About the World's Largest Terrestrial Arthropods - Coconut Crabs

Coconut crabs are the largest land-dwelling arthropods in the world.

Coconut crabs belong to a class of crab-like creatures, and are related to hermit crabs and king crabs. Though they look like crabs, and are named likewise, scientifically they fall under a different infraorder.

While 'true crabs' come under the infraorder Brachyura, hermit crabs, king crabs and coconut crabs are classified under the infraorder Anomura. These two infraorders in conjunction form the clade Meiura.

All members of the clade Meiura evolved from a common ancestor, but there are noticeable difference between Anomura and Brachyura. The most important difference is the position and size of the last pair of legs.

Both true and pseudo crabs have five pairs of legs, but the last pair of legs in pseudo crabs is quite tiny compared to the other four pairs, and is hidden under their body. False crabs don't use the fifth pair for locomotion, but for reproductive purposes.

Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Superfamily: Paguroidea
Family: Coenobitidae
Genus: Birgus
Species: B. latro

They are found on most islands in the Indian Ocean, on the Malay archipelago, and beyond Australia. They used to be found in Australia and Africa, but those populations have been eradicated.

Coconut crabs are the largest terrestrial arthropods in the world. They can reach a leg-span of up to 1 meter (though 60-70 cm is common), and can weigh as much as 4 kg.

As mentioned before, coconut crabs have 5 pairs of legs, like true crabs, but the last pair is hidden under the carapace -- their main body. Males use the legs to transfer their sperm to the females, who in turn use the legs to protect and clean the embryos. Both genders use the legs to clean the underside of their body.

Crabs on the African side of their range, especially in Seychelles, tend to be red, while most other populations are blue. Like the hermit crabs, which are their closest relatives, coconut crabs use mollusc shells to protect their soft abdomens.

However, unlike hermit crabs, coconut crabs build a hard plating over their abdomens after adulthood. The plating has to be molted annually, leaving the individual vulnerable while it is recreated.

Coconut crabs are designed for terrestrial survival. If dropped in water, they will drown in less than 24 hours! They have well-developed air-breathing organs known as branchiostegal lungs, an intermediate step between lungs and gills, and can't breathe in water except in the larval stage.

But branchiostegal lungs need moisture to function in optimum condition, so the crabs use their hidden 9th and 10th legs to moisten it. Coconut crabs also have an undeveloped set of gills that allows them to survive in water for some time.

Despite their name, coconuts are not the coconut crab's primary or favorite meal. They primarily eat the flesh, seeds, and pith of fresh fruits. In addition to fruits, they act as scavengers, feeding on carcasses, and also seek out nuts.

They do consume coconuts, but it is an occasional treat, than a staple diet. They rip off the husk, and then either use their strong pincers to crack open the tough shells, which can take days, or climb up a tree and drop the fruit from a height. They have even been observed actively hunting small animals, but this is, in all probability, a rare occurrence.

Coconut crabs have a well-developed sense of smell, and rely almost entirely on it to find food.

The breeding season of coconut crabs falls between late May and early September. Fertilization occurs on dry land, and the embryos are carried by the female on the underside of her body until the time of hatching. When the time comes, they are released into the ocean.

This is the only stage of their lives spent in water. The larvae float in the water for about a month, after which they find a shell suitable for their size, and start visiting dry land. After several changes of shells (with increasing size), the infants finally develop hard exoskeletons, at which point they crawl out of the water permanently.

They gradually lose the ability to breathe underwater. They reach sexual maturity after five years.

Coconut crab larvae are easy prey for a multitude of predators. As is the case with most species, many larvae are killed before adulthood.

Coconut crabs are killed for their meat, which is a delicacy on many islands. They have also been wiped off where their habitat coincides with human population. However, this doesn't seem to pose a significant threat to their survival.