Phantoms of the Deep
What are stingrays doing in a freshwater Florida spring?
- Peter Taylor
- Aug 01, 2001
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.
"Most rays, sharks and other elasmobranchs usually do not go into low-salinity water because their body fluids are nearly the same salt concentration as seawater," says Piermarini, who studies how these rays can tolerate such extreme variations. "In a less concentrated solution, they’d start taking up water uncontrollably due to osmosis through their gills and skin. As a result, they’d balloon up so much that they’d get stiff and die."
Piermarini has learned that the Atlantic stingray boosts its urine output tenfold to rid itself of excess water. Meanwhile, it reduces the concentration of its body fluids to minimize the osmotic flow into its body. Using this strategy, the creature can quickly acclimate to salty waters. Certain other fish, such as salmon that travel from the ocean to streams for spawning, are genetically programmed to undergo physiological adaptation to fresh water as they approach reproductive age, regardless of whether they actually enter fresh water. But the Atlantic stingray can transform at any age in response to its environment.
This species isn’t the only "marine" animal occurring in the St. Johns River system. Various oceanic fish can be found in surrounding lakes, while gobies, pipefish and killifish are among the residents of the upstream reaches. Even another ray species--the southern stingray--has been reported as a rare transient.
But the Atlantic stingray maintains a permanent, self-sustaining presence in the St. Johns River. Although exact numbers are unknown, the animals are quite abundant. "We catch them more often than catfish," says Piermarini. "On a good day we’ll catch 30 to 40 stingrays."
By sheer number and appetite, all those rays play a significant role in the ecosystem, consuming large quantities of snails, worms, insect larvae and other prey rummaged from bottom sediments. The Atlantic stingray and other rays can detect prey more than six feet away by using an elaborate system of electroreceptors on the undersides of their heads, called ampullae of Lorenzini. Such capabilities are especially handy in murky waters near the river bottom or ocean floor and enable the rays to find hidden prey buried in muddy sediments.
In the St. Johns River, adult stingrays themselves probably face few predators, apart from the occasional alligator. Also, because of their small size, Atlantic stingrays are rarely a target of recreational anglers. Nevertheless, the stingrays of the St. Johns River, like other wildlife in this river system, do face health risks from water pollution and habitat degradation.
To investigate the risks of human activities on the stingrays’ survival, Mote Marine Laboratory researchers Jim Gelsleichter and Cathy Walsh are comparing the health of stingrays from polluted and nonpolluted lakes. They are also trying to determine whether stingrays are experiencing reproductive problems. "Our research also says a lot about the health of the St. Johns River ecosystem as a whole," says Gelsleichter.
Unlike the stingrays in the St. Johns River, marine stingrays face much higher rates of predation. In the ocean, bottlenose dolphins reportedly prey on rays from time to time. But the species’ only major predators are sharks. When pursued, rays have been known to launch themselves well above the sea surface, like flying fish, to elude their attackers below.
Defense against such formidable adversaries probably spurred the evolution of the rays’ stinging weaponry. The stinger itself is a bonelike spine, often several inches long, located near the base of the tail. When threatened, a ray reflexively whips its tail to stab the aggressor and releases a toxin into the wound.
But while the sting offers protection against natural predators, rays are almost never aggressive toward humans. The defensive stinging action is usually triggered by a swimmer inadvertently stepping on a ray or a fisher attempting to remove one from a hook. Recovery time varies depending on the person and severity of the injury. Although some victims heal quickly, infection is a risk, causing others to become incapacitated for months by tissue necrosis. "They end up with a huge hole around where the wound was," says Snelson.
Ray stings are quite common among water enthusiasts on the Florida coast and in other stingray-rich locales. In Seal Beach, California, for example, 200 to 500 stingray-related injuries are reported each summer along a stretch of coastline that is less than a mile long. Last year, of the 1,200 stingray-related injuries in North America, at least 300 occurred there--more than any other place in the country.
A research team led by Chris Lowe, a professor at California State University-Long Beach, is working to understand why the animals favor that particular shore and to offer solutions for reducing stingray injuries. By tracking the rays’ movement patterns with telemetry he hopes to develop guidelines for when beachgoers ought to be especially careful. He suspects that certain human activities--perhaps the discharge of warm water from nearby power plants--may attract the rays, but that natural factors, such as tides, also influence their habits.
Lowe has also rigged up an underwater oscilloscope to watch rays deter predators or capture a meal with a zap of electricity. Pacific electric rays, for example, can discharge as much as 50 volts. "That’s a lot of juice," he says. It’s certainly enough to scare off a sea lion or stop the heart of a scuba diver who ventures too close.
Indeed, sea lions in Southern California apparently quickly learn to avoid Pacific electric rays, which grow to nearly four feet wide. "Even small rays can produce quite a jolt. We’ve seen young sea lions who just swim off as soon as they see a ray," says Lowe. "You get the impression that they’ve learned from the school of hard shocks."
Some divers aren’t quite so lucky. Each year, one to five divers mysteriously turn up dead in Southern California, showing no sign of equipment failure or trauma. Experts suspect that the deaths may be attributable to run-ins with Pacific electric rays. On the East Coast, the bigger Atlantic torpedo ray, occurring from Nova Scotia to Florida and beyond, is even more powerful. It can measure six feet across and weigh 200 pounds. Beachgoers, however, have nothing to fear from either of these species. They live in deep, cold waters and are mostly active at night. "Fishermen occasionally catch the Atlantic torpedo in their trawls," says Lowe. "Pulling up the net, their hands tingle. They know a ray is in the net before they even see it."
Other species in U.S. waters include the spotted eagle ray, a widespread creature whose dark topside features constellations of white dots, and the bat ray, which ranges from Oregon to Mexico. Both of these species gather in schools of a hundred or more animals, offering a spectacle for divers lucky enough to see them. In the Chesapeake Bay, the cownose ray is well known for preying on commercially valuable clams and oysters; come winter, it migrates southward to Florida and possibly as far as Rio de Janeiro.
Back at Florida’s St. Johns River, one friend of Piermarini’s will always remember that stingrays live there. While handling a ray for research, he got stung. "The spine got lodged in his hand and didn’t break off. They had to chop off the tail," says Piermarini. "He went to the hospital for surgery--the tail still attached to his hand."
Maine journalist Peter Taylor wrote about urban red foxes in the June/July issue.
Where To See Freshwater Stingrays
If you’re traveling to Florida in search of wildlife viewing opportunities, consider going stingray-spotting in just about any freshwater spring that flows into the St. Johns River. Pick a calm day with little boat traffic, then grab your snorkel or slip your kayak into Salt Springs, Silver Glen Springs, Rock Springs, Alexander Springs or Juniper Springs in north-central Florida. A note of caution: Although these fish are not normally aggressive and don’t attack humans on purpose, they are called stingrays for good reason. If they are accidentally stepped on or handled, the venomous barbs on their tails can inflict painful and sometimes serious wounds. As with all wild animals, observe stingrays from a respectable distance.