Coconut crab

 

Coconut crab (Birgus latro)

Image courtesy Wikimedia Commons

The coconut crab is not the largest crab in the world – that accolade goes to the Japanese spider crab (Macrocheira kaempferi) – but it is the largest terrestrial crustacean. It is also the largest land-dwelling arthropod, the invertebrate family that includes insects, spiders, and centipedes. (Arthropod means ‘those with jointed feet.’)

An adult coconut crab may weigh four and a half kilogrammes and have a leg span of up to one metre.

The coconut crab, also known as the robber crab and the palm thief, is found on Indian and Pacific Ocean islands. It has become extinct on islands with large human populations, like Madagascar and Australia. Christmas Island has the largest population of coconut crabs.

This large hermit crab spends its entire life on land, once the young have developed sufficiently to go ashore. Coconut crabs mate between May and September. The female then lays the eggs and sticks them to her abdomen, where she carries them for several months. When they‘re ready to hatch, the female travels to the seashore to release them into the water. This is a dangerous time for her, because if she falls into the water, she will drown. She cannot breathe underwater and is too heavy to swim back to land.

The small shrimp-like larvae float in the ocean for three or four weeks. They are very vulnerable, and large numbers are consumed by predators. Eventually, the survivors sink to the seabed and find a shell to live in. They then move towards the seashore, where they remain for about a month until they finally venture onto dry land as young crabs, at the same time losing their ability to breathe in water.

Coconut crab on coconut! 
Image courtesy Wikimedia Commons

As they outgrow their shells, they find larger ones. If they are unable to find suitable shells, they may use broken coconut shells. Large adults do not live in shells but develop a hard exoskeleton. They moult annually. For this they dig burrows to take shelter while their soft shell hardens, which can take up to three weeks.

Their popular name suggests that they eat coconuts and while these may form part of their diet, they also consume an omnivorous diet, including fruit, nuts, carrion, birds, baby turtles, and younger relatives. They have an acute sense of smell, allowing them to locate food. They actively hunt rats and larger sea birds, like the red-footed booby, and climb trees to reach hanging fruit, like that of the pandanus tree.

Coconut crabs climbing trees in Bora Bora, French Polynesia
Image courtesy Wikimedia Commons

They mature very slowly, and can live for 120 years, reaching their maximum size between forty to sixty years. They are sexually mature at five. Colours vary by location. and may be shades of red or purplish-blue.

The only predators of coconut crabs are other coconut crabs and humans. They are hunted extensively, and in some areas, like Guam and the Mariana Islands, it is illegal to trap egg-bearing females or adults under a certain size. A limit is placed on the number allowed to be captured. It is currently a bag limit of five crabs at one time, or fifteen in a whole season.

In the Philippines it is illegal to catch them in any category. Violation of the law can lead to a heavy fine or a prison sentence up to six years. Despite this, and in order to satisfy the tourist trade, hunting continues. Coconut crab meat is toothsome.

Numbers are declining and the coconut crab is considered vulnerable by the IUCN (International Union for Conservation of Nature)