Altered susceptibility to trematode infection in native versus introduced populations of the European green crab

Abstract

Non-indigenous species (NIS) often experience a reprieve from infection by their co-evolved indigenous parasite load when they invade novel regions. Yet absolute escape from parasites may be precluded (notably with time) by introductions of parasites from the native range, by novel parasite acquisitions in the non-indigenous range, or both. As a result, NIS infection susceptibility may differ or change in indigenous versus novel regions due to divergent coevolutionary histories and parasite selective pressures – thereby influencing host resistance and/or compatibility. To investigate this question, we reciprocally exposed native (Europe) and non-native (eastern North America) individuals of the globally-invasive green crab, Carcinus maenas, to trematode parasites from both regions. We found that infection susceptibility differed by parasite origin: native European crabs exposed to native European parasites had lower infection prevalence and abundance, and the lowest proportion of irregular trematode metacercarial cysts or encapsulated metacercarial cysts (due to host immunity), compared with a treatment demonstrating possible prior coevolutionary history (~ 200 years ago) between the host and its parasites (invaded North American crabs exposed to native European parasites). Metacercarial cyst abundance was higher in the treatment with little to no coevolutionary history between host and parasite (native European crabs exposed to native North American trematodes) compared to a treatment with recent association in the last two centuries (invasive North American crabs exposed to native North American parasites). Our study provides further evidence that infection susceptibility can differ depending on coevolutionary history, which may be rapidly influenced by altered parasite selection pressures. It also provides greater understanding of the impact of human-mediated introductions on the coevolutionary dynamics of organisms worldwide.