Coconut crabs have seen a huge decrease in population due to overharvesting and increasing demand from tourist restaurants. Strict regulations on fishing, pollution, disturbance and the implementation of marine protected areas could save
the species if measures are taken soon.
The largest terrestrial arthropod on the planet, the coconut crab, has seen a worrying decrease in population over the past decade. Coconut crabs are native to the coastal areas across the tropical Indo-Pacific, mainly found on small
islands in relatively low numbers. The crabs can reach over a metre in diameter and have powerful claws that they use to crack open coconuts—the main component of their diet.
Usually nocturnal, coconut crabs spend the daylight hours hiding in vegetation and emerge at night to feed. For years, local islanders have hunted the crabs to serve on special occasions and sell to the occasional visitor, with no
significant effects on the population; the rate of hunting was sustainable and the crabs could reproduce at a sufficient rate.
However, with the tourism boom and the increasing popularity of travel to South-East Asia and the Pacific islands, demand for coconut crab meat has increased exponentially. Due to their rich diet of coconut flesh, the crab meat has an
unusual flavour and fatty texture, and its rarity makes it even more enticing to tourists looking to indulge in exotic food.
The price of a small crab can be as high as 800 pesos, equivalent to £12.19 per kilogram. Coconut crab is now served in many restaurants as a delicacy, commonly steamed in coconut milk, taken home by tourists as souvenirs, and exported to
eateries further afield.
Scientists studying crab populations in the Indonesian Sunda and Celebes seas have found that increased pressure on coconut crabs, especially the selective hunting of larger males, has resulted in an unbalanced sex ratio within populations,
with many more females than males.
The body size of the male crabs is also decreasing, most likely due to a combination of both over-hunting of the larger individuals, overfishing of other crustaceans and fish, and habitat destruction, all of which reduce the food
availability to coconut crabs and damage their habitat through ecosystem instability.
This causes a multitude of further problems; fewer males within the population means fewer potential mates for females, resulting in a decrease in mating success. Males of smaller bodies mass-produce less sperm, which reduces the chance of
successful fertilisation of females and therefore reduces the number of viable larvae produced.
In addition, females often refuse to mate with males of small body mass, due to the natural process of sexual selection, whereby phenotypic indicators of strength and health (such as large body size) are used during the selection of a mate.
This further decreases the number of viable egg spawns, exacerbating the decline in population.
Recently, a long-term study conducted on the Aldabra Atoll in the Seychelles suggested that strict protection methods could successfully recover coconut crab populations to sustainable numbers. Aldabra is listed as a UNESCO World Heritage
Site and has been a protected area since 1982, meaning the flora and fauna of the atoll are relatively undisturbed by humans.
‘Strict protection methods could successfully recover coconut crab populations to sustainable numbers.’
A large population of coconut crabs thrives across the four islands of the atoll, including a wide range of body masses and a diverse gene pool, which could potentially be used as a source population for other nearby regions which have
suffered a decline in numbers.
The healthy population of crabs is maintained on Aldabra due to the atoll’s remote location in the Indian Ocean, 1000 kilometres southwest of the main Seychelles island of Mahé. Rough terrain and limited fresh water supply have deterred
visitors to the atoll, meaning the wildlife is pristine and does not face the same pressures as similar populations in more disturbed areas, mainly from fishing and pollution.
The conservation of coconut crabs is vital as these crustaceans play an important role in ecosystem health, as they are apex predators on many small islands, controlling populations of smaller crustaceans, fish and birds. They also consume
a variety of plant matter in addition to coconuts, helping to recycle organic matter and reduce the aggressive proliferation of certain plant species through seed and seedling consumption.
Larval and juvenile crabs provide an important food source for both marine and terrestrial animals, especially birds, and the burrowing habits of the crabs increase the aeration of soils.
With appropriate measures, such as strategic placement of marine protected areas, strict fishing regulations and educating tourists about making sustainable seafood choices, the coconut crab populations across the Pacific islands can be
restored to previous levels.
Conservation will be most effective when laws, regulations and the establishment of protected areas are integrated with community-based conservation initiatives, which focus on educating and supporting locals to protect and conserve the
coconut crab and its habitat.
The Vatuvara Foundation in Fiji is paving the way through protecting this species, alongside their coral restoration work and providing information to help raise awareness of the decline of the coconut crab. Increasing the number of similar
projects would undoubtedly give the species a much-needed boost in recovering to their natural numbers.
This will take time, funding, and collaboration between many organisations and bodies, such as fisheries, locals, restaurant owners and conservation NGOs. However, acting now is an important step towards a healthier ocean.
Yorisue T., Iguchi A., Yasuda N., Yoshioka Y., Sato T. and Fujita Y. (2020) Evaluating the effect of overharvesting on genetic diversity and genetic population structure of the coconut crab. Scientific Reports.Volume 10, Issue 10026.