Australian Island Overrun by Millions of Red Crabs in Annual Migration

by Lauren Boisvert
(Photo by James D. Morgan/Getty Images)

On Australia’s Christmas Island, the red crabs are starting their annual migration. Not to worry, this isn’t like the green crabs invading Alaska. Every year, in October or November, millions of red crabs emerge from the forests and head to the water for mating season. There are an estimated 40 to 50 million red crabs on the island, and once a year they flood the streets on their pilgrimage. Often, major roadways will close on the island due to the massive number of traveling crabs. In some places, special crab bridges have been built to allow the crabs to cross over the roadways.

The great crab migration begins with the first rainfall of the wet season on the island. This is usually in October or November, but sometimes can begin in December or January. This year, it seems like the rainy season has come a bit early. Additionally, the speed of the migration is, uniquely, determined by the phases of the moon. According to Parks Australia, the crabs always spawn “before dawn on a receding high-tide during the last quarter of the moon.” Strangely, they always seem to know exactly when to leave to make it to the water by this time.

Since the migration is determined by the moon and the rain, sometimes the crabs have to wait until the next month. If the rains come early, the crabs take their time moseying along to the ocean. If the rain was late, however, they have to hurry to get to the water to spawn. Sometimes, they miss the optimal spawning time during the first month, and must wait until the next month to migrate.

Red Crabs Fill the Streets of Australia’s Christmas Island During Annual Migration

The male crabs reach the beach first, and are later joined by the females. The crabs take a dip in the water to replenish their parched bodies from their long trek, and then the males head to the lower terraces of the island to dig burrows. Burrows are often close together, and sometimes fights break out among the male crabs. The females join the males in the burrows, where they will mate, and then the male crabs take one last replenishing dip in the water before making the journey back to the plateau and forests.

The female crabs stay behind, gathering on the beach to produce their eggs. Each female can produce up to 100,000 eggs, held in a brood pouch. Then, “when the moon reaches its last quarter,” writes Parks Australia, the female crabs release their eggs into the water before heading back to the forests. The larvae hatch from the eggs as soon as they hit the water, creating great clouds of baby crabs in the ocean.

The babies measure 5 millimeters across at first, and stay in rocky pools for the first two or three days. Then, they make their own migration back to the plateau. They will hide in rocky outcroppings and debris in the forest for the first three years of their lives.

Of course, not all the baby crabs make it out of the water. Fish, manta rays, and whale sharks take advantage of the spawning season and feast on the crabs.