Quantitative methods for the determination of abundance of mobile prey of ghost crabs, Ocypode Weber, 1795 (Decapoda: Brachyura: Ocypodidae), a top predator on sandy shores

摘要

Ghost crabs, Ocypode Weber, 1795 (Decapoda: Brachyura: Ocypodidae) are the top-benthic predators on sandy beaches in the sub-tropical and tropical regions around the world. They are omnivores and opportunistic feeders with varied feeding modes, such that they can exert cascading trophic effects on intertidal communities. Although there is a wealth of literature on the diet of ghost crabs based on qualitative reports of direct observation of predation, tracks/signs of feeding activities, or presence/absence data of prey items in foregut content analyses, there is hardly any quantitative analyses of food resources in the field that are available to these top predators on the sandy shore. While the abundance of immobile food resources can be ascertained by sediment sample assays, specialized sampling methods have been used for the collection and enumeration of mobile prey items. There is a need also to understand the period of foraging activity of the study species in order to track the predator-prey interaction at the appropriate time of the day. We outline three techniques suitable for the collection of prey types with different distribution, activity period and mobility: 1) surface-dwelling prey, 2) prey that burrow, and 3) prey that are capable of flying, a novel method that has not been utilized and described before.