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A giant coconut crab has been filmed stalking, killing and devouring a seabird. It is the first time these whopping crustaceans have been seen actively hunting large, back-boned animals, and suggests they might dominate their island ecosystems.

Coconut crabs (), also known as robber crabs, are an imposing sight. They can weigh up to 4 kilograms, as much as a house cat, and sport legs that span almost a metre. This makes them the largest invertebrates – animals without backbones – on land. The crabs live on coral atolls in the tropical Indian and Pacific oceans.

They are renowned for their tree-climbing abilities and taste for coconuts, which they crack open with their powerful claws. They do sometimes eat meat, but until now it was thought that they only obtained it by opportunistic scavenging.

Between January and March 2016, of Dartmouth College in New Hampshire visited the Chagos Archipelago, a remote series of atolls in the Indian Ocean. Chagos is ideal for studying coconut crabs: it is in pristine condition, is surrounded by and has lots of coconut crabs, making them easier to find and observe.

Attack of the killer crab

One night, Laidre saw a coconut crab slowly climb a tree and so he . The crab inched towards a common seabird called a , which was sleeping in a nest near the ground. It then lunged with a claw, pinching and breaking the bone in one of the bird’s wings and causing it to tumble out of the tree.

Breaking a bird’s wing would be easy for a coconut crab, says at the Okinawa Churashima Foundation Research Center in Japan. In 2016, he showed that the , stronger than any other crustacean and comparable to the bite force of a big predator like a lion.

“The claws of coconut crabs can generate a force 80 to 100 times the mass of their body,” says Oka. “The crab in the video seems to be about 2 kilograms, so it would be able to easily break the bird’s bones.”

After the first attack, the crab slowly descended and followed the wounded bird, breaking the other wing with its claws. “At that point, when both its wings were broken and it was on the ground, it couldn’t go anywhere,” says Laidre.

Before long, five other coconut crabs ambled onto the scene, perhaps lured by the commotion and scent of blood. They proceeded to tear the bird apart and eat it.

“It was pretty gruesome,” says Laidre.

An island ruled by crabs

If enough coconut crabs hunt prey like this, it could have substantial ecological impacts. On these small islands, adult coconut crabs are by far the largest land animals. They may rule through an “island of fear” effect. Seabirds may avoid islands with lots of the crabs, to avoid getting themselves or their progeny eaten.

In line with this, surveys Laidre carried out showed that if coconut crabs were living on an island, birds were less likely to, and vice versa.

“In areas where these guys are present and abundant, it would be a smart move, especially among ground-nesting birds, not to place eggs there,” he says.

There may be a flip side to this called a “priority effect”. If an island already has lots of seabirds, coconut crabs will find it hard to colonise, as they start off small and vulnerable.

“If you have a bunch of birds there, it’s going to be very hard to get bigger because they are going to eat you,” says Laidre.

He now plans to set up remotely activated cameras at entrances to the crabs’ burrows. This should reveal what the crabs drag back to eat and how often they hunt birds.