Birgus Leach, 1816

Genus: Birgus Leach, 1816

Species: Birgus latro (Linnaeus, 1767)

Morphology: Like Decapoda , the body of the coconut crab is comprised of cephalothorax, which has (five pairs) ten legs, and the abdomen. The first and front-most pair of legs are large and with strong chelae (claws) that are used for peeling the shells of coconuts. Generally, the left chelae are larger than the right one. The next two pairs are huge, dominant walking legs with sharp tips like hermit crabs that are employed in climbing the coconut tree vertically. The fourth pair of legs are smaller with tweezers-like chelae at the end, allowing juvenile coconut crabs to dwell inside the shell or coconut husk for protection. But in case of adult, it uses this pair for walking and climbing. The last pair of legs is very tiny with small claws that are used by the female to tend their eggs, and by the males to mate. Storch et al. (1979) described general morphology of Birgus latro in the island of Olango, Philippines.

Colour: The colour of full-grown adult crab can be varied from reddish orange to purplish blue with blue is the prime colour. However, in the Seychelles, most individuals are red and therefore, the colouration of the animal may vary due to geographical region ( Fletcher, 1993).

Size: The size of adult coconut crabs can vary, generally reaching up to 40 cm; a leg can reach more than 1.0 m. The males are normally larger than females. The length of carapace is about 78 mm; a width is 200 mm. The weight of this crab can be measured up to 4.5 kg ( Naskrecki, 2005).

Reproduction and Life Cycle: Coconut crabs display sexual dimorphism, and the males are larger than females. The presence of pleopods in female abdomen is the distinctive characteristic that helps in carrying the egg masses. Spermatophores are present in the male crab that is deposited on the abdomen of the female’s gonopore. The fertilization process occurs on the external surface of the female abdomen and the females carry the eggs until hatching on pleopods. Later the mature females move from land to the shallow intertidal water and disperse the larvae in the sea in new phase. There is no parental care by male. The zoeal stage is the initial stages of its life, occurred in marine habitat and is consists of five stages with a span of 25 to 33 days. During post larvae or Glaucothoe or amphibious phase, it shifts into a vacant shell of gastropod and migrates into the land and undergoes metamorphosis. The amphibious stage is followed by the juvenile stage. Around 5 years after the hatching process, coconut crab attains sexual maturity ( Figure 1 View Figure 1 ). Larval development was studied in laboratory by Reese and Kinzin (1968). The mode of reproduction of coconut crab have been studied by Schiller et al. (1991) in the Republic of Vanuatu. Hicks et al., (1984) studied spawning particularly in the Christmas Island.

Habits: The Coconut crab is secretive, unsociable excluding the gravid females which are observed at large densities ( Schiller et al., 1991). They are spotted as nocturnal, diurnal or both. The behavioral pattern of coconut crabs is dependent upon the factors in particular, local population density and possibly the level of human activity. Day time foraging is observed among them which may be the mechanism for reducing the number of intraspecific encounters in large populations as the crabs are highly aggressive ( Helfman, 1977). These interactions can lead to the point of physical aggression, and as a result one or both antagonists can be seriously injured. Cannibalistic behaviour has been observed among larger crabs attacking and eating smaller individuals ( Altevogt & Davis, 1975; Helfman, 1977). After foraging, the crabs immediately go back to a particular sheltering site or cracks ( Fletcher et al., 1990). Coconut crabs live near to a source of coconuts, where they acquire large quantities of husk and broken kernels are often seen near the entrances of crab’s lairs ( Reese, 1987). It explains the terrestrial habit due to the characteristics of osmotic regulation ( Gross, 1955).

Habitat: The Coconut Crab Birgus latro is typically found on island habitats within a distance of 4 km from the sea ( Fletcher & Amos, 1994). They predominantly inhabit dense forest zone but can be copious in sandy coconut groves ( Grubb, 1971).Although rarely observed physically, tracks can be seen on the dunes and the upper shore along the coastline ( Grubb, 1971). These crabs usually inhabit in shelters like crevices, caves, hollow logs, tree roots, or earth burrows during the day time. Furthermore, they are usually found within a few kilometers of the sea. But in favorable conditions, some individuals make their way further inland ( Fletcher, 1993).

Food: Most studies conclude that, the coconut crab is omnivorous and probably a scavenger in nature ( Reyne, 1939; Altevogt & Davis, 1975). Cannibalistic behavior of this crab has also been observed by Altevogt & Davis (1975) and Helfman (1977). Fruits of Pandanus, Canarium spp. , Barringtonia spp. and the bread fruit, Artocarpus spp. are the major food items consumed by the coconut crabs ( Fletcher, 1993). Coconut crabs are also known as a scavenger by eating dead birds, rats, and crabs (Coenobitids, Brachyuran), and occasionally other Birgus ( Fletcher, 1993) . Recently it was observed that a large coconut crab immobilized and consumed a red footed booby ( Buehler, 2017). It can live in divergent salinity to maintain osmotic balance of their body ( Gross, 1955).

Predators: Humans and other coconut crabs are the known predators of adult coconut crabs. It is widely hunted for consumption and it is very rarely observed on islands with a human population because of its large size and quality of meat ( Wolcott, 1988).

Relationship with Human Beings: The coconut crab is served for food item and is considered a delicacy and an aphrodisiac. Coconut crabs are protected at the Cook Islands and the Mariana Islands where the people are following the traditional belief that ancestral spirits can return in the form of animals such as the coconut crab ( Orlando, 2009).

