Assessing substrate quality by the sand-bubbler crab, Dotilla fenestrata, (Brachyura, Ocypodidae): preliminary results from a field experiment.

The sand-bubbler crab, Dotilla fenestrata (Hilgendorf, 1869), has evolved a surface deposit feeding habit and exhibits stereotyped and flexible behaviours that allow to respond to both the predictable and unpredictable elements of intertidal environments (Bigalke, 1921 ; Fielder, 1970; Hartnoll, 1973; Dray & Paula, 1998). In a mangrove swamp, field observations revealed that, at the ernergence from their burrow with the reciding tide, a portion of this crab's population abandoned the residential zone and wandered in droves (Gherardi et al., 2002). The remaining population displayed a burrow-orientated activity (Gherardi & Russo, 2000) and foraged exclusively around their burrow (Gherardi et al., 1999). If crabs are small individuals or moulting crabs or ovigerous females, the only option is to stay. For the other population categories, Gherardi et al. (2002) hypothesized that their choice will depend on the richness in edible organic content of the substrate of the residential area, as well as on the subjective balance between the costs arising from the risk of being preyed upon versus the benefits emerging from foraging over richer substrata. This paper provides preliminary results from a field experiment carried out in March 1997 in the mangrove swamp of Mida Creek (Kenya). The aim was to test the feasibility of an experimental protocol designed to verify the hypothesis that at their emergence sand-bubbler crabs assess the substrate quality and behave accordingly.