It’s hard to go wrong with a hermit crab as your child’s first pet. They’re low maintenance and kinda cute in their own way, plus they’re hypoallergenic, as PetSmart feels the need to point out. But use care when choosing your crab. Whatever you do, don’t pick up Birgus latro, which can grow to a leg span of 3 feet, climb out of your terrarium, and assault the family cat.

Birgus latro is more commonly known as the coconut crab, and it's the largest terrestrial arthropod in the world (the largest overall being the Japanese spider crab – but that's a story for another week). Also known as the robber crab due to its curious propensity for stealing silverware and pots and pans, it’s the 9-pound hermit crab PetSmart wouldn’t dare carry, no matter how conveniently hypoallergenic it is.

The coconut crab is endemic to a variety of islands in the Pacific and Indian oceans, though its populations are extremely threatened on some of these thanks to, you guessed it, human tomfoolery. It grows remarkably slowly, taking perhaps 120 years to reach full size, said ecologist Michelle Drew of the Max Planck Institute.

As an arthropod, the coconut crab wears its skeleton on the outside and must shed it as it grows, so once a year it crawls into the safety of a burrow and molts. It’s highly vulnerable once it steps out of this rigid shell, so to hasten the development of new armor it ... consumes its old exoskeleton.

It is, in effect, recycling the materials, which are in short supply in its terrestrial environment. Coconut crabs that “are disturbed before they have consumed the entire shell often have soft exoskeletons until they have time to reaccumulate the necessary calcium and other minerals,” Drew said in an email to WIRED.

The crab can grow and molt every year like this for more a century, expanding and expanding like a dying star with claws until it threatens to infringe on the very laws of physics.

Image: Wikimedia

“In a water environment you get support from the water that allows you to move with a much heavier shell,” said Drew. “But on land, gravity will play a huge role on how you can move and how heavy you can get. [Coconut crabs] are probably at the limits of what is sustainable given gravity, the weight of the shell, and resources available to them in terms of food and water.”

Feeding this incredible growth is no small task, so the coconut crab eats anything it can get its claws on. It’ll go after fruit, vegetation, and carrion: dead birds and other coconut crabs and such. It has been observed hunting other crabs, and Drew has records of them ambushing young chickens as well as – don’t hate me for this – kittens, like a far less cuddly Alf of the tropics.

But what it really loves are, of course, coconuts. Now, contrary to what Harry Nilsson sang in his 1971 hit “Coconut,” one does not simply put the lime in the coconut and drink 'em both up. Coconuts are extremely difficult to open. But as you may have noticed, the coconut crab is equipped with massive pincers. (One of Drew's friends had one clamp down on his thumb, which lost feeling for three months. She stresses, though, that the coconut crab is in fact quite gentle unless threatened.)

“They use their claws to pull away the outer fibers,” said Drew. “This can sometimes take many days and it often involves a number of crabs. They then use their longest walking leg to puncture a hole through the eyes of the coconut and then they can use their claws to pry open the shell further.”

That may sound like more trouble than it’s worth, but the average mass of crabs living in coconut-rich habitats is double that of their counterparts living in coconut-free habitats, suggesting they extract a whole lot of calories from the things. It also suggests coconut crabs are among the few creatures on this planet besides my father that would actually enjoy a Mounds bar.

Coconut crabs come in all manner of colors. This one is a lovely burnt sienna. Image: Michelle Drew, Max Planck Institute

The coconut crab finds food with its extremely well-developed sense of smell. Like an insect, it uses antennae to zero in on its vittles, but takes this to an extreme by devoting considerable brainpower to the sense.

“The neural (brain) development associated with this is massive compared to other crab species,” said Drew, “and has similarities with insect olfactory development, and is a very good example of convergent evolution associated with a land-based adaptation” – convergent evolution occurring when unrelated species arrive independently at the same adaptation.

Despite its rightful place as the world's largest terrestrial arthropod, coconut crabs begin their lives in the sea. After mating on terra firma, mom releases her fertilized eggs in the ocean, where the larvae swim about for a month. They then enter what’s known as the glaucothoe stage and find a snail shell to occupy.

At this point the coconut crab is in essence much like the hermit crab you’d buy at the pet store. But whereas commercialized crabs live out their days in a shell, forever battling for the choicest homes, the coconut crab eventually leaves that whole keeping-up-with-the-Joneses silliness behind, developing a hard belly and making its way inland. Once it’s gone fully terrestrial, a coconut crab never returns to the sea except to release its eggs. They'll drown if fully submerged.

Despite its freakish size, massive pincers and formidable armor, the coconut crab increasingly finds itself in peril. They have for millions of years lived on islands with no large mammalian predators, allowing them to reach such incredible proportions. This is changing as human encroachment has thrown their food chains into chaos.

“This is why they are disappearing throughout their range,” said Drew. “Most of the islands they live on now have things like pigs, dogs, and humans," all of which will eat them. Finding truly massive coconut crabs is thus becoming rarer. They simply can’t survive long enough to grow to their full potential.

So as much as you might love to grab one so your kid can shame all their classmates during show-and-tell, please do not disturb this remarkable gentle giant. Mother Nature, and your cat, will thank you.

Browse the full Absurd Creature of the Week archive here. Have an animal you want me to write about? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.

References:

Drew, M., et al. (2010) A review of the biology and ecology of the robber crab, Birgus latro. Zoologischer Anzeiger. 2010.03.001

Drew, M., et al. (2013) Factors influencing growth of giant terrestrial robber crab Birgus latro on Christmas Island. Aquatic Biology. 19:129-141