Coconut crabs ( Birgus latro ) are the largest terrestrial arthropods on the planet, a title they hold within the phylum that encompasses crustaceans, spiders, and insects. These fascinating creatures, also known as robber crabs or palm thieves, are found on islands throughout the tropical western Indo-Pacific Ocean, including Madagascar, Seychelles, and islands adjacent to the National Marine Sanctuary of American Samoa. Their unique adaptations to life on land and their diverse diet make them a captivating subject of study.
Physical Characteristics and Habitat
Coconut crabs can grow to impressive sizes, with a body length averaging 16 inches and a weight of up to nine pounds. Some individuals can even reach a leg span of up to 3 feet 3 inches (1 meter). Males are generally larger than females, though not significantly so. Their coloration varies depending on their habitat, ranging from deep blue to red or purplish-blue, or a combination of these hues.
Juvenile coconut crabs, like other hermit crabs, utilize mollusk shells for protection until they reach a size where they can no longer fit inside. As they mature, they develop a tough exoskeleton on their abdomens, composed of chitin and calcium carbonate, providing them with biological armor. The only part of their body not covered by this armor is the abdomen, which is protected by a thick, leathery skin with small bristles.
These crabs inhabit coastlines and live in burrows, which offer protection, a safe place to store food, and a refuge during molting periods. Adult crabs are primarily terrestrial but may occasionally venture into the water.
Omnivorous Diet: From Coconuts to Carrion
Adult coconut crabs are omnivorous scavengers, meaning they consume a wide variety of food sources. While their name suggests a primary diet of coconuts, these only form part of their diet. They feed on fleshy fruits, seeds, drupes, nuts, and the pith of fallen trees. They also consume carrion and other organic matter opportunistically. Anything left unattended on the ground is a potential source of food, which they will investigate and may carry away - thereby getting the alternative name of "robber crab".
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Coconuts: A Challenging Meal
Coconut crabs are well-known for their ability to access the meat of coconuts. They often use their strong claws to poke a hole into the soft eye of a coconut before splitting it open. In some instances, coconut crabs will drag coconuts to the top of a tree and drop them to break them open. Thomas Hale Streets discussed the behaviour in 1877, doubting that the animal would climb trees to get at the coconuts. As late as the 1970s, doubts remained about the crab's ability to open coconuts. In the 1980s, Holger Rumpf was able to confirm Streets' report, observing and studying how they open coconuts in the wild. The animal has developed a special technique to do so; if the coconut is still covered with husk, it uses its claws to rip off strips, always starting from the side with the three germination pores, the group of three small circles found on the outside of the coconut. Once the pores are visible, the coconut crab bangs its pincers on one of them until it breaks. Afterwards, it turns around and uses the smaller pincers on its other legs to pull out the white flesh of the coconut.
More Than Just Coconuts: A Varied Menu
Besides coconuts, their diet includes:
- Animal carcasses: As scavengers, they consume dead animals, contributing to the ecosystem by breaking down organic matter.
- Molted exoskeletons: They eat the molted skeletons of other crustaceans, recycling valuable minerals.
- Other crabs: Coconut crabs are known to eat other crabs, including Christmas Island red crabs and Discoplax hirtipes crabs.
- Tortoise hatchlings: They are capable of preying on vulnerable tortoise hatchlings.
- Polynesian rats: Some large coconut crabs may even kill and consume Polynesian rats.
- Birds: Some large coconuts may even feed on birds.
- Small mammals: This land crab is the world’s largest and feeds on everything from coconuts to small mammals and birds.
- Chickens and kittens: Besides eating their own exoskeletons, the crabs have been known to feast on chickens, kittens and fellow coconut crabs.
Coconut crabs often bring larger food items back to their home burrows for safe consumption and storage for later. They also eat more ahead of molting periods and will consume their own exoskeletons after shedding.
Sense of Smell
Finding these delicacies requires a bit of sniffing around. The coconut crab finds food with its extremely well-developed sense of smell. Coconut crabs have a very keen sense of smell that works differently underwater (where they smell hydrophilic molecules) and on land (where they smell hydrophobic molecules). In water, coconut crabs use their aesthetascs (organs used to determine the direction and concentration of smell) on their antennae. Coconut crabs flick their antennae as insects do to enhance their reception. Their sense of smell can detect interesting odors over large distances.
Coconut Crabs and Amelia Earhart
Some believe this excellent sense of smell, in fact, led the coconut crabs to a dead or dying Amelia Earhart. According to one theory, Earhart did not drown in the Pacific but instead crash landed on Nikumaroro, a remote atoll in the Pacific. In 1940, researchers discovered a fraction of a skeleton on the island that matched the description of Amelia Earhart. Now, even more interesting clues are arising that seem to substantiate the idea that this is where she met her demise. Back in 2007, they used a small pig carcass to assess what the coconut crabs might have done. This issue is far from settled, however. The evidence on Nikumaroro could turn out to be an odd coincidence and wishful thinking, meaning that the castaway’s bones actually belong to some other poor, stranded soul.
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Predation and Conservation
The only known predator adult coconut crabs have to worry about is humans, though juveniles may be prey for lizards, toads, and feral pigs. On populated islands, the coconut crab population is almost extinct because people hunt them for their meat, which is considered a delicacy.
The IUCN lists the coconut crab as a data deficient species, meaning we don’t yet know enough to determine its conservation status. Knowing people are the only major predator of coconut crabs, we can assume overharvesting is a potential - though not likely - threat to the species, especially since they grow so slowly. Conservation management strategies have been put in place in some regions, such as minimum legal size limit restrictions in Guam and Vanuatu, and a ban on the capture of egg-bearing females in Guam and the Federated States of Micronesia.
Adaptations for a Terrestrial Lifestyle
Coconut crabs exhibit several remarkable adaptations that allow them to thrive on land:
- Branchiostegal lungs: Instead of gills, they possess organs called branchiostegal lungs, which function similarly to lungs and enable them to breathe air.
- Strong claws: Their powerful pincers, among the strongest of all land animals, are used for cracking coconuts, manipulating food, and defense. A coconut crab weighing 9 pounds has a crushing force of 3,300 newtons. In contrast, lobsters have a claw strength of only 150 newtons.
- Hardened abdomen: The hardened abdominal terga provide protection and reduce water loss, crucial for survival in a terrestrial environment.
Life Cycle and Behavior
Coconut crabs start their lives as planktonic larvae in the ocean. They feed on other plankton and transform through several life cycles before they reach their sub-adult stage and make their way toward land. On the way to shore or shortly after arriving, young coconut crabs will find the perfect shell to call home until they outgrow it, offering them protection from any potential predators and the elements.
Coconut crabs are generally nocturnal animals and generally live alone, venturing out of their burrows to find food or mate. They don’t communicate even when they look for food. On the contrary, they have a claw-waving sign that announces their presence so that other coconut crabs keep their distance. During molting periods, coconut crabs can stay buried for as long as 16 weeks!
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