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The Great Christmas Island Red Crab Migration

August 1, 2023 by Okumura Yuka

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We’re suckers for natural phenomena and when we heard about the Christmas Island Red Crab Migration, we needed to learn more. Here are just a few stats about what we know: Christmas Island Red Crabs are known as Gecarcoidea Natalis in the science world and they’re roughly 5 inches with a lifespan of twenty to thirty years. They’re native to the Christmas and Cocos Islands and while these crabs were once numerous on the islands, because of the yellow crazy ant numbers have been devastated. 

(image via: australian geographic)

Every year, millions of crabs become “rivers of red” as they move from the island’s rainforests to the oceans to breed and lay eggs. We say rivers of red, but interestingly enough while most crabs are red, you can occasionally spot an orange or purple crab. These crabs follow the same migration path each and every year so if something is in their way they simply go over it or through it. 

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Red crabs have to keep their bodies moist, so they actually wait for a rainy season to migrate to ensure they can make the long journey. And we do mean long journey, as female crabs can lay up to 100,000 eggs. Luckily crabs are omnivores, feeding mostly on fruit, seedlings, fallen leaves, garbage, land snails, and dead animals. Should they run into famine, Christmas Island Red Crabs can cannibalize. 

(image via: island conservation) 

When it comes to red crab reproduction, things get pretty interesting. Sexual maturing takes place at about four or five years of age, and at the beginning of rain season, typically October or November, crabs travel to the coast for spawning; the timing is actually linked to the phasing of the moon. Male crabs arrive first and create burrows, think of them as little homes, and once female crabs arrive, the crabs begin to mate in these burrows. After mating, males return to the rainforest, while the females stay behind for about two more weeks. At this time, the females release their eggs into the water at the turn of high tide, of the last quarter of the moon, then they follow their male counterparts back to the forest. 

Earlier we mentioned the yellow crazy ant, and now that we understand crabs burrow in the sand, we see how these ants can pose such a threat. Ants aren’t the only threats to crabs, they can also be preyed upon by coconut crabs; and larvae, entire generations, can be eaten by fish, whale sharks, and manta rays.

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