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.