Phylogenetic affinities of Plagiocirrus Van Cleave and Mueller, 1932 with the description of a new species from the Pascagoula River, Mississippi.

Plagiocirrus loboides n. sp. (Digenea: Opecoelidae) is described from Fundulus nottii, F. dispar blairae, F. chrysotus, and Notemigonus crysoleucas from the Pascagoula River in Mississippi. Plagiocirrus loboides differs from P. primus Van Cleave and Mueller, 1932, by having a longer postcecal space (14-25% of body length vs. about 7%); a more anterior vitellarium (extending at least to the middle of the ventral sucker vs. to its posterior margin); and larger eggs (51-71 microm long by 23-34 microm wide vs. 40-55 microm long by 30-35 microm wide). Plagiocirrus loboides differs from P. testeus Fritts, 1959, by having a long postcecal space (vs. < 5% of body length); irregular, oblique, contiguous testes (vs. strongly lobed, well separated, tandem testes); and a more extensive vitellarium. Plagiocirrus loboides differs from both congeners by having an ovary comprised of 3 or 4 distinct lobes rather than having an entire ovary. Plagiocirrus wuyienensis Wang, 1981, from Hemimyzon zebroidus in Fujian Province, China, is herein considered a species inquirenda because it has a Y-shaped excretory bladder. Molecular phylogenetic analysis of 28S rDNA gene fragments from P. loboides and 17 digenean species demonstrates that Plagiocirrus belongs in Opecoelidae.