Osmoregulation in the euryhaline elasmobranch, Dasyatis sabina

1. 1. Changes in plasma solutes associated with variation in environmental salinity were examined in the euryhaline elasmobrach, Dasyatis sabina. 2. 2. Body weight increases and hematocrit decreases when stingrays are transferred to dilute sea water, but return to normal within 6 days suggesting that body water balance and blood volume are regualted at a constant level. 3. 3. Stingrays consistently regulate plasma osmotic pressure above that of the external medium. Plasma osmolarity does, however, decrease in dilute sea water; osmotic pressure of the plasma is not directly proportional to the osmolarity of the external medium. 4. 4. Urea contributes 38–40 per cent and sodium 29–30 per cent to plasma osmotic pressure between 35 and 100% sea water. 5. 5. Plasma NaCl concentration in Dasyatis are reduced to a greater extent by environmental dilution than are urea levels until the external medium osmolarity falls below 350 mOsmole/l.