A new species of freshwater fish in the family Characidae, called Hyphessobrycon myrmex, exhibits an intriguing sexual dichromatism: Adult males are a deep reddish-orange, while females and juveniles are pale yellow. The species has been described in an article published in the Journal of Fish Biology. H. myrmex is approximately 2 cm long and lives in the waters of the Formiga River, a tributary of the Juruena River in Serra dos Parecis, Mato Grosso State. The Formiga is part of the Tapajós Basin.