Evidence for a polyspermy block at the level of sperm-egg plasma membrane fusion in Urechis caupo.

The results of sperm binding experiments reveal no change in the sperm binding properties of the egg surface coat at fertilization of Urechis caupo eggs. When fertilized eggs are reinseminated, sperm continue to attach to the egg surface coat. The acrosomal tubules of supernumerary sperm are observed in the perivitelline space closely apposed to the egg membrane. Thus, the polyspermy block in Urechis eggs involves neither alteration of sperm binding sites nor inhibition of the acrosome reaction. Our results suggest that the block is at the level of sperm-egg membrane fusion.