEM: INVADERS INFECTED BY NATIVE PARASITESM. KrakauAlfred Wegener Institute for Polar and Marine Research, Wadden Sea Station Sylt, Hafenstrasse 43, 25992 List, GermanyFollowing the Enemy Release Hypothesis, a lack of predators or pathogens let benefit introduced species leading to a competitive advantage over native species. In the European North Sea region, it has been assumed that introduced species of coastal ecosystems were not infested with metazoan parasites.Recent results show that only the introduced gastropod Crepidula fornicata was found to be free of trematodes. In contrast, native parasites, dominated by the trematode Renicola roscovita, infected the introduced bivalves Crassostrea gigas and Ensis americanus. However, when comparing these introduced species with native ones from the same locality, trematode intensity was always lower in the introduced bivalves.Implications for the coastal ecosystem will be presented concerning (1) the question whether introduced species do have an advantage over native species after invasion; (2) the influence of introduced bivalves affecting parasite populations, and (3) the natural infection pattern related to different susceptibility of the host organism. These results suggest that the Enemy Release Hypothesis may have to be modified in some cases.