A new species of miniature fish of the characid genus Priocharax is described from a small lake near the rio Ipixuna, rio Purus drainage, Amazonas State, Brazil. It is distinguished from all congeners except P. pygmaeus by the lower number of teeth on the maxilla and dentary. It differs from P. pygmaeus by the presence of two postcleithra and 22–27 branched anal-fin rays (vs absence and 19–22). The new species is further distinguished from other species of Priocharax by a combination of characters involving the number of pelvic-fin rays and branched anal-fin rays, the number of postcleithra, the shape of postcleithrum 3, and the absence of the claustrum. Molecular evidence based on COI sequences of all valid species of Priocharax also corroborates the validity of this new species.

We used the DNA barcoding technique (COI and CytB markers) combined with GMYC analysis to characterize the genetics of the widely distributed Neotropical fish species Knodus moenkhausii from three different isolated hydrographic basins. Despite the fact that most of the Neotropical hydrographic basins have been isolated for millions of years, species could be shared between basins due to natural events (stream capture) or anthropogenic activities. Recent surveys, however, have shown that many widely distributed species are actually species complexes divided into previously unrecognized cryptic species. In this study, we tested the hypothesis that K. moenkhausii from three hydrographic basins represent a single panmictic species and discuss the most likely explanation of its present geographical distribution. The GMYC analysis revealed that all specimens of K. moenkhausii represent a single species: the intra- and intergroup minimum K2P genetic distances for both genes were zero and haplotypes were shared among the three hydrographic basins. This suggests there has been recent interchange of K. moenkhausii throughout the three hydrographic basins. It is likely that this is due to recent human activities, either the transposition of natural barriers or intentional introduction or accidental escape due to the ornamental fish trade.