Billions of red crabs crawl from the sea into the forest on Christmas Island.

In Australia, young crabs emerge from the sea and crawl into the forest, only to face cannibalistic adult crabs lying in wait along the way.

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

Footage from British naturalist David Attenborough's "Our Planet II" program captures the moment billions of baby crabs risk their lives scrambling past cannibalistic adult crabs on the coast of Christmas Island, Australia, Live Science reported on June 13.

The Christmas Island red crab ( Gecarcoidea natalis ) migrates once a year. An estimated 65 million crabs travel 2 km from the forests where they live on the island to their spawning grounds on the coast. The migration begins after the first rains of the wet season, usually in October or November, according to the Australian National Parks Agency. Upon reaching the sea, the male crabs dig burrows and mate with the females. The male crab transmits sperm for the female to store in her pouch and then leaves. The female crab stays behind, laying up to 100,000 eggs at a time in her brood pouch attached to her abdomen.

The egg-laying process takes place before dawn for about a week after the full moon. Female crabs release their eggs into the water as the tide begins to recede. Fertilized eggs fall into the sea and hatch immediately upon direct contact with water, according to Lucy Turner, a marine biologist at Plymouth University, England.

For over a month, going through several different larval stages, the young crabs develop to a stage called megalopa. In the video from "Our Planet II," they return to land. When they emerge from the water, they shed their waterlogged shells to become fully grown crabs with a diameter of 5 mm. As soon as the young crabs reach the beach, danger lurks. An adult crab is waiting, using its claws to catch the tiny young crabs and devour them.

"Red crabs are opportunistic predators, so they'll eat anything. I've never seen them eat immature crabs before, but I have observed them eating other dead adult crabs," Turner said.

The cannibalistic behavior is particularly surprising because adult red crabs are not typically aggressive enough to actively hunt other individuals, according to Simon Webster, a zoologist at Bangor University in England. After migrating, their muscle glycogen levels are extremely low, so they will eat anything they can find. According to Webster, they will also eat crabs that die from overcrowding along the way. Only a small number of young Christmas Island red crabs make it to the forest safely. Many are even killed before leaving the sea. Scientists estimate that only 1-10% of young crabs reach shore and survive to adulthood.

An Khang (According to Live Science )

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