When a species colonizes a new range, it can escape enemies found in its original range. Examples of enemy escape abound for invasive species, but are rare for climate migrants, which are populations of a species that colonize a new range due to climate-driven range shifts or expansions. The fiddler crab Minuca (=Uca) pugnax is found in the intertidal salt marshes of the US east coast. It recently expanded its range north into the Gulf of Maine as a result of ocean warming. We tested the hypothesis that M. pugnax had escaped its parasite enemies. Parasite richness and trematode intensity were lower in populations in the expanded range than in populations in the historical range, but infection prevalence did not differ. Although M. pugnax escaped most of its historical parasites when it migrated northward, it was infected with black-gill lamellae (indicative of Synophrya hypertrophica), which was found in the historical range, and with the trematode Odhneria cf. odhneri, which was not found in the historical range. To our knowledge, this is the first time that O. cf. odhneri has been reported in fiddler crabs. These results demonstrate that although M. pugnax escaped some of its historical parasites when it expanded its range, it appears to have gained a new parasite (O. cf. odhneri) in the expanded range. Overall, our results demonstrate that climate migrants can escape their enemies despite colonizing habitats adjacent to their enemy-filled historical range.

See Johnson et al. 2020. A climate migrant escapes its parasites. Marine Ecology Progress Series 641: 111-121 for more details.