Potamotrygonidae

River stingrays or freshwater stingrays are Neotropical freshwater fishes of the Potamotrygonidae family 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. River stingrays are almost circular in shape, and range in size from Potamotrygon wallacei, which reaches 31 cm (1.0 ft) in disc width, to the chupare stingray (S. schmardae), which grows up to 2 m (6.6 ft) in disc width. The latter is one of only two marine species in this family (the other is S. pacifica). The largest freshwater species in this family are the discus ray (Paratrygon aiereba) and short-tailed river stingray (Potamotrygon brachyura), which grow up to 1.5–1.6 m (4.9–5.2 ft) in disc width. At up to 220 kg (490 lb), by far the heaviest freshwater member of the family is the short-tailed river stingray, which among South American strict freshwater fish only is matched by the arapaima (Arapaima) and piraíba catfish (Brachyplatystoma filamentosum). In each species in the family Potamotrygonidae, females reach a larger size than the males. The upper surface is covered with denticles (sharp tooth-like scales). Most species are brownish or greyish and often have distinctive spotted or mottled patterns, but a few species are largely blackish with contrasting pale spots. Juveniles often differ, in some species greatly, in colour and pattern from the adults. Members of Potamotrygonidae are predators and feed on a wide range of animals such as insects, worms, molluscs, crustaceans and fish (even spiny catfish). Plant material is sometimes found in their stomachs, but is likely ingested by mistake. The exact diet varies with species; some are generalist predators and others are specialists. For example, Potamotrygon leopoldi mainly feeds on freshwater snails and crabs, although captives easily adapt to a generalist diet. The largest species such as Paratrygon are top predators in their habitat. The jaw joints of stingrays are 'loose', allowing them to chew their food in a manner similar to mammals. The family includes both species that are diurnal and species that are nocturnal.