Found this browsing around.. thought I would share, I know its no eagle landing 
.. Check out this pic of this super juvenile potamotrygon sp.
The unusual stenohaline freshwater stingray Potamotrygon sp. has lost most of the characteristics that are typical of marine elasmobranchs (e.g. high osmolality, urea retention, a functional rectal gland) and displays an ionoregulatory physiology superficially similar to that of freshwater teleosts. Potamotrygon thrives in the ion-poor waters of Brazil's Rio Negro, despite the presence of branchial ion transport systems that exhibit surprisingly low affinity for external sodium and chloride and virtual insensitivity to the ionic composition of the acclimation water. However, ionic permeability is very low, and the ion transporters appear to differ at a mechanistic level from those of freshwater teleosts.
scientific link;
http://jeb.biologists.org/cgi/content/short/205/19/3039
*Note - low affinity for external sodium = salt is beneficial for disease control but not detrimental for the osmoregulation of potamotrygon, thus the reasoning for large dosages of salt for health problems.
.. Check out this pic of this super juvenile potamotrygon sp.The unusual stenohaline freshwater stingray Potamotrygon sp. has lost most of the characteristics that are typical of marine elasmobranchs (e.g. high osmolality, urea retention, a functional rectal gland) and displays an ionoregulatory physiology superficially similar to that of freshwater teleosts. Potamotrygon thrives in the ion-poor waters of Brazil's Rio Negro, despite the presence of branchial ion transport systems that exhibit surprisingly low affinity for external sodium and chloride and virtual insensitivity to the ionic composition of the acclimation water. However, ionic permeability is very low, and the ion transporters appear to differ at a mechanistic level from those of freshwater teleosts.
scientific link;
http://jeb.biologists.org/cgi/content/short/205/19/3039
*Note - low affinity for external sodium = salt is beneficial for disease control but not detrimental for the osmoregulation of potamotrygon, thus the reasoning for large dosages of salt for health problems.
