A person walks past migrating red crabs on Christmas Island, Australia. Image Credit: Reuters

Millions of bright red crabs shut down roads in Australia's Christmas Island in mass migration

The first rainfall of the wet season on Australia's Christmas Island has begun, which means one thing is certain: Millions of bright red crabs are on the move once again to mate and spawn - shutting down roads and "turning up everywhere," in the words of a local conservation agency.

The large male crabs lead and the females follow, migrating from the jungle toward the Indian Ocean so that they can mate in or near burrows by the water. Together they march through towns, crossing specially-made crab bridges and covering just about anything in their path in the process.

According to the Australian government, some 50 million red crabs live on the Island - the only place in the world where they can be found. The phenomenon happens each year on the island and is determined by the phase of the moon, according to Parks Australia, which calls the mass migration "one of the most incredible natural processes on Earth."

The colorful migration usually falls between October and November but has been known to occur as late as December or January in the past.

"The crabs are turning up everywhere, including at the door of an office block," Parks Australia tweeted this week, adding that the yearly migration was "in full swing."

Migrating red crabs are seen on Christmas Island, Australia. Image Credit: Reuters

Its staff were kept busy "raking crabs off roads" and managing road closures to keep the crustaceans safe, the government agency added.

It's a significant annual event for the island - tourists and nature-lovers flock to coves and beaches so that they can bear witness to the crabs' arduous trek, while others can stay updated by tuning into local radio stations which offer frequent migration updates and travel advice.

Acting manager of Christmas Island, Bianca Priest, confirmed that workers had taken precautions to help the creatures with their annual voyage.

"Christmas Island National Park staff put up kilometers of temporary barriers, erect signs and close roads across the island to protect millions of crabs leaving their forest homes for the coast," she told Daily Mail Australia on Wednesday.

Officials have also been known to build bridges, underpasses and crossings to help the creatures migrate safely.

While those keen to see the mass migration are unable to drive on certain roads, visitors are allowed to walk among the crawling crabs although caution is advised.

The crabs are "extremely important to the island's rich biodiversity," Parks Australia says, helping to sustain the island's ecosystem by eating leaf litter which contributes to keeping the forest floor in a healthy condition.

However, an increased number of crabs have died during the annual migration due to human activities, with thousands crushed by cars each year and others at risk of dehydration when they pass areas where forest cover has been cleared, the agency adds.

Once mating has taken place inside or near to the burrows, females - which greatly outnumber males - stay behind until they are ready to produce eggs - which happens within three days. They remain in the burrows for a fortnight as the eggs develop until they are ready to dispense them into the ocean.

During the spawning process, the females swarm the shoreline so that they can release their eggs into the sea. The process can happen on 5-6 consecutive nights during this period, according to Parks Australia.

Migrating red crabs climb a bridge on Christmas Island, Australia, Image Credit: Reuters

If the larvae which hatches from the deposited eggs is not eaten by hungry fish, manta rays or giant whale sharks, baby crabs are born. Often the larvae will be devoured completely, leaving no baby crustaceans at all but experts say once or twice a decade a larger number survive, maintaining population numbers on the island.

The natural migration process is so impressive that world famous naturalist Sir David Attenborough listed it as one of the top TV moments of his career, calling it "an astonishing, wonderful sight."

"It is like a great scarlet curtain moving down the cliffs and rocks toward the sea," he said, adding that the creatures treated him as though he was just another obstacle on their path toward the sea, climbing up his legs as they continued their journey.

"That's how I discovered how difficult it is to deliver lines while several four-inch crabs, each armed with sharp claws, are advancing menacingly up your inner thigh," he joked.