With the arrival of the rainy season, Christmas Island changes completely. Roads closedthe forest floor comes to life as tens of millions of red crabs take over. The remote Australian territory, located about 190 nautical miles south of Indonesia, plays host to one of the world’s most unique natural phenomena. There, red crabs emerge from their burrows en masse each year like clockwork and migrate toward the Indian Ocean, guided by rainfall and lunar cycles. Thick-legged, hard-shelled bodies, about five inches or less wide, cover the 52-square-mile island as they move from land to sea.
“It’s a spectacular event that few people get to see,” says Brendan TiernanSenior Field Program Coordinator for Endangered Species at Christmas Island National Park. Tiernan has lived and worked on the island for 17 years, as one of approximately 1,500 residents. Yet each year he is an enthusiastic witness to “one of the largest animal migrations.”
Decades of a destructive invasive species have dented red crab populations and by 2023 there will be a unusually dry year disrupted the crustaceans’ breeding ritual. Despite these setbacks (and thanks to active management), Tiernan says the crabs are currently thriving. In 2015, there were approximately 45-50 million red crabs on Christmas Island. Now, Tiernan says, there are over 100 million. Their numbers and unique life cycle make them their home on the island.
Ecosystem engineers
Christmas Island Red Crabs (Gecarcoidea natalis) are a species with a small range. They are native to only two locations: most live on their namesake island, and a much smaller population inhabits an archipelago called the Cocos Islands, about 600 miles southeast. The crabs live in forests for most of the year. They are detritivores, living strictly on the ground and in underground burrows where they feed on decaying vegetation and young seedlings.
In parts of Christmas Island where the crabs are abundant, the forest floor is devoid of leaf litter. “It looks like it’s been raked and it’s full of holes like a moonscape with their burrows,” Teirnan says. Popular science. “You end up with a really unusual forest structure where there’s no undergrowth and you can see for miles. It’s like nothing else,” he adds. In other tropical places, such empty undergrowth might be a sign of trouble, but on Christmas Island it’s the natural state of affairs. The crabs are the ones who define their environment.
Despite their impact, the crabs are usually not particularly active or conspicuous. They may breathe air, but they are still “moisture-loving creatures,” says Tiernan. As a result, they spend most of the dry season in their burrows. But when it rains, it’s a different story.
Beach bound
In late October or November, Christmas Island usually gets its first significant rainfall of the upcoming rainy season. From there, the crabs begin their countdown. They must be in the ocean in time to spawn within a few days of the last quarter moon, when the tides are at their lowest and their young are least likely to be washed away.
Red crabs leave their forest burrows with the first rains and head for the coast. “It’s millions of crabs, crossing roads and walking over cliffs,” Tiernan says. Somehow the crustaceans are aware enough of the lunar cycle to adjust the speed of their migration to be faster or slower, depending on how long they have before the last quarter and neap tides. The uncanny ability to track the moon means that some years they “take their time and wander very slowly. Other years they’re on a tight time budget and you just get this roaring stampede as they all race toward the ocean,” he explains.
Males generally reach the coast first, where they build mating dens. When females arrive, Tiernan says they select the “biggest male with the best den” they can find, and the crabs mate. After mating, the males return to the forest and the females enter the coastal dens to incubate their eggs — about 100,000 each. The pregnant crabs leave their dens shortly after the last quarter, releasing their eggs into the ocean in a mass, coordinated, hectic up and down dancearound 4am, a few hours before sunrise.
Immediately after hitting the water, the eggs hatch into wriggling larvae called “megalops.” The crabs remain in this stage for about four weeks, until they return to land, molting into tiny crabs. Hatching success varies considerably from year to year. “Some years we get an incredible number of baby crabs coming back. Other years we get none at all,” Tiernan notes. Why is a bit of a mystery. Little is known about the marine phase of the crabs’ life cycle, he says. But in successful years, literally trillions of baby crabs make it back to shore in “this big carpet of red.”
Resilient Red Tide
Tiernan says there has been a surge in massive crab fishing in the past decade, which has significantly increased the population. “It’s a striking change in the landscape. There are more crabs,” he says. But for much of recent history, this hasn’t been the case.
The spread of invasive yellow crazy ants in the 1990s led to “invasive meltdown.” Yellow crazy ants killed millions of crabs by spraying formic acid into their eyes and joints in an attempt to defend their “supercolony” territories, which can cover hundreds of hectares. Red crab numbers declined in the 2000s and 2010s as the ants took over the island. In their wake, and in the crabs’ absence, swaths of forest were transformed, with dense undergrowth and thick leaf litter, changing the fundamental composition of the habitat and making it inhospitable to other animals native to Christmas Island. Pesticide baits helped to tackle the ant problem, but also killed more crabs.
But in 2016, the tide began to turn in the crabs’ favor with the introduction of a biological control agent. The Australian National Parks Agency bred and a tiny wasp was released. The Malaysian insect, counterintuitively, does not target the ants, but a scale insect that the ants depend on for a food source. Within a few years, monitoring revealed the wasp was at work. Crazy ants numbers dropped and crabs started to recover. “It’s made it much easier for red crabs to migrate and live on the island without being decimated by the yellow crazy ants,” Tiernan says.
The war isn’t quite won yet, he notes. The park is exploring new ways to apply pesticide treatments and additional biological controls to eliminate even more ants. For now, though, things are looking better for the crabs and the entire Christmas Island ecosystem as a result.
That’s even after last year, when the rainy season started much later the mega migration effectively brought to a halt prevention. The first rains came in February and the crabs did not migrate to the coast in anywhere near their usual numbers.
It is rare, but not unheard of, for the crabs to skip a breeding season due to unusual weather, Tiernan says. It happened once before in 1997 during a particularly intense El Niño event. In addition, Christmas Island’s climate is subject to the Indian Ocean Dipolean oscillating pattern similar to the El Niño-Southern Oscillation, which can occasionally produce exceptionally dry and wet years. Some degree of irregularity is to be expected.
Climate change, however, will make things harder for the island and its crabs in the long run. According to a recent study commissioned by the national park, Tiernan says Christmas Island will experience longer dry spells and shorter, more intense wet spells as global warming progresses. If that probability comes true, the red crabs will struggle to stay fed and hydrated during the dry season, and will face a more dangerous migration and breeding period in the wet season. The crustaceans have proven resilient so far, but there’s no guarantee they’ll make it through the next hurdle.
Tiernan expects a good turnout this year at least. “There’s no reason why we shouldn’t have a huge crab migration.” In a few months, Christmas Island will be completely covered in red again.
