The African Lungfish (Protopterus dolloi): Ionoregulation and Osmoregulation in a Fish out of Water

Abstract Although urea production and metabolism in lungfish have been thoroughly studied, we have little knowledge of how internal osmotic and electrolyte balance are controlled during estivation or in water. We tested the hypothesis that, compared with the body surface of teleosts, the slender African lungfish (Protopterus dolloi) body surface was relatively impermeable to water, Na+, and Cl− due to its greatly reduced gills. Accordingly, we measured the tritiated water (3H‐H2O) flux in P. dolloi in water and during air exposure. In water, 3H‐H2O efflux was comparable with the lowest measurements reported in freshwater teleosts, with a rate constant (K) of 17.6% body water h−1. Unidirectional ion fluxes, measured using 22Na+ and 36Cl−, indicated that Na+ and Cl− influx was more than 90% lower than values reported in most freshwater teleosts. During air exposure, a cocoon formed within 1 wk that completely covered the dorsolateral body surface. However, there were no disturbances to blood osmotic or ion ...