Coconut crab

The coconut crab is a type of hermit crab that is scientifically known as Birgus latro. It is one of the largest crabs in the world and the largest that spends its whole life living on land. These giant crabs can measure up to 40 inches (1 meter), weigh about 10 pounds (4.5 kilograms), and have the strength to lift objects around 50 pounds (22.7 kilograms). They also have incredible grip in their pincers to grab and crack open coconuts. However, the coconut crab has also been known to prey on small mammals and birds. Coconut crabs live in the islands of the Indian and Pacific oceans, where the coconut palm grows.

Background

The coconut crab is also known as the robber crab, drover crab, or palm thief. These names are from the translation of the crab’s name from the indigenous languages of its habitat. The coconut crab falls under the phylum of arthropod, invertebrate animals with an exoskeleton mainly of chitin, which is molted as they grow larger. Their exoskeleton is also segmented and has paired jointed appendages. The term arthropod means “jointed feet.” The subphylum that the coconut crab belongs to is crustacea, similar to other crabs; however, it is the only species of the genus Birgus, although it is related to other types of land-dwelling hermit crabs.

Western science has been familiar with the coconut crab since explorers such as Francis Drake documented their voyages to the islands in the South Pacific and Indian Oceans in the sixteenth century. English Explorer and naturalist William Dampier also documented the crab in the late seventeenth century. However, Indigenous people living on the islands where the crabs are found have been using them as a food source for generations.

The German botanist Georg Eberhard Rumphius, who was a merchant for the Dutch East India Company in the mid-eighteenth century, identified the coconut crab as the Cancer crumentatus. In 1816, English zoologist and marine biologist William Elford Leach renamed the coconut crab Birgus latro and founded the genus Birgus to fit the crab. The genus was classified in the family Coenobita, which includes other similar terrestrial hermit crabs.

Overview

The coconut crab lives up to sixty years and spends the entirety of its adult life on land. Females carry their eggs on their undersides and mate during the summer months. The fertilized eggs are released into shallow seawater where the larvae live on floating objects such as coconuts or logs for four to six weeks. After this, the young crabs, called glaucothoe, find an empty shell on the seabed to further develop and begin to make their way back to the land. At this point, juvenile coconut crabs resemble hermit crabs, but as they become adults, they develop an exoskeleton and no longer live in a borrowed shell. Adults reach sexual maturity at about five years old and then begin the reproduction cycle.

From adulthood on, the crabs live on land and can even drown if submerged in water for a prolonged period. They breathe air using branchiostegal lungs, which are specialized respiration organs that function similarly to both gills and mammalian lungs. Gill-like tissues absorb oxygen from the air and can be expanded and inverted to expand their surface area. The coconut crab uses its small hind legs to moisten and clean their lungs since these types of lungs require moisture from water to function properly.

Since coconut crabs require a ready supply of water, they live in burrows or rock crevices near the sea or other sources of water. They live alone and will use one of their large claws to function as a door to their burrow and to maintain climate control. They also will take the fibrous lining of a coconut husk to line their borrows for insulation and comfort. Usually, they only come out in the evening to hunt for food.

The coconut crab primarily eats fruit, nuts, and seeds that have fallen on the ground. These include coconuts. If nothing is readily found on the ground, they can use their curved claws to climb trees. Coconut crabs use their strong pincers and claws to remove the fibrous husk and bore holes in a coconut, but since coconuts have a very dense and impenetrable husk, the coconut crab has also been observed climbing a nearby tree to drop a coconut from a height of 10 to 30 feet (3 to 9 meters) to crack it open. To prevent other coconut crabs from stealing their meal, they will then fall out of the tree after it, landing uninjured on the ground nearby. Coconut crabs are omnivores, meaning they also will eat other animals along with plants. They will prey on smaller crabs, tortoise hatchlings, rats, birds, and carrion. They have been recorded to attack small animals who venture near their burrows and to ambush nesting seabirds.

Coconut crabs have no natural predators, aside from other coconut crabs and humans. Because of their size, the crabs have been historically hunted for their meat. In some heavily populated areas, such as Australia and Madagascar, the coconut crab has become extinct. The Christmas Islands have the largest population of coconut crabs. Conservation efforts and harvesting regulations are seeking to improve their population in other areas. After many years of insufficient data to estimate the population, in 2018, the International Union for Conservation of Nature (IUCN) determined that the species was “vulnerable.”

Bibliography

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Freund, Cassie “‘A Bigger Deal Than It Sounds’: Coconut Crabs Are Vanishing, Island by Island.” Mongabay, 1 Feb. 2022, news.mongabay.com/2022/02/a-bigger-deal-than-it-sounds-coconut-crabs-are-vanishing-island-by-island/. Accessed 10 Aug 2023.

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Guarino, Ben. “Scientists Discover the Stupendous Force (740 lbs.) of Coconut Crab’s Claws, Get Pinched in Process.” The Washington Post, 28 Nov. 2016, www.washingtonpost.com/news/morning-mix/wp/2016/11/28/scientists-discover-the-remarkable-strength-of-coconut-crabs-claws-get-pinched-in-process/. Accessed 10 Aug 2023.

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