Formation of the block to polyspermy in ascidian eggs: Time course, ion requirements, and role of the accessory cells

Abstract The eggs of Ascidia nigra become incapable of fertilization by a second sperm about 21 seconds after fertilization by the initial sperm. Removal of the follicle cells slows the rate and lowers the percentage of fertilization but has no influence on the block to polyspermy. Similar numbers of sperm are found in the perivitelline spaces of polyspermic and monospermic eggs subjected to high sperm concentrations, suggesting that the block to polyspermy is at the egg surface. Like other polyspermy blocks in marine invertebrates, that of ascidians requires Na+ for formation.