The exchangeable ionic space,and salinity effects upon ion,water, and urea turnover rates in the dogfish Poroderma africanum

Using radio-isotopes, the turnover rates of sodium, chloride, water, and urea were measured, at different salinities, in the pyjama shark, Poroderma africanum (Gmelin). The exchangeable ionic space was also measured. Sodium turnover was 0.24%/h in normal sea water (467 mM Na/l, 550 mM Cl/l, 1020 m0sm/l), and under similar conditions, chloride, water and urea showed turnover rates of 2.47, 97, and 0.08%/h, respectively. Chloride and water turnover showed maximal values in normal sea water, and declined with variation of the medium away from this salinity, the decline in chloride turnover rate being more marked. Exchangeable ionic space was calculated, using Chloride-36, and was found to be 34.4% body volume or 32.4 ml/100 g fresh weight. Present results show that chloride turnover is about 10 times that of sodium; a relationship previously observed in other elasmobranchs. Water turnover rates are similar to those of other elasmobranch species, although urea turnover is somewhat lower. Salinity has a pronounced effect upon chloride and urea turnover rates.