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Saturday, June 25, 2005

Invertebrates: Coconut Crab


The coconut crab (Birgus latro) is the largest terrestrial arthropod in the world. It is a derived hermit crab which is known for its ability to crack coconuts with its strong pincers in order to eat the contents. It is sometimes called the robber crab or palm thief (in German, Palmendieb), because some coconut crabs steal shiny items such as pots and silverware from houses and tents. Another name is the terrestrial hermit crab, due to the use of shells by the young animals (although terrestrial hermit crab also applies to a number of other hermit crabs - see Australian land hermit crab). The coconut crab also has different local names as for example ayuyu in Guam, or unga or kaveu.

Reports about the size of Birgus latro vary, and most references give a weight of up to 4 kg (9 lb), a body length of up to 400 mm (16 in), and a leg span of 1 m (3 ft), with males generally being larger than females. Some reports claim weights up to 17 kg and a body length of 1 m. It is believed that this is near the theoretical limit for a terrestrial arthropod (but when the body is supported by water, larger sizes are possible; see Japanese spider crab). They can reach an age of up to 30-60 years (references vary).

The body of the coconut crab is, like all decapods, divided into a front section (cephalothorax) and an abdomen, which has 10 legs. The front-most legs have massive pincers used to open coconuts, and these claws (chaelae) can lift objects up to 29 kg (64 lb) in weight. The next three pairs have smaller tweezer-like chelae at the end, and are used as walking limbs. In addition these specially adapted limbs enable the coconut crab to climb vertically up trees (often coconut palms) up to 6 m high. The last pair of legs is very small and serves only to clean the breathing organs. These legs are usually held inside the carapace, in the cavity containing the breathing organs.


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