50 million crabs marching to the sea in synchrony. Their timing is set by the moon; rainfall. And an internal clock scientists still don't fully understand.
A Crimson Tide on Land
Each year, the floor of the Christmas Island forests begins to move. The trigger is the first rainfall of the wet season, typically in October or November. This moisture allows the island's residents, the Christmas Island red crabs (Gecarcoidea natalis), to begin their long trek to the sea. For most of the year, these land crabs live solitary lives in individual burrows dug into the forest soil. The wet season awakens a collective impulse, and millions of crabs emerge simultaneously to start their migration.
The males lead the migration, a trek that can take a week or more. Females join the procession along the way. The sheer density of the crimson river of crabs forces the closure of many of the island's roads. To aid the migration, park staff have constructed special "crab bridges" and underpasses, allowing the animals to cross roads safely. Upon reaching the coast, the males excavate and defend mating burrows. After mating, the males begin their return trip to the forest, leaving the females behind.
Spawning by the Moon
The females retreat into the burrows, where they remain for about two weeks. During this time, each female develops up to 100,000 eggs, which she holds in a brood pouch on her abdomen. The final stage of this process is governed by the moon. The crabs' spawning is synchronized with the lunar cycle, always occurring just before dawn on a receding high tide during the last quarter of the moon.
On the predicted nights, the egg-laden females gather at the water's edge, sometimes reaching densities of 100 individuals per square meter. They move into the sea to release their eggs, which hatch the instant they contact the salt water. The resulting larvae are swept out to sea, where they spend three to four weeks developing through several stages. They eventually transform into prawn-like animals called megalopae. These gather near the shore for a day or two before becoming fully-formed crabs, only 5 millimeters across. These tiny crabs then begin their own nine-day march inland, hiding in forest debris for their first three years of life before they are old enough to join the annual migration.
💡Fun Facts
The red crab is endemic to Christmas Island and the Cocos (Keeling) Islands and is found nowhere else on Earth.
The invasive yellow crazy ant (*Anoplolepis gracilipes*) has killed millions of red crabs by spraying them with formic acid. A biological control program using a Malaysian micro-wasp to target the ants' food source is helping the crab population recover.
Most years, very few or no baby crabs survive the larval stage in the ocean, where they are eaten by whale sharks and manta rays. The population is sustained by massive, successful returns that happen only once or twice a decade.
Red crabs are primarily diurnal but avoid direct sunlight to keep their gills moist; during the dry season, they seal themselves inside their burrows for up to three months to maintain humidity.
The migration is a natural event that typically begins with the first rains of the wet season (Oct-Jan). Spawning dates are predictable and tied to the lunar cycle. Check the Christmas Island National Park website for predicted dates.
Admission
Free to observe. Access to some areas may be restricted by road closures to protect the crabs.
Accessibility
Key viewing areas like Flying Fish Cove, Drumsite, and Ethel Beach are generally accessible. The island is rugged, and visitors should be prepared for walking. Some roads will be closed to vehicles.