Christmas Island Red Crab

The Christmas Island Red Crab (Gecarcoidea natalis), is a species of terrestrial crab endemic to Christmas Island and the Cocos (Keeling) Islands in the Indian Ocean. Although restricted to a relatively small area, it is estimated that up to 120 million red crabs may live there, making it the most abundant of the 14 terrestrial crab species on Christmas Island.

The carapace is up to 4.5 inches long, rounded, and encloses the gills. The claws are usually of equal size, unless one becomes injured or detached, in which case the limb will regenerate. During that time, it will be the smaller of the two. The male crabs are generally larger than the females, while adult females have a much broader abdomen and usually have smaller claws. The broader abdomen of the female Christmas Island Red Crab only becomes apparent in the third year of growth.

Christmas Island Red Crabs live in burrows, in order to shelter from the sun. Since they still breathe through gills, the possibility of drying out is a great danger for them. They are famous for their annual migration to the sea in order to lay their eggs in the ocean. During the migration, the crabs cover the routes to the coast so densely that they can be seen from the air. Volunteers shovel the crabs off the roads and, although no harm is intended, some of the countless millions of crabs inevitably get injured or killed.

The large population size of this species is most likely due to the extinction of Maclear’s Rat (Rattus macleari) in 1903. It is possible that these endemic rats kept the crab’s population in control. However, in recent years, an exploding population of the Yellow crazy ant, an invasive species accidentally introduced to Christmas Island and Australia from Africa, is believed to have killed 15-20 million red crabs.

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