Piabucus melanostoma, sometimes called the chin tetra, is a small species of freshwater fish from the rivers of South America. It is the second-largest species of the genus, but still only reaches about 12 cm (4 in) long. Occasionally found in captivity, it has an appealing iridescent-silver coloration. It is amongst the largest of fish still considered "tetras" in the aquarium hobby. While it is commonly referred to as P. melanostoma, it is also easily found under the name P. melanostomus. Its baisonym is P. melanostoma, and it is more often listed as such, but both are used by the scientific community. Description Piabucus melanostoma reaches a maximum of 11.5 cm (4.1 in) in SL (standard length). It is second-largest out of the three species in its genus; Piabucus caudomaculatus is the smallest, at 9.6 cm (3.8 in) SL, and Piabucus dentatus the largest, at 12.9 cm (5.0 in) SL. The scales are an iridescent silver-yellow, with a stripe of brighter silver down each side; this stripe ends in a patch of black on the middle caudal-fin rays. The lateral stripe may also display a touch of green or yellow. One of P. melanostoma's most distinctive characteristics is its lower jaw, which is colored black. Notable morphometric characteristics include 44 to 46 anal-fin rays, 11 dorsal-fin rays, and 82 to 92 lateral-line scales. P. melanostoma also has some aspects similar to species in the genus Creagrutus; specifically, the structure of the fifth ceratobranchial is reminiscent. (The ceratobranchial is the longest bone in the branchial arches of fish, or the gill arches.) However, this feature (and related features) are the only similarity between P. melanostoma and any Creagrutus. Sexual Dimorphism All three species of Piabucus develop hooks and lappets (fleshy extensions) on the first few rays of the anal fin, but further sexual dimorphism specifically in P. melanostoma is unknown. Taxonomy Piabucus melanostoma has largely retained its baisonym, which was prescribed by Argentine natural historian Eduardo Ladislao Holmberg in 1891. The genus Piabucus was referred to as Piabuca at the time, but otherwise it has remained the same. Research using DNA barcoding has discovered that P. melanostoma is made up of two distinct lineages, though they are not different species. The lineages are partially the result of the restricted population flow presented by a wide range combined with a low dispersion capacity - that is, isolated groups far apart from each