ABSTRACT. Floodplains systems present complex biodiversity and upper Paraná river floodplain retains a wide variety of terrestrial and aquatic species. Serrasalmus marginatus (nonindigenous species) and Serrasalmus maculatus (native species) are part of this biodiversity component and can be infected by trophic transmission. In order to understand the ecological processes within the upper Paraná river floodplain, endoparasite fauna of those two hosts species were analyzed, considering prevalence, abundance, intensity and richness variables to determine this relationship. So, abundance of acantocephalan Echinorhynchus sp. was positively correlated to S. marginatus length, while Kritskyia annakohnae (Monogenea) prevalence and S. marginatus length presented a negative correlation. S. marginatus relative condition factor (Kn) was negatively and significantly associated to the abundance of K. annakohnae, and Kn of infected fish by that species is lower compared with the non-infected fish. Since hosts are in the context of the introducing of species, the research of endoparasites ecological variables allows to understansd infracommunity and component community as a way of analyzing the consequence of these parasites distribution in the native and non-native species.Keywords: neotropical floodplain, parasite ecology, invasive species, piranha.Fauna endoparasitária de Serrasalmus spp. (Characidae: Serrasalminae) de uma planície de inundação neotropical RESUMO. Sistemas de inundação possuem uma complexa biodiversidade. Por causa desta característica, a planície de inundação do alto rio Paraná abriga uma gama muito grande de espécies aquáticas e terrestres. Serrasalmus marginatus (espécie introduzida) e Serrasalmus maculatus (espécie nativa) compõem a ictiofauna desta área, e estes hospedeiros podem ser infectados por endoparasitos pela transmissão trófica. A fauna endoparasitária das duas espécies de peixes foi analisada por meio da prevalência, abundância, intensidade e riqueza para averiguar a relação destas com as variáveis dos hospedeiros. Observou-se que a abundância do acantocéfalo Echinorhynchus sp. está positivamente correlacionada com o comprimento de S. marginatus, enquanto existe uma correlação negativa e significativa entre a prevalência de Kritskyia annakohnae (Monogenea) e o comprimento de S. marginatus. O fator de condição relativo (Kn) de S. marginatus está relacionado negativa e significativamente com a abundância de K. annakohnae, e o Kn dos peixes parasitados por esta mesma espécie é mais baixo se comparado com os peixes não parasitados. Como os hospedeiros estão no contexto de introdução de espécies, as investigações das variáveis ecológicas dos endoparasitos possibilitam o conhecimento de cada infracomunidade e comunidade componente, como forma de delimitar a atuação dos parasitos na mediação da relação ecológica entre a espécie nativa e não nativa.Palavras-chave: planície de inundação neotropical, ecologia parasitária, espécie invasora, piranha.

Species richness and composition are central themes in community ecology of helminths because they improve the understanding of factors that determine community structure. Floodplain ecosystems and the environmental alterations induced by hydrological cycles are factors that maintain biodiversity over time, as observed in the Upper Paraná River floodplain, where the individuals of Brycon orbignyanus were collected. The objective of this study was to evaluate whether variables such as the richness and abundance of endoparasites are related to the fish size during different hydrological periods (high and low waters). A total of 163 endoparasites belonging to 13 species were collected in the 104 fish examined. Fish size had a positive correlation with mean diversity and abundance of the endoparasite infracommunity, supporting the idea that larger hosts are able to harbor a more abundant and diverse parasite infracommunity. The diversity of habitats provided by the floodplain can be considered the main factor explaining the differences between these attributes of parasite species. Variations in parasite richness between different hosts provide not only a good model for studies on community diversification but are also of great interest in species conservation. All parasite species found in B. orbignyanus were recorded for the first time in this host, especially in the Upper Paraná River floodplain.

The construction of dams causes several impacts on aquatic environments, altering the flow of rivers, environmental variables, and all biota present, including parasites. Little is known about how the parasitic community can be influenced in the long term by environmental changes. In this study, it was expected that the impacts caused by environmental disturbances will be directly reflected by the composition of the parasite populations. We evaluated the change in the structure of the Prochilodus lineatus endoparasite community between two periods sampled 15 years apart in the upper Paraná River floodplain. There was a significant difference in the weight–length relationship of P. lineatus between these periods and a total of 15 species of parasites were found: 11 species in Period 1 and nine species in Period 2 and five species occurred in both periods. The species richness and diversity were higher in Period 1, and we observed that the correlation of descriptors (richness, diversity and evenness) increased with fish length in this period. In both periods, digeneans numerically dominated the parasitic community, and we verified changes in the composition of parasites between periods. Both the host and the parasites were possibly affected by the environmental impacts resulting from the construction of dams over time, and it is noteworthy that complex life cycle parasites such as Digenea and Acanthocephala require intermediate hosts to complete their life cycle, and the population responds to fluctuations in the face of modified environments.