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Although the role of macroinvertebrates in the nutrient cycling of the Indo-Pacific mangrove ecosystems is now a paradigm of ecological research, most of the research and scientific assumptions deal with decapod crustaceans alone. However, decapods are not the only representatives of mangrove macrobenthos and in certain mangals they are surely outnumbered by gas-tropods, especially by potamidids of the genus Terebralia. This paper aims at bridging the gap in the knowledge of the ecological role of the largest and most abundant potamidid species of the Indo-Pacific mangals, the semiterrestrial T. palustris. In particular we studied size structure and distribution in relation to micro-environmental factors and feeding ecology, both in terms of diet composition and grazing rate, of two populations of this gastropod colonising a Kenyan mangrove creek. In contrast to what has been described by various authors concerning other mangals, in our study site juveniles and adults of T. palustris were present throughout the whole forest, from the landward belt to the seaward fringe, locally reaching very high densities at various levels. Both life stages showed a preference for patches characterised by soil with high organic content; thus the habitat of juveniles largely overlapped with that of the adults, although they are known to occupy different ecological niches. Indeed, gut-content analysis confirmed that juveniles are detritivorous and adults are mainly leaf-litter consumers. In situ experiments on the grazing activity of the adult snails showed that, in contrast to the litter-consuming crabs, they feed in every light and tidal condition, for an hourly consumption rate of ~0.65 g per snail. Considering an average density of about 10.5 adults [m.sup.-2], it is estimated that a total of 10.5 leaves [m.sup.-2] are removed by adult snails in each tidal cycle (i.e. about every 6 h). T. palustris is the only leaf consumer capable of searching for food under water during high tide and is responsible for the removal of a great quantity of mangrove litter. The results of the present study strongly suggest that T. palustris plays a major role in the food web, mangrove litter degradation and nutrient cycling of the Kenyan mangal.

In a Kenyan mangrove, we studied the interactions between the gastropod Terebralia palustris and the crab Neosarmatium smithi when foraging on decaying mangrove leaves. Interactions are considerable on account of their wide overlap in zonation (Rhizophora mucronata belt), food items (mangrove leaves) and activity window (diurnal low water). The snails find a leaf by a systematic transecting of the platform and eat the leaf after crawling on to it, usually en masse. The crabs rarely venture beyond 80 cm from their burrows, and once they find a leaf they quickly drag it back into their burrow, probably to reduce the strong intra-specific competition. A crab is able to drag a leaf away even if a number of snails are already feeding on it, by strongly pulling the leaf away or by pushing the snails off the leaf. The success of the 'thief' depends on both the crab's size and the number of snails on the leaf.

Results point out that the sesarmine crabs of the East African mangroves show a spatial distribution in relation to their size, male or female, food habits, and activity profiles.