Coconut Crab 

Coconut Crab Summary

The coconut crab is a species of hermit crab belonging to the genus Birgus. It occurs on islands throughout the Indian and Pacific Oceans, making its home in burrows in the soil. It is most famous for its ability to climb trees in pursuit of coconuts, which it splits open with its powerful pincers. Its massive size leaves it with few predators. Despite this, its numbers worldwide are declining.

5 Coconut Crab Facts

• The largest terrestrial arthropod: These crabs are the biggest land-dwelling arthropods in the world with a leg span of up to 3.3 feet. They hold a Guinness World Record for the heaviest terrestrial crustacean. However, the title for the world’s largest crab goes to the Japanese spider crab with a leg span of up to 12.5 feet.
• Cannibals: These crabs are not only giants, they also occasionally prey on each other in addition to their other food sources.
• They eat coconuts: True to their name, these crabs climb trees to reach coconuts or find them lying on the ground. After breaking them open with their claws, they feast on the white flesh inside.
• Slow developers: These crabs are long-lived and slow to grow, only reaching sexual maturity after about five years. They live as long as 60 years in the wild.
• Endangered: This fascinating species is vulnerable to extinction due to habitat loss and overharvesting.

Coconut Crab Scientific Name

The scientific name for the coconut crab is Birgus latro. Alternate names for this species include the robber crab and palm thief. It is the only member of the genus Birgus. It further belongs to the family Coenobitidae (terrestrial hermit crabs) and the order Decapoda (includes crabs, hermit crabs, lobsters, shrimps, and crayfish). These arthropods (invertebrates with an exoskeleton) fall within Malacostraca, the largest class of crustaceans.

Coconut Crab Appearance and Behavior

Coconut crabs are the largest terrestrial arthropods in the world with a leg span ranging from three to 3.3 feet. They weigh an average of 5.5 pounds with the heaviest specimens weighing up to nine pounds. Adults range from light violet to dark purple or brown, though young adults are brown with black leg stripes. As decapods, they have ten legs, including a front pair that ends in a powerful set of serrated pincers. Despite being hermit crabs, these arthropods shed their shells as juveniles, relying on their tough exoskeletons for protection.

This species is both solitary and nocturnal, spending most of the day inside individual burrows. At night, it comes out to hunt and feed. It is a ruthless predator but rarely attacks humans unless cornered or provoked.

Coconut Crab Evolution and History

The ancient ancestors of modern decapods, including coconut crabs, may have appeared as far back as the Late Ordovician Period about 455 million years ago. Originally, scientists thought decapods evolved from the later Devonian Period. Diversification resulted in over 15,000 extant species and 3,000 fossil species in 233 families.

Hermit crabs evolved during the Jurassic Period (206 to 144 million years ago) and likely came from a lobster-like ancestor. Most species evolved asymmetrically with abdominal musculature to help them hold onto the spirals of their appropriated shells. Unlike most other hermit crabs, coconut crabs abandon their shells by adulthood. This means that their shells do not limit their growth, enabling them to grow to extreme sizes.

Coconut Crab Habitat

Coconut crabs primarily inhabit small islands throughout the Indo-Pacific region, infrequently occurring on the coasts of larger landmasses. Their north-south range generally lies between the tropics of Cancer and Capricorn. They prefer coastal forests with rock crevices and soil, which are useful for digging burrows up to two feet deep.

Below is a table of the countries, islands, and regions with populations of this species. Each listing may include further nearby or surrounding islands.

Coconut Crab Predators and Threats

Coconut crabs are omnivores, consuming both plant and animal matter. They are fierce predators with few rivals. Despite this, they are in danger of dying out. As of 2018, the IUCN includes this species on its Red List of Threatened Species with the status of Vulnerable due to its decreasing populations. Current threats to its existence include habitat loss, hunting, invasive diseases, and climate change. Its long lifespan and slow reproduction rate make it difficult for this species to repopulate.

What Do Coconut Crabs Eat?

As scavengers and opportunists, this species feeds on a variety of plant and animal matter. Besides coconuts, it consumes fallen fruit, the pith of fallen trees, nuts, seeds, and drupes. Specific items in its diet include custard apples, candlenut, Lister’s palm, Pandanus elatus (a tropical plant), and Ochrosia ackeringae (a species of flowering plant).

As an omnivore, it also preys on animal flesh, including that of its own species. In addition to eating the meat of other crabs, it also consumes their shells for calcium. Besides this, the species is famous for its ability to scale trees and attack birds in the branches, breaking their wings and dragging them back to its burrow. It also feeds on tortoise shells, freshly hatched turtles, Polynesian rats, and carrion.

This species flicks its long antennae to enhance its incredible sense of smell. It is especially attracted to the odors of rotting meat, bananas, and coconuts.

What Eats the Coconut Crab?

The only two predators of the coconut crab are humans and members of its own species. Its aggression, tough exoskeleton, and sheer size discourage other predators from attacking.

Coconut Crab Reproduction, Babies, and Lifespan

Coconut crabs mate between May and September with an increase in activity in July and August. The male deposit his spermatophores on the abdomen of a mature female, thus fertilizing her eggs. After laying the eggs, the female glues them to her abdomen and carries them with her for a few months. After this, she transports them down into the water and releases them at high tide. She risks drowning as she cannot breathe underwater. Many female crabs congregate at the same time to release their eggs with the larger females laying as many as 250,000 eggs. A female’s clutch may weigh up to one-third of her body weight.

The eggs hatch into zoeae (the larval form of crabs) and disperse on floating coconuts or other objects for about four weeks. After this, they become glaucothoe (shrimplike creatures) and seek out discarded shells for protection on the seabed or shore. This lasts another four weeks. If they cannot find an appropriate shell, they use broken pieces of coconut. When they outgrow their shells, they also lose the ability to breathe underwater and begin living entirely on land as young adults.

This species lives as long as 60 years and attains sexual maturity at around five years old.

Coconut Crab Population

Although the exact coconut crab population worldwide is unknown, scientists have observed serious population declines in most areas. As noted, the IUCN considers this species to be vulnerable to extinction. According to their entry on coconut crabs, the species has already disappeared from Australia, Chile, French Polynesia, Madagascar, Mauritius, Réunion, and Tokelau. It may or may not also be extinct in India.

Related Animals:

• Hermit Crab
• King Crab
• Rock Crab

1. Brittanica, Available here: https://www.britannica.com/animal/robber-crab
2. San Diego Zoo Wildlife Alliance Library, Available here: https://ielc.libguides.com/sdzg/factsheets/coconut-crab#:~:text=Range%3A%2045%2C000%20to%20250%2C000%20eggs,%3B%20Sato%20and%20Yoseda%202008).
3. Natural History Museum, Available here: https://www.nhm.ac.uk/discover/coconut-crabs-bird-eating-giants-on-tropical-islands.html
4. American Oceans, Available here: https://www.americanoceans.org/facts/can-you-eat-coconut-crab/
5. Guinness World Records, Available here: https://www.guinnessworldrecords.com/world-records/115789-heaviest-crustacean-on-land
6. IUCN Red List, Available here: https://www.iucnredlist.org/species/2811/126813586
7. Schwab, I.R.; Nilsson, Dan-E, Available here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1955607/
8. Cunningham, C.W.; Blackstone, N.W.; Buss, L.W., Available here: https://www.nature.com/articles/355539a0