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A PRELIMINARY SURVEY OF THE COCONUT CRAB BIRGUS LATTRO ON HOGA ISLAND

Abstract: 

The distribution and habitat preference of adult Birgus latro on Hoga Island, Turkangbesi Archipelago, Indonesia, were assessed during the months of July and August 2002. The sites assessed were situated in dense woodland up to 1 km inland, entering the path north of Le Baba, at the beach, along the western side of the Island. Preliminary results suggest, that adult Birgus latro are nocturnal and preferably inhabit single limestone holes and crevices per specimen. On a single occasion, a female and male were located in a single coral cave. The size of captured individuals (large claw to unfolded abdomen) ranged between 10 and 50 cm with a weight range of 83 to 2500 g, where females were larger than males.  

Introduction:  

Birgus latro, or commonly known as the coconut or robber crab, and belonging to the Family Coenobitidae, is a world-wide protected species and on the ‘Red List’ since 1983 (IUCN, 1983). The crabs inhabit oceanic islets and atolls as well as on the coasts of islands in the tropical Indo-Pacific area (Chauvet and Kadiri-Jan, 1999). With intensified harvesting of coconuts and environmental change a rapid decline in the number of Birgus latro has been recorded in the past two decades (IUCN, 1983; Bruggren and McMahon, 1988; Fletcher et al., 1990). Despite the effort undertaken to manage the stocks, to date little is known on the natural populations (Chauvet and Kadiri-Jan, 1999).

Originally, it was thought that this species was absent from the Tukangbesi Archipelago, but it was discovered during the previous years on Hoga Island. This initiated the survey of the crab’s distribution and habitat preference on Hoga during July and August 2002.

Method: 

Two local guides were trained and introduced to the humane capture, the measuring and marking of adult specimens of Birgus latro and their subsequent release at the location of capture. A developing understanding of the importance of this species and the interest in its protection are crucial aspects for the success of this ongoing project, as Birgus latro is a delicacy among the local people of the Turkangbesi Archipelago.

The distribution and habitat preference of adult Birgus latro on Hoga Island, Turkangbesi Archipelago, Indonesia, were assessed twice weekly on Wednesdays and Sundays between 7 and 9 pm during the months of July and August 2002, since animals were not located during day time baiting the areas with coconuts. The sites assessed were in dense woodland up to 1 km inland, entering the path north of La Baba (05°28.12’ S,  23° 45.69’ E) , at the beach, along the western side of the Island. After baiting the areas with chopped coconuts at 3 pm The crabs were captured and the sites were recorded by GPS, the time noted and crabs taken back in individual bags to the wet laboratory at Operation Wallacea base for subsequent analysis.

Individual specimens were weighted, measured by taking the whole length (tip of large claw to unfolded abdomen) and size of the inner larger claw. Sex was recorded and individuals marked with permanent paint on the second segment of the upper carapace or abdomen. The animals were then returned to the site of capture the same night by the guides.

Results and Discussion: 

Lianas, palm tree branches and debris in the understorey of Hoga Island hindered heavily any systematic survey and the crabs were not easily located in the torch beam as described by Chauvet and Kadiri-Jan (1999). Single adult Birgus latro were found and captured in limestone holes, 1 –2 m deep or small caves and crevices and only during one occasion a pair was found in a singe limestone cave (Table 1).  

Table 1: A record of distribution, sex, weight, size and marking of Birgus latro near Le Baba, Hoga Island, Turkangbesi Archipelago, Indonesia.

*large claw

Similarly the majority of Birgus latro were discovered or recorded in dense vegetation of the Taiaro Atoll (Chauvet and Kadiri-Jan, 1999). Preliminary results indicated, that Birgus latro are nocturnal and preferably inhabit single limestone holes and crevices per specimen. On a single occasion, a female and male were located in a single coral cave. This is in opposition to islands, where the crabs do not form part of the diet of their only preditor, man, such as the Christmas Island (Ecology of Sulawesi, other refs). So, for instance, in the Pacific, on the Christmas Island, crabs  are commonly found during daylight in populated areas by man and islands, where found on the menu of humans, this species became nocturnal (Ecology of Sulawesi).

So far, the size of captured individuals (large claw to unfolded abdomen) ranged between 10 and 50 cm with a weight range of 83 to 2500 g, where females were larger than males. It is possible that due to predation by the local population, as observed with dogwhelks and other marine species, the animals do not reach the much larger sizes reported from Zanzibar, Africa and other pacific islands (pers. comm. with Richard Barnes).

Evidence exists that coconuts form the predominant part of the crabs diet. In several limestone holes the coconuts had been stripped of the fibres and claw marks were visible in crushed areas of these nuts.   

References:  

Chauvet, C. and Kadiri-Jan, T. (1999). Assessment of an unexploited population of coconut crabs, Birgus latro (Linne, 1787) on Taiaro atoll (Tuamotu archipelago, French Polynesia). Coral Reefs 18: 297-299.