Perivitelline and Vitelline Potentials in Teleost Eggs as Influenced by Ambient Ionic Strength, Natal Salinity, and Electrode Electrolyte; and the Influence of these Potentials on Cadmium Dynamics within the Egg

The differences in perivitelline potential (pvp) recorded from eggs, in dilute ambient salt solutions, when recorded from NaCl- or KCl-filled pipettes are considered to be due mainly to differences in diffusion potentials generated within pipettes. Experiments with Evan's blue (a large, impermeable anion) suggest that anion permeability does not influence the pvp response of freshwater eggs to changes in ambient ionic strength. The pvp of the cod (Gadus morhua) egg became more positive as the ambient medium was diluted rather than more negative, as occurs with eggs of freshwater species. The cod pvp response is attributable to diffusion potentials generated by movement of salt across the chorion. The Atlantic salmon (Salmo salar) perivitelline fluid (pvf) concentrates Cd2+in accordance with the levels predicted by the magnitude of the pvp. The slow rate of pvf Cd2+accumulation is attributed to the presence of two large sinks for Cd2+adsorption bounding the pvf: the embryo and the chorion. The potential difference between the yolk and pvf (vitelline potential) is much less responsive to ambient pH than is the pvp, a probable reflection of the low permeability of the periblast. The above results indicate that pvp may be used to predict equilibrium cation concentration differences between pvf and the ambient medium for freshwater eggs. In marine eggs, the influence of perivitelline colloids on the pvp is negated by the high perivitelline salinity (essentially in equilibrium with ambient salinity).