New species of Plagioporus Stafford, 1904 (Digenea: Opecoelidae) from California, with an amendment of the genus and a phylogeny of freshwater plagioporines of the Holarctic.

Plagioporus hageli n. sp. is described from the intestine of Oncorhynchus mykiss (Walbaum) collected from the River Yuba, California, USA. Of the accepted, nominal species of Plagioporus Stafford, 1904 from the Nearctic, the new species is morphologically similar to three intestinal species from the western USA parasitising diadromous fishes, including Plagioporus shawi (McIntosh, 1939), Plagioporus kolipinskii Tracey, Choudhury, Cheng & Ghosh, 2009 and Plagioporus siliculus Sinitsin, 1931, and is also similar to Plagioporus serotinus Stafford, 1904 from catostomids from eastern Canada. Plagioporus hageli n. sp. is distinguished from the former three species in lacking a dorsal vitelline field and from the latter species in having a consistent interruption in the distribution of the vitellarium at the level of the ventral sucker. The new species is also morphologically similar to an unnamed species of Plagioporus and a species misidentified as 'Plagioporus angusticolle' that were collected from California, but it is easily distinguished from both in its shorter body length. To estimate the placement of the new species within Plagioporus and within the Opecoelidae Ozaki, 1925, we conducted a Bayesian inference (BI) analysis of partial 28S rDNA sequence data that included sequences from Plagioporus hageli n. sp., five other species of Plagioporus, three species of Neoplagioporus Shimazu, 1990, including the type-species, Neoplagioporus zacconis (Yamaguti, 1934), two species of Urorchis Ozaki, 1927 (including the type-species, Urorchis goro Ozaki, 1927) and sequences of 42 opecoelid species obtained from GenBank. Our phylogenetic analysis revealed (i) plagioporines parasitising freshwater hosts form a monophyletic group; (ii) Plagiocirrus loboides Curran, Overstreet & Tkach, 2007 nested within the rest of the members of Plagioporus; (iii) the new species was closer to Plagiocirrus loboides than to Plagioporus shawi, the other salmonid parasite included in our analysis; (iv) P. shawi was the poorly supported sister to its congeners; (v) Neoplagioporus elongatus (Goto & Ozaki, 1930) Shimazu, 1990 was closer to the two species of Urorchis than to the other two species of Neoplagioporus; and (vi) the paraphyly of the Plagioporinae Manter, 1947 was reinforced. Based on 28S rDNA sequence data and our BI analysis, we propose Plagioporus loboides (Curran, Overstreet & Tkach, 2007) n. comb., and amend Plagioporus accordingly. This analysis represents the first phylogenetic study of the opecoelids that estimates the interrelationships of the Plagioporinae that includes a member of Plagioporus.