Metynnis is a genus of silver dollar fish in the family Serrasalmidae — the same family as piranhas and pacus — native to South America, with atlas records from Brazil, Bolivia, Venezuela, Colombia, Guyana, Paraguay, Peru, and French Guiana. The 16 species in the atlas run roughly 2.5–8 in and share the deep-bodied, laterally compressed, disc-like silhouette that gives the group its common name, along with the small, closely packed teeth of a mostly herbivorous serrasalmid built for grazing on plant matter and fallen fruit rather than the blade-like dentition of predatory piranhas in the same family.
Metynnis is a long-established aquarium genus: silver dollars such as Metynnis argenteus, M. hypsauchen, and M. lippincottianus are popular community fish for larger planted or semi-planted tanks, valued for their peaceful, shoaling temperament and bright, flashing silver flanks, though their plant-eating habits mean they are a poor match for heavily planted aquascapes. Metynnis maculatus, the spotted metynnis, adds a distinctive dark-spotted pattern to an otherwise mostly silver genus. Most assessed species are Least Concern, and the genus is not considered a conservation concern, though several species remain Not Evaluated or Data Deficient across its wide Amazonian and Orinocan range.