(Shin-ichoro Oka/PLOS.org)

The largest hermit crabs on the planet cannot help but attract speculation and curiosity. The Indo-Pacific creatures have been blamed, dubiously, for scampering off with Amelia Earhart’s bones. They are said to eat kittens. They go by many names: the robber crab, the palm thief, Birgus latro and, most popularly, the coconut crab.

The truth is that coconut crabs are immense — up to 18 inches long and nine pounds — and their oddities do not end with bodily superlatives. Even though female coconut crabs release their eggs into the sea, full-grown crabs are strict landlubbers. Their gills have evolved into spongy lung-like tissue that absorbs oxygen from the air. Unlike most hermit crabs, adult coconut crabs do not tuck soft posteriors into sea shells. As they age, their skin hardens into tough, pebbly sheathes, which they will molt as they grow (and then they consume the discarded exoskeleton). They like to climb trees.

That would all be strange enough. But among the crabs’ outsize traits, the most impressive is the power of their claws. They can heft items up to 60 pounds. And, according to a recent study, the coconut crab pincer generates up to an estimated 740 pound-force — a force about 90 times their own body weight. It is also four to five times as strong as the force the human jaw can produce.

“The force is remarkably strong,” study author Shin-ichiro Oka, a zoological researcher at Japan’s Okinawa Churashima Research Center, said in an email. “The range of their relative pinching force was greater than the maximum force of any animals within their body-size range.”

In a study recently published in the journal PLOS One, the process to measure the crab’s claws sounded straightforward enough. The scientists collected 29 crabs from Okinawa, Japan; then they placed a stainless steel force-measuring stick between the animals’ pincers and let the coconut crabs crush down.

The crush forces, the strongest of which was a 400 pound-force, were correlated with body size. (Oka told Reuters that if his hands were as strong as coconut crab claws, he could exert six tons of crushing force.) When extrapolated to the biggest crabs, the scientists say the coconut crab’s claw force could nearly rival a lion’s bite. Few other animals can generate such power — though there are notable exceptions, including the extraordinarily strong alligator jaws.

In practice, though, Oka’s research was a brief sojourn into the world of arthropod-inflicted pain.

Crab pincers snagged the fleshy part of his palm during the study. Twice. “When I was pinched, I couldn’t do anything until they unfastened their claws,” Oka said. “Although it was a few minutes, I felt eternal hell.”

As for why the crab would need so mighty a pincer, Oka offered a few explanations. “They have no shell to protect from their enemies,” he said, “so the powerful claw and large body would be useful as a weapon to protect themselves.” He pointed out the crabs do occasionally eat coconuts, which only a strong claw could tear open.

Although the researchers examined the muscle fibers and geometry of a crab claw, the study authors were not quite sure how the animals developed their impressive clamp maneuver. As they wrote in the paper, “The mystery of the massive force remained unsolved.” Oka said he hoped other specialists would examine the claw to see whether the animals’ biochemistry or physiology held additional clues.

For an animal who has been the object of scientific curiosity for hundreds of years — Charles Darwin tried to capture one in a biscuit tin, but it escaped — the crab still has its secrets. Although it can be found on several islands across the Indian and Pacific oceans, biologists do not have sufficient data to estimate the world population. 

Despite the threat of a few minutes of hellish pain, Oka and his colleagues remained undeterred. “We are going to continue the study of their life history,” he said.

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