Arthropods, including crustaceans, insects and arachnids, are by far the largest group of animals that exist on Earth, comprising over 80% of all known species of animals. And the single largest land-dwelling arthropod is the coconut crab (Birgus latro). Found on tropical islands in the Pacific and Indian oceans, these monsters may reach a leg span of well over six feet.
I recently did a four-part series here on hermit crabs. As you can see from the second image in that diary, a hermit crab removed from its shell looks very much like one of these terrestrial coconut crabs. They are closely related, so much so that young coconut crabs inhabit the empty shells of land snails, conveniently betraying their ancestry. Adolescents, before becoming large enough to hold their own against any natural predators, run out of snail shells large enough to accommodate their size. These individuals often use hollowed out coconut shells as a temporary home.
As the coconut crab ages the soft abdomen becomes hardened with exoskeleton material (mostly chitin and other proteins), so the protective snail or coconut shell can be discarded. And here it parts ways with its relatives behaviorally. Instead of hunkering down in larger and larger shells, the coconut crab takes to the trees.
This crab is incredibly powerful. An adult specimen is capable of lifting 75 pounds, over eight times the weight of its own body. As I mentioned in the hermit crab diaries, the last two pairs of legs are adapted to hold the shell in place. These legs are so strong that any attempt to pull a hermit from its shell will likely result in tearing the animal in two. These two back pairs of legs on the coconut crab are put to good use as tools to scale twenty-foot high palm trees in order to fetch their favorite foods.
Most crustaceans are carnivorous, but coconut crabs have evolved to live off the flesh of the palm seed: coconuts, the other, other white meat. The claws are powerful enough to cut holes through the tough covering of the coconut seed, and this crab has the distinction of being the only known animal able to do so. It is a common myth that coconut crabs purposely cut the fruit loose so that it can be eaten back on the ground. It is doubtful that they are able to think so far ahead, and most dropped fruit is the result of attempts to open them up in the tree.
Although coconuts provide the main source of nourishment for these crabs, they are opportunistic enough to feed on pretty much anything they can catch. They are even known to feed on newly hatched sea turtles as the baby reptiles try to make it to the surf after hatching. They’ll even raid garbage cans.
Even though they live on land, coconut crabs still need moisture to breathe like all crustaceans do. The breathing apparatus is a transitional stage between a gill and a lung. This organ is called a branchiostegal lung, and although it functions out of water, it still must be kept wet in order to work. This is accomplished in two ways: One is to periodically trek to the waters edge to wet the lungs. The other is to conserve moisture by burrowing underground during the day and feeding only at night. The entrance to the burrow is blocked by the larger of the two claws, trapping moist air inside and allowing the lungs to function throughout the day.
Coconut crabs are edible, and their populations on many inhabited islands are threatened due to over-harvesting. It doesn’t help that in some Pacific Rim cultures the animal is considered to be an aphrodisiac. Although young ones are preyed upon by introduced mammals, such as rats and feral pigs, the only enemy of an adult is humans.
Reproduction provides another glimpse into this animal’s ancestry. Like all crabs, the larval stages are planktonic. Coconut crabs mate on land and the female carries the fertilized eggs beneath her curled abdomen. When they are about to hatch she migrates to the ocean’s edge, turns her back to the waves and extends her tail to release the young. The larvae drift for several weeks and then return to the beach to live out their lives on land.
Other diaries in this series can be found here.