The coconut crab (birgus latro), also called the robber crab, is the largest living land invertebrate, although there seems to be differing views on exactly how large. Most webpages and books give a figure of around 5 kilograms with a three foot leg span, but I have heard estimates of 10 or 15 kilograms. I am sure that somewhere deep in the jungles of the South Pacific, someone has seen a coconut crab larger than a human being. However, the best estimates for coconut crab size are between 10 and 20 pounds.
The coconut crab is not a true crab, but is a false crab, and is indeed nothing more then an overgrown hermit crab. However, unlike other hermit crabs, the coconut crab has a hardened abdomen, and therefore does not need to collect the shells of mollusks, which is just as well, since there are no mollusks that grow large enough.
The coconut crab is a modified sea creature, which means it needs to return to the sea to mate, and to give birth to its larva, which are born as tiny zygotes and live as plankton until they make it to adulthood and shore. Also, although it lives on land, the coconut crab does have gills that must be kept wet if it wishes to breathe. For that reason, it can only survive in humid climates where the gills will not dry out. Since the crab lives on small islands in the south pacific, this is relatively easy to do.
Much like its smaller cousin, the hermit crab, the coconut crab is extremely omnivorous, eating vegetables and carrion. As its name suggests, the crab is particularly fond of coconut, and will climb trees to knock coconuts down. It then uses its powerful claws to open the coconuts up.
With the exception of certain parasites, the crab has only one natural predator: humans, who consider the crab meat to be a delicacy. This is perhaps a very efficient recycling of food, since the crab eats much material such as carrion that would be lost otherwise.
The existence of the coconut crab is very interesting, to say the least, even discounting many of the apocryphal stories that surround them. It is the world's largest crustacean, a crab that loves to climb trees to get coconuts. It makes the Tree Octopus sound somewhat plausible.