Adaptive changes of the water permeability of the teleostean gill epithelium in relation to external salinity.

Fresh-water teleosts, whose blood is hypertonic to the external medium (about 330 m-osmoles/kg. vs. a few m-osmoles/kg.), face the problem of osmotic entry of water and loss of electrolytes, while sea-water teleosts live in an environment (1150 m-osmoles/kg.) which induces osmotic loss of water and a diffusional influx of electrolytes. Many data are available on the electrolyte balance of fish (see reviews by Smith, 1932; Krogh, 1939; Parry, 1966; Motais, 1967; Maetz, 1968; Potts, 1968). Much less work has been devoted to water equilibrium since Smith (1932) suggested the contrasting ways by which this equilibrium is achieved in fresh-water and sea-water teleosts. The fresh-water fish balances the net influx of water through the gill by excreting large quantities of hypotonic urine, the drinking rate being considered as negligible, while the sea-water fish balances the gill outflux by drinking sea water, absorbing monovalent ions together with water in the gut and simultaneously excreting excess ions through the gill. While urine flow rates have been measured in numerous fishes including euryhaline fishes in low and high external salinity (see reviews by Potts, 1968 or Maetz, 1968), drinking rate studies have been resumed only recently (Motais & Maetz, 1965; Evans, 1967a, 1968, 1969; Potts & Evans, 1967; Potts et al. 1967; Hickman, 1968; Maetz & Skadhauge, 1968; Oide & Utida, 1968; Lahlou, Henderson & Sawyer, 1969; Lahlou & Sawyer, 19696). The surprising observation is that the drinking habit occurs also in fresh-water fish. The quantification of the components of the Smith model requires measurements of urine flow and drinking rate in the same species, and the assumption that the difference between them balances the osmotic water flow (positive in fresh water, negative in sea water) across the permeable boundary of the fish. Therefore the osmotic permeability of this boundary can only be determined indirectly. The present report is an attempt to evaluate this permeability for a few fresh-water, marine and euryhaline teleosts. The euryhaline fishes have been studied in both environments in order to measure the permeability of the same membranes when the water flow occurs in opposite directions. More recently a few investigators have also studied the diffusional