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Lower parasite pressure in invasive freshwater bivalves than in sympatric native Unionidae mussels in southern European lakes
AbstractThe Enemy Release Hypothesis (ERH) proposes that the success of bioinvasions is attributable to lower enemy pressure on invasive species compared to native ones, giving a competitive advantage for invaders. In line with the hypothesis, we previously observed in northern European bodies of fresh water that invasive bivalves were subject to lower parasite pressure than sympatric native mussels. Here, we investigated ERH in three southern European lakes, where the native mussels are rapidly declining and being replaced by non-native bivalves. In total, 679 bivalves (n of individuals per species per lake varying from 12 to 187) were collected during 2016–2018. Ten parasite taxa were found. The mean lake-specific number of parasite taxa in the native mussels (Anodonta exulcerata, A. cygnea, A. anatina and Unio elongatulus) was 2.6 times that in the invasive bivalves (Dreissena polymorpha, Corbicula fluminea and Sinanodonta woodiana). Similarly, the mean lake-specific sum of prevalences of infection by different parasite taxa in the native mussels was 3.4 times that in the invasive bivalves. Notable was the complete lack of parasites in C. fluminea. Thus, the results supported the Enemy Release Hypothesis 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. As parasites are usually harmful, this may contribute to the observed successful invasion of non-native freshwater bivalves in Europe.
Enemy release: loss of parasites in invasive freshwater bivalves Sinanodonta woodiana and Corbicula fluminea
Invasive freshwater bivalves harm native species, ecosystems and biodiversity, and incur economic costs. The enemy release hypothesis posits that invasive species are released from enemies during the invasion process, giving them a competitive advantage in the new environment. We compared parasitism in two invasive freshwater bivalves, Sinanodonta woodiana and Corbicula fluminea between their original range (China) and invaded range (Europe). For S. woodiana , the average sample‐size‐standardized population‐specific parasite taxon richness was 2.1 times as high, and sum of prevalence was 3.0 times as high in the native range (3 populations, 81 individuals studied) as in the invaded range (6 populations, 210 individuals studied). For C. fluminea , the average standardized population‐specific parasite taxon richness was 1.3 and sum of prevalences was 27.5 in the native range (4 populations, 749 individuals studied), whereas all European C. fluminea were free of parasites (7 populations, 418 individuals studied). The results demonstrate loss of parasites as a result of invasion. Previous studies have shown that parasite pressure on S. woodiana and C. fluminea in the invaded range in Europe is, on average, lower than on sympatric native freshwater mussel populations. Together, these results support one aspect of the enemy release hypothesis: invasive bivalves experience reduced parasite loads as a result of invasion, which may contribute to their success, given the costs typically imposed by parasitism.