The extremely high concentration of urea found in elasmobranch blood (approx. 400 mmol 1–1) is secondary to nearly complete reabsorption (90–95%) of this solute in the renal tubules (Forster, 1967), and a branchial urea permeability which is approximately 1% that of the toad bladder (<10 × 10–8cm s–1, Payan, Goldstein & Forster, 1973). Evans, Oikari, Kormanik & Mansberger (1982) have recently shown that the premature spiny dogfish (Squalus acanthias) pup is capable of osmoregulation in sea water, despite an initial reduction in plasma urea concentration (13% in 3 days). This fall could have been secondary to a reduced synthetic rate, or caused by an increased efflux across surfaces such as the gills, renal and/or rectal gland epithelia, or yolk sac. The present study was therefore undertaken to measure the rate of urea efflux from intact spiny dogfish pups, and to determine the effect of stress on that efflux. Stress was induced by the experimentally common procedure of handling, MS-222 anaesthesia, and weighing.

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