A new species of Rhinebothroides Mayes, Brooks & Thorson, 1981 (Cestoda: Tetraphyllidea) from the ocellate river stingray in Argentina, with amended descriptions of two other species of the genus.

A new species of the tetraphyllidean genus Rhinebothroides Mayes, Brooks & Thorson, 1981 is described from the spiral intestine of specimens of the ocellate river stingray Potamotrygon motoro (Müller & Henle) collected in the Río Colastiné, Argentina. R. campbelli n. sp. can be distinguished from its congeners particularly in worm size (up to 7 mm long), the number of segments (5-7), the number of testes per segment (26-41), the extension of the ovarian lobes and the absence of darkly-staining cells surrounding the distal end of the cirrus-sac. Comparison of tapeworms matching the original description of R. mclennanae Brooks & Amato, 1992 with the type-specimens of R. glandularis Brooks, Mayes & Thorson, 1981 showed that these are two distinct species. Consequently, R. mclenannae is considered a valid species rather than a synonym of R. glandularis, as suggested by Marques & Brooks (2003). R. mclennanae can be distinguished from R. glandularis by the morphology of the ovary, the number of medial loculi in the bothridia, the number of testes per mature segment and cirrus-sac size. It is concluded that variation in microthrix form and density may prove to be of taxonomic utility for distinguishing species of Rhinebothroides. Some comments on characters noted by Marques & Brooks (2003) in their revision of the genus are also provided, such as the absence of an external seminal vesicle in R. mclennanae and R. glandularis, the absence of microtriches in the genital atrium, intraspecific variation of bothridial shape and ovarian shape.