Apareiodon affinis

The darter characine is a small, fusiform fish growing to a maximum length of 14.3 cm (5.6 in). Like other characids, it has a mouth on the underside of the head with a poorly developed upper lip. There are no dentary teeth and the pectoral fins have a single, unbranched fin-ray which the fish uses to prop itself up on the substrate.[2]

The darter characine is an open water fish,[3] and was one of several fish species in a reservoir on the Paraná River to thrive when large submerged macrophytes were removed.[4] The diet consists of diatoms, green algae and the periderm of aquatic vegetation, perhaps removed accidentally while the fish scrapes off the algae.[5]

The karyotype of this fish varies between populations. In the Upper Paraná basin, the sexes have distinct diploid numbers, the males showing 2n = 54 and the females 2n = 55. They have a multifactorial ZW sex-determination system where the female is determined by ZW₁W₂ and the male by ZZ. In the separate Cuiabá River system, all individuals show 2n = 54.[6] This is a non-migratory species and its reproductive strategy involves external fertilisation and a lack of parental care of eggs or young.[7]