Enemy release in the invasive New Zealand mud snail in eastern North America

The enemy release hypothesis is one of the best supported hypotheses to explain the success of invasive species. This hypothesis suggests that invaders are successful, in part, because they experience fewer natural enemies (i.e. predators and parasites) in their invaded range compared to their native range. The New Zealand mud snail (NZMS), Potamopyrgus antipodarum , is a world-wide invader that is highly infected by digenetic trematodes in its native New Zealand. Here we compared infection prevalence of NZMS from multiple locations in the eastern US to published infection prevalences in the native range, and we also compared the infection prevalences of NZMS to infection prevalences of coexisting native snails where coexisting natives were found. We found no NZMS infected with trematodes at any site. In the two locations with coexisting natives, we found at least some natives infected with trematodes, and in one of the locations, we found both natives and NZMS associated with the annelid, Chaetogaster limnaei . C. limnaei can exist as a parasite of mollusks in the renal organ, or it can be found in the mantle cavity where it can act as a mutualist, consuming parasites (i.e. miracidia) that may be trying to infect the snail. We found that NZMS were generally less associated with both forms than native snails, but the ectosymbiotic form was more prevalent than the endoparasitic form in NZMS (and most natives). This creates the possibility that the symbiont may be a net benefit to NZMS and could positively influence invasion success.