Rivulid killifishes are among the most threatened components of freshwater biodiversity in the Brazilian Atlantic Forest, yet their biogeographic patterns remain poorly documented. This study provides the first comprehensive biogeographic assessment for rivulids across the Southeastern and Eastern Atlantic basins of Brazil. We compiled distribution records for 54 species, analyzing patterns of endemism, similarity between hydrographic regions, and sampling coverage. Our results reveal patterns of hyper-endemism, with 31 species (57.4%) restricted to single basins and a highly aggregated distribution. Faunal similarity between regions was negligible, indicating strong historical isolation, with only 3.7% of species shared between the Eastern and Southeastern Atlantic basins. Seventeen bioregions were delimited across the study area, with minimal faunal overlap. We identified that 50.0% of basins lack any rivulid records, despite the region having been extensively sampled for more than 150 years (including since the Thayer Expedition of 1865–1866). This study area contains the highest concentration of research institutions in Brazil, suggesting that the absence of records reflects genuine absence rather than a sampling artefact. Based on endemism values, we highlight 16 priority basins for urgent conservation management. These findings reveal that small coastal basins harbor a disproportionately high, imperiled, yet overlooked diversity. We argue that effective conservation of this unique evolutionary heritage requires targeted research in unsampled areas, basin-scale management, and expanded protected area mosaics to incorporate these high-endemism micro-basins. Our results provide the biogeographic foundation for evidence-based strategies to prevent the extinction of these fishes.
SARMENTO-SOARES et al. (2026) studied this question.