the Legacy continues…………………….
Gregory R. Mann, Ph.D. {ret.}

Coconut Crab

“Birgus latro”

The Coconut Crab is a type of land crab with a spectacular appearance and intriguing biology. Able to grow to relatively gigantic proportions, the Coconut Crab is probably the largest terrestrial arthropod in the world. Charles Darwin described the Coconut Crab as “monstrous” when he encountered it on the Keeling Islands during the voyage of the H.M.S. Beagle. Unlike most other Hermit Crabs, only the very small Coconut Crab juveniles find and use gastropod shells to protect their soft-skinned abdomen as they develop. Larger juveniles abandon the shell-carrying habit and instead their abdomen develops a hard skin the exoskeleton, as over the rest of the body. This protects the Coconut Crab, reduces water loss and does not restrict its growth allowing it to reach up to a meter in size toe-to-toe. This huge crustacean is well adapted to life on land with long strong legs with large muscular claws used for husking coconuts and opening the shell to eat the flesh. This is a unique behavior among crabs and explains why this species is called the Coconut Crab. The claws are in fact so powerful they can lift objects such as vegetation or rocks weighing up to 28 kilograms. Its stalked eyes are red and this crab’s body color varies between islands from purplish-blue to orange-red. Studies show that male Coconut Crabs are considerably larger than females. The Coconut Crab is almost entirely terrestrial and has adapted so well to living on land that it actually drowns in water. That said, it does still breathe through modified gills. The gills are surrounded by spongy tissues which need to be kept moist. The Coconut Crab does this by dipping its legs into water and passing them over the gills. The Coconut Crab does require some contact with the sea as it often drinks the water to maintain its salt balance and females need to return to sea to release eggs.

By day, the Coconut Crab inhabits burrows where it is protected from desiccation and intruders and by night it goes in search of food. As its name suggests, this crab feeds on coconuts and is actually able to climb coconut palms where it is thought to pinch off coconuts with its powerful claws when coconuts are not already available on the ground. If the coconut does not break open on its fall, the Coconut Crab husks the coconuts by pulling back the husk from the end that was formerly attached to the palm and evidence indicates that they then pierce the “soft eye” with a pointed walking leg, before gradually enlarging the hole by breaking off sections of the shell until they can reach in to scoop out the flesh. This crab feeds on more than just coconuts however and will scavenge for anything organic from fruit to leaves. It also feeds on the molted exoskeletons of other crustacean species, which are thought to provide calcium for its own growth. Courtship between Coconut Crabs is quick, simple and infrequent. Mating occurs on land and the female carries the fertilized eggs beneath her abdomen, held in place by three specialized appendages. When the eggs are ready for hatching, the female walks down to the edge of the sea during high tide and releases the larvae. The larvae are pelagic and remain floating in the sea for up to 28 days while they develop.

This is followed by an amphibious stage of 21-28 days when the young Coconut Crabs occupy gastropod shells and are able to migrate on to the land. This shell-living habit serves to protect the juveniles from desiccation and predation during this early and vulnerable life stage. When they are 2 to 3 years old and still less than 2 centimeters long, they abandon the shell, harden their skin and transform into a miniature of the adult Coconut Crab with a thoracic length of just 5-10 millimeters. Their exoskeleton is molted regularly to allow the crab continuous growth. Molting occurs in the safety of a burrow and takes around 30 days, after which the Coconut Crab eats the cast-off exoskeleton. These crabs are slow-growing and there is good evidence that they live to be more than 40 years after which they don’t increase in size, although they might live for many more years. The Coconut Crab is found on oceanic islands and small offshore islets adjacent to large continental islands across a broad geographical range in the tropical Indo-Pacific region, with reports stretching from the Aldabras Islands in the Indian Ocean to the Pitcairn group and Easter Island in the Pacific Ocean. The Coconut Crab inhabits rock crevices & sand burrows along the coastline, though preferences vary between islands depending on the habitat available. For example on Olango Island in the Philippine Islands, the Coconut Crab lives in burrows in coral rock and thick undergrowth while on Guam Island, it establishes burrows within the porous limestone substrate.