The Coconut Crab, Birgus latro, sometimes known as the Robber Crab, is the world's largest terrestrial Arthropod. It is a form of Hermit Crab, Coenobitidae, reaching over 4 kg in mass and over 1 m in leg span found on small islands and coral atolls across the Indian and Pacific oceans
In a paper published in the Raffles Bulletin of Zoology on 4 January 2023, Neil Cumberlidge of the Department of Biology at Northern Michigan University, Tim Caro of the School of Biological Sciences at the University of Bristol, Victoria Watson-Zink of the Department of Evolution and Ecology at the University of California, Davis, Tohru Naruse of the Tropical Biosphere Research Center of the University of the Ryukyus, Peter Ng of the Lee Kong Chian Natural History Museum and the Department of Biological Sciences at the National University of Singapore, Max Orchard of the Christmas Island National Park, Dwi Rahayu and Daisy Wowor of the Research Center for Oceanography of the Indonesian National Research and Innovation Agency, Darren Yeo, also of the Lee Kong Chian Natural History Museum and the Department of Biological Sciences at the National University of Singapore, and Tim White of the Hopkins Marine Station of Stanford University, collate available data on the Coconut Crab's distribution, threats, population trends, and protection levels, and use this data to update the species status on the International Union for the Conservation of Nature's Red List of Threatened Species.
Until 2020, the Coconut Crab was listed on the Red List as 'Data Deficient', this was subsequently updated to 'Vulnerable'; for the reasons that Cumberlidge et al. put forth in their study. Cumberlidge et al. believe the species warrants this change, as it is globally threatened, and because it is evolutionarily distinct species with no close relatives.
Coconut Crabs are found across a wide area of the Indian and Pacific oceans, inhabiting small islands and Coral atolls, and more rarely continental shores. They are found across a huge biogeographical range, encompassing the Afrotropic, Indo-Malayan, Australasian, and Oceanic realms. In the Pacific Ocean the species is found north of the Tropic of Cancer in the Ryukyu Islands of Japan, and almost reaches south to the Tropic of Capricorn in French Polynesia. Coconut Crabs have been reported in Hawai'i, but this appears to have been an ill advised (and unsuccessful) attempt at Human introduction. In the Indian Ocean its range is more limited, reaching 11.5° north in the North Sentinel Islands (part of the Andaman and Nicobar Islands) and 12.6° south at Quisiva Island off the coast of Mozambique; historically Coconut Crabs have been reported further south in the Western Indian Ocean, from Juan de Nova Island in the Mozambique Channel and the Mascarene Islands, but they now appear to be locally extinct in this region.
This wide range has been achieved by the planktonic distribution of the Coconut Crab larvae. Adult and juvenile Coconut Crabs are terrestrial in nature, with adults rapidly drowning if they become immersed in water, but the larvae are pelagic members of the plankton, taking four-to-six weeks to mature to mature to the point when they are ready to return to land. When they reach this stage, they metamorphose into an actively swimming stage, the glaucothoe, which migrates till it reaches shallow water, then sinks to the bottom, finds a Mollusc shell to inhabit, then crawls ashore. This enables the larval Coconut Crabs to be dispersed across vast areas of ocean, depending on favourable currents, whereas the terrestrial juvenile and adult stages will probably only move at most a few kilometres inland, enabling them to return to the sea easily when it is time to spawn.
In order to assign a Red List status to a species, it is necessary to know its geographic range, the proportion of that range actually inhabited by the species, and the number of locations where the species is known to occur. The Coconut Crab has a geographical range of 81 056 813 km³, one of the widest ranges of any species known. However, more than 81 000 000 km³ of this area is ocean, while the terrestrial range available to the species is actually less than 50 000 km³.
