Roads on Christmas Island have been closed as they turn scarlet with thousands of red crabs emerging from the forest to begin their annual migration to the ocean.

Video footage from Parks Australia shows large numbers of red crabs huddling to climb a crab bridge built to help the creatures cross the road on the island off the coast of Western Australia, while multiple roads were also closed to protect the animals.

Parks Australia says the annual migration starts with the first rainfall of the wet season, usually in October or November, but can sometimes be as late as December or January.

Red crabs all over the island, up to the millions in numbers, leave their homes at the same time and start marching towards the ocean to mate and spawn, creating a flame red spectacle on the island, the conservation agency said.

After mating, the male crabs will journey back to the jungle first as the females stay behind in the burrows for about two weeks to lay eggs.

Each female crab can produce up to 100,000 eggs, which she holds in a brood pouch.