Integrative taxonomy: Morphology and ancient DNA barcoding reveals the true identity of Astyanax taeniatus, a tetra collected by Charles Darwin during the Beagle's voyage

Many of the early descriptions of Characidae species are very short and mainly vague, lacking essential information for a reliable diagnosis and determination of species. That is the case of Astyanax taeniatus. In a break from his journey on board the HMS Beagle, Charles Darwin wandered the interior of Rio de Janeiro State, and collected two small fishes that were briefly described in 1842 by Jenyns as Tetragonopterus taeniatus, and later transferred to Astyanax by Eigenmann. Since then, both the identity and the type-locality of the species have remained somewhat elusive, generating taxonomic confusion with other congeneric species and misidentifications. A redescription of A. taeniatus in 2001 perpetuated the problem, since the type-material was not examined and the geographic region covered by that study was relatively limited, maintaining the taxonomic uncertainty to the present day. In this study, in order to retrieve taxonomic data and to warrant an accurate identification for A. taeniatus henceforth, we integrated molecular and morphology data obtained from syntypes and new specimens collected in the type-locality. Additionally, information from Darwin's journal and from a Brazilian project to retrieve Darwin's travels in Brazil ("Caminhos de Darwin"), allowed to accurately locate the A. taeniatus type-locality at Conceição de Macabú municipality, northern Rio de Janeiro State. The two syntypes of T. taeniatus were examined and a tissue sample was extracted from one of them to compare genetically to other Astyanax species. Then, the Cytochrome Oxidase Subunit I (COI) sequence of this syntype was included in a matrix with 30 characid species from Brazilian coastal rivers, including those collected in the Córrego das Aduelas at type-locality. The smallest values of p-distance (0.01) were found between the sequences of the syntype and specimens of Astyanax from Córrego das Aduelas, Rio Macaé basin, in “Fazenda Socego”. The anatomical and meristic information of the modern specimens collected in the type-locality also agree with data from the syntypes. Integrating morphology and molecular techniques for ancient DNA, we succeeded to definitely solve the taxonomy of A. taeniatus and also identify modern topotypes in Córrego das Aduelas at Conceição de Macabu (RJ). Based on the syntypes and on a collection of A. taeniatus, including topotypes, the species is herein redescribed and a lectotype is designated. Besides, the accurately delimitation of A. taeniatus allowed us to detect and describe a new species, Astyanax keronolepis n. sp., which has been occasionally misidentified as A. taeniatus.