SERUM COMPOSITION OF FRESHWATER STRINGRAYS (POTAMOTRYGONIDAE) ADAPTED TO FRESH AND DILUTE SEA WATER

Abstract

1. Juvenile freshwater stingrays of the family Potamotrygonidae are unable to survive in salinities in excess of 20.6‰ when gradually acclimated. 2. No differences were observed in blood pH or hematological parameters when fish adapted to a salinity of 14.5‰ were compared with freshwater controls. 3. Significant increases were found in serum sodium (21%), chloride (21%), calcium (48%), and magnesium (51%). Increases in total CO2 (16%), potassium (21%), and inorganic phosphorus (35%) were not significant on account of high variance. Serum osmolarity increased 69%. 4. There was no apparent effect of salinity on serum total cholesterol, organically bound phosphorus, or total carbohydrates. Glucose contributed only 25% of the latter. 5. Serum urea was low (1.1 mM/1) as previously reported, and the trend to increase in a saline environment was not osmotically significant. 6. Freshwater stingrays are unique among elasmobranchs in possessing significant amounts of a protein with the electrophoretic mobility of human serum albumin. There was no significant change in this fraction or in serum total protein in fish adapted to a saline medium.