Parasites are important and abundant members of communities; thus it is critical to understand their influences within and among bioregions. Specifically, understanding how parasite diversity patterns are affected by anthropogenic dispersal of their hosts and the effect differing vectors may have on those patterns is largely unexplored. We therefore assembled a comparative study of trematode parasite diversity in two snail hosts (Ilyanassa obsoleta and Littorina saxatilis) introduced to the west coast of North America from native east coast populations. Although these snail species have overlapping geographic ranges on both North American coasts, they have highly differentiated invasion histories: I. obsoleta was introduced to the west coast via intentional (failed) oyster transplantation from the east coast in the early 1900s, while L. saxatilis was introduced via the live seafood/baitworm trade in the 1990s.

Results/Conclusions

Results of our analyses showed that both snail species had significantly lower total and average site-level trematode richness and prevalence in the introduced versus native regions. However, L. saxatilis escaped about 1.8 times more parasites than I. obsoleta, which we attributed to several potential, non-mutually exclusive mechanisms: time since introduction, introduction vectors, propagule pressure, parasite prevalence in source populations, and available hosts. In both species, but especially I. obsoleta, we found close links between native source populations and introduced populations, especially for non-native San Francisco Bay populations, which probably represent one of the oldest non-native founding areas. Host availability and parasite-specific life cycle strategies were also found to have important influences on resulting parasite diversity patterns in introduced populations, and definitive host availability was found to be especially important. On the whole, understanding our hosts’ particular ecological and invasion histories was integral for comprehending the parasite biogeographic patterns we observed across native and introduced ranges. We also demonstrated that time since introduction, vector pathways, source information, host availability, and propagule pressure all played strong roles in determining parasitism in introduced locations. Because both snail species, and particularly L. saxatilis, have escaped many parasite species in their introduced region, they may possess an advantage in competitive interactions and overall success on the west coast just by the fact that their health and reproductive fitness are less hampered by these castrating parasites. Further research over time can help reveal community level (including native biota) impacts of these introduced parasite systems on the US west coast.