Absence of parasites in non-native fish from a Neotropical floodplain: evidence for the enemy release hypothesis

The cichlid Geophagus sveni, native to the Tocantins-Araguaia basin, was introduced into the floodplain of the upper Paraná River and has achieved great adaptive success, with high abundances there. In order to test whether the enemy release hypothesis is happening in the species invasion process, we collected 29 individuals in the Tocantins River (native range) and 29 in the Paraná River (non-native range) to compare their parasite fauna. In the Tocantins River, 17 fish were parasitized by at least one parasitic species, totaling eight species, comprising 54 individuals in total, while in the non-native fish from the Paraná River, we found only one representative of a parasitic species. We found differences in the weight-length relationship, where individuals from the Paraná River showed a greater investment in weight compared to individuals from the Tocantins River. Furthermore, we observed a significant positive relationship between weight and abundance of parasites in native fish. Our results indicate that the hypothesis of enemy release may be occurring in G. sveni, as the quantitative difference in endoparasites between sites shows that hosts from the Paraná River obtained a competitive advantage when arriving at the site.