The female digs a pit near the high water mark and lays about 200-300 eggs. The males release sperm over the eggs and the nest is covered. They may come back again at the next high tide and a female may lay a total of 2,000-30,000 eggs. Eggs hatch at the next full moon when the tide is at its highest again. The larvae burrow into the sand and after a few moults begins to develop its tail.
Humans are the main threat to horseshoes now.
Habitat loss and overharvesting have seriously depleted horseshoe populations. The shallows that they need are easily polluted. Because their eyes are large and they have a large optic nerve. Horseshoe crab chitin is very pure, unlike other arthropods. So horseshoe crab chitin is processed to produce chitosan used in contact lenses, skin creams, and hair sprays.