New species of Ergasilus von Nordmann, 1832 (Copepoda: Ergasilidae) from the gills of freshwater fishes in Patagonia, Argentina.

A new parasitic copepod, Ergasilus yandemontei n. sp., is described based on 10 adult females collected from the gills of the atherinid silverside Odontesthes hatcheri (Eigenmann), in Lake Pellegrini, Neuquén Province, Patagonia, Argentina. This new copepod species is characterized by having: (i) a 2-segmented endopod on leg 1; (ii) a semi pinnate seta on the terminal segment of the exopod of leg 1; (iii) a reduced leg 5 with a single seta; (iv) aesthetascs on antennule, 1 aesthetasc on the sixth segment and 1 on the fourth segment. Ergasilus yandemontei n. sp. represents the first species described from Patagonian freshwaters. Ergasilus sieboldi var. patagonicus Szidat, 1956 described from Lake Pellegrini, should be considered a synonym of the new species described herein.