Black tetra

The black tetra (Gymnocorymbus ternetzi), also known as the petticoat tetra, high-fin black skirt tetra, black skirt tetra, black widow tetra and blackamoor, is a species of freshwater ray-finned fish belonging to the family Acestrorhamphidae, the American characins. It is native to the Paraguay River basin of south-central Brazil (mainly Pantanal region), Paraguay and northeast Argentina, but there are also populations in the upper Paraná and Paraíba do Sul Rivers that likely were introduced. It was formerly reported from the Guapore River, but this population is part of G. flaviolimai, which is found throughout the Madeira River basin and was described in 2015. The black tetra is often kept in aquariums. The black tetra can grow up to 7.5 cm (3.0 in) in length. It has a roughly tetragonal body shape and is greyish in colour, fading from near black at the tail to light at the nose. Two prominent, black, vertical bars appear just posterior to the gills. It is easily distinguished from all of its congeners by the presence of a dense field of dark chromatophores spread homogeneously over the posterior half of the body, unlike the lack of such pigmentation in all congeners. The black widow tetra is a shoaling fish that feeds on small crustaceans, insects, and worms.