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 in disc width. At up to , 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.<ref name=Carvalho2016a>{{cite journal | author = Carvalho M.R.d. | year = 2016 | title = Potamotrygon rex, a new species of Neotropical freshwater stingray (Chondrichthyes: Potamotrygonidae) from the middle and upper rio Tocantins, Brazil, closely allied to Potamotrygon henlei (Castelnau, 1855) | journal = Zootaxa | volume = 4150 | issue = 5| pages = 537–565 | doi=10.11646/zootaxa.4150.5.2| pmid = 27615813 }}</ref> Juveniles often differ, in some species greatly, in colour and pattern from the adults. Behavior Feeding 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. Breeding Like other Elasmobranchs, male freshwater stingrays are easily recognized by their pair of claspers, modifications of the pelvic fins used when mating. Mating occurs in a ventral-to-ventral position and the females give birth to live young. While still in the mother's uterus, the developing embryo feeds on histotroph, a secretion produced by trophonemata glands. Depending on exact species, the gestation period is 3 to 12 months and there are between 1 and 21 young in each litter. The breeding cycle is generally related to flood levels. Relationship with