PEERING THROUGH SHALLOW, crystalline waters, Peter Piermarini is astonished at the number of stingrays he sees blanketing the bottom. Occasionally, one darts, kicking up swirling powder, its long, spiny tail trailing behind. What's unusual is that Piermarini isn't snorkeling in the Caribbean or some other land of blue water and multicolored fish. He's even dozens of miles from North America's salty coastal shores. A short distance downstream, the gurgling spring empties into the murky, tannic waters of Lake George on the St. Johns River in northern Florida. It is here that these stingrays-the only ones in North America able to live their entire lives in fresh water- glide out of sight. Sometimes even the locals don't know the creatures live there.
"It's amazing the number of people who see me out doing research that are shocked to learn there are more than alligators to watch out for in the river," says Piermarini, a zoology graduate student at the University of Florida. Indeed, the creature's signature means of defense can leave a "nasty puncture wound that looks awful and bleeds terribly," says Franklin Snelson, a University of Central Florida biology professor who has studied the freshwater rays and inevitably been stung himself.
The Atlantic stingray, a species more conventionally found in coastal waters from the Chesapeake Bay to Central America, was first recorded- as "the dreaded stingaree"-in Florida's pristine springs by naturalist William Bartram in the late 1700s. But it's really not surprising that so many people still don't know about this remarkably adapted bat- winged fish. Although common in every ocean, rays-all 400 or so known species-have attracted scant attention even from scientists. Their notorious shark cousins have stolen much of the limelight. But recent studies revealing the species' unusual physiological adaptations to extreme variations in salinity have prompted researchers to take a longer look at these winged phantoms of the deep.
Rays belong to the group of fish known as elasmobranchs because their skeletons are made of cartilage, not bone. Evolving from their shark forebears some 200 million years ago, rays have radically flattened, disc-shaped bodies and extremely wide pectoral fins that form broad "wings" used for propulsion. The largest rays-the manta rays, found in tropical areas throughout the world-boast wingspans of more than 22 feet and weigh a ton. Shaped like Stealth aircraft, the animals soar through the ocean with tremendous speed, grace and dexterity. Most, but not all, rays defend themselves with a sharp, toxin-laced spine near the base of the tail. Some species can also deter predators and stun prey by generating powerful electric shocks.
Resembling rays in body is another group of elasmobranchs called skates. However, skates can't sting, and the two groups reproduce differently. While rays give birth to live young, which as embryos have been nourished by a "milk" produced in the uterus, skates produce eggs encased in small, black leathery pouches that, when found washed ashore, are known as "mermaids' purses."
About 50 ray species throughout the world regularly venture into fresh water. More than two dozen species are capable of surviving their life cycles in fresh water. At the extreme, several ray species in the Amazon River Basin have become so rigidly adapted to river life that they cannot survive salt water. Other rays, including the Atlantic stingrays of the St. Johns River-with their modest two-foot wingspans- demonstrate a remarkable tolerance for variations in salinity.
Some four dozen species of rays and skates are found in North America's coastal and offshore waters, but only the Atlantic stingrays of the St. Johns River live year-round in fresh water. Scientists believe that these rays travel infrequently, if ever, between river and ocean. Nonetheless, individuals taken from the river and placed in salt water- or vice versa-can quickly adapt and survive. …