Retroculus acherontos, a new species of cichlid fish (Teleostei) from the Rio Tocantins basin.

Retroculus acherontos is described from the Rio Tocantins basin, Brazil. It is distinguished from all congeners by the presence of a nuchal hump in large specimens, an irregular black blotch located on the middle of the anterior soft dorsal-fin rays, fewer pectoral-fin rays (15-16), a deeper body, a convex ventral profile, a more transversely convex ventral surface of the body, and by chest scales similar in size to the scales on the side. An expanded diagnosis for the genus Retroculus is provided.