| Summary: | 1. The fluxes of 3H 2 O, 22Na and 36Cl were simultaneously measured in yolk-sac larvae of cod ( Gadus morhua L.) in 34 sea water at 4.5°C. 2. The rates of turnover of all three isotopes were higher than in adult fish. Diffusional permeability coefficients, which relate ion fluxes to surface area, were however lower, indicating that larvae are less permeable than adults. Furthermore, there is close agreement between the diffusional and osmotic permeability coefficients, which supports a previous hypothesis that relatively low drinking rates in marine fish larvae are a consequence of low integumental permeability. 3. Estimates of the sodium and chloride concentrations derived from the equilibrium levels of 22Na, 36Cl and 3H 2 O indicate that yolk-sac larvae of cod regulate their body fluids hypotonic to sea water. Also, the ionic concentrations of the tissues of yolk-sac cod larvae are similar to those of adults.
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