Spectacular migration of the Red Crabs of Christmas Island


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Christmas Island is a small Australian island in the Indian Ocean, 2,600 kilometers northwest of the city of Perth, that is home to many species of animal and plant. The island is particularly noted for its prodigious populations of Christmas Island red crabs, a species of land crab that is endemic to the island, and their spectacular migration from the forest to the coast each year during the breeding season.

Christmas Island in Southeast Asia sets the stage for the most epic of nature’s journeys: the synchronised mass movement of 65 million crabs walking up to 8 km in just 5 days. When the wet season kicks in, and the tide is right, the crabs make their move, emerging from their solitary burrows in the tall rainforest, and walking as one to the sea. Nothing gets in the way of these single-minded crustaceans, neither shops, nor golf course nor busy roads. Even cliff faces are climbed down with apparent ease.
The millions of bright red crabs that set off each year – normally in October or November – are driven by a clear purpose: to breed and spawn. So how are such big creatures able to walk a kilometre a day during the migration, when usually they can’t crawl for more than 5 minutes at a time? Scientists recently found that they are fuelled by the release of a hormone that produces a sugar-rush; giving the crabs the energy they need for their marathon journey.
Current estimates place the population of red crabs on Christmas Island at a staggering 120 million – dwarfing the human population of just 1600 – but their numbers are under threat. Human activity has had a major impact. Crabs are at risk of drying out when forced to traverse areas cleared of forest cover. Also, thousands are crushed by vehicles while crossing roads – a situation that has led to road closures, traffic detours and crab crossing tunnels being built under highways for crabs to pass through.An even deadlier menace for the crabs has appeared in the form of an insidious biological invasion: the yellow crazy ant.

Accidentally introduced to Christmas Island from Africa, the crazy ants prey upon the red crabs after first squirting them with poison. The super-colonies are believed to have killed 15–20 million of the crabs in recent years, and the population of these ants is exploding amid climatic changes already threatening the red crabs through the late arrival of the monsoon. If not killed by these ants or by vehicles on roads, male crabs reach the sea first to dehydrate before retreating to the lower terraces to dig and fight over burrows where they mate with female crabs that arrive soon after the male ones.Crackling noise, a chorus of clicking and the march has begun ! The sight is unbelievable as the entire forest floor, and even the roads that run through it turn into a sea of red. So thickly do the crabs blanket the routes to the shoreline that they can easily be seen from air. It is for all these reasons unique that the annual migration of the red crabs is one of the most spectacular animal migrations on the planet.

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