Emigration behaviour during sea-to-land transition of the coconut crab Birgus latro: Effects of gastropod shells, substrata, shelters and humidity

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Populations of the coconut crab, Birgus latro, have been severely depleted on most inhabited islands throughout the Indo-Pacific because of over-harvesting and environmental degradation. Coconut crabs emigrate from the sea as megalopae carrying gastropod shells. Better knowledge of emigration behaviour can assist in conservation of coconut crab populations through understanding and protecting the habitats indispensable for completing their life cycle. We examined the behaviour of coconut crabs during sea-to-land transition by culturing megalopae and juveniles in containers that hold sea-water and hard substrate simulating an intertidal microhabitat in nature. The test environmental factors were availability of gastropod shells and substrata (small and coarse coral sands) including shelters on land, and humidity (relative humidity, 76% and 98%). Megalopae matured their nature for acquiring shells and emigration from the sea in around 10 days. Humidity and shell availability affected animal behaviour. Low humidity decreased the proportion of animals on land, and burrowing behaviour was stimulated in low humidity and shell-less conditions. Animals in low humidity had a low survival rate. Shelters and shells stimulated the proportion of animals on land at low humidity, and animals concentrated under the shelters on land. Shells, burrows and shelters should act to protect animals from desiccation by keeping microclimates around animals on high humidity. Thus, coconut crab megalopae and juveniles preferred a humid environment, and high-humidity conditions stimulated emigration from the sea. Our findings highlight the importance of a humid environment around the seashore in sustaining the recruits for coconut crab populations.