The freshwater fish faunas of the Palaearctic and Nearctic regions show similarities but also notable differences, resulting in diverse distributions of their parasites. Relatively few parasite species occur in both regions and fewer still have been examined using molecular data. We report a rare example of ‘amphi-Pacific' distribution, involving the common Palaearctic parasite, the caryophyllidean cestode Caryophyllaeides fennica (Schneider, 1902), in the chiselmouth Acrocheilus alutaceus Agassiz and Pickering, 1855, an endemic cyprinid in northwestern Nearctic, Oregon. Available information on nonnative fish species in Oregon suggests that the parasite is native to the area and not introduced. Molecular data (18S ribosomal [r]DNA, 28S rDNA, internal transcribed spacer 2, and cytochrome c oxidase subunit 1 gene) indicate very little genetic divergence between representatives from the Palaearctic and Nearctic, and possibly a relatively more recent colonization of the Nearctic region by this cestode via the Beringian land bridge. This is remarkable considering that Acrocheilus has reportedly been in Oregon since the Miocene.

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