Determining the actual area inhabited by Coconut Crabs is more complicated. Assuming that every recorded instance of the Crabs represents an area of 4 km³ centred on that location, then the Crabs occupy 546 km³, but this is likely to be an underestimate, as the total range of the species includes a huge number of tiny islands, which have never been examined for the presence of Coconut Crabs. An alternative approach considered was to take the known coastline length of every landmass where the species is known, and multiply this by 3 km³, to allow for the average distance which the species moves inland (Coconut Crabs are known to move as much as 5-6 km inland on Christmas Island). This gives the species a range of about 75 000 km³, though this is likely to be a considerable overestimate, as the species will avoid areas with rocky shorelines and cliffs, and needs to find an undisturbed, humid rainforest within migrating range of a suitable beach. This gives the species a potential range of somewhere between 546 km³ (which would merit it being treated as Vulnerable) and 75 000 km³, which would make the species of Least Concern.
The number of locations actually inhabited by the species groups together occupied locations which are sufficiently close that they could conceivably be wiped out by the same threat. Coconut Crabs are known to occur at a minimum of 200 localities, although grouping close-together localities reduces this to 135 locations, potentially few enough for the species to be placed in a threatened category on the Red List.
Cumberlidge et al. believe it likely that where the species has previously been reported at a location, but not relocated following several surveys, then it should be regarded as having been locally extirpated. Coconut Crabs have historically been reported from the coastal waters of East and southern Africa, Madagascar, Mauritius, the Indian subcontinent, mainland Asia, large islands in Indonesia (Borneo, Sumatra, Java, Sulawesi, and Indonesian Papua), and mainland Australia, but is no longer found on any of these landmasses. This absence from suitable habitats on larger landmasses is likely to be a result of hunting pressure from Humans, as well as other large Animals, which tend not to be present on very small islands. The species may also have come under pressure from land-use changes by Humans, which could have removed suitable environments where they were once able to survive.
In order to fully understand the geographic range of the Coconut Crab, it is necessary to look at the environments inhabited at different stages of the species life-cycle. The species only inhabits the open ocean during its larval stage, which lasts four to six weeks, the shell-carrying juvenile Crabs inhabit coastal environments, but the majority of the Crab's 60 lives are spent inland as adults. Most studies into the conservation of the species have looked at the habitats used during this long terrestrial part of the life-cycle, but the Crabs need to be able to access all three environments to survive.
Adult Coconut Crabs are well-adapted to a terrestrial environment, and will drown rapidly if immersed in water. Unlike the juvenile stages, which resemble other Hermit Crabs in being soft-bodied and using Mollusc shells for protection, the exoskeleton of adult Coconut Crabs is hard and waterproof. These Crabs consume fallen fruit, seeds, and nuts (including Coconuts), as well as occasionally consuming Plants and carrion. Their favoured environment is the floor of shade-providing, humid, tropical forests, close enough to the shore for them to return to beaches to spawn. They are typically active during the day (although where they share their environment with Humans they often become nocturnal), but shelter in burrows during the hottest part of the day.
Within this range of preferred environmental conditions, the Crabs can be quite adaptable, inhabiting different different environments on different islands. On Christmas Island, an Australian territory in the Indian Ocean, they are mostly found in dense tropical rainforest, 3-5 km from the shore, and at altitudes of up to 300 m, but they tend to avoid the drier coastal stands of Pandanus and Coconut Palms. On Lifou Atoll, in New Caledonia, Coconut Crabs are again found in rainforest, but also inhabit sparser woodlands, coastal rubble, and Coconut groves. On Pemba Island, on the coast of Tanzania, and Okinawa Island, Japan, Coconut Crabs are strictly nocturnal, spending the day in holes and crevices in Coral rag (limestone rubble derived from ancient Coral reefs) in coastal forests. On Nuie in the Marshall Islands, Coconut Crabs are found in the humid, dense coastal forests, but avoid more open environments. On Christmas Island and Vanuatu, juvenile Coconut Crabs spend the day in the gaps in Coral rubble on the shores, but venture inland to forage at night.
