Metynnis also commonly known as the silver dollar is a genus of freshwater ray-finned fishes belonging to the family Serrasalmidae, which includes the pacus, piranhas and related fishes. The fishes in this genus are found in tropical and subtropical South America. They are herbivorous or omnivorous, and inhabit a wide range of freshwater habitats, ranging from rivers and streams (both fast- and slow-flowing), to floodplains, flooded forests, lakes, pools and reservoirs. They generally reach 8–15 cm (3–6 in) in standard length, although a few species may reach up to 18 cm (7 in). The genus contains many of the species known as silver dollars in the aquarium trade.

Though part of the subfamily Serrasalminae, members of this genus are not commonly called piranhas, due to their largely herbivorous diets.

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