These killifishes from the Paraná River Basin were once thought of as variants of single polymorphic species, Melanorivulus pictus, but further field studies have concluded that these fishes are actually many distinct species, currently M. vittatus; M. scalaris; M. linearis; M. faucireticulatus; M. egens; M. nigropunctatus; M. proximus; M. rutilicaudus; M. ofaie; M. nigromarginatus; M. formosensis. More new species will likely be discovered within the M. pictus group as research advances.
The three new species below are described in ZooKeys.
Melanorivulus linearis has sharply delineated thin, red chevron lines.
Melanorivulus nigromarginatus is easily identified by its black anal and lower caudal fin margins (hence its name).