Millions of Red Crabs Have Overrun a Tiny Island on Their Annual Migration

Millions of Red Crabs Have Overrun a Tiny Island on Their Annual Migration
Red crabs are seen walking in a drain on November 23, 2021 in Christmas Island. (Photo: Parks Australia, Getty Images)

Folks living on Christmas Island, an Australian territory in the Indian Ocean, were greeted with a familiar but bizarre sight this week: thousands of bright red crabs scuttling en masse down roads, across yards, and over special bridges set up for them to cross busy streets.

These crabs (species name Gecarcoidea natalis) live only on Christmas Island, which has an estimated 120 million of them. (That gives the crabs a more than 91,000-to-1 ratio with the island’s human inhabitants.) This week, they’re making their annual journey to the coastline for the start of the mating season.

“This year’s migration has just been absolutely epic,” Christmas Island National Park natural resource manager Brendan Tiernan told Reuters. “The roads have been a seething mass of red crabs. It’s caused traffic jams on this small island and people having to get out of their cars and rake them out the way.”

That Crabs Are Taking a Yearly Trip

Photo: Parks Australia, Getty Images Photo: Parks Australia, Getty Images

The crabs migrate every year, in tandem with the start of the rainy season, which usually begins in October or November but can lag to December or even January. Around 40 to 50 million of the island’s crabs trek across the island to reach the shoreline for mating season. While most people associate crabs with the ocean, Christmas Island is home to several terrestrial crab species that live in the lush rainforests of the 52-square-mile (135-square-kilometre) island, including the red crabs.

The crabs actually set their biological clocks for this migration to coincide with a very specific set of environmental circumstances as well as the lunar calendar. After the first rain of the rainy season, the crabs know that their optimal time for mating is during the last quarter of the moon, before dawn as the tide is going out. According to Parks Australia, this timing can change the makeup of the trip; if rains come close to the ideal spawning date, the crabs will rush across the island, but they’ll take a more leisurely road trip if they know they have some time.

A Dangerous Journey for Crustaceans

Crabs walking over a crab bridge. (Photo: Parks Australia, Getty Images) Crabs walking over a crab bridge. (Photo: Parks Australia, Getty Images)

The journey to the shore can be perilous. Crabs are forced to migrate across large stretches of human infrastructure, including roads and highways. The island has built special “crab bridges” to help the critters cross the busiest stretches of road.

Christmas Island also has an invasive ant population, known as the “yellow crazy ant,” or Anoplolepis gracilipes. These ants were accidentally brought to the island between 1915 and 1930 and contain an acid that can blind the crabs. Researchers estimate that the ants may have killed tens of millions of red crabs since their arrival on the island.

Climate change could also pose a threat to the crabs. Because the crabs’ migration is so delicately balanced between lunar cycles and the start of the rainy season, research has shown that delays in rain or otherwise erratic rainfall precipitated by changing weather patterns and climate change could have a real impact on the crabs’ migration. During one particularly dry season, researchers found, the crabs didn’t migrate at all.

Mating And Spawning in the Ocean

Photo: Parks Australia, Getty Images Photo: Parks Australia, Getty Images

When they get to the ocean, after taking a little swim to get some of their moisture back, the male crabs will dig burrows on the shoreline, often fighting with each other for the best spots. They’ll then invite the females over to mate in or around the burrows.

After the migration, the male crabs will pop back to the forest, while females will stay in the ocean for another two weeks to produce eggs. Each female can carry up to 100,000 eggs, which she holds in a brood pouch for those two weeks. Over the course of five or six days during the last quarter phase of the moon, before sunrise, the females will release their eggs into the water.

Tiny larvae will hatch out of the eggs, and the petite hatchlings will make their own journey back to the forest where their parents came from. Crabs can live for more than 12 years, so these younger crabs will make their own migration to the shore around the ages of four to five.

Some People Are Getting ‘Covered in Crabs’ (Willingly)

Members of the public and Parks Australia staff rack away thousands of red crabs off a road. (Photo: Parks Australia, Getty Images) Members of the public and Parks Australia staff rack away thousands of red crabs off a road. (Photo: Parks Australia, Getty Images)

Australia’s Parks division says the crab migration is the island’s biggest tourist attraction. The island closes some roads to make crab passage safer, while information posted on notice boards and local radio stations give updates during the migration.

“Some people were quite freaked out by the fact that they’re surrounded by millions of crawling arthropods, whereas other people are just immersed — basically [they] do a little red crab angel,” Tiernan said. “They’ll lie on [the] ground and let themselves get covered in red crabs.”