Writer: Aaron Hoover
GAINESVILLE, Fla. — Only a few of the world’s stingrays venture into freshwater, and most are threatened by overfishing, pollution or both.
Now, a University of Florida researcher hopes to help save these rare and little-studied fishes through broadening knowledge of a unique stingray found in a Florida river.
The stingray found throughout much of the St. Johns River has the distinction of being the only stingray in North America able to spend its entire life in freshwater. UF zoology doctoral student Peter Piermarini is probing this capability in hopes of giving scientists an easy way to discern exclusively freshwater stingrays from ones that can tolerate both fresh and salt water. It is important to identify the freshwater rays first because they are more vulnerable to fishing and pollution than their more adaptable counterparts, Piermarini said.
"We can’t save everything, so which ones are we going to prioritize?," Piermarini said. "If a stingray is restricted to a freshwater environment, it should be prioritized over stingrays that move back and forth between fresh and salt water."
Scientists have identified over two dozen species of freshwater rays in Latin America and another dozen or more rays that may venture in to freshwater worldwide, said George Burgess, a UF ichthyologist. These include the giant freshwater stingray, a ray with a reported 12-foot wingspan that lives in parts of Thailand and the Ganges River in India, Piermarini said. They also include several species that live in coastal rivers or lakes in impoverished, unstable regions in West Africa and Southeast Asia. As a result, studying these rays may be time-consuming and difficult, Piermarini said.
By contrast, the ray Piermarini is studying Dasyatis sabina, is common in the St. Johns River. "It’s one of the only places in the world where these rays spend their entire lives in freshwater and where you can study them easily," he said.
The ray is brown on the top, white on the bottom and can reach a little more than 2 pounds. It is the same species of ray common along both coasts of Florida known to temporarily enter fresh water. Although the ray can live exclusively in freshwater, it normally lives in brackish water throughout its range in the St. Johns.
Most fish are confined either to salt or freshwater because of a chemical process known as osmosis: Fluids containing salts or other solutes move through semi-permeable membranes from an area of higher concentration to one of lower concentration.
Because the concentration of salt in saltwater fishes’ bodies is not as high as salt water, they are in danger of losing fluids and gaining salt. As a result, their bodies have evolved to gain water and excrete salt. Freshwater fish, by contrast, absorb water and lose salt because their bodily fluids are saltier than the surrounding freshwater. Their bodies have evolved to get rid of the extra water and absorb salts from the environment.
A small number of fish, and an even smaller number of sharks, skates and rays, can migrate between fresh and salt water or live equally well in both. The object of Piermarini’s research is to understand how the St. Johns River ray manages this feat. Scientists know fish use their gills to regulate the salt and water concentrations in their bodies, and much of Piermarini’s work involves examining the biochemistry and cellular composition of gill material from rays under powerful microscopes.
Piermarini is also studying a South American ray that can live only in freshwater. When both studies are completed, he expects to have data that other scientists can use to identify a given ray as restricted or unrestricted to fresh water.
Piermarini said rays’ plight is particularly urgent because rays, like sharks and skates, are live-bearing and give birth to only a few young.
"It takes a lot of time to do the experiments in the places where these animals are threatened," he said. "It would be better if we could just go there, take a little bit of gill tissue and bring that back to the lab and analyze it."
Piermarini’s research is being funded with a three-year, $102,000 graduate research fellowship from the Environmental Protection Agency.
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Source: University Of Florida. February 2000