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Introduction: Species of the genus Anacanthorus Mizelle & Price, 1965 represent commonly known parasites of Neotropical Characiformes. Anacanthorus is one of the most specious Neotropical monogenean genus, currently comprising 73 nominal species., Materials and Methods: Fish were obtained from the Miranda River, Mato Grosso do Sul, from the Xingu River, Pará, from the Mogi Guaçu River, São Paulo and from the Upper Parana River, Paraná, Brazil. The monogeneans found were examined in a light microscope and partial sequences of the mitochondrial cytochrome c oxidase 1 gene were obtained for most species., Results: Two new species of Anacanthorus were retrieved from Brazilian serrasalmids and are described herein. Anacanthorus paraxaniophallus n. sp. from Serrasalmus maculatus Kner has great similarity to A. xaniophallus Kritsky et al. (J Helminthol Soc Wash 59:25-51, 1992) by the general morphology of the copulatory complex, but it can be easily differentiated by a larger accessory piece with a slightly curved point, by having a basal flap in the male copulatory organ and by the morphology of hooks, with flattened thumb. Anacanthorus myleusi n. sp. from Myloplus schomburgkii (Jardine) is mainly distinguished by its unique copulatory complex, possessing an inverted J-shaped MCO with terminal flare and a hook-shaped accessory piece. Additionally, new host records are provided to A. lepyrophallus, geographical records are provided to four species of Anacanthorus (A. lepyrophallus, A. maltai, A. sciponophallus and A. thatcheri), which are being recorded in the Paraná River basin for the first time, and we also provide molecular data for six species of Anacanthorus.

This study investigates the ecology of monogenean gill parasites of Aslyanax altiparanae Garutti & Britski, 2000 and Rhamdia quelen (Quoy & Gaimard, 1824) in a stretch of the Sao Francisco Verdadeiro River, Parana, Brazil. Statistical and ecological indices were used to examine observed levels of parasitism in relation to host and environmental characteristics. A. altiparance and R. quelen had infestation intensities of 2.8 and 23.1 parasites per fish, respectively. The only significant environmental influence was observed at the upstream station for R. quelen. For both host species, parasitized and non-parasitized individuals presented similar weight-ength relationships. Parasitized individuals had dispersed K,, values indicating abnormal conditions. The low levels of parasitism observed in this study suggest that the environment is relatively undisturbed. Additional studies should compare these two species and their respective parasites following completion of the hydroelectric headquarters planned for construction in this stretch of the Sao Francisco Verdoadeiro River.