At up to 1 m (3 ft) long, coconut crabs are not simply the largest land crabs. They are the largest arthropods, the group that includes insects, spiders and crustaceans, that live on land. The Japanese spider crab is bigger, but it has the advantage of living underwater where its weight is supported.

Despite their size, little is known about the state of their populations. As a result they are classed as Data Deficient by the International Union for Conservation of Nature, meaning we do not know enough to say whether or not they are threatened.

They live on small islands in the tropical Indian and Pacific oceans. Despite their name they eat a varied diet, including lots of fleshy fruit, and they also prey on smaller crabs.

When they decide to eat a coconut, they rip strips of husk off with their pincers, then hit it repeatedly until it breaks open.

They live in underground burrows, lined with fibres from coconut husks. Unlike most crabs, they are almost entirely land-based, only returning to the sea to lay their eggs.

Coconut crab populations have never been properly surveyed, so we do not know how many there are or if they are in danger.

"Historically, these crabs were hunted by people but not for several decades," says Heather Koldewey of the Zoological Society of London in the UK. Nowadays it seems they are being hunted by rats, which have been introduced to some of their islands.

Koldewey and her colleagues have now begun the first survey of coconut crab populations. They searched for crabs on three atolls in the Chagos Archipelago in the Indian Ocean.

It's too early to say how the crabs are doing, Koldewey says. It will take repeated surveys over a few years to determine that.

It does seem that rats are a problem on some islands. On small islands with little vegetation, the rats seem to hunt coconut crabs more, probably because there isn't much else to eat.

That being said, the crabs do sometimes turn the tables. There are several observations of coconut crabs eating rats.

In the Chagos at least, the crabs should be relatively safe. "Chagos is a fully-protected marine reserve and therefore these crabs are currently well-protected," says Koldewey.

And those vexatious rats may also be on the way out. "The Chagos Conservation Trust started a programme of rat eradication this year," she adds.