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Effect of low environmental salinity on plasma composition and renal function of the Atlantic stingray, a euryhaline elasmobranch

¹Grice Marine Laboratory, College of Charleston, ²Marine Biomedicine and Environmental Science Center, ³Medicine, ⁵Anatomy and Cell Biology, and ⁶Cell and Molecular Pharmacology and Experimental Therapeutics, Medical University of South Carolina, and ⁴Ralph H. Johnson Veterans Affairs Medical Center, Charleston, South Carolina

Submitted 25 January 2006 ; accepted in final form 20 March 2006

Marine elasmobranchs maintain internal osmolality higher than their external environment, resulting in an osmotic gradient for branchial water uptake. This gradient is markedly increased in low-salinity habitats. The subsequent increase in water uptake presents a challenge to volume homeostasis. The Atlantic stingray is a marine elasmobranch that inhabits a remarkable range of environmental salinities. We hypothesized that the ability of these stingrays to regulate fluid volume in low-salinity environments is due primarily to a renal glomerular and tubular functional reserve. We tested this hypothesis by measuring renal excretory function after a rapid and sustained 50% reduction in the osmolality of the external medium. Atlantic stingrays were maintained in harbor water [control salinity (CS) 850 mosmol/kgH₂O] for 1 wk. Rays were then either transferred to diluted harbor water [low salinity (LS) 440 mosmol/kgH₂O] or maintained in CS for a further 24 h. Renal excretory function was markedly higher in the rays subjected to low salinity. Glomerular filtration rate was threefold higher and urine flow rate ninefold higher in the LS group. The clearance of solute-free water was greater, and solute-free water comprised a significantly larger proportion of the urine output for the stingrays transferred to dilute harbor water. We conclude that 1) the kidneys of Atlantic stingrays have a remarkable glomerular and tubular functional reserve, and 2) the marked increase in renal function attenuates the increase in fluid volume when these fish move into low-salinity habitats.

free water clearance; glomerular filtration rate; tubular urea reabsorption

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