Larval Growth of the Coconut Crab Birgus Latro with a Discussion on the Development Mode of Terrestrial Hermit Crabs
Abstract With intensified harvesting and environment deterioration during the past two decades, a rapid decline in the number of coconut crabs, Birgus latro, which is a protected species listed in the IUCN Invertebrate Red Data Book, has occurred on many islands. Thus, it is important to protect this species by establishing conservation areas and/or replenish natural population by larval cultivation. In this study, the development modes were analyzed and the effect of enriched diet on larval growth and survival were examined. Two types of zoeal development patterns were found. In general, zoeae took 29-33 days to complete five zoeal stages and metamorphose to glaucothoes. However, some zoeae directly metamorphosed from the 3rd zoeal to glaucothoe stage in 25∼28 days. Morphologically, these zoeae with accelerated development had thoracic appendages that appeared like the fifth stage zoeae, but with the telson, antennule, and antenna similar to those of the third stage zoeae. When fed Artemia nauplii enriched with nutritious substances, the zoeae had significantly greater survivorship and sizes, particularly at the fourth and fifth zoeal stages. Accelerated development may suggest early adaptation to a terrestrial lifestyle. The adaptation of larval development related to glaucothoe size and zoeal life span is also compared and discussed for eight terrestrial hermit crab species of Coenobitidae. The comparison suggests four adaptive modes of larval development and these are described as mangrove adaptation, larger glaucothoe adaptation, smaller glaucothoe adaptation, and hypersaline adaptation. The selective advantage of each mode may reflect a response to the uniqueness of each specific habitat.