The giant crustaceans use their mighty claws to scavenge, hunt
A big coconut crab snaps its outsized left claw as hard as a lion can bite, new measurements suggest. So what does a land crab the size of a small house cat do with all that pinch power?
For starters, it protests having its claw-force measured, says Shin-ichiro Oka of the Okinawa Churashima Foundation in Motobu, Japan. “The coconut crab is very shy,” he says. It doesn’t attack people unprovoked. But wrangling 29 wild Birgus latro crabs on Okinawa and getting them to grip a measurement probe inspired much snapping at scientists. Oka’s hand got pinched twice (no broken bones). “Although it was just a few minutes,” he says, “I felt eternal hell.”
The strongest claw grip the researchers measured squeezed with a force of about 1,765 newtons, worse than crushing a toe under the force of the full weight of a fridge. For comparison, a lion’s canines bite with 1,315 newtons and some of its molars can crunch with 2,024 newtons, a 2007 study calculated. Because grip strength increases with body size, crabs bigger than those measured in the study might surpass the bite force of most land predators, Oka and colleagues proposed last year in PLOS ONE.
Coconut crabs, however, start life about as scary as a soggy grain of rice. Fertilized eggs hatch in seawater and bob around planktonlike in the western Pacific and Indian oceans. The crabs eventually return to land, where they spend most of their long lives, up to 50 (or maybe 100) years, as landlubbers that will drown if forced back into water for too long. Yet females have to risk the ocean’s edge each time they lay the next generation of eggs.
Both moms and dads grow a powerful left claw, handy for dismembering whatever the omnivorous scavengers find: roadkill and other dead stuff, innards of palm trees and nuts. The crabs can break open coconuts, but the job “takes hours,” says Jakob Krieger of the University of Greifswald in Germany. Cracking open a red crab, however, takes seconds.
Coconut crabs not only scavenge red crabs but also hunt them on Christmas Island in the Indian Ocean, Krieger says. Only the strictest vegetarian would ignore the 44 million or so red crabs scuttling around, and even small coconut crabs get a taste. Krieger watched an underpowered coconut crab grab hold of and wrestle its prey. The red crab abandoned its trapped limb and fled. But the little coconut crab scored a crab-leg dinner.
S.-i. Oka, T. Tomita and K. Miyamoto. A mighty claw: pinching force of the coconut crab, the largest terrestrial crustacean. PLOS ONE. Published online November 23, 2016. doi: 10.1371/journal.pone.0166108.
J. Krieger et al. Notes on the foraging strategies of the giant robber crab Birgus latro (Anomala) on Christmas Island: evidence for active predation on red crabs Gecarcoidea natalis (Brachyura). Zoological Studies. Vol. 55, 2016. doi: 10:6620/ZS.2016.55-06.
R. Cheung. Size doesn’t matter for crayfish’s one-two crunch. Science News. Vol. 181, April 21, 2012, p. 15.
P. Christiansen and S. Wroe. Bite forces and evolutionary adaptations to feeding ecology in carnivores. Ecology. Vol. 88, February 2007, p. 347. doi: 10.1890/0012-9658(2007)88[347:BFAEAT]2.0.CO;2.