Roads, bridges, gardens invaded by swarms of thousands of marching crabs. The scene seems straight out of a science fiction movie. For Christmas Island, however, there is nothing unusual about it. It even occurs every year on this Australian territory located in the Indian Ocean some 300 kilometers off the coast of Java.
This island is indeed famous for its red crabs of the species Gecarcoidea natalis which is endemic. According to estimates, the population numbers no less than 50 million individuals who embark on a spectacular migration each year. The phenomenon generally starts at the beginning of the rainy season, in October or November.
On this occasion, the crustaceans which spend most of their time in the forest move by the thousands in order to reach the ocean before reproducing and laying their eggs. And all roads are good to reach the coast. For several days, the crabs thus invade the streets and roads of Christmas Island, including in residential areas.
This year, the invasion started around November 1, as evidenced by the photos and videos posted by Parks Australia, the institution that manages the six Australian national parks including Christmas Island.
“With the red crab migration in full swing on Christmas Island, crabs are everywhere, including at the door of an office building!“, wrote the institution in another tweet showing several specimens stacked in front of an entrance. If the inhabitants are accustomed to this mass migration, it requires a little organization.
For days, the authorities must indeed regulate traffic and close roads to be able to let the crabs move without hindrance. Temporary barriers as well as warning signs must also be installed in certain places and it is not uncommon for residents to find themselves stuck in their homes because of the phenomenon.
“Residents of Drumsite on Christmas Island were unable to leave their neighborhood on Sunday due to large numbers of crabs crossing the roads“, confirmed Parks Australia in another tweet. The spectacle of these thousands of crabs crossing the island is nevertheless a popular attraction that attracts many tourists every year.
Egg laying scheduled for November 28-29
It usually takes at least a week for the crustaceans to reach their goal. The migration also depends on the phases of the moon, specialists estimate that the spawning should take place this year around November 28 and 29. The eggs will thus be released into the ocean at high tide between the last quarter and the new moon.
Once mating is complete, the males return to the forest while the females stay for a few more weeks to incubate their eggs before releasing them. It is then up to them to make their way back while the larvae begin their development. The young crabs will not come out for several weeks to reach the mainland.
species G. natalis normally has few natural predators and competitors on Christmas Island. The population is however threatened by the yellow crazy ant (Anoplolepis gracilipes), an invasive species accidentally introduced into the territory. According to a study published in 2003, the insect had already killed ten to fifteen million crabs in previous years.
Hence the importance of seeing these millions of crabs continue to spring every year and facilitate their journey to the ocean. “They call it the best time of the year…and we’re not talking about the countdown to Christmas. It’s Christmas Island National Park crab migration season!“, rejoiced Parks Australia on Twitter.
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