Coconut Crabs: The Bird-Eating Behemoths Thriving On Isolated Tropical Islands

 

Coconut crabs are the world's largest land-dwelling crab ©

You may be familiar with hermit crabs, the adorable and often tiny crustaceans that totter along the beach toting their bodies in seashells.

Meet their enormous relatives, coconut crabs. Their leg span is up to a metre, they have incredible grip strength and they can lift objects the weight of a 10-year-old child.

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. 

Coconut crabs only protect their bodies with shells whilst they are juveniles

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.

Curved legs and an inward grip make coconut crabs excellent climbers. 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.

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. Coconut crabs have a varied diet including fruits, seeds and animals such as rats and seabirds

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.

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.