Coconut Crabs mate between May and September, when they gather in large numbers at the top of the tidal zone, and the females deposit hundreds of thousands of fertilized eggs into the incoming sea. The larval Crabs live in the surface waters, among floating logs, coconuts, or rafts of vegetation and disperse over a wide area. After about 42 days they migrate to coastal waters on night-time flood times, where they sink to the bottom and metamorphose into benthic shrimp-like glaucothoe, which seek out Gastropod shells to protect their soft bodies, then migrate to the shore. After reaching the land, they spend about four weeks living around the high tide line. Over the next two years they expand their territory to include more of the shoreline, including the faces of cliffs, growing steadily, and replacing their shell home each time the molt. Eventually they reach a stage where their pleon hardens, and they no longer need to find shells for protection. At this point they become obligate air-breathers, unable to survive in water, and migrate away from the coast.
Coconut Crabs are an exceptionally slow breeding species compared to any close relative, with males taking about six years to reach sexual maturity, while females take between seven and nine years. At this stage their carapace is about 2.5 cm long. The Crabs continue breeding until they are about 30 years old[ although they can live to be about 60, older females rarely breed. This gives a generation time of between nine and thirty years. with an average of twenty.
Any terrestrial species with a planktonic larval stage is reliant on the regular recruitment of larvae back into the terrestrial population. Millions of larval Coconut Crabs are shed into the plankton, where they are vulnerable to predation, as well as fluctuations in seawater temperature and pH. Recruitment of larval Crabs back into the adult population is therefore irregular, and is not guaranteed to occur every year. A long generation time, combined with a complex life-cycle with different stages occurring in different environments makes Coconut Crabs particularly vulnerable to environmental disturbances, and populations that suffer heavy losses can take many years to recover, even where the Crabs are protected.
On most islands, Coconut Crabs come in two different colour variations, either predominantly orange and red or predominantly blue. However, in some populations only one colour variant is present, such as Aldabra Island, in the Seychelles, where all the Crabs are red, or on Okinawa or New Guinea, where all the Crabs are blue. The reasons for this colour polymorphism are unclear.
Almost all of the threats faced by Coconut Crabs are anthropogenic in origin. One of the biggest threats the species faces is the modification of coastal environments for farming or commercial development. The species is also threatened by the loss of its terrestrial (inland) environment, and the shrinking of small islands due to rising sea levels. The long generation time of the species makes it harder for the Crabs to respond quickly to rapid environmental threats. In addition to environmental degradation, Crabs are vulnerable to predation by Humans and Human-introduced species, and to road vehicles.
Humans have consumed Coconut Crabs wherever the two species have come into contact. This has become more of a problem for the Crabs (and many other island species) as Humans have expanded their range to include ever more remote islands in the Indian and Pacific oceans. The large size and slow movement of the adult Crabs makes them particularly vulnerable to Human predation, as little effort and no specialist equipment is needed to capture them. Furthermore, the Crabs are generally considered to be particularly tasty, and are therefore easy to trade, providing a ready source of cash to those who capture them. The shells of the Crabs are sometimes sold to tourists as trophies, and on some Pacific islands the meat of the Crabs is considered to have aphrodisiac properties.
Juvenile Coconut Crabs are vulnerable to natural predators such as Seabirds and Monitor Lizards, as well as Human-introduced species, such as Pigs and Rats. Furthermore, on Christmas Island they have been observed being attacked and killed by invasive Yellow Crazy Ants, while on Palmyra Atoll in the Pacific they have declined in areas colonised by invasive Scale Insects. In Vanuatu the Crabs have been intentionally killed by Coconut farmers, who believe they feed on Coconut seedlings, and in many places within their range Coconut Crabs are regularly killed by road vehicles.
The population of Coconut Crabs appears to have collapsed on Sorol and Yap Atolls in the Pacific, despite an abundance of suitable habitat, apparently due to over-harvesting by Humans and predation by introduced Animals which target juvenile Crabs. In the Cocos-Keeling Islands in the Indian Ocean have been almost totally wiped out by over-harvesting for Human consumption, although on North Keeling Island, where the Crabs are protected, they remain both common and unafraid of Humans,
The threat of rising sea levels due to global warming posses a particularly severe threat to Coconut Crabs, as many of the islands and atolls where they are found are both small and low-lying. The global sea level is currently rising at about 3 mm per year, which means that many of the islands where Coconut Crabs are found could be lost within 10-20 years. This would not only remove huge areas of land inhabited by the Crabs, it would greatly reduce the amount of larvae entering the plankton, and make it harder for the remaining plankton to find suitable environments. The rising global temperature is also causing the oceans to become more acidic, which has been shown to affect the behaviour of pelagic Brachyuran Crab larvae, and seems likely to have a similar impact on the larvae of Coconut Crabs.
