The Force is with the Claw of Land Crabs

By Dave Armstrong - 24 Nov 2016 14:20:0 GMT
The Force is with the Claw of Land Crabs

This land crab weighs in at 4.1kg (9lb), has gigantic crushing claws and will pinch you painfully and mercilessly, given the chance. So why do we want to study them? Great guard dogs for your coconut trees!

Coconut Robber image; Credit: © Shin-ichiro Oka

The crabs are often forgotten, except for edible examples, when you remain in the western world. However, freshwater and land crabs prove to be as successful as some of their crustacean relatives. The ultimate example is the Pacific Island crab called the coconut crab, Birgus latro This omnivorous giant still needs to return to water for breeding, but has achieved what several species seek to do on land in that it reaches a size that dominates almost all predators into submission, including me, for example! Even a protective shell is found unnecessary in the evolution of this monster from an ancestral Coenobita hermit type.

The latest paper on this incredible species measures the claw (chela) force on a bite- measuring device and explores the reasons for such an evolved weapon. Shin-ichiro Oka of the Okinawa Churashima Foundation in Japan and his colleagues, Taketeru Tomita and Kei Miyamoto release their useful results in PLoS One today, with a most valuable read available under the title of A Mighty Claw: Pinching Force of the Coconut Crab, the Largest Terrestrial Crustacean.

29 wild crabs from the Ocean Expo Park in northern Okinawa were briefly captured, with many dangerous pinches on various parts of the researchers anatomy. The strength in their chelae was far beyond that predicted by the muscle cross-sectional area. A force of up to 3300N was estimated, greater than that exerted by any other crustacean. It also exceeds any animals bite strength apart from alligators. The alligator presumably can apply greater leverage with its lengthy jaw. Gender differences also did not appear in the results, which contradicts other measurements on Christmas Island.

As the land crab species evolved, they lost their need for shelter. In contrast, hey still maintains very aggressive territorial defence using those mighty claws They take their sustenance from fruit, tree pith and carrion. They employ the claw on both coconuts and the hard cases of animal prey. Exactly how the force is applied through muscle sarcomeres now remains a further challenge. We wait eagerly for the research that will determine whether the muscle sarcomere length is indeed as long as it needs to be to exert the necessary force.

Another land crab with a parallel niche in Hawaii seems to have disappeared with Polynesian landings on the islands thousands of years ago. Unrelated to the Birgus genus, it has been named Geograpsus severnsi. The story is at Hawaiian crab driven to Extinction.