Invertebrates
Christmas Island Red Crab
Gecarcoidea natalis · also called Christmas Island red crab, red crab
Christmas Island, a speck of rainforest in the Indian Ocean, is run by crabs. Around 50 million bright red land crabs live in its forest, recycling the leaf litter — and once a year, on a cue of rain and moonlight, they all walk to the sea at once, in one of the great animal migrations on Earth.
Overview
The Christmas Island red crab is endemic to Christmas Island (an Australian territory) and the nearby Cocos Islands. It lives in the island’s rainforest, sheltering in burrows on the forest floor, and it is an omnivore that eats mainly fallen leaves, fruit, flowers, and seedlings — the island’s principal litter recycler. At 40–50 million individuals on 135 square kilometres, it is the island’s dominant land animal.
The migration
Sealed in their burrows through the dry season, the crabs emerge with the first heavy rains of the wet season (October–December), timed to a specific moon phase so that spawning coincides with the smallest tidal range. Adults then walk to the coast, up to several kilometres, in a moving red mass. Males dig shore burrows, mate, and head back inland; females brood eggs for about two weeks and release them into the sea at a pre-dawn high tide. The larvae spend about a month at sea — usually almost all eaten, but every few years vast numbers survive and pour back onto land as a carpet of tiny crabs.
An island built around them
During the migration, Christmas Island closes and diverts roads, and the national park has built crab underpasses, tunnels, and a 5-metre “crab bridge” to get the animals across safely. Even so, vehicles kill thousands each year.
A keystone species
At its numbers the red crab controls the rainforest floor — litter depth, seedling density, soil turnover. Where it has been wiped out, the forest changes completely and other invaders move in.
Status
The red crab is Least Concern but has fallen by a third to a half since the 1990s, almost entirely because of the invasive yellow crazy ant, which forms supercolonies and sprays formic acid that blinds and kills the crabs — an estimated 10–15 million dead in one wave. Australia fights the ants with baiting and a biological-control wasp.
Christmas Island Red Crab: common questions
What triggers the Christmas Island red crab migration?
Rain and moon. The crabs spend the dry season sealed in humid burrows on the forest floor, largely inactive. The first substantial rains of the wet season, usually between October and December, prompt them to emerge, and the migration is also tuned to the lunar cycle — spawning must happen at a specific point in the tidal cycle, when the difference between high and low tide is smallest, so that females can release their eggs into the sea from a low rocky ledge without being swept off or stranded. If the rain comes at the wrong moon phase, the crabs may wait, and in a dry year the migration can be small or delayed.
How does the island cope with millions of crabs on the move?
With considerable effort. During the peak weeks, Christmas Island's roads are closed or diverted, staff and volunteers rake crabs off the tarmac, and the national park has built dedicated crab infrastructure — underpasses, tunnels, and a 5-metre-high "crab bridge" over a main road — to let the animals cross safely. Residents sweep crabs off driveways with leaf blowers and keep them out of houses. Even so, many thousands are killed by vehicles each year, and drivers are asked to slow right down or stay off certain roads entirely.
Why are the crabs important to the forest?
The red crab is a keystone species. At tens of millions of individuals, it is the main consumer of fallen leaves, fruit, flowers, and seedlings on the rainforest floor, and by eating and burrowing it controls the leaf litter depth, the seedling density, and the soil turnover across the whole island. Where invasive yellow crazy ants have wiped the crabs out in "supercolonies," the forest floor changes dramatically — litter piles up, a different set of plants takes over, and other invasive species move in. The crab is doing a job that nothing else on the island can replace.
What is threatening the red crab?
One thing above all — the invasive yellow crazy ant. Accidentally introduced to Christmas Island, the ant forms enormous multi-queen "supercolonies" and sprays formic acid into the eyes and joints of the crabs, blinding and killing them. Between the 1990s and 2000s, crazy ant supercolonies killed an estimated 10 to 15 million red crabs, and the population has still not recovered. Australia has fought back with targeted ant baiting and, since 2017, the release of a tiny Malaysian wasp that parasitises the scale insects the ants farm for honeydew, cutting the ants' food supply. Vehicle strikes during the migration are a smaller, ongoing loss.