Polyspermy block of Spisula eggs is prevented by cytochalasin B.

The eggs of the surf clam Spisula solidissima have a built-in mechanism that prevents polyspermy: the eggs show a 70 percent decrease in sperm receptivity 5 seconds after fertilization, and become completely resistant to sperm by 15 seconds. When the outer egg coat (vitelline layer) was removed, there was no change in fertilizability or the timing of the block to polyspermy. This suggests that the alteration occurs in or at the plasma membrane. Such changes in the egg surface were sensitive to low concentrations of cytochalasin B.