Trematode prevalence and an invasive freshwater snail: fewer infections and parasites likely contribute to the success of an invasive snail

The enemy release hypothesis postulates that non-native species establish and become abundant because coevolved enemies from the native range are missing or greatly reduced in the introduced range. We assessed whether the invasive New Zealand mud snail, Potamopyrgus antipodarum, is released from castrating trematode parasites by comparing prevalence and diversity of trematode parasites in P. antipodarum and co-occurring native snails in the western United States. Consistent with the enemy release hypothesis (1) P. antipodarum was not infected by trematodes at 80% of the sites and, relative to native snails, had low prevalence (4–5%) at the remaining sites, (2) across all sites, mean prevalence of trematodes was nine times lower in P. antipodarum than in co-occurring native snails and, (3) P. antipodarum were infected by half or fewer of the trematode taxa that infected co-occurring native snails. Taken together, our results suggest that fewer trematode infections and fewer trematode taxa in P. antipodarum could contribute to the success of this invasive snail. However, despite the large geographic scale of our survey (32 rivers sampled) and the large number of sites that we visited (n = 82), few sites had either enough snails or trematode parasites in native snails to be informative. Thus, our conclusions are based on a small number of sites (n = 10).