Daytime Activity and Habitat Preferences of two Sympatric Hermit Crab Species (Decapoda: Anomura: Coenobita)

The beach environment is extremely dynamic in space and time. Abiotic factors like tides, sun exposure or sediment structure are defining the ecology of the beach-associated fauna. Among the most common beach-dwelling organisms of tropical and subtropical shores are the hermit crabs of the genus Coenobita (Latreille, 1829) (Decapoda: Anomura). They utilize gastropod shells to protect against predators, to avoid desiccation and disruption by wave action and further show behavioural adaptations, like burrowing in the substratum to withstand the abiotic stressors of coasts. Little is known, however, if the abiotic factors of the beach habitat influence the daytime activity and habitat preferences. We therefore analysed the changes in abundance during daytime, at different tidal times and in different coastal habitats in a community of two sympatric Coenobita species, C. rugosus and C. perlatus. We hypothesized that habitat, daytime and tidal time influenced the overall abundance. Here, we showed that hermit crabs became largely absent during midday, while their highest diurnal activity laid in the 2 h before low tide until absolute low tide. Structurally more complex beach types were preferred over pure fine sand or rock beaches. These behaviours and preferences of the investigated hermit crabs are adaptive as they aid in avoiding desiccation, while becoming most active when food availability is highest during low tide. Heterogenous beach habitats are probably favoured over homogenous sandy beaches, because accumulation of marine debris, a major food source, is increased. This emphasizes how physically controlled the distribution of beach-dwelling organisms is and demonstrates how abiotic stressors can become major drivers for behavioural adaptations in beach crustaceans.