Amazing Migration of Red Crabs


Image from Flickr is by Peter from Perth

Christmas Island, a small island in the Indian Ocean, is home to more than 100 million red crabs, which migrate from the forest to the coast each year during the breeding season. As per



At the start of the annual rainy season (the last quarter of each year), fully grown red crabs begin their migration from their burrows in the forest to the coastal areas of the island. It is believed that biological programming triggers the migration, and almost the entire adult population of red crabs (about 50 million) starts moving almost simultaneously to try to reach the coast at the time of high tide during the last quarter of the moon in the rainy season. The red crabs have an innate ability to sense environmental cues that signal the start of their migration. These cues include changes in humidity, temperature, and the presence of rainfall. As the wet season approaches on Christmas Island, there is an increase in humidity, and the first heavy rains occur. These changes in the environment serve as triggers for the crabs to initiate their migration.



At the coast, each female crab releases several their eggs into the water, and once the eggs hatch, the young crabs, along with the adults, start their trek back to the forest. During the migration, the crabs cover the routes to the coast so densely that from the air, the entire island looks like a creepy crimson carpet. The to-and-fro migration takes about 3–4 weeks, and during this period, the crabs are almost everywhere: on the road, in the house, on the lawns, on the beaches, and even in the public lavatories.




Image from Flickr is by vincshuang



Image from Google Maps is by Nay Say




Image from Flickr is by CuriousJM. Note: This is a digitally enhanced image of a screenshot of Google Street View.



During migration, some roads are closed to vehicular traffic, and national park rangers and volunteers erect temporary barriers to control the juggernautic movement of red crawlies.




Image from Google Maps is by Michael Klamm



The authorities have even built crab bridges over roads to protect the crabs from their road crossings.




Image from Google Maps is by DUC THO LE



The entire event is often described as one of the world's most impressive wildlife migrations and has to be seen to be believed. Perhaps the following videos can show, to some extent, the massive scale of the migration.




An amazing video of baby red crabs:


Google Street View



Source of Information: christmas.net.au



Note: This post has been updated on June 4, 2023. The text has been updated, and the dead links and missing pictures have been removed or replaced.

2 comments:

  1. Animals are driven to migrate by the sun. The only thing that stays constant is where the sun will be. Crabs, whales, penguins and a lot of other animals migrate because of the placement of the sun.

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  2. Dawn - Thank you for your info. My understanding is that the Crab follow the lunar cycle during the rainy season. I am reproducing below some relevant information from http://www.christmas.net.au/parks/crabs/

    Quote:
    At the beginning of the wet season (usually October / November), most adult Red Crabs suddenly begin a spectacular migration from the forest to the coast, to breed and release eggs into the sea. Breeding is usually synchronized island wide. The rains provide moist overcast conditions for crabs to make their long and difficult journey to the sea. The timing of the migration breeding sequence is also linked to the phases of the moon, so that eggs may be released by the female Red Crabs into the sea precisely at the turn of the high tide during the last quarter of the moon. It is thought that this occurs at this time because there is the least difference between high and low tides. The sea level at the base of the cliffs and on the beaches, where the females release their eggs, at this time varies the least for a longer period, and it is therefore safer for the females approaching the water's edge to release their eggs. Sometimes there are earlier and later migrations of smaller numbers of crabs but all migrations retain this same lunar rhythm.
    Unquote

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