Attack of the Bird-Eating Crabs!

Crabs are not generally thought of as dangerous or frightening animals. True, many species can give a painful pinch when threatened, we usually think of them as seafood rather than anything to worry about. The majority are detrivores that spend there time scavenging whatever organic material they come across. However, the coconut crab has made the quantum leap from scavenger to predator and has become a hunter of sea birds.

Killer Crabs

The coconut crab, also called the robber crab or palm thief, is the largest terrestrial invertebrate in the world, weighing as much as a domestic cat and having a leg span exceeding three feet. They are related to hermit crabs but only use empty snail shells when young, as older crabs grow secrete chitin and chalk to harden their exoskeleton. Coconut crabs can be found in Indonesia and on islands in the Indian and Pacific Oceans. Unlike most other crab species, the coconut crab is highly adapted for a terrestrial lifestyle and will drown if submerged for too long. They rely on a set of special organs known as branchiostegal lungs for respiration rather than gills. When not out foraging for food, which they locate with their well-developed sense of smell, they spend most of their time in burrows. Coconut crabs are excellent climbers and will readily ascend trees to feed on fleshy fruits. As their name suggests, they do eat coconuts which they crack open with their powerful claws. A large coconut crab can generate over 700 pounds of crushing force with its pincers, which is comparable to the bite force of a big cat like a jaguar. These powerful tools allow coconut crabs to also feed on nuts, seeds, carrion, smaller crab species, and the occasional small vertebrate. While they are incapable of swimming as adults, coconut crabs start their lives as part of the oceanic plankton after the females release their eggs into the sea. The larvae drift on currents for several weeks before coming ashore. It takes approximately five years for a coconut crab to reach sexual maturity and 40 to 60 years before it reaches its full size.

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Tastes Like Chicken

In the winter of 2016, scientists visiting the Chagos Archipelago managed to document an astounding and horrifying bit of coconut crab behavior. They observed a large crab climbing a tree to access a red-footed booby which was sleeping in its nest. The crab attacked the unfortunate bird, easily breaking one of its wings and knocking it to the ground. It then descended the tree and broke the bird’s other wing, rendering it immobile. Several other crabs showed up on the scene shortly, possibly attracted by the scent of blood, and proceeded to dismember and devour the booby. This is the first time such predation has been observed in coconut crabs and up until now it was believed that any meat consumed was obtained via scavenging. This behavior may be more common than previously thought and may have significant ecological impacts. Islands with large populations of coconut crabs are less likely to have colonies of sea birds due to the risk of predation to their eggs and chicks. In an opposite affect, islands with lots of birds would be harder for crabs to colonize since when they first emerge from the sea they are small and extremely vulnerable to predation themselves.

Birgus latro

Crabs and Humans

Despite looking a lot like the face-hugger from the Alien movie franchise, coconut crabs have much more to fear from humans than we do from them. Their meat is a prized delicacy and some island cultures believe it to possess aphrodisiac powers, so they are hunted throughout their range. They typically do not occur on human-inhabited islands and are now extinct in mainland Australia and Madagascar. Coconut crab meat is safe to eat, but they do sometimes sequester toxic chemicals from the fruits they consume and there are documented cases of people being poisoned after eating them. In some islands, the coconut crab is venerated as an ancestor spirit known as taotaomo’na.

Birgus latro (Bora Bora)


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For info on crabs, check out our Crustacean Center!

For info on arthropod evolution, check out our Arthropod Center!