The Coconut Crab, an amazing species to save |06 January 2014

Coconut Crab, (Birgus latro) also named Robber Crab or ‘Sipay’ in Creole, is an endangered species very vulnerable to human predation. The animal is a Seychellois symbol represented on local currency (the R25 note). Extinct from the main Seychelles islands many locals have and will never get the privilege to observe this remarkable crab which is protected under the Seychelles Wildlife Protection Act.

A crab remarkable by its size, power and ability

The Coconut Crab belongs to the terrestrial hermit crab family Coenobitidae. It is the largest land crab of the world weighing up to 4kg and having a leg span up to 1m for the biggest individuals. The Coconut Crab is capable of climbing the trunk of the coconut tree to select a coconut and cut it free using its pinchers. Its claws are so powerful it can remove the husk from the coconuts. Then, the crab pounds the coconut against a rock until it breaks open. It may take hours or even two days to open the nut. This unique behaviour among crabs is at the origin of its name. But coconuts are not its first diet, the crab is omnivorous feeding primarily on fleshy fruits and seeds, it can catch turtle hatchlings and as scavenger can eat animal carcasses including crabs.

A unique free-living hermit crab

The crab spends the first stage of its life at sea. The pelagic larval development takes 3-8 weeks, after that time the young crab migrates on land and, like the other members of its family, it occupies a gastropod shell to protect its soft abdomen that it will need to change as it grows. But at its adult stage the crab differs from the other hermit crabs by developing a tough exoskeleton on its abdomen allowing it to be a free-living hermit crab by abandoning the protection of the shell. The species can leave until 40 to 60 years and reaches sexual maturity after five years. The species is sexually dimorphic with the male being bigger and the female developing three short legs on the ventral surface of the abdomen to carry eggs.

An amazing clever behaviour to optimise larvae survival

The female releases the mature larvae on the beach at the waters’ edge. They generally time their larval release on the spring tide during the new moon to maximise the chances of the larvae being carried off the reef into the open ocean and to avoid predation. During spring tides the water exchange on the reef flat is at its greatest which maximises the chances of the larvae being carried off the reef to open ocean where predation may be reduced and where a more constant supply of food (phyto- and zooplankton) will be available for the developing larvae. At times of full moon tidal ranges are also high, but the moon's illumination may increase the vulnerability of the larvae to predators.

Conserve coconut crab for enjoyment of future generation

Dr Poupin is a French scientist specialising in the study of arthropods. He has crossed the Indian and Pacific oceans to search for the charismatic crab that has become particularly sparse, a victim of human predation and loss of habitat. Island Conservation Society and Dr Poupin working for BIORECIE research programme, conducted by IRD/La Réunion (P. Chabanet), have worked together to update the distribution of the crab in Seychelles and pointed out the urgent need to strictly protect the species. In spite of being protected under the Wildlife Protection Act, the crab is threatened in the Seychelles where populations are generally not large enough to be viable while on the main islands the crab is considered to be already extinct. Only Aldabra Atoll, a Unesco World Heritage Site, still supports an abundant population of Coconut Crabs well protected by Seychelles Islands Foundation. The Island Conservation Society based on Aride, Silhouette, Desroches and Alphonse takes part in the reinforcement of the law alongside other Seychellois conservation organisations. As for our natural heritage, we all have the responsibility towards future generations to conserve this charismatic species.

Aurélie Duhec
Island Conservation Society