Species: Birgus latro

Habitat: Coastal forests of islands in the Indian and Pacific Oceans

It's paradise with a twist. As the sun sets over the beautiful Palmyra atoll, south of Hawaii, Earth's largest land arthropod emerges from its lair.

Coconut crabs can reach a length of 40 centimetres, with a leg span of 90 centimetres, and weigh 4 kilograms. Most such monster arthropods – the group that includes insects, spiders and crustaceans – live in the sea, where the water helps support their heavy bodies. To survive on the land, coconut crabs have had to evolve a suite of strange adaptations. But their exceptional lifestyle has also put them at great risk, and conservationists are only now working out how to protect them.

Coconut crabs – also known as robber crabs for their habit of stealing food from inattentive campers – are not true crabs. They are members of the group that includes hermit crabs, known for living in discarded mollusc shells. Coconut crabs do this when they are young, but as they grow they give up the habit.

They begin their lives in the sea. Female crabs carry their fertilised eggs around on their bodies, and release them into the sea when they hatch – generally when the moon is new. The juveniles develop in the sea for about a month, after which they head out onto land.

There they face a problem: how to breathe. Aquatic crabs have gills, but these are inefficient out of the water. To get round this, coconut crabs have developed organs called branchiostegal lungs, which are essentially sets of gills turned inside out. They have only been able to develop these strange organs because, unlike other hermit crabs, they perform the slightly obscene-sounding trick of exposing their abdomens to the surrounding air.