Coconut Crabs: Crustaceans on Steroids

coconut crab, robber crab
The coconut crab (Birgus latro) is the world's largest terrestrial crab, and is a cousin of the hermit crab. The largest population of coconut crabs live on Christmas Island, Australia. David Stanley/Flickr (CC By 2.0)

Advertisement

As a child, you may have had a pet hermit crab. Maybe you got it on the boardwalk in New Jersey and took it home, gave it a variety of gaudily-painted shells to slip in and out of and a sponge full of water to drink, and forgot about it. Maybe it nipped you with its claws a time or two. Is that hermit crab still alive? You have no idea because it was small, boring and mildly dangerous. Possibly your mother cared for it for many years without your knowledge. Maybe she's still caring for it.

Well, a close cousin of your hermit crab — the coconut crab (Birgus latro) — would not have been so easily ignored. Native to the islands in the Pacific and Indian oceans, coconut crabs are the world's biggest land-dwelling arthropods. Japanese spider crabs (Macrocheira kaempferi), the biggest arthropods, skitter around on the bottom of the ocean without bothering anybody, but coconut crabs hang out on land, and therefore are able to startle us with their size: They can weigh up to 9 pounds (4 kilograms), with a leg span of around 3 feet (1 meter). And although the coconut crab is a close relative of your childhood pet hermit crab, they have been known to eat kittens, rats, chickens and even each other. They've even been proposed as a possible culprit in the disappearance of the body of Amelia Earhart.

"The coconut crabs have no shell as protection from enemies, so they have powerful claws and a large body to protect themselves," says Shin-ichiro Oka, chief research scientist at the Okinawa Churashima Research Center in Japan and the co-author of a 2016 paper about the strength of the coconut crab's claws. "In addition, their mighty claws let them monopolize the terrestrial hard foods, including coconuts, which other animals are unable to get into."

How strong are the claws of a coconut crab?

"We could find that coconut crabs can generate the pinching force of 90 times of their body weight," says Oka. "The calculated pinching force of the largest coconut crab is almost equal to the bite force of the adult lions."

So, these overgrown, coconut-smashing, kitten-eating crustaceans are basically hermit crabs on steroids, but they actually do spend some of their life cycle in the same manner as their daintier cousins.

Coconut crabs spend most of their lives on land, but they start out in the ocean. On the new moon, a female coconut crab deposits larvae (which she's been carrying around in her abdomen since they were just fertilized eggs) into the ocean, and the babies float around in the currents for a month or so before dropping to the seafloor and finding nice, cozy snail shells to move into. Just like your childhood hermit crab friend, coconut crab babies move in and out of shells as they bulk up and get used to living on land. After about a year, the teens of the species eventually find there are no shells left on the beach large enough to accommodate their bulk, so they move out altogether. From here on out, they live the rest of their lives out of the water — they'll drown if totally submerged.

An adult coconut crab's body has a calcium-based exoskeleton, which hardens up once they mature, and they're free to grow to as monstrous a size as they can manage. Every few months, they molt their too-tight exoskeleton in favor of a larger one — they eat the old one after they shed it. In fact, they'll eat almost anything: all kinds of fruit, plant matter, dead animals they find lying around, other crab species or even their own friends (actually, coconut crabs don't have friends — they're pretty solitary). They have an excellent sense of smell, which makes them great at finding rotting carcasses.

But their most important source of food is coconuts, because coconuts seem to be what allows them to achieve the gigantism they're known for.

As you might imagine, size for a coconut crab is a real boon. The bigger they get, the better they are at ripping into coconuts, which aren't their only food option, but they're important — a 2010 study found that the crabs that have access to coconuts are likely to have around double the mass of those living in coconut-free environments. So, if they're lucky enough to have coconuts around, and if they're strong enough to can-opener their way into a coconut, the size ceiling becomes a whole lot higher.

But just because they might be capable of devouring the body of the world's most famous female aviator doesn't mean they're not vulnerable. Coconut crab numbers seem to be declining, probably because the islands they live on aren't what they used to be — introduced species like dogs, pigs and humans eat the adults, and invasive rats gobble up the smaller, more vulnerable babies. They are extremely slow growing and can live to be about 50 years old. Coconut crabs have been listed as Data Deficient by the International Union for Conservation of Nature, meaning nobody really knows enough about these animals to understand their conservation needs. Although it's likely they're not doing well, their status was last assessed in 1996, so more research is needed to understand where they stand.