River stingrays or freshwater stingrays are Neotropical freshwater fishes of the family Potamotrygonidae in the order Myliobatiformes, one of the four orders of batoids, cartilaginous fishes related to sharks. They are found in rivers in tropical and subtropical South America (freshwater stingrays in Africa, Asia and Australia are in another family, Dasyatidae). A single marine genus, Styracura, of the tropical West Atlantic and East Pacific are also part of Potamotrygonidae. They are generally brownish, greyish or black, often with a mottled, speckled or spotted pattern, have disc widths ranging from 31 to 200 centimetres (1.0–6.6 ft) and venomous tail stingers. River stingrays feed on a wide range of smaller animals and the females give birth to live young. There are more than 35 species in five genera. They are native to tropical and subtropical northern, central and eastern South America, living in rivers that drain into the Caribbean, and into the Atlantic as far south as the Río de la Plata in Argentina. A few generalist species are widespread, but most are more restricted and typically native to a single river basin. The greatest species richness can be found in the Amazon, especially the Rio Negro, Tapajós, and Tocantins basins (each home to 8–10 species). The range of several species is limited by waterfalls. Freshwaters inhabited by members of Potamotrygonidae vary extensively, ranging from lacustrine to fast-flowing rivers, in blackwater, whitewater and clearwater, and on bottoms ranging from sandy to rocky. In at least some species juveniles tend to occur in shallower waters than adults. Most species are strictly freshwater, but a few may range into brackish estuarine habitats in salinities up to at least 12.4‰. In 2016, two fully marine species formerly included in Himantura were found to belong in Potamotrygonidae, and moved to their own genus Styracura. These are S. schmardae from the tropical West Atlantic, including the Caribbean, and S. pacifica from the tropical East Pacific, including the Galápagos. Potamotrygonidae are the only family of rays mostly restricted to fresh water habitats. While there are true freshwater species in the family Dasyatidae, for example Himantura chaophraya, the majority of species in this family are saltwater fish.
