Extensive urine production in euryhaline red stingray for adaptation to hypoosmotic environments.

Maintaining water balance is a prerequisite for all organisms. Euryhaline elasmobranchs face the severest water-influx potential in fresh water (FW), as they retain high concentrations of urea even in hypotonic environments. To elucidate how they overcome this osmotic challenge, we assessed urine output in conscious euryhaline red stingrays (). Following acclimation to 5% diluted seawater, the stingrays increased urinary output by 87-fold-the greatest change observed in vertebrates-partly due to 6.8-fold increase in glomerular filtration rate (GFR). In the nephron, expressions of (, , and were strongly downregulated in FW, indicating that tubular diuresis bridges the gap between GFR and final urine volume. Meanwhile, FW-acclimation upregulated and in the distinct bundle structure, which promotes urea reabsorption. Euryhaline elasmobranchs resolve the huge osmotic challenge of FW by excreting massive amounts of water and retaining osmolytes including urea through coordinated regulation of GFR and expressions.