These red crabs make mass migrations on Christmas Island, in the Indian Ocean. It’s one of earth’s most spectacular sights, says Sir David Attenborough. About a hundred million red crabs live on the island— and only on this island.
A Long Journey to Reproduce
Like many animals that migrate, the reason for reproduction. When the first rains begin on the island, the crabs begin their journey out of the forest, where they live in burrows during the worst of the dry season. “Red crabs all over the island leave their homes at the same time and start marching towards the ocean to mate and spawn. Male crabs lead the migration and are joined by females along the way.” The trip can be as long as five miles one-way. That’s a long way for a small 4.5-inch-wide crab!
When they reach the shore, each male digs a burrow for the best spot to beat the competition. The females arrive later, and the mating begins. After mating the males start the trek back to their forest homes. The females remain in the moist burrows where their fertilized eggs start to develop, in about three days. They stay for approximately two more weeks until their eggs become larvae ready to hatch.
Females Read the Tides
The female crabs always spawn (release their eggs) on a receding high tide during the moon’s last quarter. Carrying as many as 100,000 developing eggs, they leave their burrows at night to deposit them in the sea’s high waters. Timing is everything for the female crabs. They need to deposit the eggs when the tide is high enough to carry the eggs out to sea, but still allow the females to get safely back on dry land. Once back on shore, the female crabs head back to the forest. The spawning event may last five to six days.
A Tough Road for Tiny Crabs
The crab larvae hatch from the eggs almost immediately upon being released into the ocean. Then, like all marine crabs, they go through a series of larval stages. The larvae spend three to four weeks changing and growing into miniature crabs near the shore. Most don’t make it back to the island. The millions of larvae and tiny crabs are a feast to many marine animals like whale sharks and manta rays that gather near the island each year.
Sometimes the conditions are just right for millions of the baby crabs to make it to shore and head back to the forest. Christmas Island is populated with people so this is a perilous journey that can last a week. (watch this video). Enough crabs make it back to the forest over time to maintain the island’s crab population.
Watch “Spy crab” in the middle of the migration.
