Lower parasite pressure in invasive freshwater bivalves than in sympatric native Unionidae mussels in southern European lakes

Abstract

Abstract Enemy Release Hypothesis (ERH) proposes that the success of bioinvasions is contributed by lower enemy pressure on invasive species compared to native ones, giving a competitive advantage for invaders. In line with EHR, we earlier observed that the invasive bivalves had a lower parasite pressure than the sympatric native freshwater mussels in northern European waterbodies. Here, we investigated ERH in three southern European lakes, where the native mussels are rapidly declining and replaced by exotic bivalves. Examination of 679 bivalves (n of individuals per species per lake varying from 12 to 187) collected 2016-18 revealed in total 11 parasite taxa. On average, 2.6 times higher number of parasite taxa and 3.4 times higher sum of infection prevalences was observed per lake among the native mussels (Anodonta exulcerata, A. cygnea, A. anatina and Unio elongatulus) than in the sympatric invasive bivalves (Dreissena polymorpha, Corbicula fluminea and Sinanodonta woodiana). Notable was the complete lack of parasites from C. fluminea. Thus, results supported ERH and were in accordance with previous results from northern Europe, suggesting, on average, a lower parasite pressure in invasive bivalves than in sympatric native mussels. This may contribute to the observed successful invasion of exotic freshwater bivalves in Europe.