By Linda Orlando
The coconut crab, a type of hermit crab that can grow to gigantic proportions, is the largest terrestrial arthropod in the world. Most hermit crabs live their entire lives inside the shells of other sea creatures, to protect their soft bodies from predators. However, unlike other hermit crabs, only very small coconut crabs use the shells of other animals to protect their soft-skinned abdomens as they develop. Once they reach the juvenile stage, they abandon those shells and their abdomen develops a hard exoskeleton over the rest of their body. This hard skin protects the crab, reduces water loss, and continues to grow along with the crab. This crab species can grow to as large as a meter in size.
Although most crabs live near and swim in water, the coconut crab cannot swim. Even small specimens will drown in water. But this gigantic crustacean is well-adapted to living on land. It has long and strong legs, and large, muscular claws that are so powerful, they can lift heavy objects such as coconuts and vegetation weighing several pounds. The large claws are used for husking coconuts and opening the shell to eat the flesh, which explains why the species is called the coconut crab.
It is also called the 'palm thief' or 'robber crab', because some people have claimed that it steals shiny objects such as silverware and pans from houses and tents. The coconut crab also has various local names among the islands where it lives, such as ayuyu on Guam, and kaveu or unga in the Cook Islands. Some people call the coconut crab a taotaomo'na, because they think the crab might be an illusion brought about by ancestral spirits called the taotaomo'na.
The coconut crab's body color varies depending on the island where it lives, ranging from orange-red to purple or blue. Males are considerably larger than females. Its stalked eyes are red, and the antennae are quite different from those of other crabs, in that they look like the smelling organs of insects. Although insects and the coconut crab do not evolve in the same way, they both need to detect smells in the air, and therefore have evolved with remarkably similar organs. The coconut crab flicks its antennae as insects do, to enhance its reception, and it has an excellent sense of smell. It can detect odors over large distances, particularly the smell of rotting meat, bananas, coconuts, and other potential food sources.
The coconut crab is also different from other crabs, in that it uses a special organ called a 'branchiostegal lung' to breathe. This organ is one of the most significant adaptations of the crab to its habitat, because the way it works is something between gills and lungs. The chambers of the branchiostegal lung are located in the rear of the thorax and contain tissue similar to gill tissue, but more suited for the absorption of oxygen from the air, rather than water. A coconut crab uses its smallest pair of legs at the back of its body to clean these breathing organs and to moisten them with sea water, which it requires to function. This crab may also drink salt water by using its smallest pair of legs to transfer the water to its mouth.
It uses its strong claws to cut holes into the shells of coconuts and eat the moist fruit inside, a behavior that is unique in the animal kingdom. If the coconut is still covered with a husk, the crab uses its claws to rip off strips of husk, starting from the end with the three small germination holes. Once the pores are open, the crab bangs its pincers on one of them until the coconut breaks open. Afterward, the crab turns around and uses the smaller pincers on its other legs to tear out the flesh inside the coconut and transfer it to its mouth. It eats mostly fruit, although it will eat nearly anything organic, including leaves, rotten fruit, dead animals, tortoise eggs, and the shells of other animals. It may also eat live animals that move slowly, such as freshly-hatched sea turtles. One coconut crab has even been observed catching and eating a Polynesian Rat. Crabs often try to steal food from each other and will pull food into their burrows to keep it from being stolen.
The coconut crab is admired for its strength and holds a special place in the local culture of many areas where it lives. Villagers in some places use it to guard their coconut plantations, because the crab may attack a person if it feels threatened. Adolescent crabs are sold as pets in Tokyo and other places, but they must be kept in a cage that is strong enough that the animal cannot use its powerful claws to escape. If a frightened crab pinches a human, not only is the pinch painful, it is difficult to dislodge the powerful claw.
Uses and Threats
Adults have virtually no natural predators, yet this fascinating creature is threatened with extinction in some areas because significant numbers are eaten by people. In some areas, populations are declining because of over-harvesting, and in remote areas, the crab has become a valuable commodity. It is considered a delicacy and is either purchased and eaten privately, or sold in restaurants.
So, like many of nature's most interesting and lovely creatures, the coconut crab is in danger of vanishing because humans descend and exploit it. However, some groups have been formed specifically to work toward preventing the extinction of this unique animal.