Status: The statistics shows that the coconut crab populations in several areas are decreasing or becoming locally extinct due to human predation as well as habitat loss ( Brown & Fielder 1991; Fletcher, 1993; Schiller, 1992; Lavery et al., 1996). In many of the countries the crab is effectively extinct ( Schiller, 1992). Hence, it is protected internationally and nationally.

Globally: In 1981, it was categorized as a vulnerable species by the IUCN Red List. But later it was relegated to ‘Data deficient’ category due to lack of biological data ( Eldredge, 1996).

India: Coconut crabs are listed as Schedule - I, Part - I under Wildlife Protection Act, 1972.

Distribution: Globally: Coconut crabs have a wide distribution range that extend over the tropical Indo-Pacific region that starts from Mauritius in the western Indian Ocean to the Tuamotu Archipelago in the eastern Pacific region ( Reyne, 1939; Davis & Altevogt, 1978; Harries, 1983; Robertson, 1991). They have not been recorded from east Africa, the Indian sub-continent, mainland Asia or Australia and the reports are only from island habitats ( Brown & Fielder, 1988). The studies on Coconut crabs in several islands were made by researchers namely Amesbury (1980) from Guam ( Mariana Islands); Fletcher (2003) from Vanuatu; Fletcher and Amos (1994) from Australia; Anagnostou and Schubart (2014) from Christimas Islands; Kadiri-Jan and Chauvet (1998) from Lifou, New Caledonia; Chauvet and Kadiri-Jan (1999) from Tuamotu archipelago, French Polynesia; Vogt (2004) from Diego Garcia; Sato and Yoseda (2008) from Hatoma, Japan; Helagi et al. (2015) from Niue Island, New Caledonia; Storch et al. (1979) reported from Olango Island, Philippines; Matamaki et al. (2016) from Cooks Islands, New Caledonia; Serosero et al. (2018) from Indonesia.

Andaman and Nicobar Islands: At most few studies have been carried out on Coconut crab in Andaman and Nicobar Islands. Distribution of coconut crab from south sentinel was reported ( Alcock, 1905; Davis & Altevogt, 1976) and Nicobar Islands ( Hume, 1874; Alcock 1902, 1905; Man, 1938; Bhaskar & Rao, 1992). Daniel & Premkumar (1968) studied about the feeding and breeding behavior in the Great Nicobar Island. Altevogt and Davis, (1975) reported on the observation of habitat, activity patterns, feeding bahaviour in South Sentinel, Andaman Islands. First post-tsunami sighting of the coconut crab in the Nicobar Islands was by Patankar (2007).Recently, Patankar and D’Souza (2012) reported31 individuals of coconut crabs from Menchal and Camorta Islands. Under the present study, status, distribution, abundance and relative density of Coconut crab, sex ratio and morphometric characteristics of coconut crabs and size variations of male and female were estimated, and the distribution map of Coconut crab was prepared.

Study Area

This study was carried out from April 2015 through March 2018 in Little Andaman and the Nicobar group of Islands (Great Nicobar Island, Little Nicobar Island, and Nancowry group of Islands).

Little Andaman

Location and Topography: Little Andaman is the southernmost island of the Andaman Archipelago. It is the southernmost island of the South Andaman District. It is separated by the Duncan Passage from the Andaman Island Group in the North and separated from the Nicobar Island Group by the Ten Degree Channel in the South, situated approximately 120 km away from Port Blair. Little Andaman has underlain by marine sedimentary group of rocks, other igneous rocks and coralline limestone rocks. This island is considered as an incredible part of biodiversity with the presence of quantum of lives with its natural habitat and phenomenon, pristine sandy beaches of the islands, presence of several ecosystems such as evergreen forests, deciduous forests, and mangrove forests.

Nicobar Islands

Location and Topography: Geographically , the Nicobar group of Islands is an archipelagic chain in the eastern Indian Ocean. It is pertained of a great island arc formed by the collision of the Indo-Australian Plate with Eurasia. They are located south of Port Blair , and around 150 km north of Aceh on Sumatra , and it is separated from Thailand to the east by the Andaman Sea. In the year of 2013, the UNESCO has declared the Great Nicobar Island as one of the World Network of Biosphere Reserves ( UNESCO, 2013). These groups of Islands are flat, fertile islands with un-spoilt seas all around. The climate in the Nicobar group of islands is warm and tropical, and the temperatures ranging from 22 to 30° C. Heavy annual rainfall of 3000 to 3800 mm is recorded in Nicobar group of islands due to annual monsoons. The vegetation of the Nicobar is typically classified into the coastal mangrove forests, evergreen, deciduous, tropical and subtropical moist broadleaf forests. In addition, many of the islands contain extensive grasslands and coconut plantations. The presence of pristine sandy beaches of the islands, several vegetation’s and suitable climates make this group of oceanic islands as incredible habitat for coconut crabs .

Geography: The area of Nicobar group of Islands is 1,841 km 2. They consist of three distinct groups, namely, Northern group, Central group and Southern group.

Southern Group: Great Nicobar, Kondul Island, Little Nicobar, and Pulo Milo, Meroe, Trak, Treis, Menchal, Kabra, Pigeon and Megapod.

Central Group: Chowra, Bompuka, Teressa, Katchal, Camorta, Nancowry, Trinket, Laouk and Tillangchong.

Northern Group: Car Nicobar and Battimalv.

The objectives of this survey are

• To assess present status and distribution of Coconut crab population

• To estimate the sex ratio

• To measure morphometric characteristics of male and female coconut crab populations

• To establish conservation and monitoring action plan which will be the most beneficial to Coconut crab populations in Nicobar group of Islands.