Effects of environmental dilution on body fluid regulation in the yellow stingray, Urolophus jamaicensis

Adult yellow stingrays ( Urolophus jamaicensis), collected off the southeast Florida coast, were maintained in filtered and re-circulated synthetic sea water (33‰) for 5–13 days at 30°C. Animals exposed to 82%, 74% and 66% SW in gradual steps exhibited rapid and significant weight gains followed by recovery to pre-dilution levels in 2–6 days. Acclimated animals at each salinity [100% ( N=12), 82% (7), 74% (4) and 66% SW (3)] were anesthetized (MS222) and bled from the caudal vein. In 100% SW, stingray plasma was slightly hypo-osmotic to the external medium. Plasma osmolality decreased with stepwise dilutions, but became increasingly hyperosmotic to the bathing media. Plasma [Na] and [Cl] each decreased by approximately 13%, 23% and 16%, respectively, in 82%, 74% and 66% SW. Plasma [urea] decreased by 21%, 25% and 59%, respectively. Changes in plasma [K] and [Ca] were minor. Mean corpuscular [Hb] measurements suggest that stingray red cells swelled less at each dilution than predicted for a passive erythrocyte osmometer. RBC [K] decreased by 12%, 36% and 29%, respectively, in 82%, 74% and 66% SW. Quantitatively, the other measured electrolytes (Cl, Na and Ca) changed by lesser amounts. Results suggest that for mild and moderate dilutions (82% and 74% SW), yellow stingrays release both ions and urea from intracellular and extracellular compartments. With further dilution (66% SW), the elasmobranchs retain electrolytes at the expense of urea.