ADAPTIVE RESPONSES OF AGLOMERULAR TOADFISH TO DILUTE SEA-WATER

Plasma and urine of toadfish (Opsanus tau) in sea water and 10% sea wa ter were analyzed to assess responses of an aglomerular fish to hypoos motic challenge. Following transfer to 10% sea water, plasma osmotic p ressure decreased slowly from 318 to 241 mmol . kg H2O-1, over a perio d of 10-15 days. Urine osmotic pressure decreased in parallel from 299 to 207 mmol . kg H2O-1, leaving urine/plasma ratios of osmotic pressu re essentially unchanged. In contrast, the volume and composition of u rine changed rapidly following transfer to 10% sea water. Urine flow r ate increased 110% from 3.0 to 6.3 mu l . 100g(-1). h(-1) and Na+ excr etion increased 346%, while excretion of Mg2- and SO42- decreased 81% and 90%, respectively. Excretion rates for Cl- were low in seawater to adfish and decreased further in 10% sea water. An unknown sulfur-conta ining anion, present in the urine of seawater toadfish, contributed si gnificantly to the composition and ionic balance in urine of toadfish in 10% sea water. These results suggest that the inability to produce strongly dilute urine obliges toadfish to lose salt in order to excret e water, in hypoosmotic media. The decrease in plasma osmotic pressure may be both a strategy to reduce osmotic and ionic gradients in dilut e media and a consequence of the kidney's inability to excrete water w ithout salt.