The migration begins with the first substantial rains, usually in October or November, when tens of millions of crabs leave their burrows and move toward the coast. Males arrive first, dig burrows near the shore, and mate there; females then remain in the burrows for around twelve days while the eggs develop, held in a brood pouch beneath the abdomen.
Spawning is precisely timed. Females release their eggs into the sea before dawn on a receding high tide during the last quarter of the moon, when the difference between high and low water is smallest and the sea at the shoreline is least violent. The eggs hatch on contact with seawater, and the larvae spend around a month in the ocean before surviving juveniles emerge and walk inland.
Outcomes vary enormously between years. Larval survival can be near total failure for several consecutive seasons and then produce a mass recruitment. Meanwhile the introduced yellow crazy ant has killed large numbers of adult crabs, prompting a long-running biological control programme, and roads on the island are closed or bridged during the migration.