The giant coconut crab

21/08/2014 07:32

Written by Philip Briggs

Looking for all the world like a refugee from an improbable science fiction B-movie, the giant coconut crab is the world’s largest terrestrial crustacean, attaining a mass of 5kg, a length of up to 50cm and a leg span of 1m.

The foremost of its five leg-pairs terminates in deadly pincers capable of scything straight through a wooden broomstick, or of lifting an object six times its body weight. Pairs two to four, meanwhile, are tipped by smaller pincers that enable the crab to clasp tightly on to the trunks of the vertical palm trees it habitually ascends.

Giant coconut crab Tanzania by Ariadne Van Zandbergen, Africa Image LibraryThe giant coconut crab is the world's largest crustacian © Ariadne Van Zandbergen, Africa Image Library

The first recorded description of this massive decapod was penned by the 17th-century Dutch naturalist Georgius Rumphius, who noted that it was ‘always on land, without ever getting into the water’ and that ‘it climbs Coconut Trees, and pinches off the nuts, and then searches under the tree for the ones that were thrown down’.

More than a century later, the ‘monstrous’ terrestrial crab and ‘wonderful strength’ of its pincers so captured the imagination of Charles Darwin that he devoted a full page of his landmark Voyage of the Beagle to describing how an associate ‘confined one in a strong tin box… the lid being secured with wire; but the crab turned down the edges and escaped [and] actually punched many small holes through the tin!’

Looking for all the world like a refugee from an improbable science fiction B-movie, the giant coconut crab is the world’s largest terrestrial crustacean.

Darwin was the first to document the crab’s ability to break open a coconut using its mighty pincers: ‘The crab begins by tearing the husk, fibre by fibre, always from that end under which the three eye-holes are situated; when this is completed, the crab commences hammering with its heavy claws on one of the eye-holes till an opening is made. Then turning round its body, by the aid of its posterior and narrow pair of pincers it extracts the white albuminous substance.’

Unusually for a crustacean, the coconut crab only reaches sexual maturity at the venerable age of five years, and some individuals live to be at least 30. The young are amphibious but the adult is a confirmed landlubber that would drown were it to be submerged for any time. Several idiosyncrasies are associated with this terrestrial lifestyle. It has an acute sense of smell thanks to a ‘nose’ that most closely resembles those of terrestrial insects, a textbook example of convergent evolution.

Its remarkable capacity to climb smooth palm trunks to a height of 6m is encapsulated by Dr Karen Burns’ evocative recollection of a ‘big adult coconut crab dining on a dead rat that he had hauled high up onto a tree limb leopardstyle’. Then there is its magpie like propensity for wandering back to its daytime lair with a booty of shiny household objects, as alluded to in its German name palmendieb (palm thief) as well as in the Latin binomial Birgus latro (robber crab).

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