Coconut crabs: the bird-eating behemoths thriving on isolated tropical islands

Coconut crabs (Birgus latro) might seem unusual: they're absurdly large, land-dwelling, bird-hunting crustaceans. But on their isolated island homes, their peculiar traits work incredibly well for them.

What is a coconut crab?

To say that coconut crabs are big would be an understatement.

'They're a mega-crab, really,' explains Miranda Lowe, Principal Curator of Crustacea at the Museum. 'They range, but they can be huge - up to a metre leg span.'

They're not quite the world's largest crab - that would be the Japanese spider crab (Macrocheira kaempferi), which can reach a whopping 3.7 metres from claw to claw.

But the coconut crab is the largest crustacean that spends all its adult life on land, with a Guinness World Record to prove it. It's also the biggest land-dwelling arthropod, the group of invertebrates that also includes insects, spiders and centipedes.

Coconut crabs can live up to 60 years, reaching sexual maturity at about five years old. They mate between May and September and females release their eggs into the water.

When they hatch, the larvae disperse on floating coconuts, logs or other rafts for four to six weeks. They then transform into shrimplike creatures called glaucothoe and sink to the seabed to find a suitable gastropod shell for protection. They will then begin to migrate towards the shore, spending another four weeks around the high-tide mark before they become juvenile crabs. 

As adults they no longer use gastropod shells and instead rely on their tough exoskeleton to protect them from predators.

Although they spend their multiple larval stages in water, when coconut crabs eventually become adults, they can't swim. In fact, they drown if they end up underwater for a prolonged period. This is because instead of gills, they have branchiostegal lungs that allow them to breathe air.

Coconut crabs are found across the Indo-Pacific, from islands off the coast of Africa near Zanzibar to the Gambier Islands in the Eastern Pacific Ocean. They are typically found in coastal forests with plenty of rock crevices and soil for digging burrows. On some islands, they can be found up to six kilometres from the shore.

What do coconut crabs eat?

Their name might give it away, but coconut crabs are known for cracking into green coconuts to feast on the white flesh inside.

'Their walking legs are kind of curved and clawlike, and they have an inward grip so they can climb palm trees and other trees,' explains Miranda.

Coconuts aren't all they'll munch on, however. Coconut crabs will eat fallen fruit, nuts and seeds. 

Perhaps unexpectedly for crabs, they are also rather ruthless predators.

'Most other crabs live exclusively on the water's edge or in the ocean, and their food source is dead animals - marine worms, the flesh of other dead crabs, that sort of thing. They're not exactly vicious hunters.'

Coconut crabs, however, are known predators of rats, others of their own species and even large migratory seabirds, such as boobies that they find nesting on their islands. They have been spotted mounting attacks in the dark of the night and grabbing unsuspecting prey that pass too close to the crabs' burrows.

Miranda says, 'It's an adaptation because of their food source. On land, some of the time they have to find something else other than coconuts.'

Coconut crabs' broad diets have even led some to suggest that the reason famed aviator Amelia Earhart was never found when she disappeared mid-flight over the Pacific Ocean was because she was devoured by coconut crabs after perishing on Nikumaroro island. 

How strong is a coconut crab?

If you have ever tried to open a coconut, you'll know it can be a bit of a challenge. Not so for a coconut crab.

Armed with two large and powerful pincers, coconut crabs can pound and rip their way through the tough exterior of a coconut with relative ease.

Miranda says, 'People may be familiar with crab pincers if they try and pick the animals up from the front and not the back. Coconut crabs' pincers have a serrated edge, which are commonly known as teeth because they do act as teeth on the front claws. They're used to crack open the coconuts.'

The squeeze of a coconut crab's pincers can be much stronger than a human's 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.

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 haven't been many large-scale studies on the populations of coconut crabs. We don't accurately know how many there are, but the International Union for Conservation of Nature lists them as a Vulnerable species.

Coconut crabs don't have many predators, thanks to their often isolated island homes. However, one of the biggest threats they face is from humans.

Miranda explains, 'They're such a large crab, so obviously they've got lots of crab meat in them. It's really difficult to tell islanders that you should protect the species when it's their food source.

'Communities rely on them for food, but also to sell. It can have a major impact on the species.'

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.

There are regulations to help protect coconut crabs across many of the Pacific islands they inhabit, though the specifics of these are determined by each country.

Minimum harvest sizes are used on many Pacific islands - in some locations, females carrying eggs on their underside are also specifically protected. Some governments have set quotas for the amount of crabs that can be caught, and permits may be required to export this species, although in other areas exporting coconut crabs is entirely prohibited.

Another threat to the species is the total loss of their island homes. Many populations of coconut crabs depend on low-lying island atolls. Sea level rise caused by climate change is a big threat to these types of habitats, with some at risk of being entirely lost to the ocean.