Using integrative taxonomy to understand the evolutionary relationship of isolated lineages of Scleromystax barbatus (Siluriformes: Callichthyidae) in Atlantic Forest rivers

Scleromystax barbatus is a Callichthyidae catfish inhabiting coastal streams of the Atlantic Forest from Rio de Janeiro to Santa Catarina. Despite its broad distribution, the species is restricted to isolated river basins, making it a promising model for understanding how historical isolation and geomorphological complexity influence evolutionary diversification in Neotropical fishes. We investigated patterns of molecular and morphological variation across 165 individuals from 10 populations throughout their range. Mitochondrial (Cytb, ND4) and nuclear (RAG1) DNA sequences were analysed to assess genetic structure, phylogenetic relationships, and lineage delimitation, while geometric morphometrics based on 15 anatomical landmarks were used to evaluate morphological variation morphological variation between sexes and populations. Genetic data revealed pronounced divergence among populations, whereas morphological differentiation was comparatively low. Nevertheless, both approaches consistently identified four distinct evolutionary lineages. These results indicate that river basin isolation within the Atlantic Forest has promoted significant genetic structuring in S. barbatus, even in the absence of marked morphological change. Our findings provide insights into evolutionary processes in Callichthyidae and highlight the role of historical landscape dynamics in shaping biodiversity patterns in the Atlantic Forest.