Fertilization of sea urchins needs magnesium ions in seawater.

When sea urchin eggs were inseminated in seawater free of magnesium, the fertilization rate was very low. Spermatozoa that had been treated with egg jelly to induce the acrosome reaction also failed to fertilize eggs in seawater free of magnesium. These results indicate that magnesium is indispensable for some process or processes at fertilization, such as membrane, fusion or sperm penetration.