 Arthropods, living organisms with hard skeletons on the outside of their bodies, include insects, spiders, lobsters and crabs. Of those living on land, which one do you think grows the largest?  The answer is a species of crab called the 'Coconut Crab', Birgus latro, named for its ability to crack open coconuts. It is indeed the largest land-living arthropod in the world, with a body length of up to 40 cm, a weight of up to 4.1 kg, and a leg span of more than 90 cm. (Some specimens have been reported to have reached a body length of almost 2 metres). Coconut crabs can live more than 30 years.The body of the coconut crab, a decapod, has two sections; the front, with 10 legs, and the abdomen. The front pair of legs have large claws, which are used to capture and tear open food. The next two pairs of legs are powerful, allowing this crab to climb trees. The Coconut crab is a type of hermit crab. Young ones use discarded snail shells to protect their soft abdomens; as they get older they may also use broken coconut shells. Unlike other hermit crabs, however, adult coconut crabs don't carry shells, but harden their abdominal shells by depositing chitin and chalk. These shells have to be moulted at periodic intervals; after moulting, it takes about 30 days for the exoskeleton to harden, during which time the crab's body is soft and vulnerable; the crab stays hidden to protect itself.  Coconut crabs can't swim, and even small ones will drown in water. They use a special lung, an organ that is half-way between gills and a true lung, to breathe. Coconut crabs use their smallest pair of rear legs to clean this breathing organ and to keep it moist so that it can absorb oxygen from the air. These crabs also have a rudimentary set of gills, which are too small to be of use in water; they are an evolutionary remnant from an earlier era when these creatures were aquatic.
The coconut crab can smell, but this sense works differently than in its aquatic cousins. Because it lives on land, the smell sensors on its antennae have evolved to work in air, making them look like those of insects. Insects and the coconut crab originated from different evolutionary paths, but the need to detect smells in the air led to the development of remarkably similar sense organs, making this a good example of convergent evolution. Coconut crabs can detect odours like the smell of rotting meat, bananas, and coconuts over large distances. To reproduce, male crabs deposit spermatophores on the abdomen of the female, and the female's eggs get fertilized as they are extruded through this spermatophore mass. She carries them glued to the underside of her abdomen for several months; when they hatch, the larvae are released into the ocean. The new-born larvae will eventually find a discarded shell for protection and crawl back onto land, where they lose the ability to breathe in water. Four to eight years after hatching, a mature crab is ready to reproduce.   The diet of coconut crabs, which are omnivorous, consists of fruits, coconuts, seeds, dead animals, and even other crabs. They will steal food from one another, and they tend to eat in rock crevices or burrows where they can do so undisturbed. Their burrows often are lined with fibres from coconut husks. Coconut crabs live in areas around the Indian and central Pacific Ocean. Christmas Island in the Indian Ocean has the largest population of coconut crabs in the world. Adult coconut crabs have few natural predators; they are mostly only killed (and eaten as a delicacy) by humans. On many islands where they are found they are often protected by law. |