Background: Lernaea cyprinacea was introduced in South America with cyprinids. To the best of our knowledge, this ectoparasite has not been reported in Santa Fe province (Argentina). Objective: To report Lernaea cyprinacea presence in Rhamdia quelen under hatchery conditions in Santa Fe province (Argentina). Methods: In May 2014, samples of L. cyprinacea attached to the fins and flanks of R. quelen were obtained in the Chronobiology Laboratory of the Veterinary Sciences School at Esperanza (Santa Fe, Argentina) where they were collected and identified. Results: A description of L. cyprinacea is presented along with detailed records previously reported in this and other regions. Conclusions: Presence of this parasite constitutes evidence of the introduction of exotic species through commercialization of their natural hosts and the potential pathogen colonization of natural environments, which constitutes a threat to the integrity of aquatic ecosystems.

Prochilodus lineatus has been considered an ecosystem engineer in the Neotropics given its infl uence on important ecosystem processes, and it is therefore relevant to understand their inter-specifi c relationships. The association of P. lineatus with parasitic helminths was studied in two isolated shallow lakes of the alluvial plain of the Middle Paraná River. Only two species of gastrointestinal nematodes were found:Spinitectus asperus (Cystidicolidae) and Klossinemella iheringi (Atractidae), with S. asperus having higher prevalence and mean intensity values. This is the fi rst record of K. iheringi in the Middle Paraná River, Argentina, as well as the southernmost citation of S. asperus.

We studied gill parasites of Prochilodus lineatus in the San Javier River, which is connected to the Middle Paraná System (Santa Fe, Argentina). In 25 specimens, the parasite prevalence in branchial organs was 92% and the average intensity was 8.3 parasites/infested fish. The parasite community showed no dominance of any taxon, but the family Dactylogyridae represented 60% of the community. We found a significant association between Tereancistrum curimba and Dactylogyridae specimens. The prevalence of the parasites T. toksonum and T. curimba was higher than what has been recorded in the floodplain of the Upper Paraná River, Brazil, demonstrating that the geographic distribution of parasites belonging to Tereancistrum genus is thus expandeing.Keywords: Parasite prevalence, Sábalo, San Javier River, Dactylogyridae, Tereancistrum. ResumoForam estudados os parasitos das brânquias de Prochilodus lineatus, do rio San Javier, Sistema Paraná Médio (Santa Fe, Argentina). Em 25 espécimes de peixes analisados, a prevalência de parasitos foi de 92% e a intensidade média de 8,3 parasitos/peixe parasitado. A comunidade parasitária não mostrou predomínio de um taxón, embora tenha sido observado que 60% da comunidade foi representada pela Família Dactylogyridae. Verificou-se associação significativa entre Tereancistrum curimba e indivíduos da família Dactylogyridae. Os índices de prevalência parasitária de T. toksonum e T. curimba se destacam como sendo superiores aos registrados na planície de inundação do rio Alto Paraná, no Brasil, demonstrando desta maneira expansão da distribuição geográfica de parasitas do gênero Tereancistrum.Palavras-chave: Prevalência de parasitos, Sábalo, Rio San Javier, Dactylogyridae, Tereancistrum.