First record of the parasite fauna of Serrasalmus geryi Jegú & Santos, 1988 (Characiformes: Serrasalmidae) and ecological implications of its invasion in a Neotropical floodplain.

Biological invasions can alter ecological interactions, including parasite-host dynamics, with effects that are often gradual and difficult to predict. Serrasalmus geryi, native to the Tocantins-Araguaia basin, was recently recorded in the upper Paraná River floodplain (PIAP), Brazil. This study aimed to characterise, for the first time, the parasitic fauna of this species at an early stage of its introduction and to explore potential interactions with congeneric species. A total of 55 non-native individuals were collected between 2021 and 2024 and parasitological analyses included identification of ecto- and endoparasites. Additionally, a literature review was conducted to compare parasite assemblages with those of the native Serrasalmus maculatus and the established invasive Serrasalmus marginatus. Eleven parasite species were recorded, all reported for the first time in S. geryi, with three representing new records for the PIAP. Overall, parasite prevalence and abundance were low, and the assemblage was dominated by generalist taxa already present in the system. These findings are consistent with expectations for early invasion stages, suggesting that S. geryi is currently experiencing reduced parasitic pressure, possibly due to enemy release. The limited overlap with congeneric species indicates that parasite sharing and community restructuring are still incipient processes. This study provides a rare early-stage baseline of parasite-host interactions during a biological invasion in a natural system. The results contribute to invasion ecology by enabling future assessments of parasite acquisition, co-introduction, and transmission dynamics, and are relevant for understanding long-term ecological impacts on Neotropical floodplain ecosystems.