Piabucus melanostoma, sometimes called the chin tetra, is a small species of freshwater fish from the rivers of South America.

[3][5] P. melanostoma also has some aspects similar to species in the genus Creagrutus; specifically, the structure of the fifth ceratobranchial is reminiscent.

[6] 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.

[7] Piabucus melanostoma has largely retained its baisonym, which was prescribed by Argentine natural historian Eduardo Ladislao Holmberg in 1891.

[10][11] 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 other, with very little way to come in contact.

Genetic flow between populations is aided by wetland flood cycles, which offer temporary transportation for relevant species.

[11][12] The genus name "piabucus" is a Latinization of the Brazilian Portuguese word "piaba", which is and was used to refer to various small characiform fishes.

[1] P. melanostoma is found especially easily in the Pantanal wetland region, which is largely within Mato Grosso, Brazil.

[20] Researchers have warned that the cumulative effect of various minor stressors could lead to a more catastrophic collapse if usage of the Pantanal is not more strictly regulated.