The Natural History of Freshwater Stingrays
All stingrays belong to the class Chondrichthyes, which includes all 850 species of sharks, rays, skates, and chimaeras. These fishes have cruised the oceans since the Paleozoic era, but the basic dorsoventrally compressed body plan of the ray is thought to have originated during the Jurassic period, approximately 100 Mya. This design, coupled with the lack of a swim bladder and powerful jaws designed for crushing hard shells of molluscs such as clams and muscles, has made the ray the successful benthic predator it is today.
CLASS CHONDRICHTHYES (cartilaginous fishes)
There are many different theories as to the origins of South America's freshwater stingrays. We do know that they are most closely related to the genus Himantura (family Dasyatidae), which has representatives in both the Caribbean and the Pacific, so it is difficult to determine exactly when or where the progenitors of the Potamotrygonids entered what would become the Amazon basin.
The evolutionary history of the Amazon's fishes is as complex as the numerous geological events that forged the South American continent. During the early part of the Cretaceous period, the river flowed into the Pacific Ocean rather than the Atlantic, and like many present-day marine fish, ancient stingrays most likely made frequent trips into freshwater and then back to the ocean. However, the formation of the Andes mountain system around 90 mya eventually cut off the route to the Pacific and caused the river to flow in the other direction, towards the east. Over the years, some evolutionary biologists have postulated that the stingrays inhabiting the river became trapped and either died or adapted and moved further upstream, where they evolved independently of their saltwater relatives.
However, new evidence derived from molecular techniques including mitochondrial DNA analysis suggests that the Potamotrygonids diverged from their saltwater ancestors much later than was previously believed, sometime during the Miocene between 12 and 22 mya. In addition, the incursion appears to have come from the eastern coast of the continent, which contradicts earlier speculations (Lovejoy, Bermingham & Martin, 1998).
The majority of stingrays in the family Potamotrygonidae belong to the genus Potamotrygon, which consists of approximately 20 known species, and many more that are still either undiscovered or unclassified. The other two genera, Paratrygon and Plesiotrygon (antenna rays), are both represented by only one species.
Contrary to popular belief, there is no such thing as a true freshwater stingray outside of the family Potamotrygonidae. However, there are several species belonging to the family Dasyatidae that are classified as euryhaline, which means that they have the ability to move back and forth between freshwater and marine habitats without disrupting their internal osmotic concentration. Some populations of dasyatids are known to inhabit freshwater ecosystems throughout their lives, but upon closer examination, their physiology shows little if any difference from their marine counterparts.
Although the Atlantic stingray is most commonly found in marine environments, entirely freshwater populations of these fish have recently been located in Florida's St. John's River and have been the subject of much research. In addition, this stingray has become rather popular recently as an "alternative" freshwater stingray for aquarists who, for one reason or another, chooses not to keep potamotrygonids.
Although the Dasyatis sabina of the St. John's River reproduce and spend their entire lives in freshwater, they are nearly identical in physiology to saltwater populations. However, they are significantly smaller. This could be due to the fact that it is energetically costly for a euryhaline elasmobranch to persist in a freshwater environment, so less energy is allocated for growth in these populations.
It is estimated that freshwater Atlantic stingrays have been separated from their marine brethren for a little under a million years, which is not very long in terms of evolutionary time. Their osmoregulation strategy is the same one that is employed by other euryhaline elasmobranchs; although they are able to significantly reduce the amount of urea in their bodies while they are in freshwater, they are never able to completely eliminate it as the Potamotrygonids have done. Consequently, they are not considered a true stenohaline freshwater species, and are physiologically capable of returning to a marine habitat should the need arise.
Interestingly enough, however, the ratio of body weight to rectal gland weight was found to be significantly lower in the freshwater population compared with marine populations. Even when the freshwater stingrays were fully acclimated to salt, the weight of the rectal glands remained the same.