Sperm-engulfing response of sea urchin egg surfaces inseminated with acrosome-reacted starfish sperm

Sperm-egg interaction was examined in two interclass combinations of sea urchin (Strongylocentrotus nudus and Hemicentrotus pulcherrimus) eggs and starfish (Asterina pectinifera and Asterias amurensis) sperm. Cross-fertilization was unsuccessful between these combinations. When the vitelline coat-free sea urchin eggs were mixed with acrosome-reacted starfish sperm, the elongated microvilli on the egg surface wrapped the sperm head. This sperm-engulfing response observed on the denuded egg surface was induced only in sperm immediately after initiation of the acrosome reaction. Further fertilization events, such as gamete membrane fusion or discharge of cortical granules, did not proceed. These observations suggest that acrosome-reacted sperm can induce a local response on the heterologous egg surface, that is independent of gamete membrane fusion and egg activation.