Horseshoe Crab

Breeding
During the breeding season, horseshoe crabs migrate to shallow coastal waters. Males select a female and cling onto her back. The female digs a hole in the sand and lays her eggs while the male fertilizes them. The female can lay between 60,000–120,000 eggs in batches of a few thousand at a time. Many shore birds eat the eggs before they hatch. The eggs take about 2 weeks to hatch. The larvae molt six times during the first year.
It has proven to be difficult to raise horseshoe crabs in captivity. There is reason to believe that mating only takes place in the presence of the sand or mud in which the horseshoe crab eggs were hatched. It is not known with certainty what in the sand is being sensed by the crabs nor how they sense it.