Marine cartilaginous fishes keep their body fluids close to seawater by retaining urea and trimethylamine oxide, and dump excess salt through a rectal gland. Some rays and sharks move into rivers by adjusting those systems temporarily; the bull shark and the sawfishes do exactly this.
The Potamotrygonidae went further. Their blood carries little urea, and the rectal gland is reduced to a functionless remnant. The physiology is now tuned to dilute fresh water, and adult river stingrays cannot tolerate full seawater. It is the only family of rays for which the return to the sea is closed.
The ancestors are thought to have entered South American drainages from the Caribbean side during Miocene marine incursions, then diversified into more than thirty species. The consequence for people is direct: buried in shallow warm sand, these rays are the commonest cause of serious stingray injury in the Amazon basin.