Meet the crab with the ‘mighty claw’

By Helen Briggs for BBC News.

The claws of coconut crabs have the strongest pinching force of any crustacean, according to research.

What’s more, their maximum crushing force is stronger than the bite force of all land animals, except the alligator.

Coconut crabs are remarkably strong, lifting up to 28 kilograms (62lb) – the weight of a small child.

They use their claws to fight and defend themselves, as well as to crack open coconut shells.

At up to one-metre (3 ft) across, coconut crabs are also the largest of all land-based arthropods – the group that includes insects, spiders and crustaceans.

They live on small islands in the tropical Indian and Pacific oceans.

Useful weapons

Researchers in Japan measured the squeezing force of 29 wild coconut crabs living on Okinawa Island using a sensor.

They calculated that a large crab could be expected to exert a squeezing force of more than 3000 newtons, which exceeds other crustaceans, such as lobsters.

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“The mighty claws of these crabs are useful weapons to deter predators and competitors,” say scientists from the Okinawa Churashima Foundation.

“In summary, coconut crabs have the ability to exert the greatest force among almost all terrestrial animals.”

Coconut crabs are well adapted to life on land.

Unlike most crabs, they only return to the sea to lay their eggs.

They can also climb trees and cut coconuts down.

Despite their size and strength, little is known about coconut crabs and whether or not they are a threatened species.

‘Monstrous’

Charles Darwin described the coconut crab as of “monstrous size” when he saw them on the Cocos (Keeling Islands) in the Indian Ocean during the voyage of the Beagle.

He wrote: “To show the wonderful strength of the front pair of pincers, I may mention that Captain Moresby confined one in a strong tin box, which had held biscuits, the lid being secured with wire; but the crab turned down the edges and escaped.

“In turning down the edges it actually punched many small holes quite through the tin!”