RESUMO
Gyrodactylus laevisoides n. sp. is described from the gill rakers of red belly dace, Phoxinus eos Cope (Cyprinidae), from Nova Scotia, Canada. Gyrodactylus laevisoides n. sp. is the second species of Gyrodactylus Nordmann, 1832 described from this host and is characterised by weakly curving hamuli, a small ventral bar lacking anterolateral processes, stout dorsal bar, small marginal hooks with sickles larger proximally than distally and having a small circular process on the heel, a MCO with spines arranged in two arched rows, and lack of obvious excretory bladders. The new species most closely resembles Gyrodactylus laevis Malmberg, 1957, a Eurasian species whose principle host is Phoxinus phoxinus (L.). The two species are separated by Gyrodactylus laevisoides n. sp. having less divergent and longer hamulus root and marginal hook sickle toe with a steeper continuous angle and heel that is less prominent. The morphological description is supplemented with sequences of the 18S gene (449 bp, including the V4 region) and of the ITS region (821 bp). Gyrodactylus sedelnikowi Gvosdev, 1950 infecting Barbatula barbatula (L.) and Gyrodactylus neili Leblanc, Hansen, Burt & Cone, 2006 infecting Esox niger Lesueur are the most genetically similar species on GenBank for the 18S rRNA gene and ITS regions respectively (c.96% and c.92%). Gyrodactylus laevisoides n. sp. belongs to Malmberg's subgenus Gyrodactylus (Gyrodactylus) and phylogenetic analysis of the ITS region groups this species with other members of the subgenus. The phylogeny has two main clades, one comprised of Eurasian species and the other of North American species, specifically Gyrodactylus laevisoides n. sp. and Gyrodactylus neili. It is suspected that this lineage, which is seemingly underrepresented in North America, likely colonised the new world with an ancestral species of Phoxinus via the Bering land connection around the time of the Pliocene.