Overview
Formally described in 2016 from the Tocantins River Basin of Brazil, Potamotrygon rex is one of the largest strictly freshwater stingrays in the Americas. Fittingly named ‘rex’ (king) due to its monumental proportions and majestic golden-spotted dorsal pattern, it commands the sandy beds of one of South America’s most modified aquatic systems.
Biology and Physical Characteristics
Boasting a massive circular pectoral disc exceeding one meter in diameter, the Great River Stingray features a blackish-brown to dark brownish-gray background covered with clusters of intense yellow and golden-orange ocelli centered with dark cores. Its stout muscular tail bears one to two serrated venomous caudal spines backed by proteinaceous toxins capable of delivering excruciating defensive stings.
Behavior and Ecology
A solitary benthic predator, it glides gracefully across sandy flats and buries its body beneath loose silt, leaving only elevated spiracles and periscopic eyes exposed to watch for prey. It uses specialized ampullae of Lorenzini to detect bioelectric fields generated by hiding crabs and catfish, excavating prey with hydraulic jetting from its mouth.
Conservation and Human Impact
Assessed as Data Deficient by the IUCN, but facing severe ecological fragmentation. The Tocantins River is heavily dammed by monumental hydroelectric projects (such as the Tucuruí Dam), which impede seasonal ray movements, alter thermal regimes, and drown natural shallow sandbanks essential for foraging and pup rearing.
Interesting Factoid
Unlike oceanic stingrays that maintain salt balance through urea retention, Potamotrygon rex is fully adapted to freshwater; its kidneys have lost the ability to retain urea, meaning it would perish if placed in saline ocean water.
Last updated: October 2026