Phagocytosis of sperm heads lacking the acrosomal process by unfertilized starfish oocytes

In sperm of the starfish Asterina pectinifera, the acrosomal process and the flagellum were mechanically separated from the sperm head with a disperser. The sperm head fraction was then used to examine the direct interaction between the sperm head and the egg surface. Sperm heads lacking the acrosomal process and the flagellum did not fertilize oocytes, even after removal of the vitelline coat. Transmission electron microscopy showed that each denuded oocyte engulfed the sperm head without gamete membrane fusion. The sperm‐engulfing response, similar to phagocytosis, was induced without the mediation of the acrosomal process. The present results suggest that the process of sperm incorporation consists of two independent events, acrosomal process‐egg surface fusion and the phagocytotic movement of the egg surface.