Commercial exploitation has been shown to present a threat to Coconut Crabs in eastern Indonesia, where the Crabs are sold for US$2-5, to be exported in packages of up to 80 Crabs to more other parts of Indonesia, where they can typically be sold in seafood restaurants for up to US$20 each, with the largest females selling for as much as US$35. This means that, despite the Crabs being in theory protected, there has been a sharp decline in the population of large members of the species. In Japan adult Crabs are also sold for food, with the juveniles also being targeted for the international pet trade; Coconut Crab enthusiasts in Singapore have been known to pay US$150-300 for individual Crabs, depending on their size. Zoological gardens and public aquariums also frequently seek Coconut Crabs for their collections.
The global population of Coconut Crabs is almost impossible to determine. There are large and apparently thriving populations on many remote, uninhabited, and protected islands in the Indian and Pacific oceans. However, there are many more locations where the Crabs formerly thrived but are now absent or severely depleted, with this decline even extending to many uninhabited islands. The Crabs have completely vanished from mainland areas of Australia and Madagascar.
In almost all areas where the population of Coconut Crabs has been assessed, a decline in the number of Crabs has been recorded, with the majority of locations having recorded a decline of about 30% of adult Crabs over the past 15-20 years. This has been linked to over-harvesting of the Crabs by Humans, combined with a loss of habitat, and decline in the quality of the remaining habitat. The highest populations of Crabs are now apparently found on uninhabited islands, with almost all populated areas suffering sharp population declines, even where the Crabs are in theory protected.
Coconut Crabs remained abundant on Christmas Island when they were last surveyed. Two thirds of the island is covered by the Christmas Island National Park, and the Crabs even appeared to be thriving outside the park. However, the population here was last assessed before the invasive Yellow Crazy Ant took hold and began to form supercolonies on the island. These Ants are now severely impacting the Coconut Crab populations here, with the total number of Crabs on the island likely to have been greatly reduced. Parts of the island have also seen forests cleared for mining or Human habitation, which is likely to have impacted the Crabs, and the species is known to be vulnerable to road traffic on the island.
Although the total area inhabited by Coconut Crabs is vast, the species has been wiped out from much of this range. Coconut Crabs were once found on the coast of East Africa from Somalia to Mozambique, but have now vanished from almost all of this range. The Crabs are now very rare on the populated East African islands of Unguja and Pemba, and while they are found on smaller, uninhabited islands in the Zanzibar region, these islands are often visited by fishermen who harvest the Crabs.
Coconut Crabs were once common across the 115 islands and atolls of the Seychelles, but have suffered a rapid decline over the past 15-20 years, with losses estimated at about 80% of the population. This has been linked to both over-harvesting of the Crabs and the loss of suitable habitat. The only place in the Seychelles where they remain common is on Aldabra Atoll, a protected UNESCO World Heritage Site.
All of the main islands of the Comoros have suffered declines in their Coconut Crab populations of about 50%, The species may be extinct on Mayotte, where surveyors have been unable to locate it for over a decade. The surviving populations in the Comoros are suffering from a declining and fragmented habitat.
Coconut Crabs have suffered a population decline of at least 80% in the Glorioso Islands (a French Overseas Territory to the northwest of Madagascar) in the past 15-20 years, due to over-harvesting and habitat loss, and are considered vulnerable to local extirpation. The species may still be present on Grande Glorieuse Island, but is extremely rare, and is largely protected only by the presence of a military base, which discourages illegal collecting. The species appears to have disappeared from Ile du Lys, where it was last recorded in 1882, and has not been relocated during recent surveys.
The Crabs are now extremely rare on Juan de Nova Island, a French territory in the Mozambique Channel, and probably extinct on the nearby Europa Island (also French), as well as Madagascar and the Mascarene Islands, where numerous extensive surveys have failed to locate a single specimen for some years.
