The World’s Largest Land Crab Is Fierce—Coconut Crab Under Threat
It could be a scene from a science fiction movie: giant, omnivorous crabs—the size of small dogs—feasting on coconuts, which they crush with their claws, and climbing trees in search of live prey. But the enormous crabs are themselves vulnerable—to human predators. “They grow to monstrous size,” Charles Darwin noted when he encountered them in his travels in the 1830s, and “are very good to eat.”
As their numbers dwindle, these “invertebrate dinosaurs” need to be better understood, says Mark Laidre, an assistant professor of biological sciences. With a research grant from the National Geographic Society, he has been studying the crabs’ biology, behavior, and ecology in one of their few remaining strongholds, the Chagos Archipelago in the Indian Ocean.
“These crabs may weigh up to nine pounds, have a leg span of more than a meter across, and live more than 60 years,” says Laidre. Strong and well armored, they are also skilled hunters. Laidre has a paper, which will be published later this year, describing gory details of coconut crabs preying on other animals.
But despite their intimidating size, coconut crabs are in trouble, he says. “People are eating them or destroying their habitat. With sea levels rising, in another 10 to 20 years many of the coral atolls on which they live are going to get drowned.”
The crabs’ longevity is another factor working against their survival as a species. “It’s not hard to totally decimate populations, since it might take 50 to 100 years to fill the void left by the harvested adults,” Laidre says.
Although the coconut crab has not been declared a threatened species, the International Union for Conservation of Nature has classified the species as “data deficient.” This means that “even though the species has become locally extinct on many islands, no one has compiled information over their range throughout the Indo-Pacific,” Laidre says. With a network of collaborators, he is trying to change this, launching an effort to conduct systematic census taking and collecting data on populations’ genetic diversity.
Laidre is spreading his message of protection and preservation, in part by sharing his enthusiasm with schoolchildren and sponsoring an art exhibit, free and open to the public, at Dartmouth’s Class of 1978 Life Sciences Center. “The exhibit includes preserved specimens of the humongous crabs as well as artwork inspired by these extraordinary creatures,” says Laidre. Woodcarvings, paintings, watercolors, glass sculptures, and even sections from a science fiction trilogy (all inspired by coconut crabs) are on display on the third floor of the Life Sciences building.
Mark Laidre has assembled a collection of coconut crab art that’s on display at the Class of 1978 Life Sciences Center. In his hand is a glass sculpture that artist Robert DuGrenier made from an impression of a crab in sand (see detail below). (Photo by Robert Gill)Recently he visited the Frances C. Richmond Middle School in Hanover, N.H., where he spoke with students and gave them an up-close look at a preserved coconut crab. “They seemed to really get a kick out of the crabs,” Laidre says. “That’s my new favorite animal,” said one middle-schooler. “Oh, that’s gross and scary,” exclaimed another. “But overall, they loved them and some had quite insightful questions about behavior and scientific experiments,” Laidre says.
This term, he is in Costa Rica, teaching a Dartmouth tropical ecology field course that focuses on animal behavior and ecology. Laidre has conducted research in Costa Rica focusing on the social life of terrestrial hermit crabs, close evolutionary relatives of coconut crabs.
In the spring, he and fellow biologist Thomas Jack will co-teach a new course they developed, “Animal Minds,” which will explore how animals, from invertebrates to primates, make decisions.
The decision Laidre thinks most important is one humanity as a whole will have to make about conserving coconut crabs: “Only if we work together will we stand a chance of protecting these amazing animals for generations to come.” source
The mighty pinch of the coconut crab
The coconut crab, or “robber crab” (Birgus latro), is the largest terrestrial arthropod in the world, with individuals weighing up to four kg (about 9 pounds). They have a wide range:
But because of their size and the fact that they’re tasty, they’ve been largely driven extinct by humans on populated islands. Here’s how big they are:
They come in different colors, often red or blue. Here’s a beautiful blue one:
Two facts about these species. First, they’re fairly close relatives of the hermit crab, and in fact are descended from hermit crabs that occupied shells throughout their lives. But this species is sufficiently large as to have few predators besides humans, and so only the very young and small coconut crabs live in shells (sometimes bits of coconut shells). They’re also completely terrestrial, and will die if put in salt water.
