Billions of red crabs crawl from the sea into the forests of Christmas Island

Australia Young crabs emerging from the sea and crawling into the forest face cannibalistic adult crabs lurking along the way.

Adult red crabs wait to eat young crabs. Photo: Live Science — Adult red crabs wait to eat young crabs. Photo: *Live Science*

A film from British naturalist David Attenborough's "Our Planet II" program captured the moment billions of baby crabs risked their lives to scurry past cannibalistic adult crabs on the coast of Christmas Island, Australia, Live Science reported on June 13.

Christmas Island red crabs ( Gecarcoidea natalis ) migrate once a year. An estimated 65 million crabs travel 2 kilometers from the forests where they live on the island to their coastal breeding grounds. The migration begins after the first rain of the wet season, usually in October or November, according to the Australian National Parks Service. Once at sea, male crabs dig burrows and mate with females. The male transfers sperm to the female, who stores it in a pouch, and leaves. The female stays behind, laying up to 100,000 eggs at a time in a brood pouch attached to her abdomen.

Egg-laying occurs before dawn within a week of the full moon. Female crabs release their eggs into the water as the tide begins to recede. The fertilized eggs fall into the sea and hatch upon direct contact with water, according to Lucy Turner, a marine biologist at the University of Plymouth in England.

Over the course of a month, through several larval stages, the baby crabs develop into what’s known as megalopa. In this video from “Our Planet II,” they return to land. As they emerge from the water, they shed their water-soaked shells to become fully fledged crabs, about 5 millimeters in diameter. As soon as the baby crabs reach the beach, danger lurks. An adult crab is waiting, grabbing the tiny baby crabs with its claws and eating them.

"Red crabs are opportunistic predators and will eat anything. I've never seen them eat juvenile crabs before but I've seen them eat other dead adult crabs," Turner said.

The cannibalism is particularly surprising because adult red crabs are not usually aggressive enough to actively hunt others, says Simon Webster, a zoologist at Bangor University in England. After their migration, their muscle glycogen levels are extremely low, so they will eat whatever they can. They will also eat crabs that have died because they were jostled along the way, Webster says. Only a small number of Christmas Island red crabs make it safely to the forest. Many are killed before they even leave the sea. Scientists estimate that only 1 to 10 percent of the crabs reach shore and survive to adulthood.

An Khang (According to Live Science )

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