We present an annotated and updated checklist of freshwater fish species occurring in the Northeastern Mata Atlântica ecoregion (NMAF), Eastern Brazil. A total of 305 native freshwater fish species are documented for this ecoregion, of which 210 are endemic. Distribution maps of 298 species are provided. There is a general pattern of increased sampling intensity in coastal zones, gradually diminishing towards the west as elevation increases. A bioregionalization algorithm identified four biogeographic units in the NMAF ecoregion: Northern Bioregion, Central Bioregion, Coastal Tablelands Bioregion and Southern Bioregion. The heterogeneous nature of the ichthyofauna in this area, along with the significant faunal overlap observed between NMAF and adjacent ecoregions, suggests that the approach of ecoregion delimitation may not be fully congruent with the current faunal compositions. This underscores the need for novel regionalization approaches that reinforce outcomes consistent with the diversity patterns evident within the ecoregion.

The large number of less-known coastal drainages in the Atlantic Forest still represents an important gap in the knowledge of ichthyofauna. It is evaluated the taxonomic diversity, frequency and constancy of occurrence, and the distribution of species in the various segments of the Perequê-Açu river basin, state of Rio de Janeiro, Brazil. Twelve sampling campaigns resulted in 13,423 individuals, belonging to seven orders, 10 families, 21 genera, and 23 native species. Cyprinodontiformes and Siluriformes represented about 87.2% of the total abundance, mainly due to the contribution of Phalloceros anisophallos and Schizolecis guntheri, which account for 63.2% of the total sampled individuals. The analysis of constancy of occurrence reveals that 78.3% of the drainage species are residents, with approximately 93% of the residents of the Carrasquinho River sub-basin. The Sertões River sub-basin had the largest number of species in total: 18. The rarefaction curve did not reach an asymptote, although Chao 2 and bootstrap estimate the richness of 23.9 species and 23.7 species, respectively. The results of PERMANOVA, PCoA, and SIMPER indicate that the fish composition along the catchment is not homogeneous. The conservation status of species is also discussed, regionally, nationally, and globally.

The fish fauna of Rio de Janeiro has been extensively studied, resulting in a comprehensive database of species collected over more than three centuries. This study aimed to provide a checklist of species, to identify patterns of diversity and the distribution of freshwater ichthyofauna, to delineate biogeographic units, and to explore changes in faunal composition among different areas. Analyzing data from ichthyological collections and the literature on original species descriptions revealed 206 freshwater fish species: 183 native and 23 allochthonous. The checklist includes updated species names. The sampling effort in Rio de Janeiro is extensive, especially in coastal lowlands. The findings indicate that inventory work is still needed in some areas, particularly within the Rio Paraíba do Sul basin. Seven bioregions of freshwater ichthyofauna were identified, including a major region of higher species richness and smaller areas with higher endemism of restricted-range species. This biogeographic assessment underscores the diverse and distinctive freshwater fish fauna in the basins of Rio de Janeiro, with well-defined biogeographic units.

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.

In light of the growing loss of aquatic biodiversity amid the current freshwater biodiversity crisis, systematic conservation planning is crucial for developing conservation policies to establish new protected areas that effectively address the complexity of freshwater environments. This study aims to identify priority basins for conservation of threatened, near-threatened, and restricted-range endemic freshwater fish species in the Northeastern Mata Atlântica freshwater ecoregion, highlighting irreplaceable and critical locations for conservation and impact mitigation. Through spatial prioritization, three scenarios with distinct conservation features were considered: (a) 35 threatened species, (b) 118 near-threatened and restricted-range species, and (c) all 153 species. A total of 50 and 89 basins were prioritized in the first two scenarios, respectively, and 135 basins in the third scenario, with 36 classified as critical due to environmental stressors, particularly in the Rio Doce basin. Our results suggest that the current protected area network is insufficient for safeguarding most target fish species. This study presents an initial framework for prioritizing areas that can provide a basis for further, more refined assessments and to support conservation policies such as national or regional action plans, protecting basins and species.