Restoration of the Coconut Crab Birgus latro (L.) in Green Island: Artificial Cultivation of Larvae, Investigation of Juvenile Habitat and Community Education

Chang-Po Chen, Fang-Lin Wang
Research Center for Biodiversity, Academia Sinica, Taipei, Taiwan115, Republic of China.


Introduction

 The coconut crab Birgus latro (L.), a monospecific genus Birgus within the family Coenobitidae, is found throughout many of the islands of the Indo-Pacific, including, the Green Island in Taiwan . Females release their larvae into the sea in the new moon phase, and the larvae live with the planktonic community for about a month. Then as postlarvae, or glaucothoes, settle to the bottom, and carry a small snail shell, and then emerge onto the shoreline as a part of hermit crab communities (Reese, 1968). When they reach a size of roughly 1 cm across the carapace, coconut crabs give up the shell-carrying habit of their hermit crab ancestors.

 With intensified harvesting and habitat deterioration, a rapid decline has happened in the number of Birgus latro. It has been on the Red list of the IUCN since 1983 (Wells et al., 1983), and is the only protected crustacean species designated by the Wildlife Conservation Law in Taiwan since 1995. Successfully conserving the habitat of coconut crabs will not only protect the coastal zones and nearby waters, but also land areas. It is an umbrella species of Green Island, and the best indicator for monitoring the quality of this island's ecosystems. In order to restore and manage the vitality of this threatened species, we have implemented three comprehensive lines of works, including artificial culture, habitat protection, and community education.


Three actions for coconut crab conservation

1. Larval culture of coconut crabs

Fig. 1 Larval sampling of Birgus latro. (a) gravid female in the vessel with sea water to make eggs hatching. (b) the first stage zoea.

 

Fig. 2 Larval culture devices of Birgus latro under the condition of 24-25℃ and 34-35 ppt (a) zoeal-reared aquarium (b) glaucothoel-reared glass container (c) aquarium simulating intertidal environments for rearing glaucothoes. Zoeae were fed with hatched Artemia nauplii.

 

Fig. 3 Survival rates of larval Birgus latro reared in this study in 2003 and 2004.

 

Fig. 4 The mean survival rates of enriched Atemia (EA) groups were significantly higher than unenriched Atemia (UA) those in the forth and fifth zoeal stages (t-test, P<0.05).

 

Fig. 5 There were 20 Zoeae metamorphosed to glaucothoes on the 27th to 34th day and 6 carrying the shell on the 37h to 43rd days of its zoeal life in our study. This picture shows a glaucothoe of coconut crab hiding in a snail shell.


2. Investigation of Juvenile Habitat

 Four juvenile coconut crabs without carrying shell were found in coastal shrubs and farmland which were away from shoreline 500m after sampling 6 sites in Green Island where a total of 52 adults were encountered in 2003 and 2004. The results of low recruitments reinforced the necessity of replenishing natural population by artificial cultivation and preserving habitat diversity.

 

Fig. 6 The juvenile and its habitat of coconut crab in Green Island: (a) a juvenile crab was found at farmland; (b) and (c) shows the habitats of juveniles with the characters of abundant rock crevices and food resource, such as botanic tuber and roots.


3. Local community empowerment through education

Fig. 7 The population investigation work has been team up with local people in Green Island in 2005: (a) transferring local hunter of coconut crab into protector by inviting them to join the work of population census for coconut crab; (b) teaching local resident the way of recording individual size of a coconut crab; (c) measuring the cephalothoracic length; (d) female and male size frequency distribution. By the method of mark-recapture, we estimate the number of the population of Birgus latro is 82 in 2005.


Future works

1.Ecological corridors for crabs will be built to separate the crab’s migration routes from the road encircling Green Island through community-based works including the study of the distribution of population, migrating behavior of crabs and environmental education of local residents.

2.Environmental education and local communication will be promoted effective through the coconut crab conservation team work.

3.The research on larval mass cultivation will be continued in 2006.

4.The study of genetics of Birgus latro in the Indo-Pacific region will apply to plan the appropriate conservation strategies for coconut crab.


Reference

Reese, E.S. & R.A. Kinsie 1968 The larval development of the coconut or robber crab Burgus Latro (L.). in the laboratory (Anomura, Paguridae) Crustaceana Suppl. 2, 117-144.

Wells, S.M., Pyle, N.M. and Collins, R.M. 1983. Coconut or robber crab. In: IUCN Invertebrate Red Data Book. International Union for Conservation of Nature and Natural Resources, Gland, Switzerland

 

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