Coconut Crabs, Christmas Island

The coconut crab, Birgus latro, is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest land-living arthropod in the world, and is probably at the upper size limit for terrestrial animals with exoskeletons in recent Earth atmosphere, with a weight of up to 4.1 kg (9.0 lb). It can grow to up to 1 m (3 ft 3 in) in length from leg to leg. It is found on islands across the Indian Ocean and parts of the Pacific Ocean as far east as the Gambier Islands mirroring the distribution of the coconut palm; it has been extirpated from most areas with a significant human population, including mainland Australia and Madagascar.

The coconut crab is the only species of the genus Birgus, and is related to the terrestrial hermit crabs of the genus Coenobita. It shows a number of adaptations to life on land. Like hermit crabs, juvenile coconut crabs use empty gastropod shells for protection, but the adults develop a tough exoskeleton on their abdomen and stop carrying a shell. Coconut crabs have organs known as “branchiostegal lungs”, which are used instead of the vestigial gills for breathing. They cannot swim, and will drown if immersed in water for long. They have developed an acute sense of smell, which has developed convergently with that of insects, and which they use to find potential food sources. Mating occurs on dry land, but the females migrate to the sea to release their fertilised eggs as they hatch. The larvae are planktonic for 3–4 weeks, before settling to the sea floor and entering a gastropod shell. Sexual maturity is reached after about 5 years, and the total lifespan may be over 60 years.

Adult coconut crabs feed on fruits, nuts, seeds, and the pith of fallen trees, but will eat carrion and other organic matter opportunistically. The species is popularly associated with the coconut, and has been widely reported to climb trees to pick coconuts, which it then opens to eat the insides. While coconut crabs can climb trees, and can eventually open a coconut collectively, coconuts are not a significant part of their diet. Coconut crabs are hunted wherever they come into contact with people and are subject to legal protection in some areas. In the absence of precise information the IUCN lists the species as Data Deficient.

The Territory of Christmas Island is a territory of Australia in the Indian Ocean, composed solely of that island. It has a population of 2,072 residents who live in a number of “settlement areas” on the northern tip of the island: Flying Fish Cove (also known as Kampong), Silver City, Poon Saan, and Drumsite. The majority of the population are Chinese Australian. It is called “Christmas Island” because it was discovered on Christmas Day (25 December 1643).

The island’s geographic isolation and history of minimal human disturbance has led to a high level of endemism among its flora and fauna, which is of interest to scientists and naturalists. 63% of its 135 square kilometres (52 sq mi) is an Australian national park. There exist large areas of primary monsoonal forest.

Phosphate, deposited originally as guano, has been mined on the island for many years.

Coconut Crabs, Christmas Island

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