Fluctuations in two osmoregulatory components, urea and sodium chloride, of the clearnose skate, Rafa eglanteria bosc 1802—II. Upon natural variation of the salinity of the external medium

Abstract

1. 1. Clearnose skates, Raja eglanteria, were captured from water in which salinity ranged from 18.0 to 33.2% and temperature extremes were 19.0 and 27.0°C. 2. 2. Although serum chloride values ranged from 216 to 284 mmoles/l, no definite depression of chloride values related to dilution of seawater could be demonstrated. 3. 3. Urea values varied from 316 to 427 mmoles/l and were depressed at lower salinities although urea values and the resultant serum osmotic pressures were higher at low salinities than expected. 4. 4. It is suggested that at low salinities, the relatively high serum osmotic pressure may increase hydration and glomerular filtration beyond the capacity of the kidney tubules to effectively absorb certain significant metabolites and therefore prevent this skate's successful habitation of salinities less than 18.0%o.