If you encyst: evidence of parasite escape and host-switching among three co-occurring crabs

Biological invasions influence species interactions around the globe, including host and parasite communities. We evaluated trematode parasite diversity and the potential for host-switching of parasites in 3 co-occurring crabs in the Northeast USA, including 1 native species (Cancer irroratus) and 2 non-natives (Carcinus maenas, Hemigrapsus sanguineus), of which the former represents a historical and the latter a contemporary invader. At 7 sites from Maine to Rhode Island, we surveyed crabs for trematode infection prevalence and abundance, and the influence of parasitism on host body condition. We also conducted DNA sequencing using the 18S rRNA barcoding marker to determine species composition, diversity, and gene flow of trematode lineages among the co-occurring hosts. While the native host, C. irroratus, and the historical invader, C. maenas, exhibited no statistical difference in trematode prevalence, we found that C. maenas had a greater abundance of metacercarial cysts than the other 2 hosts, and the contemporary invader, H. sanguineus, was rarely infected. Crab condition did not vary with infection abundance, although infected females of all species had higher reproductive investment than other groups. Genetic analyses revealed that the microphallid trematodes consisted of 3 main clades, representing over 50 haplotypes, with evidence of host-switching by native parasites utilizing the non-native hosts. Given the importance of crustaceans to parasite life cycles, the introduction of novel hosts to these systems alters both free-living and host-parasite community interactions and could ultimately affect community structure and function. Future studies should continue to investigate host-parasite diversity and demographics following invasions to better understand impacts on native marine communities.