Oceans

The strange ritual migration of 50 million red crabs on Christmas Island

Once a year on Christmas Island something incredible happens: millions of crabs cross the whole island to reach the ocean, where they drop their eggs.

Around fifty million red crabs (Gecarcoidea natalis) are estimated to live on Christmas Island, located in the Indian Ocean off the northwestern coast of Australia, which it is a part of. Between October and December, at the beginning of the wet season, these animals start an incredible journey across the island, leaving their homes in the inland to go to the seaside and lay their eggs.

When does the migration take place?

Female crabs have to drop their eggs into the water (this is known as spawning) “before sunrise on spring tides during the last quarter of the moon”, the Australian government reports. The migration begins when crabs are sure they can complete the journey, mate and then brood the eggs for two weeks before spawning. This year the possible spawning dates are 22-24 November and 21-23 December, during the Austral Spring. Four or five weeks before these dates crabs are expected to begin their migration to the ocean across forests, cliffs and roads.

The stages before new crabs are born

Once they’ve reached the seaside, crabs dip in the ocean to replenish body moisture and salts. Then males dig holes in the sand where the mating occurs, and after that they start their journey back. Females remain inside the burrows and brood the eggs for two weeks before dropping them into the water and leaving the beach too. The eggs hatch into larvae which will soon evolve into baby crabs, whose carapace is only 5 millimetres long. They immediately start their parent’s same journey to the inland in order to find a place to live among the rocks. After four or five years they will become adults and will be ready to mate.

A street sign warns of crabs on road
Christmas Island’s inhabitants take a lot of safety measures to protect crabs during their migration © Parks Australia

Christmas Island’s inhabitants try to protect crabs during their migration

People on the island do everything they can to avoid crabs’ accidental death. So they build bridges and underground tunnels to allow them to cross streets safely. During the peak of migration some roads are closed and traffic is diverted away from where you can actually see a “red carpet” moving on the road surface.

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