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Cool Critter: Coconut Crab

February 9, 2009

The coconut crab—a.k.a. the giant robber crab—is the world’s largest terrestrial arthropod (animal with jointed limbs, a segmented body and an exoskeleton). Granted, other terrestrial arthropods include spiders and centipedes so the competition isn’t especially fierce…

Nevertheless, the coconut crab is one big-ass crab!

Even ‘big-ass’ is an understatement. These crabs are freakishly huge. They can weigh up to nine pounds with a body length of 16 inches—that’s about the size of a Shih Tzu. And, if an adult coconut crab were to lay flat with its legs fully extended (which would be unlikely for a living crab), it would have a leg span of more than three feet. (Baseball bats max out around 34 inches, FYI.)

Alas, big ass-ness will only get a critter so far in this world. To truly succeed an animal needs to have a talent of some sort. For instance, manakins are known for their dancing skills and cuttlefish can create body doubles. Coconut crabs, well, they’re just really strong mo’ fo’s. They can use their front claws, called “chelae,” to easily cut through a broom handle. These same claws can lift objects weighing up to 64 pounds—that’s about the average weight of an 8-year old child, according to the CDC.

Fortunately, there haven’t been any reports of coconut crabs actually lifting small children with their claws.

In fact, coconut crabs are rather shy. They hide alone in underground burrows during the day and emerge at night to find food. Coconut crabs can and will eat anything, but they really like fruit. They eat guava and figs and nutmeg, but they looove coconuts (hence the name “coconut crab”). Locals believe that the crabs climb palm trees, cut the coconuts down and then scamper down the tree to de-husk and open the ripe coconut. Scientists disagree. Numerous experiments have shown that coconut crabs can only open coconuts that are already damaged.  

When they’re not eating fruit, coconut crabs munch on dead fish and rats and even attack land-crabs. When the comparatively wimpy land-crab sticks out its large front claw to defend itself, the coconut crab grabs it and holds on. Eventually the land-crab casts off its front claw so it can escape and the coconut crab is left with a fresh claw to eat at its leisure.

Ahhh, life is pretty good for the big-ass/bad-ass coconut crab. But this big, bad mo’ fo’ can’t swim. In fact, an adult coconut crab will drown almost immediately if it ends up in the ocean. (Yeah, we know you’re thinking, “What kind of crab can’t swim?” And we know that query may have led you to think of another kind of “crab,” but please, this is not that kind of blog!)

Adult coconut crabs can’t swim because they can’t breathe underwater—they breathe through a lung-like organ that absorbs oxygen from the air rather than water, the way gills do. Coconut crab larvae, however spend their first few months in the ocean. The mama coconut crab releases hundreds, thousands, maybe even millions of eggs into the ocean at a time. The eggs hatch immediately and the larvae spend about a month floating near the surface. Then, they drop down to the ocean floor and search for an empty snail shell. (That’s right, coconut crabs are a type of hermit crab.) They hang out on the bottom for another month or so, switching shells as they grow, and then they haul themselves (and their shell du jour) out of the water.

Juvenile coconut crabs live as hermit crabs in the intertidal zone for a few more months, until they can no longer find a shell that fits them. Then, they dig themselves a burrow and start molting (shedding their outer layer and developing a bigger, better exoskeleton). They molt and molt and molt until they become full grown, which probably takes about 40 years.

After 40 years, they’re big and strong and pretty much dominate their environment (which includes most of the islands of the Indian and Central Pacific Oceans). But “pretty much” is not the same as totally. Juvenile coconut crabs can fall victim to large mammals or, on Christmas Island, yellow crazy ants, but the big ‘uns have only one predictable predator: humans. It turns out that coconut crabs are rather tasty—so tasty, in fact, that the species is threatened on some islands.


To see what a coconut crab looks like, click here. Check out our facebook page for more photos of this cool critter.



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