The parasitic copepodGauchergasilus euripedesi(Montú, 1980) Montú & Boxshall, 2002 was described from plankton samples and specimens found in four fish species from the estuarine area of Patos Lagoon, state of Rio Grande do Sul (RS), Brazil. Later, one different fish species was reported parasitized withG. euripedesiin the same locality. Species ofAstyanaxBaird & Girard, 1854 (Astyanax henseliMelo & Buckup, 2006 andAstyanax lacustris(Lütken, 1875)) andPsalidodonEigenmann, 1911 (Psalidodon eigenmanniorum(Cope, 1894) and Psalidodon aff. fasciatus (Cuvier, 1819)) were collected in two environments (Pintada Island, municipality of Porto Alegre and Itapeva Lagoon, municipality of Terra de Areia, RS) to investigate their parasites. The copepods found in the gill arches were counted, processed, mounted in permanent slides, and photographed using light microscopy, or processed for observation in scanning electron microscopy. Specimens ofP. eigenmanniorumfrom Pintada Island,A. lacustrisand P. aff. fasciatus from Itapeva Lagoon, were parasitized byG. euripedesi, with prevalences of 29.03% (A. lacustris), 10.34% (P. eigenmanniorum), and 9.68% (P. aff. fasciatus). Measurements obtained for specimens ofG. euripedesiwere similar to those found in the literature, except for egg sacs which were larger in the specimens examined in the present study. In addition to being the first report ofG. euripedesiparasitizing species of fish (A. lacustris,P. eigenmanniorum, and P. aff. fasciatus), the results presented here also extend the known geographic distribution of the copepod species.
