Florida's Freshwater Stingrays
The most common reaction I receive from people when I mention that I conduct research on freshwater stingrays in Florida is “I didn’t know there were freshwater stingrays in Florida.” I’ve found this response to be so predictable and so consistent, ranging from people with Ph.D.s to G.E.D.s, that sometimes I question it myself. However, the truth is that stingrays are quite abundant in Florida’s St. Johns River, and they are living in fresh water.
After starting my research on the salinity tolerance of stingrays, I soon realized why people were so amazed to hear of a freshwater stingray. Stingrays belong to a group of cartilaginous fish known to biologists as elasmobranchs, which includes all known species of sharks, skates and rays. Unlike the bony fish (that is, bass, grouper, trout, etc.), elasmobranchs have a skeleton composed entirely of cartilage, a material that makes up the ends of our noses and ear lobes. Overall, elasmobranch fish do not venture into freshwater environments; there are approximately 1,000 elasmobranch species, and only about 50 have been reported to occur in fresh water.
The freshwater stingray in Florida happens to be on of those 50 species. Calling the stingray that lives in the St. Johns River “the Florida Freshwater Stingray,” however, would be a misnomer. In actuality, it is the same species of stingray that is found all around Florida’s coastline (except parts of the Keys). It is the same species that inspires cursing from fishermen and groans from beachgoers who shuffle their feet across the sandy bottom to prevent being stung. The Atlantic stingray (Dasyatis sabina) is one of the most commonly encountered rays along Florida’s coastal waters and is known to foray into freshwater rivers during the warm summer months in other parts of its range. However, the St. Johns River populations are unique because they are the only known populations of the Atlantic stingray that reproduce and complete their life cycle in a freshwater environment.
How Did They Get There?
This is one of the obvious questions that people have about the freshwater stingrays. While you may expect a simple answer, we actually don’t know. I do have a couple theories, one more imaginative than the other.
The first is what I call the “Stork” theory, and I credit it to a person I’ll call “Grady,” whom I met at a fish camp near Sanford (north of Orlando). Grady knew I worked on stingrays and mentioned to me that he had seen birds (species not mentioned) flying around with stingrays in their beaks, and every once in a while a baby stingray would fall into the river.
Now, I’m not sure whether birds actually prey on stingrays, but the phenomenon of baby stingrays (pups) falling out of pregnant female rays is quite common. Stingrays give birth to fully developed, live young, not egg cases like skates do. When they are near term, even the slightest stress - like dangling from a hook or bird’s mouth - can cause them to release their young
So, Grady put 2 and 2 together and hypothesized that the stingray invasion of freshwater was initiated by a bird with a pregnant stingray (from the ocean) in its beak. When the bird was over the St. Johns River, the stingray released her young into the river, and stingrays have been going strong ever since.
I admit the “Stork” theory sounds more like myth than reality, but it is interesting nonetheless.
Another theory, which is more likely, is based on the geologic history of the St. Johns River and the salinity tolerance of the Atlantic stingray. The formation of the St. Johns River has occurred relatively recently. In the late Pleistocene (less than 100,000 years ago), when seawater levels were higher than they are today, the land mass from the east coast of Florida to the east coast of the St. Johns River was underwater. As sea levels gradually declined, the land mass became more exposed and is thought to have formed a brackish water lagoon (similar to the Indian River Lagoon). As the sea level continued to recede, the river became more isolated from the ocean (except for its mouth) and gradually became the freshwater system it is today
Because the Atlantic stingray is prevalent in brackish waters throughout its distribution (like the Indian River Lagoon), it is likely that this species was living in the brackish water that eventually became the St. Johns. The stingray also had an ability to handle fresh water, so the gradual transition to living in fresh water (over thousands of years) was not a huge hurdle to overcome.
What Do They Eat?
This is another common question asked about the freshwater stingrays, and we have a fairly good idea of how they are obtaining calories. First of all, they are bottom feeders, and they have a highly derived sensory system to detect electrical and chemical impulses produced by prey living in mud and sand.
Stingrays’ food-handling ability is quite amazing. I have fed captive stingrays whole medium-sized shrimp and, after a few seconds of munching, seen an almost intact exoskeleton spit out
Snails compose about 50% of the rays’ food source, and the rest of their diet is a mixture of insect larvae and other small invertebrates, including freshwater shrimp and an occasional crab. Like most people who enjoy escargot, these rays have no use for the shell. Amazingly, the only piece of the snail that ends up in their stomachs is the meat. The shell is crushed to pieces by their jaws and is filtered out by their gill slits.
Overall, they are probably opportunistic feeders (like alligators) and may shift their food preferences for whatever is most abundant on the river bottom. The take-home message for fisherman is: If you don’t want to catch stingrays, then don’t put your bait on the river bottom
Prove to Me They Really Are in Fresh Water
The only way to convince yourself (if you are still skeptical) is to see these creatures in the wild with your own eyes. Unfortunately (for you, not for the stingrays), the water of the St. Johns River is black, with only a few inches of visibility. Fortunately (for you, not for the stingrays), the stingrays are also commonly found in many of the freshwater springs that run into the St. Johns River.
For example, I have seen many stingrays along the run of Salt Springs and have heard many anecdotal reports of stingrays in Silver Glen Springs, Alexander Springs, Juniper Springs and Rock Springs. Basically, any spring that has a connection to the St. Johns will more than likely have stingrays. Because spring water is usually crystal clear, you can often see them from a canoe, or even better by snorkeling. Your best chance to see them would be when the springs are least crowded (weekdays), as human activity tends to decrease visibility and may scare off the rays.
A note of caution: Although these fish are not aggressive and don’t attack humans on purpose, they are called stingrays for good reason. If they are accidentally stepped on or if they are handled, they can in defense inflict a rather painful and sometimes serious wound with a venomous barb on their tail. So, if you encounter a stingray, I would not recommend “petting” it or moving too closely to it. It is best to observe from a distance because their behavior in the wild can’t be predicted. Likewise, I would not recommend trying to fish for these animals to get a closer look. Removing them from fish hooks without getting stung can be difficult if you don’t use the proper equipment and technique.
A Rarity to Enjoy
The Atlantic stingray is a common and prevalent component of Florida’s aquatic wildlife and ecology. Although many may consider this species a nuisance, having a resident freshwater species of an elasmobranch is quite rare and should be appreciated by nature enthusiasts and laypeople alike. I encourage you to learn more about this unique species and hope you will be lucky enough to observe one in its natural habitat.
Peter Piermarini is a graduate student in University of Florida’s department of zoology. Atlantic stingrays are the focus of his doctoral research. His research has been published in several scientific journals and has earned him several awards. Peter also took the photos shown here.
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