More than any other fishes - including piranhas - freshwater stingrays
are feared by people who live along the Amazon and its tributaries. Stingrays
bury themselves in the sand or mud bottoms of river shallows. Anyone who
accidently steps on one stands a good chance of being lashed with the
sharp, serrated spiine that grows out of the stingray's tail. The spine
carries painful venom attacks the nervous system and heart.
These stingrays' nearest relatives live in the Pacific Ocean. Their ancestors
were marine stingrays that lived in a seaway that connected with the Amazon
River east of the Andes. Some would have been trapped in teh river's estuary
as sedimentation closed the connection. Adaptations to a freshwater habitat
included a change in the salt concentrations in their bodies.
Amazon Rising: Seasons of the River. Shedd Aquarium