Closely related to the saltwater stingray, freshwater stingrays are cartilaginous fish classified into two different families based on their geographic locations. Also called river stingrays, the Potamotrygonidae rays are found in the rivers of tropical and subtropical South America, being mostly concentrated within the Amazon Basin. These beautiful rays are generally small compared to their saltwater cousins, growing up to 46 cm (18 inches) wide with long tails, are very rounded, and have colorful spots of different sizes against a dark background of black, brown, or gray, depending on the species.