The crab migration on Christmas Island

Follow the journey of millions of crabs as they make their way to the beach for an annual get-together

Image credit: Chook keeper

As the rainy season descends upon the beaches of Christmas Island in the Indian Ocean, resident red crabs realise that it’s time to emerge from their forest burrows and begin a mass-migration to the beach. Flooding the island’s roads and paths, a cascade of crimson crustaceans march in their millions to congregate on the shore to mate.

Typically occurring in October or November, the phases of the migration are tied to the phases of the moon. For a successful spawning, females must drop their eggs before the high tides on the last quarter of the moon (one week after a full moon). This is believed to be because of the small difference between the shoreline’s low and high tide.

Leading the six-legged army are male red crabs, with the females not far behind. Arriving at the shoreline, males take a quick dip in the waters to rehydrate their tired shells, then move back to the land to dig their breeding burrows.

Once couples have met and mated, the female holds 100,000 eggs in a brood pouch on the underside of her body. Hauling her offspring out of the beach burrow and into the water, each female crab offloads its offspring over a period of five or six nights, before returning to the forests.

Over the next month, larvae will develop and hatch as shrimp-like megalopae, before growing into small, soft-shelled crabs in one or two days. Only five millimetres wide, many of these tiny crabs will not make the journey to the forest, thanks to the many ocean predators waiting in the waves. Those that survive the predators emerge in their millions on the island’s beaches and make a nine-day journey into the forests.

This article was originally published in How It Works issue 132

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