Fish assemblages and endemism in blackwater aquatic habitats of theCoastal Plain in northern Santa Catarina, southern Brazil

Abstract

Abstract Blackwater streams are unique environments characterized by dark, acidic, and nutrient-poor waters, supporting highly specialized aquatic communities. This study presents an inventory of the ichthyofauna of such systems in the northern region of Santa Catarina state, based on surveys conducted between 2020 and 2024. A total of 22 sites were sampled, including perennial rivers, intermittent streams, and temporary pools, located both within and outside protected areas. In total, 30 fish species were recorded, representing 13 families and five orders. The order Characiformes showed the highest richness and was represented mainly by small-characins of the family Acestrorhamphidae. Nine species were identified as endemic to blackwater environments of the Atlantic Forest, five of which officially listed as threatened. The guppy Poecilia reticulata was recorded for the first time in blackwater environments of the region, as well as Brachyhypopomus cf. draco, representing the first record of Hypopomidae (knifefish) in the region. DNA barcoding (using COI) provided molecular information, in addition to the morphological identification of 12 species, providing new local sequences for them. Results indicated a high similarity between the ichthyofauna of this region and that of blackwater streams in the southern coast of São Paulo. Endemic and threatened species were recorded both inside and outside legally protected areas, highlighting gaps in current conservation efforts. Furthermore, intermittent streams and temporary pools proved to be critical habitats for maintaining species with extremely restricted distributions. The continued degradation of riparian vegetation, combined with climate change, poses an additional threat to the persistence of these communities. These findings underscore the urgent need to expand conservation actions targeting blackwater ecosystems along the southern Brazilian coastal plain.