ABSTRACT
A substantial part of our understanding of early animal development comes from studies of sea urchin eggs and embryos. To date, there have been no reports of sea urchin eggs having steady ionic currents, similar to those reported for plant and other animal embryos, where the currents have been shown to have developmental significance (Jaffe, 1981). The lack of such reports is probably because the sea urchins studied in North America and Europe have small eggs in the range, 40–100µ m, that would make the measurement of steady ionic currents difficult. An australian sea urchin, Holopneustes purpurescens, is a direct developing sea urchin with eggs approximately 400µ m in diameter. Their especially large size has opened the way for a study of steady ionic currents in sea urchins. Steady ionic currents were measured around these eggs using the vibrating probe apparatus (Jaffe and Nuccitelli, 1974). Steady ionic currents up to 2.4µ A/cm2 were measured around the unfertilized eggs of H. purpurescens. The patterns of currents tend to be grouped as areas of inward or outward currents. The patterns are not mosaics. The data fits a model in which current enters half the egg and leaves the other half. The current patterns appear not to be associated with the area of gray pigmentation of the eggs or with the orientation of the egg with respect to gravity. The ability to measure steady ionic currents is limited by a low signal to noise ratio caused by the high conductivity of seawater. Ours is the first report of the measurement of steady ionic currents around a marine animal.
