Millions of Red Crabs Flood Christmas Island in a Fiery March to the Sea
Every year on a small Australian territory in the Indian Ocean, the forest floor turns scarlet and starts to move. Tens of millions of Christmas Island red crabs pour out of the rainforest and head for the coast to breed. Local radio tracks road closures. Bridges lift crabs over traffic. The island becomes a living river of claws and legs, as seen in coverage from BBC News.
The red crab migration on Christmas Island involves more than 100 million crabs.
The First Switch: Rain
The migration’s opening act is water from the sky. The first heavy rains of the wet season break the long dry, softening leaf litter and cooling ground. That downpour triggers a synchronized departure from burrows across the island, with males leading and females following, according to Christmas Island National Park. In some years the rains arrive late; then many crabs wait and migrate the following month to keep their young on the best timetable, park rangers note via the national park guidance.
The Second Switch: The Moon and the Tide
Rain starts the march. The moon sets the date. Females only release eggs before dawn on a receding high tide during the last quarter of the lunar cycle — timing that helps sweep larvae offshore with the same cadence, Discover Wildlife reports. For 2025, park managers forecast major spawning around November 15–16, with a second wave in mid-December, per Christmas Island National Park.
Rain triggers the start of the journey from forest to shore.
A March Shaped by Weather — And Climate Signals
This year’s migration began a little earlier than usual, linked to a negative Indian Ocean Dipole and warmer-than-usual waters that prime the region for rain, The Guardian reports.
“We haven’t won the war” against threats, Brendon Tiernan, who helps oversee threatened species programs on the island, told the Guardian.
Bio-control has “had a significant impact” after invasive yellow crazy ants killed vast numbers in the 2000s and early 2010s. Recovery years for baby crabs can be “bumper,” though survival at sea is at the mercy of currents and predators, the Guardian notes.
The event occurs annually between October and December.
On the Ground: Burrows, Bridges, and Leaf Blowers
Once at the coast, males dig dense networks of burrows. Females arrive, mate, and incubate as they wait on the lunar window. Each can carry up to 100,000 eggs, the park service explains. Residents help shepherd the throng: rangers rake roads; special overpasses funnel traffic; and when fingernail-sized juveniles return a month later, staff switch to leaf blowers to spare the tiny crabs, acting park manager Alexia Jankowski told ABC News (AP).
“Some people might think they’re a nuisance, but most of us think they’re a bit of a privilege,” she said.
Why So Many? And Why It Matters
With few native predators and abundant forest food, red crab numbers can top 100–180 million in strong years, according to The Guardian. Their synchronized journey is not just a spectacle; it is a keystone process that fertilizes forests, feeds marine giants during spawning, and binds a community that has built an island around the rhythm of rain and the pull of the moon.
