Searching for the Sea

A red tide flows from the forest to the sea every fall on Christmas Island in the Pacific Ocean. That’s when millions of Christmas Island red crabs migrate. They skitter to the sea to release their eggs into the ocean. How do they know what to do?

Their trek is triggered by the rainfall. During the island’s wet season, beginning in October, the crabs abandon their rain forest burrows. The timing of their migration is also linked to the phases of the moon.

Males travel first, in great numbers. They are undeterred by obstacles. They frequently cross island roads, essentially shutting down traffic for any “migrating” humans.

The trip usually takes about a week. Once oceanside, the males dig deep burrows and wait for the females to arrive. Mating takes place in or near the burrows. The males return to the forest. Precisely at the turn of the high tide during the last quarter of the moon, the females release their eggs into the ocean. Then they, too, return to their homes—driven by the same forces that brought them to the sea. The young grow up in the sea and come to land as adults.