Morphological and Histometric Alterations in Gonadal and Neural Tissues of Native and Non-Native Fishes From the Lower Doce River Basin in Southeastern Brazil.

The ichthyofauna of the lower Doce River basin has been exposed to multiple anthropogenic stressors. Among these, the collapse of the Fundão mining tailings dam in Mariana, Minas Gerais, Brazil, in 2015 resulted in massive fish mortality and widespread contamination of aquatic environments. In this context, morphological and histological analyses of gonadal development and larval malformations are valuable tools for assessing biological responses to environmental degradation. This study was designed as an ecosystem-level biomonitoring assessment involving native and non-native species to evaluate gonadal maturation stages (SGM), gonadal histopathology, and craniofacial deformities in fish larvae. Fish were collected from lakes, tributaries, reservoirs, and the main channel of the lower Doce River during rainy and dry seasons using gill nets and cast nets. Histological analyses revealed reproductive alterations, including accumulation of interstitial proteinaceous fluid, oocyte atresia, and yolk liquefaction in several species. Non-native species exhibited a higher frequency of spawned gonadal maturation stages than native species in the main channel of the Doce River and its tributaries. Morphological and histological analyses of the telencephalon and optic tectum of Pygocentrus nattereri and Prochilodus costatus larvae revealed tissue eversion and abnormal displacement of neural structures associated with craniofacial deformities. Histometric analyses demonstrated significantly higher nuclear intersection counts and reduced inter-nuclear distances in normal tissues compared with malformed tissues (p < 0.05). The occurrence of reproductive and neural alterations across multiple species suggests ecosystem-level biological impairment and highlights the value of fish assemblages as indicators of environmental quality in the lower Doce River basin.