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

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.”

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-kilometer) 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.

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.

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.”