I planned to blog about these crabs for a long time ago, but for some reason it’s taken me over 400 posts to get to them. The poor Christmas Island red crab got lost in all the animals I had lined up for posts, and it’s been a few years since I first heard of them. But I knew today’s post had to be about a crab, and suddenly I remembered Christmas Island red crabs! Now they finally get their time in the limelight.

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Christmas Island red crabs are a wonderfully vibrant shade of red. They’re quite pretty, don’t you think? Image by DIAC images, CC BY 2.0, via Wikimedia Commons

Christmas Island red crabs can get to be reasonably big, reaching over eleven centimetres in diameter. Males are larger than females, and also have larger claws. Despite their name, not all red crabs are actually red. Most are, but they can also be orange or even purple.

These crabs are only found on Christmas Island, which is a small little island in the Indian Ocean. A few have also been found on North Keeling Island, but these are the result of larvae drifting over from Christmas Island, and are not an established breeding population. Christmas Island crabs are land crabs, and spend the majority of their time hanging around the island’s forests.

One of the problems about being a land crab is that crabs need to stay wet to breathe. This is because like aquatic crabs, Christmas Island red crabs use gills for breathing, and gills don’t work too well (or at all) when they dry out. To stay moist, red crabs stay in the cover of the forest, where direct sunlight can’t reach them. They spend a fair bit of time inside burrows which they dig themselves. During the dry season, red crabs will cover the entrance to their burrows with leaves to keep moisture inside.

The crabs are omnivores, feeding on leaves, seeds, fruits and dead animals. They have basically no competition for food. Image by David Stanley from Nanaimo, Canada, CC BY 2.0, via Wikimedia Commons 

It’s when the wet season comes that Christmas Island red crabs get really exiting. They have to travel to the ocean to breed, so all the crabs leave the forest and head on an epic migration. Millions of crabs make the week long migration to the coast, creating a crimson tide of crustaceans. During their migration, red crabs have to cross multiplenroads, which can be both a danger to the crabs and to people. Driving over a bunch of crabs might not seem like a big deal, but their hard shells can puncture tires and cause accidents.

To prevent such incidents, special walls are constructed near crab-heavy roads. This stops the crabs getting on to the roads, but they still need to be able to reach the other side. So the walls funnel the crabs into special tunnels called ‘crab grids’ that go beneath the roads. Though not all casualties are prevented this way, these efforts have gone a long way to ensure that both people and crabs stay safe during their migrations.

Being crushed under a moving vehicle isn’t the only danger to red crabs. During their larval stage, when they drift around the ocean before coming to land, red crabs fall victim to manta rays, whale sharks, and other filter feeders. Once on land, things get a bit safer: adult Christmas Island red crabs have no natural predators. However, an introduced species, the yellow crazy ant, has had a devastating effect on red crab populations.

Crab movements disturb the ants, which then feel threatened, and spray formic acid on the crabs. The crabs are often blinded, and attempt to flush the formic acid from their bodies. This dehydrates the crabs, and they eventually die from dehydration or exhaustion. As the bodies decay, the ants are able to feed on the crabs’ flesh, so at the very least the meat isn’t wasted. Still, an estimated ten to fifteen million crabs have been killed by yellow crazy ants, and conservation efforts are ongoing to try and keep the crazy ants under control.

A tunnel beneath a road to help Christmas Island red crabs travel safely during their migrations. Image by David Stanley from Nanaimo, Canada, CC BY 2.0, via Wikimedia Commons

Mating in red crabs occurs on the coast, with male crabs digging burrows and defending them fiercely from other male crabs. When the females arrive, the crabs mate in their temporary burrows. The males then leave, heading back to the forest while the female crabs finish their job. For the next two weeks the female red crabs stay in their burrows, incubating their eggs.

When this is done, they head to the ocean and release their eggs. The eggs hatch immediately, and the larval crabs spend three to four weeks going through a number of larval stages before returning to shore and heading inland. At this point, the young crabs look like adults, except that they’re only five millimetres across. The crabs reach sexual maturity at around four or five years of age.

The current population of Christmas Island red crabs is estimated to be about 43.7 million. The extinction of Maclear’s rat in 1903 may have led to this population boom – other than introduced ants and humans, Christmas island red crabs have no predators and no competition for food. It’s a bit strange to think of crabs dominating an entire ecosystem, but that’s why island ecosystems are so fun to study!

Cover image by John Tann from Sydney, Australia, CC BY 2.0, via Wikimedia Commons