Burrowing activity in the sand-bubbler crab, Dotilla fenestrata (Crustacea, Ocypodidae), inhabiting a mangrove swamp in Kenya

Abstract

Several species of crabs belonging to the family Ocypodidae are burrowers; they inhabit tropical sandy and muddy shores, including mangrove swamps, exposed to extreme conditions of intertidal areas. Dotilla fenestrata, the only East African representative of the subfamily Scopimerinae, exhibits high flexibility in its burrowing activity. Only part of the population has an exclusively burrow-orientated activity, whereas most individuals form dense aggregations or droves, that during the phase of diurnal low water move away from the normal distribution zone. Burrows are built according to two different architectural designs, the feeding-trench burrow and the igloo, which then constitute their territory. A resident will lose a contest for the burrow when the attacker is larger. When the ground is uncovered at low tide, emergence can be delayed or even fail to occur, and some crabs remain buried during the entire low water period. Only 50% of crabs remain faithful to the original burrow, while the others either leave it to occupy an empty one or dig a new burrow, or join the wandering drove. The spatial distribution of burrows from the high water neap level (our reference point) to the sea, differs during both spring and neap tides, and both diurnal and nocturnal tides. The differences may be due partly to the change of physical conditions, such as drainage and substrate grade (although within a mangrove swamp these do not follow a consistent gradient), but mostly may be related to the trade-off between the benefits and costs of living at different intertidal levels. With its burrowing and feeding activity leading to the re-deposition of excavated and pseudofaecal pellets, Dotilla plays a role as a sediment mover in this very specialized habitat. The results obtained from this study are compared with those reported in other ocypodids.