You may have seen hermit crabs before. They are cute and often very small crustaceans that walk along the beach carrying seashells around with them.
Meet their enormous relatives, coconut crabs. They have amazing grip strength and can lift things that are as heavy as a 10-year-old child. Their legs can reach up to a meter in length.
Coconut crabs (Birgus latro) might seem unusual: theyre absurdly large, land-dwelling, bird-hunting crustaceans. But on their isolated island homes, their peculiar traits work incredibly well for them.
How Big is a Coconut Crab? A Look at the Largest Land Arthropod
Coconut crabs are truly larger-than-life creatures. As the largest land-dwelling arthropod in the world, these hefty crustaceans grow to incredible proportions. Their massive size and formidable pincers strike awe and inspire curiosity in those who encounter them. Just how colossal do coconut crabs become? Let’s take a closer look at the astounding dimensions of these monster crabs!
First off, coconut crabs are big – like, really big. They measure an average of 16 inches (40 cm) from leg to leg, but can span over 3 feet (1 meter) from pincer tip to pincer tip when fully grown. Their bodies alone can reach up to 16 inches (40 cm) long Now that’s a huge crab!
These behemoths can weigh up to 9 pounds (4 kg). To put that into perspective, that’s about the same as a gallon of paint or a medium sized bowling ball. Lifting a coconut crab must feel like lifting a dumbbell!
The claws are particularly massive, sometimes reaching an alarming 6 inches (150 mm) in length. Coconut crabs can crack open coconuts and other hard-shelled fruits with the crushing force of their claws, which exert an incredible 1300 pounds per square inch (PSI). That’s the same biting force as an adult lion – yikes!
Males tend to be larger than females, giving them an advantage in confrontations over food and mates. The biggest coconut crabs are often mature males at the peak of their growth. They reach full size after around 40-60 years. These aged crabs truly represent the upper limits of the species’ astounding growth.
Coconut crabs grow steadily throughout their long lives, molting their exoskeletons to accommodate their increasing girth. Each time they molt, they hide away in burrows for weeks while their new shells harden. As juveniles, they use shells vacated by snails and hermit crabs for protection.
Baby coconut crabs, called larvae, are tiny – less than half an inch (1 cm) long. They grow into golf ball-sized glaucothoe crabs before transitioning fully to land. While all crustaceans molt to grow, no other species comes close to the sheer immensity attained by coconut crabs!
Just how do coconut crabs get so colossal compared to other crabs? Their size is attributable to two key evolutionary adaptations:
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Larger abdominal muscle mass for increased power. Coconut crabs developed enlarged abdominal muscles to aid in climbing and crushing prey. More muscle means more power for walking, pinching and lifting heavy objects!
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Structural support for weight on land. Specialized abdominal armor and modified limb anatomy allows their heavy bodies to move and sustain themselves in a terrestrial environment.
In short, coconut crabs are built sturdy! Their unique biology allows them to not only survive, but thrive at sizes beyond the scope of other land crabs.
The sight of a fully grown coconut crab creeping through the brush is sure to give pause, as it looks more like a small lobster than an average crab. Give such a behemoth a wide berth should you have the fortune of encountering one! While their fearsome claws can inflict painful injury, they rarely attack humans unless severely provoked.
Nonetheless, it pays to appreciate coconut crabs from a respectable distance. Let them pass undisturbed as they lumber through their tropical island habitats. The age of discovery may have ended, but for those who meet a coconut crab for the first time, it’s sure to feel like encountering a monster from the ancient past! These titans of the crab world never cease to amaze with their larger-than-life proportions.
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How strong is a coconut crab?
People who have tried to open a coconut before know how hard it can be. Not so for a coconut crab.
Coconut crabs have two big, strong pincers that they can use to easily pound and tear through the tough shell of a coconut.
If people try to pick up crabs from the front instead of the back, Miranda says, they may know what crab pincers are. When a coconut crab’s front claws are closed, the pincers have a serrated edge that looks like teeth. Theyre used to crack open the coconuts.
The squeeze of a coconut crabs pincers can be much stronger than a humans grip. Scientists tested the force exerted by 29 wild coconut crabs, unintentionally getting themselves pinched several times  in the process, and found a maximum force of a little over 1,765 newtons. For reference, humans have a maximum bite force of between 1,100 and 1,300 newtons, based on a study using virtual models of human skulls.
There is a link between mass and pincer force in coconut crabs. The bigger the crab, the stronger their pincers are. © John Tann via Flickr (CC BY 2.0)
The bigger the mass of the crab, the greater the pincer force. When scaled up for a fully grown, four-kilogram coconut crab, the maximum force would be around 3,300 newtons.
Coconut crabs can also lift up to about 30 kilograms, approximately the weight of a 10-year-old child. Their powerful claws and strength are essential for accessing their various food sources.
Coconut crabs and humans
There havent been many large-scale studies on the populations of coconut crabs. We dont accurately know how many there are, but the International Union for Conservation of Nature lists them as a Vulnerable species.
Coconut crabs dont have many predators, thanks to their often isolated island homes. However, one of the biggest threats they face is from humans.
Miranda explains, Theyre such a large crab, so obviously theyve got lots of crab meat in them. Its really difficult to tell islanders that you should protect the species when its their food source.
Communities rely on them for food, but also to sell. It can have a major impact on the species.
Coconut crabs are at risk from being overharvested by humans as well as facing threats from climate change © Drew Avery via Flickr (CC BY 2.0)
Coconut crab populations have been depleted on inhabited islands from overharvesting, but also habitat loss. Their slow population growth rate will add up to be a problem for the species survival.
Many of the Pacific islands where coconut crabs live have laws in place to help protect them, but the specifics of these laws vary from country to country.
On many Pacific islands, there are minimum harvest sizes, and in some places, females carrying eggs on their undersides are also protected. In some places, governments limit the number of crabs that can be caught, and you may need a permit to export this species. In other places, it is illegal to export coconut crabs at all.
Another threat to the species is the total loss of their island homes. Many populations of coconut crabs depend on low-lying island atolls. Many of these habitats are in danger because of rising sea levels caused by climate change. Some may even be washed away by the ocean. Â.