As It Happens

Why millions of crabs are taking over Australia's Christmas Island

Residents of the island have been finding crabs on every corner of the island since the start of this month.

This is what life looks like on the island during crab migration season

Migrating red crabs are seen on Christmas Island, Australia, in this still image from undated video obtained via social media. (Parks Australia/Reuters)

Crabs by the ocean. Crabs on the sidewalk. Crabs on the road. 

This is what life looks like on Australia's Christmas Island during crab migration season — when the crustaceans cover almost every inch of the island's surface. 

The island's crabs begin a yearly migration from forest to ocean around this time of year, and millions of the little red creatures dominate even the strangest of places. 

One of those places is inside Tanya Detto's home.

"I'm up on the second story of a three-storey building. They'll pretty commonly climb up into my house," Detto, an invasive species program co-ordinator at Christmas Island National Park, told As It Happens host Carol Off. 

"They sometimes fall off the roof, so they get up to the top and they can't find their way down." 

But the most bizarre part of Detto's day-to-day during migration season might be what she calls "crab directing." 

"Once they do start migrating, especially through the township, we have to do a fair bit of traffic management."

Detto says the work that goes into that management is extensive. It includes putting up fences for the crabs and clearing the underpasses they migrate through. 

But they get plenty of help from locals, Detto said, especially when residents begin taking their boats out for the weekend. This includes kids raking crabs out of the way on local roads when boats come back in. 

Mass crab migration underway on Christmas Island

14 days ago
An 'epic' migration of crabs is underway on Australia's Christmas Island, as millions of red crabs make their way to the ocean to spawn. (Park Australia/Reuters) 0:59

The elements that trigger these crabs to migrate vary. One is their spawning date, which Detto says can happen any time between October and January. 

"The sort of turning tide at four o'clock ... in the morning is when they need to spawn, and then everything sort of works backwards from there," she said. 

"So the females and males have to get down to the coast two weeks before that date so that they can mate because the females need to incubate their eggs for two weeks, and they need at least a week to get down to the coast."

A dangerous journey 

Once they have babies, it's another journey up to the forest. Once their eggs are released into the water and are hatched, Detto says the babies return to shore — but this last moult can be a dangerous time for them. 

"At that point, they can't swim, and if they end up getting washed back into the water, they'll actually drown," she said.

"But when they safely get onto land, they have the breeding trek back up into the forest. And a lot of years, we don't get any babies come back. So it just depends on the tides and the currents."

A sign indicating a road closure for red crab migration. (Parks Australia/Reuters)

She described one year where they had such a large return of those babies, the critters essentially painted the island red. 

When that made it impossible to walk outside without stepping on them, Detto said people had to do a "special little crab dance."

"You kind of stand on one foot and wave your other foot around so that the crabs can clear a little patch so you can step down," she said. "So it makes for a very easy walk through town."

Written by Keena Alwahaidi. Interview produced by Sarah Jackson. 

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