Coconut Crabs also appear to be in serious decline in the Andaman and Nicobar Islands, due to over-harvesting, despite being protected by both local taboos and the Indian Wildlife Protection Act, as well as loss of suitable habitat to agriculture. The Crabs appear to have disappeared completely from Katchal, Trinket, and Car Nicobar islands, where recent surveys have been unable to locate any specimens, and it is considered likely that the species may soon be extinct across the archipelago.
Large numbers of Coconut Crabs are still present on Christmas Island, where about 70% of the population live within the protected area of the Christmas Island National Park, but can be found wandering across the whole of the island throughout the year. Nevertheless, the population on Christmas Island is considered to have dropped sharply between 1979 and 2012, due to habitat loss, over-harvesting, roadkill incidents, and invasive species, notably the Yellow Crazy Ant. The population is being actively protected both in the Christmas Island National Park, and along the migration routes that connect the park to the sea, and it is considered likely that without that protection the population would decline rapidly.
It has been claimed that the Coconut Crabs of Christmas Island are larger than those on islands of the West Pacific, but Cumberlidge et al. interpret this as probably implying that populations of Coconut Crabs across the West Pacific are suffering from over-harvesting, which tends to remove the largest individuals from the population. Studies of unregulated harvesting in Indonesia have revealed that hunters disproportionately target larger Crabs, and also select by sex, although this seems to result in the over-harvesting of male Crabs on some islands (such as Halmahera), while on others (such as Siompu) females are disproportionately taken.
In the Philippines, Coconut Crabs have protected by legislation which prevents their harvesting or sale since 2001, but this legislation is widely flaunted, and the Crabs are now listed as locally endangered on the islands of Batan, Sabtang, and Itbayat in Batanes Province.
On Teraina Atoll in Kiribati the population of Coconut Crabs has fallen by at least 80% in the last 15-20 years, due to habitat destruction and over-harvesting. The Crabs here are considered particularly vulnerable due to a low population size, small area of available habitat, and a lack of conservation measures.
Coconut Crabs are found in the KentingNational Park on the southern coast of Taiwan, as well as on the offshore islands of Lanyu and Lyudao. The Crabs remain fairly common on Lyudao Island, but their average size has dropped, and few specimens larger than 1 kg can now be located. The population on Lyudao has fallen about 30% over the past 15-20 years, due to over-harvesting, habitat loss, roadkill incidents, and introduced predators. The population here is again thought to be vulnerable due to the small size of the island. Coconut Crabs are protected by Taiwan's Wildlife Conservation Law, and poachers can potentially be gaoled.
The population on the island of Guam has also declined by about 30% over the past 15-20 years, due to over-harvesting, road kills, introduced predators, and widespread coastal habitat destruction along migration routes. In the 1990s Coconut Crabs could be obtained in markets on Guam for US$20-30, but tightening of legislation has largely stopped this, although the population is still considered st high risk of local extinction due to over-harvesting and habitat loss.
Coconut Crabs cannot breed till they reach about 9 years old, and thereafter reproduce only once per year. This applies to both sexes, so over-harvesting can cause problems not just by removing adult Crabs, but also by skewing the sex ratio. This appears to have happened in the Ryukyu Islands of Japan, where there is a distinct shortage of males with carapace lengths of 4 cm or greater. This inhibits breeding by the Crabs, as females will not typically mate with males smaller than themselves.
Coconut Crabs enjoy varying degrees of protection in Japan, Taiwan, the Philippines, Indonesia, Guam, Vanuatu, Tuvalu, Funafuti Atoll, Christmas Island, Aldabra, Nuie, and the Tanzanian Islands, but are still unprotected across the majority of their range. Most of these unprotected populations are declining, due to the ease with which the Crabs are captured, and the slow rate at which the population recovers from losses. Cumberlidge et al. recommend that all future protection policies put into place for the species take into account this slow recruitment and recovery rate, and also involve local communities in the development of protection plans. Harvesting can be sustainable if hunting bans, no-take areas, catch-size limits, minimum size limits, and a ban on the capture of egg-bearing females form part of the management strategy. Such sustainable management would ensure that Crabs remain available for future generations to consume, as well as offering other benefits in the form of tourism revenues etc.
