Factors affecting the distribution and behaviour of the crab Dotilla fenestrata on East African shores

Abstract

Dotilla fenestrata, an ocypodid crab which burrows intertidally on sandy substrates, was studied near Dar es Salaam on the East African coast. It is restricted to areas where the surface drains and becomes firm at low water and in which the median particle diameter lies between 0.16 and 0.44 mm (fine to medium sands). Within such areas it occurs between m.l.w.n. and m.h.w.n., and is most abundant from m.l.w.n. to just above m.t.l. with small specimens concentrated at the lower levels. The crab remains buried whilst its habitat is covered by the tide and only emerges an hour or more after the tide has receded. It then commences to feed by sorting the organic content from the surface layer of the sand, in the course of which it systematically excavates a sector of the substrate adjacent to its burrow. Apart from the largest males, which wander freely over the Dotilla zone, individuals remain in the immediate neighbourhood of their burrows. About ten minutes before the flooding tide covers them the crabs retreat into their burrows and plug the openings. The burrows do not descend to the water table, so for the bulk of the feeding period Dotilla must depend upon interstitial water. This is collected with the aid of a tuft of specialized setae on the fourth abdominal segment, an arrangement which is unique to Dotilla.