You can see the hermit-crab ancestry in the reduced and curled abdomen behind the carapace:
Hermit crab without shell:
I’ve written two earlier posts on coconut crabs (here and here), including photos by reader Dennis Hansen from the Indian Ocean of Aldabra. There they invade the field station, making off with leftover food from the table, and they can also open garbage cans, as they have great climbing skills:
They’re HUGE! Second Fun Coconut Crab fact: they do indeed open coconuts, and can climb trees to get them, though they’re omnivores and coconuts are not a main part of their diet. Wikipedia gives the details (my emphasis):
The diet of coconut crabs consists primarily of fleshy fruits (particularly Ochrosia ackeringae, Arenga listeri, Pandanus elatus, P. christmatensis), nuts (coconuts Cocos nucifera, Aleurites moluccanus) and seeds (Annona reticulata), and the pith of fallen trees. However, as they are omnivores, they will consume other organic materials such as tortoise hatchlings and dead animals. They have been observed to prey upon crabs like Gecarcoidea natalis and Discoplax hirtipes, as well as scavenge on the carcasses of other coconut crabs. During a tagging experiment, one coconut crab was observed killing and eating a Polynesian Rat (Rattus exulans). Coconut crabs may be responsible for the disappearance of Amelia Earhart’s remains, consuming them after her death and hoarding her skeletal remnants in their burrows.
The coconut crab can take a coconut from the ground and cut it to a husk nut, take it with its claw, climb up a tree 10 m (33 ft) high and drop the husk nut, to access the coconut meat inside. They often descend from the trees by falling, and can survive a fall of at least 4.5 metres (15 ft) unhurt. Coconut crabs cut holes into coconuts with their strong claws and eat the contents, although it can take several days before the coconut is opened.
Thomas Hale Streets discussed the behaviour in 1877, doubting that the animal would climb trees to get at the nuts. In the 1980s, Holger Rumpff was able to confirm Streets’s 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 will use 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 will bang its pincers on one of them until they break. Afterwards, it will turn around and use the smaller pincers on its other legs to pull out the white flesh of the coconut. Using their strong claws, larger individuals can even break the hard coconut into smaller pieces for easier consumption.
Clearly, these animals must be strong. Indeed, they can lift up to 28 kg (62 pounds)—at least seven times their own weight.
This is all leading up to a new paper published in PLOS ONE by Shin-Ichiro Oka et al. (reference and free download below), in which the authors measured the pinching force of 29 coconut crabs. As you might expect, they pinch hard, exerting more force per gram of body weight than nearly any other animal, including terrestrial carnivores. source
Come See a Coconut crab – at the San Francisco Zoo
- Coconut crabs are a type of hermit crab and are the world’s largest terrestrial arthropod (up to 3 feet across!).
- Unlike most other hermit crabs, only juveniles use shells to protect their abdomens; older coconut crabs develop a hard skin.
- They cannot swim, and will drown if immersed in water for long.
- They have large muscular claws to help open coconuts.
- Coconut crabs are hunted extensively for food, which can be problematic due to their long lifespan and slow growth rate.
- They can live to be over 60 years old!
Coconut crabs live in rock crevices and burrow along the coast. They are nocturnal and feed on coconuts, fruit, and leaves. If coconuts aren’t already available on the ground, the coconut crab can climb trees to cut them down.
Status in the Wild
Data Deficient – IUCN 1996
Found throughout tropical Indo-Pacific oceanic islands and offshore islets.
Location in the Zoo
Invertebrates Zone of the Sculpture Learning Plaza