Coconut crabs on Fanjove Island
The coconut crab (Birgus latro) is also known as the robber crab or palm thief. It is the largest of the terrestrial crabs and the largest land-living arthropod in the world: not being constrained by the confines of living in a shell allows this species to grow up to 40 cm in length and up to 4 kg in weight. The leg span can reach 1 metre.
It is an Indo-pacific species whose reach mirrors the spread of the coconut palm. It is present nowadays only on islands such as Chumbe and Fanjove, off the Tanzania coast. It has disappeared from mainland Australia and Madagascar, and in many other minor islands. The abundance of this species is unclear except to say that it has disappeared from many areas of its former distribution because it is considered a delicacy by local populations.
On Fanjove Island there is a natural population, whose dimension and status is unknown: for this reason, a collection of data was started, where measurements and individual markings are recorded.
Males are larger than females and they have 10 pairs of legs: the last pair of legs is very small and used by females to tend their legs. The left cheliped leg is larger than the right one. They mate from May to September; the female holds the fertilized eggs for a few months before releasing them into the ocean. This is normally done in the night from rocky shores during the high tide. After 2 or 3 weeks of planktonic life, it metamorphoses and seeks a gastropod shell and moves onto land where it buries in the substrate until it molts to the first juvenile stage, abandoning the shell forever. The adult develops a tough exoskeleton on the abdomen and lives in burrows, holes and cavities in root systems.
The coconut crab cannot swim. It breaths through branchiostegal lungs and they drown if immersed in water. Sexual maturity at the age of 5, the life span may be over 60 years.
It has no natural predators, apart from other coconut crabs and people and It forages by night, feeding on plants and coconuts but also scavenges on carcasses or injured animals, and on turtles’ eggs and hatchlings. It has a very good sense of smell, comparable to that of an insect.