Adaptive Changes of the Water Permeability of the Teleostean Gill Epithelium in Relation to External Salinity

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Abstract
Cannulation of afferent and efferent branchial vessels in the eel permitted studies of tritiated water clearance. It was observed that most of the diffusional water flow occurs through the gills. Diffusional and osmotic water flows have been measured in a fresh-water (Carassius), a marine (Serranas) stenohaline fish and in two euryhaline species (Platichthys and Anguilla) adapted to either fresh water or sea water, and are found to be lower than in any comparable epithelia so far studied. The diffusional water flow deduced from THO turnover is significantly smaller in the sea-water fish. The osmotic water flow, determined indirectly by measuring drinking rate and urine flow, is smaller in the sea-water fishes despite a greater osmotic gradient across the gills. Attempts to compare diffusional and osmotic permeabilities for the gill are hindered by our ignorance of the extent of solute (salt)-solvent interaction in the epithelium. It is suggested that the gill of the fresh-water-adapted fishes is semipermeable, while that of the sea-water teleosts may not be, because of the very high ionic exchange across the gill. The surprisingly low diffusional and osmotic permeabilities of the gill epithelium in sea-water fish may be possibly related to the absence of water-filled pores.