It wasn't until a few years after the 1903 extinction of the predatory endemic species, the Maclear's Rat, that the people of Christmas Island started to experience what has developed into a yearly phenomenon of great awe and inconvenience – being directly in the migration path of millions of red crabs. Who is more inconvenienced, the islanders or the crabs, is hard to say. Either way, for the amount of time it takes for the army of decapods to march to their mating grounds, the island belongs to them.
Every year in the fall, Christmas Island is overcome by a scarlet wave of migrating crustaceans, on a mad dash from the forest to the sea. Terrestrial crabs that are unique to this area, their little red crablings must be born in the ocean or they will not survive. As soon as the humidity is just right, around October or November, they swarm over the island, across roads, through homes and schools, under every foot, every tire, inside unattended shoes and covering every surface available, all 100 million of them, give or take.
The locals are very patient and protective of their sideways-scuttling neighbors, and are careful to minimize driving, hang “Crabs Crossing” signs on the roads, and watch their feet during the migration, but a few million become casualties every year to vehicles, other animals, and clumsy tourists not accustomed to tip-toeing about. Once the masses reach the ocean and go about their business, they return to the forest, first the males, then the females, exhausted and spent, to wait for their offspring to return.
That's when it gets really weird.
A few weeks later, the larval hatchlings have grown into their land legs, and it's time for them to make their journey across the island to their woodland home. At 100,000 hatchlings per female, the sheer number of teeny tiny crablets is hard to fathom as they wash back across the roads and buildings like a sea of scarlet spiders, swarming over everything in their path. The islanders do their best to help them along and keep them out of shoes and beds, but it's a losing battle. There is nothing to do but work around them as much as possible, and try to ignore the skin crawling invasion.
If you wish to see this overwhelmingly massive migration, you may be running out of time, as it seems may be the case as for these fascinating creatures. The accidental introduction of the aggressive and aptly named “yellow crazy ant” has recently begun to decimate the numbers of the red crabs at an alarming rate. The highly invasive insect has wreaked havoc on the ecology of the island, their excretion of formic acid poisoning the crabs as well as several other species unique to Christmas Island. Over a quarter of the red crab population has been wiped out so far, and the tenacious crabs now face ant-induced extinction.