The function of wandering in the sand-bubbler crab, Dotilla fenestrata.

The sand-bubbler crab, Dotilla fenestrata (Hilgendorf, 1869), has evolved a surface deposit feeding habit and exhibits stereotyped and flexible behaviours that allow this species to respond to both the predictable and unpredictable elements of intertidal environments. In a mangrove swamp of Kenya, field studies revealed that, at the emergence from burrows with the receding tide, a portion of this crab's population abandoned the residential zone and wandered in droves. The rest fed and engaged in other surface activities exclusively around their burrow (either a feeding-trench burrow or an igloo). Laboratory analyses were carried out to assess the functional significance of the choice of D .fenestrata between these opposing spatial strategies. If crabs are small individuals or ovigerous females, the only option is to engage in burrow-orientated activities. For the other population categories, the choice will depend on the richness in edible organic content of the substrate in the residential area and on the balance between the risk of being preyed upon and the benefits of foraging over richer substrata.