Release of acid and changes in light-scattering properties following fertilization of Urechis caupo eggs.

The release of a fertilization acid, monitored by measuring the pH of egg suspensions, begins within 10 sec of insemination of Urechis caupo eggs. This is 4 min before the vitelline layer begins to elevate and is apparently unrelated to that process. The eggs of two molluscs, Mytilus californianus and Acmaea incessa, do not form a fertilization acid. The acid of Urechis eggs is not accompanied by release of “fertilization” carbohydrate, sulfate, or a nonvolatile weak acid into the seawater. The light-scattering properties of Urechis eggs change during the first 10 min after insemination. A decrease in light scattering begins by 10 sec and is complete by 1 min (Phase I). This is followed by a further decrease (3–6 min, Phase II) and an increase (6–10 min, Phase III). In striking contrast to an overtly similar situation in sea urchin eggs (fertilization acid and coincident light-scattering decrease), the release of acid and the initial light-scattering change are not the result of cortical granule discharge, and the acid, at least, is not related to the changes in shape or surface area which the eggs undergo. The processes underlying these rapid events are not yet known.