Xingu River ray
The Xingu River ray, white-blotched river stingray, or polka-dot stingray ("Potamotrygon leopoldi") is a species of freshwater fish in the family Potamotrygonidae. It is endemic to the Xingu River basin in Brazil and prefers rocky bottoms. It is sometimes kept in aquaria. The Xingu river ray is a venomous stingray that contains venom localized at its dentine spine in its tail. While the ray's venom composition does not change with maturation, the venom toxicity decreases as rays get older. On the other hand, rays’ jaw shape, stiffness, and mineralization are strengthened with age, which  allows mature rays the ability to consume hard-shelled invertebrates. These changes in traits with maturation reflect the different pressures rays experience in terms of functions such as feeding and avoiding predation during different maturity stages. The fish is named in honor of King Leopold III (1901–1983) of Belgium, who sponsored many scientific studies at the Institut Royal des Sciences Naturelles de Belgique. Appearance and relatives. "Potamotrygon leopoldi" reaches up to in disc width, in total length and in weight. Females grow larger than males. It is closely related to the similar "P. henlei" from the Tocantins River basin and  P. albimaculata from the Tapajós River basin. Compared to "P. henlei", "P. leopoldi" is deeper black above and its underparts are mostly brownish-dusky (large white center to underparts of "P. henlei"). Compared to "P. albimaculata", "P. leopoldi" has fewer and larger yellowish-white spots above.