Red Crab Migration

The annual red crab migration on Christmas Island is one of the most extraordinary natural events in the animal kingdom, a spectacle in which an estimated fifty to one hundred and twenty million bright red land crabs emerge from the plateau forest simultaneously and march in a slow, inexorable column down to the sea to breed. The migration is triggered by the first rains of the wet season, usually in October or November, and once it begins it transforms the island's roads, paths and gardens into a continuous river of orange-red crustaceans that can reach densities of one hundred crabs per square metre on the most heavily travelled routes.

The migration route follows paths worn smooth by centuries of previous marches, leading from the crab burrows high on the plateau down through the rainforest and across the island's single road network to the eastern and southern shorelines where breeding takes place. Males typically arrive at the coast first and dig breeding burrows in the sand above the high-water mark, where they wait for females to make the journey and mate with them in the burrows over a period of several days before the females release their eggs directly into the sea.

The females carry the fertilised eggs in a brood pouch beneath their abdomen for two to three weeks before emerging at the shoreline during the high tide of the last quarter moon, releasing thousands of larvae in a synchronized spawning event that takes place over just a few nights. The larvae develop in the ocean for around a month before returning to the shore as miniature crabs, a process that has fascinated marine biologists since the migration was first studied in detail in the 1980s and that continues to produce new research on the environmental cues that trigger the exact timing of the release.

The return journey of the adult crabs from the coast back to the plateau is less dramatic than the downhill march but equally important for the survival of the population, and road-crossing infrastructure including purpose-built crab tunnels and temporary barrier fences has been installed along the most heavily used routes to reduce the mortality caused by vehicles. During peak migration the island's roads are closed to traffic for several hours each evening and rangers work through the night to help crabs cross safely, a level of logistical commitment that reflects both the ecological significance of the migration and its importance to the island's identity.

Christmas Island red crabs are found nowhere else on Earth and represent a single genetically connected population of extraordinary density, the result of millions of years of evolution in an isolated island environment free from the terrestrial predators and competitors that control crab populations elsewhere. The migration itself is a fixed evolutionary behaviour that cannot adapt to new obstacles quickly enough to prevent population decline in the face of introduced species, which has been the central conservation challenge for the species since the catastrophic impact of the yellow ant, an invasive species accidentally introduced in the early twentieth century that devastated the island's crab population before its own collapse.

For visitors who arrive during the migration, the experience of watching thousands of crabs moving in coordinated waves through the rainforest at night, their shells clicking against the leaf litter and their claws raised in territorial display against each other, is unlike anything else in the natural world and a compelling reason to choose Christmas Island as a travel destination at the right time of year. The spawning release on the beach, when thousands of egg-carrying females emerge simultaneously into the shallows in a scene that turns the water orange-red with emerging larvae, is one of the most remarkable wildlife spectacles accessible to ordinary visitors anywhere in the world.

It is one of the largest animal migrations on Earth by biomass, involving tens of millions of crabs moving simultaneously across the entire island.

The breeding release on the beach, where egg-carrying females simultaneously flood the shallows, is a globally unique spectacle found nowhere else.

The migration has driven significant conservation infrastructure including crab tunnels and nightly road closures that make visitor access to the event well-organised and safe.