Adaptive responses of aglomerular toadfish to dilute sea water

Plasma and urine of toadfish (Opsanus tau) in sea water and 10% sea water were analyzed to assess responses of an aglomerular fish to hypoosmotic challenge. Following transfer to 10% sea water, plasma osmotic pressure decreased slowly from 318 to 241 mmol . kg H2O-1, over a period of 10-15 days. Urine osmotic pressure decreased in parallel from 299 to 207 mmol . kg H2O-1, leaving urine/plasma ratios of osmotic pressure essentially unchanged. In contrast, the volume and composition of urine changed rapidly following transfer to 10% sea water. Urine flow rate increased 110% from 3.0 to 6.3 mu l . 100g(-1). h(-1) and Na+ excretion increased 346%, while excretion of Mg2- and SO42- decreased 81% and 90%, respectively. Excretion rates for Cl- were low in seawater toadfish and decreased further in 10% sea water. An unknown sulfur-containing anion, present in the urine of seawater toadfish, contributed significantly 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 excrete water, in hypoosmotic media. The decrease in plasma osmotic pressure may be both a strategy to reduce osmotic and ionic gradients in dilute media and a consequence of the kidney's inability to excrete water without salt.