An interesting aspect of the Atlantic Stingray's biology is its ability to handle variations in environmental salinity. In general, elasmobranchs are considered to be stenohaline (salinity restricted) marine species; less than 5% of all elasmobranch species have been found in fresh water. The Atlantic Stingray, however, is a glaring exception. It is often found in estuaries and far up rivers during the warm, summer months. The populations that I am interested in have gone to the extreme by living year round and reproducing in freshwater lakes of the St. Johns River, FL. Although an extensive tag and release program has not been conducted, it is unlikely that these animals are making consistent migrations back and forth between marine and freshwater environments.
Even though it is not likely that the freshwater stingray populations in the St. Johns River migrate to the ocean, they have not lost the physiological ability to do so. I have conducted experiments that demonstrate freshwater D. sabina from Lake Jesup can acclimate to sea water (30 ppt salinity). They are able to do this because even in fresh water their kidneys reabsorb urea. Unlike the stenohaline freshwater South American Stingrays of the family Potamotrygonidae, D. sabina still has the neprhon segment associated with urea retention. So, in essence these animals are "primed" for return to salt water by reabsorbing more urea when faced with a salinity increase.
This urea retention, however, does have some tradeoffs. In fresh water, the stingrays still retain about 50% of their urea leading to a total osmotic pressure that is 15 times greater than their environment. So these animals face a very large osmotic uptake of water. To compensate for the influx of water, these stingrays have a considerable urine flow rate which is almost 10 times higher than those found in marine individuals.
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