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¯exibility 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 bene®ts 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.

Activity and feeding ofDotilla fenestrata(Brachyura: Ocypodidae) in a warm, temperate South African estuary

“…In tropical habitats, Dotilla fenestrata and D. myctiroides are active during diurnal low tides, emerging from their burrows to commence feeding activity once it becomes light. They remain buried at night during high and low tides (Hartnoll 1973, Gherardi et al 1999, Bradshaw and Scoffin 1999, Gherardi and Russo 2001, Gherardi et al 2002a. Although Gherardi et al (1999) did observe some D. fenestrata opening their burrows at night, these crabs showed no other surface nocturnal activity.…”

“…Dotilla fenestrata (Hilgendorf 1869), commonly known as the sand-bubbler crab, is a small species (Dray and Paula 1998, Gherardi et al 2002a, Flores et al 2005. It is distributed along the East African coast from Kenya to South Africa, and also occurs on Madagascar and the Comoro Islands (Barnard 1950, Hartnoll 1973, Vannini and Valmori 1981, Dray and Paula 1998, Gherardi and Russo 2001, Gherardi et al 2002a, Flores et al 2005, Litulo et al 2005. In South Africa, D. fenestrata is most abundant north of Durban (29°51′ S), but small numbers do exist in warm temperate regions as far south as the Breede River estuary (34°24′ S) (Day 1974, 1981, Branch et al 2010.…”

“…In South Africa, D. fenestrata is most abundant north of Durban (29°51′ S), but small numbers do exist in warm temperate regions as far south as the Breede River estuary (34°24′ S) (Day 1974, 1981, Branch et al 2010. Although D. fenestrata can inhabit a range of coastal biotopes, including intertidal sandflats or mudflats and mangroves (Macnae 1968, Hartnoll 1973, Dray and Paula 1998, its preferred habitat is sheltered sandflats, and this is where it reaches the greatest densities (Hartnoll 1973, Gherardi and Russo 2001, Gherardi et al 2002a, Flores et al 2005. For example, Dray and Paula (1998) recorded a maximum average density of 466 crabs m -2 on the tidal flats of Inhaca Island, Mozambique, and Hartnoll (1973) recorded 564 crabs m -2 on tidal flats at Dar es Salaam, Tanzania.…”

“…The burrow provides refuge from disturbance, predation and thermal extremes, marks the centre of the crab's feeding range, and assures replenishment of lost moisture by accessing interstitial water through capillarity (Gherardi et al 1999, Gherardi andRusso 2001). During the diurnal high tide and the nocturnal high and low tides, D. fenestrata remains within its burrow (Hartnoll 1973, Dray and Paula 1998, Gherardi and Russo 2001, Gherardi et al 2002a).…”

“…Dotilla fenestrata feed by sorting the organic detritus from the upper few millimetres of sand (Hartnoll 1973, Dray and Paula 1998, Gherardi and Russo 2001, Gherardi et al 2002a. They scoop up the substratum with their chelae, sort it with their mouthparts, and then discard the inedible material in the form of pseudofaecal pellets (Hartnoll 1973, Zimmer-Faust 1987, Gherardi et al 2002a.…”