Freshwater Stingrays in Florida
The Atlantic stingray, Dasyatis sabina, is one of the most commonly spotted rays in Florida's coastal waters; but is also encountered in many of the freshwater springs and lakes of the St Johns River.
Atlantic stingray, Dasyatis sabina, in Silver Glen Springs. Photo by Danielle Shmalberg
Some other family members (bull & tiger sharks) venture into freshwater environments for a brief time, but the St. Johns River stingray populations are unique as they are the only known populations of elasmobranchs (sharks, skates & ray family members) that can reproduce and complete their entire life cycle in a freshwater environment. They survive quite happily in zero percent salinity but can re-adapt to salt water if transferred to there!
Scientists have discovered exactly how these rays manage to tolerate such a range of salinities. Most elasmobranchs usually do not go into low-salinity (brackish or fresh) water because their body fluids are almost the same salt concentration as seawater. In a less concentrated solution (ie freshwater), they start taking on water uncontrollably due to osmosis across their gills and skin, to a point where the animal swells up and dies.
The Atlantic stingray reduces the concentration of its body fluids to minimize the osmotic flow into its body. They also boost their urine output almost tenfold to rid its body of any excess water. Using this strategy, they can quickly respond and cope with a change in salinity at any point in their lives!
Atlantic stingrays play an important role in the St. Johns ecosystem, consuming large quantities of snails, worms and insect larvae. They have a highly developed sensory system on the undersides of their heads, called ampullae of Lorenzini, that detects electrical and chemical impulses produced by prey living in mud and sand. Even in murky water, they can detect prey more than six feet away!
Snails compose almost half of the stingrays diet, but they are picky, and don't like to eat the shell! The ray crushes the snail with their jaws, filters the shell pieces out through their gill slits so the meat ends up in their stomachs.
Although the exact stingray population number in the St Johns system is unknown, they are quite abundant. Due to their bottom-feeding habits, many fishermen consider them a nuisance, but having a resident freshwater elasmobranch species is extremely rare and should be very much appreciated.
Atlantic stingray, Dasyatis sabina, in Silver Glen Springs. Photo by Danielle Shmalberg
Freshwater stingrays have been found in all the lakes (Lake George, Lake Monroe and Lake Jesup) and most the freshwater springs (Silver Glen Springs, Juniper Springs, Salt Springs and Rock Springs ) associated with the St. Johns River system.
Eager stingray-spotters should concentrate their search on the springs & spring runs, as this water is usually crystal clear. Quiet days, with less boat and human traffic, are best to see them.
This species isn't the only marine animal that lives in freshwater springs....
Coming soon: An article on other Marine visitors to freshwater springs!!
Denise Byrne
Marine Biologist/OW, Tech and Cave Instructor
denise@dayo.com
Orlando, Florida