Each year on Christmas Island, over 100 million red crabs scurry at once from the forest to the sea to breed and spawn.
Each year, up to about 120 million red crabs (Gecarcoidea natalis) scuttle to the sea from the forest across Christmas Island in an effort to mate. It is a commute which overwhelms the island – the 135sq.km (52 sq miles) Australian-owned island, located in the Indian Ocean 2600km (1615 miles) northwest of Perth, has just over 2000 human inhabitants.
The bright red land crabs leave their burrow homes on Australia’s Christmas Island and start a long, laborious trek toward the sea. They descend cliffs, climb banks and maneuver around obstacles to reach the shoreline and lay their eggs, eventually returning to the island’s central plateau with their offspring in tow.
At the beginning of the wet season (usually October – December), most adult red crabs suddenly begin a spectacular migration from the forest to the coast to breed and spawn (release their eggs into the sea). The breeding migration is usually synchronised island-wide. The crabs need moist overcast conditions to sustain them during their often long and difficult journey to the sea.
The crabs time their migration to align with the phases of the moon. It all leads up to the female crabs spawning on a receding tide before dawn, during the last quarter moon phase.
The crabs are aiming for an anticipated spawning date around 6 January so if the rain persists they have plenty of time. If the rain keeps up they will continue their slow march, but if the rain stops so will the migration.
The males lead off the migration and are joined progressively by females. When they reach the sea they will dip to replenish moisture and salts and the males will dig breeding burrows. After mating, the males will begin their return migration. The females will remain in the burrows to brood their eggs for 12-13 days, before emerging from the burrows to spawn.
For the crabs to meet the spawning date they will have to have completed their mating by Christmas.
The eggs hatch into free swimming larvae immediately after they are dropped into the sea. The larvae grow through several stages in the ocean for more than four weeks before emerging from the sea as tiny crabs. The return of baby crabs is not guaranteed; many years none or very few emerge from the ocean but when they do return in high number it is a spectacular event.
Every year Christmas Island National Park rangers work with the local community to keep the crabs safe and to clear a path for them on their journey to and from the beaches by closing roads, maintaining ‘crab crossings’ and controlling the threat of crazy ants.
Usually, these island crabs lead solitary lives, burrowing into the forest floor and defending their dens from intruders, but the beginning of the wet season marks a period when they abandon burrow and travel to the beach to spawn. During this time, the whole island is transformed into a seething red mass, with roads and pavements coming to life.
The crabs will cross any path and climb any obstacle, but road safety isn’t among their list of attributes. In recent years, the numbers crushed underneath cars has become enough of an issue to gain the crabs their own special crab-crossings.
Christmas Island, discovered December 25, 1643, is just a spec of land in the Indian Ocean. The annual red crab migration at the beginning of the rainy season, however, is so massive it can be seen from the air. It has been named a wonder of the natural world. The video below follows this terrestrial arthropod from its rainforest burrow, across dangerous landscape to the ocean to mate.