1. Comparative effects of temperature on the permeability of the gill to water and to sodium were studied in the marine sea perch Serranus and the freshwater goldfish Carassius.

2. The acclimation Q10 for the water fluxes is higher in the freshwater fish than in the marine fish.

3. In the goldfish the osmotic permeability (Pos) is greater than the diffusional permeability (Pdlf) at all acclimation temperatures, suggesting the presence of ‘waterfilled channels’ in the branchial membrane. In the sea perch, on the other hand, Pos/Pdlf is approximately 1, indicating that water movements probably occur by simple diffusion.

4. The permeabilities to water and to sodium are similar in the sea perch but very different in the goldfish. Considering these results together with those from a similar study on an elasmobranch, it would seem that the more perfect the semipermeability of the gill membrane and the weaker the transepithelial osmotic gradient, the greater is the branchial porosity.

5. Assuming that temperature changes do not cause modification of the branchial surface or relative permeabilities to water and to sodium the independence of the temperature-coefficient variations for water and for sodium indicates a certain dissociation between the movements of salt and of water, in the sea perch.

In the goldfish, assuming a constant branchial surface and in view of the fact that Pos > Pdif the high temperature coefficients for the water fluxes suggest that the water in the ‘water-filled channels’ is in a highly organized state.

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