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Species introduced to novel regions often leave behind many parasite species. Signatures of parasite release could thus be used to resolve cryptogenic (uncertain) origins such as that of Littorina littorea, a European marine snail whose history in North America has been debated for over 100 years. Through extensive field and literature surveys, we examined species richness of parasitic trematodes infecting this snail and two co-occurring congeners, L. saxatilis and L. obtusata, both considered native throughout the North Atlantic. Of the three snails, only L. littorea possessed significantly fewer trematode species in North America, and all North American trematodes infecting the three Littorina spp. were a nested subset of Europe. Surprisingly, several of L. littorea's missing trematodes in North America infected the other Littorina congeners. Most likely, long separation of these trematodes from their former host resulted in divergence of the parasites' recognition of L. littorea. Overall, these patterns of parasitism suggest a recent invasion from Europe to North America for L. littorea and an older, natural expansion from Europe to North America for L. saxatilis and L. obtusata. [ABSTRACT FROM AUTHOR]

Early invasions of the North American shore occurred mainly via deposition of ballast rock, which effectively transported pieces of the intertidal zone across the Atlantic. From 1773-1861, >880 European ships entered Pictou Harbor, Nova Scotia, as a result of emigration and trade from Europe. The rockweed Fucus serratus (1868) and the snail Littorina littorea ( approximately 1840) were found in Pictou during this same period. With shipping records (a proxy for propagule pressure) to guide sampling, we used F. serratus as a model to examine the introductions because of its relatively low genetic diversity and dispersal capability. Microsatellite markers and assignment tests revealed 2 introductions of the rockweed into Nova Scotia: 1 from Galway (Ireland) to Pictou and the other from Greenock (Scotland) to western Cape Breton Island. To examine whether a high-diversity, high-dispersing species might have similar pathways of introduction, we analyzed L. littorea, using cytochrome b haplotypes. Eight of the 9 Pictou haplotypes were found in snails collected from Ireland and Scotland. Our results contribute to a broader understanding of marine communities, because these 2 conspicuous species are likely to be the tip of an "invasion iceberg" to the NW Atlantic from Great Britain and Ireland in the 19th Century.

Even after decades of investigation using multiple sources of evidence, the natural histories of some species remain unclear (i.e. cryptogenic). A key example is Littorina littorea, the most abundant intertidal snail in northeastern North America. Native to Europe, the snail's ecological history in North America has been debated for over 100 years with no definitive resolution. To resolve its cryptogenic status, we used molecular genetics from a novel combination of the snail and a highly associated trematode parasite, Cryptocotyle lingua. Based on mitochondrial sequences of 370 L. littorea and 196 C. lingua individuals, our results demonstrate a significant reduction in genetic diversity in North America vs. Europe, North American haplotypes nested within European haplotypes, and mean divergence estimates of approxiamtely 500 years ago from Europe for both host and parasite--thus supporting a recent introduction of both host and parasite to North America from Europe. Our study therefore resolves not only a specific cryptogenic history, but it also demonstrates the success of our approach generally and could be used in resolving difficult invasion histories worldwide.