As an example of how good management can be achieved, Cumberlidge et al. cite the situation in southern Japan, where there are protected areas with complete bans on harvesting, and in other areas there is a ban on collecting ovigerous females, as well as a bans on collecting Crabs in certain months, and on the collection of juvenile Crabs.
In Taiwan Crabs are protected under the Wildlife Conservation Act, and are protected while they are in their forest habitat or on their annual migration to the beaches to spawn. Taiwan has also introduced a captive breeding program for Coconut Crabs, and a public education campaign.
On the island of Guam, a United States territory, harvesting of Crabs is strictly controlled, with restrictions on what size of Crabs can be taken and a complete ban on the harvesting of ovigerous females, as well as complete protection for the Crabs on the island's military base and some other locations.
Christmas Island is thought to host the world's largest surviving population of Coconut Crabs, with most of the island being covered by the Christmas Island National Park, where the Crabs are protected from harvesting, but vulnerable both to roadkill and attacks by Yellow Crazy Ants, with conservation efforts now being concentrated on resolving these problems.
On the island of Nuie, only females larger than 8 cm (a length reached at 14-45 years old) can be harvested, with smaller adult females (which make up 95% of the female population) protected.
In the Northern Marshal Islands, Coconut Crabs may be collected with a licence, but this only allows the collection of five Crabs per day, and these must be larger than 7.6 cm, and must not be egg baring; although the number of Crabs that may be collected rises to 15 per day from September to November.
In Tuvalu Coconut Crabs are protected on Funafuti Atoll as part of the Funafuti Marine Conservation Area.
In Indonesia Coconut Crabs are protected by law, although hunting is allowed with permits in North Maluku Province.
Other measures which Cumberlidge et al. recommend include the mapping of terrain used by Coconut Crabs on inhabited islands by attaching trackers to individual Crabs, combined with a more general mapping of suitable environments. This has been tried on Palmyra Atoll, where it has been used to demonstrate that that the loss of native forests has been directly connected to a drop in the population of Coconut Crabs. Habitat monitoring has the potential to provide insights into the conservation status of the Crabs, as well as their population levels. Potentially areas of perfectly good Coconut Crab habitat are completely devoid of Crabs, making studies of the habitat where Crabs are present more valuable. Imposing moratoriums on the collecting of Crabs is unlikely to benefit the species if there is a lack of suitable environment for a recovering population to inhabit.
In order to better protect Coconut Crabs in the future, Cumberlidge et al. believe that conservationists should: (1) Establish community-based conservation initiatives aimed at protecting the species and its habitat; (2) establish sanctuaries to protect coastal and inland forest habitats with primarily native vegetation (as opposed to only Coconut Palms); (3) close major migration routes and spawning areas to public access during the breeding season; (4) monitor and eradicate (or at least suppress) introduced species that impact Coconut Crab populations; (5) monitor Coconut Crab populations and carry out detailed research on crab ecology and genetics; (6) in places where harvesting is legal, implement a closed harvesting season (together with a permit/licensing system), set very small bag limits, and ban the collection of egg-bearing females for at least part of the year; (7) where relevant, implement a ban on the export of Coconut Crabs; (8) introduce public awareness campaigns informing of the need to arrest declines in Coconut Crab populations and habitat quality, and the need to restrict harvesting and regulate coastal land development; and (9) recommend/propose adding Coconut Crabs to CITES Appendix III.
Based upon the collected data, Cumberlidge et al. feel able to carry out a Red List assessment of the Coconut Crab, concluding that the species should be classified as Vulnerable, having suffered a population decline of at least 30% over the past 15-20 years at more than 12 sites where the species has been surveyed. The species has an impressive distribution, but is coming under increasing presure throughout its range, due to pressure from growing Human populations, leading to habitat loss and over-harvesting of the Crabs.
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