Sodium and Chloride Regulation in Uca Adapted to 10% Sea Water

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Abstract
Aspects of Na⁺ and Cl⁻ regulation in fiddler crabs adapted to 10% sea water were examined. Total body turnover rate averaged 8% h⁻¹ for Na⁺ and 19% h⁻¹ for Cl⁻. Urine was produced at the rate of 5% body weight day⁻¹. Based on the assumption that blood and urine were isotonic, the renal route represented 3%-4% of total efflux of both ions. The intestinal route provided a negligible influx of a salt as the rate of drinking was 0.6 μl g⁻¹h⁻¹. The transepithelial potential (TEP) across the crabs' body surface averaged +4.7 ± 0.7 mV. In the absence of Na⁺ the TEP -7.5 ± 1.3 mV; in the absence of Cl⁺ it was +8.9 ± 0.5 mV. Deletion of Na⁺ from the bathing medium caused little change in efflux of this ion. Deletion of Cl⁻ from the medium caused approximately 50% reduction in Cl⁻ efflux. However, Na⁺ efflux was also reduced in Cl-free medium. Flux ratio analysis indicated that both Na⁻ and Cl⁻ were actively absorbed by fiddler crabs in dilute sea water. Possible changes in osmotic and salt permeability are discussed in relation to the salinity of the environment.

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