Studies on the permeability to water of selected marine, freshwater and euryhaline teleosts.

Recent studies of the fluxes of sodium and chloride across euryhaline teleost fish have shown that acclimation to fresh water involves a sharp reduction in the flux of both ions (Evans, 1967 c, 1969; Motais, 1967; Maetz et al. 1967 a, b; Potts & Evans, 1967; Ports et al. 1967). It has been shown, at least in Fundulus heteroclitus (Potts & Evans, 1967), Tilapia mossambica (Potts et al. 1967), Xiphister atropurpureus (Evans, 1967 c) and Pholis gunnellus (Evans, 1969), that part of the reduction of the sodium flux is due to a change in the body's permeability to this ion. The permeability to chloride also declines in the latter two species, but to a lesser degree. On the other hand, it appears that the flux of tritiated water either does not change with salinity (Evans, 1967 c, 1969) or actually increases when the fish is acclimated to fresh water (Potts et al. 1967). Since it can be calculated (Evans, 1967c) that drinking and urine flow amount to only 1 % of the gross flux of tritiated water, these data indicate that in euryhaline teleosts the permeability changes to sodium, chloride and water do not parallel each other. Since the body fluid concentration of fish is approximately 350 m-osmoles/1. while the concentration of sea water and fresh water is approximately 1000 and 1 mosmole/1. respectively, in sea water there is a net loss of water from the fish and in fresh water there is a net gain of water. If the osmotic flow occurs entirely by diffusion, the net flux (computed from the gross flux of tritiated water) will be a function of the diffusion permeability (Pd) and will be equal to the osmotic permeability (P^: e.g Gutknecht, 1967). In the case of teleosts, the net flux (Pd) in any salinity can be calculated if the gross flux of tritiated water and the relative mole fractions of water in the fish and the various media are known. According to the accepted model for teleost osmoregulation the drinking rate in sea water and the urine flow in fresh water are functions of the osmotic permeability or PM. If water is moving by simple diffusion across the permeable surfaces of the fish then the drinking rate should equal the calculated net flux outward in sea water and the urine flow should equal the net influx in fresh water. The rate of ingestion of the medium in sea water will in fact be somewhat higher than the calculated efflux because renal loss of water will add to the passive loss of water. In addition, the urine flow in fresh water will also be somewhat higher than the calculated net influx because of some drinking of the medium (Evans, 19676;