The claws of coconut crabs can pinch with the strongest force of any crustacean, also exceeding the bite force of most land animals, according to Japanese scientists in a study published in the journal PLOS One.
Coconut crabs (Birgus latro) are the largest terrestrial crustaceans and are remarkably strong, able to lift nearly 30 kg (66 pounds). They use their prodigious claws to capture and manipulate food items with hard exteriors (including coconuts), as well as to fight other crabs and defend themselves.
Lead author Shin-ichiro Oka from the Okinawa Churashima Foundation says coconut crabs look like giant versions of hermit crabs without their shells. In fact, coconut crabs share a common ancestor with hermit crabs. While juvenile coconut crabs still possess shells for protection, the adults are shell-less and protected by their hard, calcified bodies.
It’s known that crabs can exert a greater force with their claws than many animals can with other appendages, but the pinching force of coconut crabs had never been measured.
Oka and his colleagues captured 29 wild coconut crabs from Okinawa Island, Japan, and measured the pinching force of their claws. During the field study, the researchers learned first-hand just how powerful these crabs are.
“I was pinched two times,” says Oka. “When I was pinched, I couldn’t do anything until it unfastened its claw. Although it was only a few minutes, I felt eternal hell.”
The researchers found that the pinching force of the coconut crab was extremely strong and increased with the size of the crab. According to a previous study, the largest coconut crab known weighed 4 kg (8.8 pounds). Based on the data from this new experiment, a crab that large would have a pinching force of 3300 Newtons. This force greatly exceeds both the pinching force of other crab species and the bite force of most land predators, with the exception of alligators.
These mighty claws provide several advantages to the coconut crab’s land-lubbing lifestyle. They serve as weapons to drive off predators and competitors, as well as tools that allow them to hunt and eat a wide variety of foods, including coconuts, which other animals are unable to access.
Oka S-i, Tomita T, Miyamoto K (2016) A Mighty Claw: Pinching Force of the Coconut Crab, the Largest Terrestrial Crustacean. PLoS ONE 11(11): e0166108. doi:10.1371/journal.pone.0166108.