Small Fishes, Big Issues: Species Delimitation in Hemigrammus Marginatus , Gill, 1958 (Acestrorhamphidae: Pristellinae) from Brazilian Coastal Basins Based on Integrative Genetics.

The small characins represent a systematic puzzle in the Neotropical ichthyofauna as a result of independent miniaturization processes, adaptive convergence and lack of diagnostic characters for several genera. In order to diminish the taxonomic uncertainties and the evolutionary pathways in Hemigrammus , we carried out an integrative genetic analysis in the putatively widespread Hemigrammus marginatus Ellis, 1958 by combining cytogenetic and molecular data based on the mitochondrial Cytochrome C Oxidase subunit I (COI). Specimens of H. marginatus from the type locality in Itapicuru River basin and other two populations from coastal rivers in northeastern Brazil were analyzed and compared with the available data from other regions in South America. Conspicuous macro and microkaryotypic differences were detected between the samples from northeastern and southern Brazil (Upper Paraná River basin). Likewise, the DNA barcoding and species delimitation analyses recovered distinct Molecular Operational Taxonomical Units within H. marginatus . Therefore, the population from the type locality should be referred to as H. marginatus stricto sensu , representing a restricted characin taxon from coastal drainages (including the São Francisco River basin) along northeastern Brazil, while other populations of this small characin fish need to be taxonomically revised and managed as unique lineages.