(Potamonautes spp.) in rivers draining Mt Kenya, East Africa

With 4 figures and 3 tables Abstract: Little is known about population densities of freshwater crabs, or their ecological importance, in Afri-can rivers. This study aimed to quantify crab abundance in rivers draining Mt Kenya. Invertebrates were sampled from 21 sites on 19 rivers. Sample sites were divided into: open sites in agricultural land from which trees were mainly absent; shaded sites, in agricultural land, with cultivated catchments but heavy shading by riparian trees; and forest sites, still dominated by natural vegetation. Crabs, mainly Potamonautes odhneri, were recorded from 14 of the 21 sites, including all forest sites, and were significantly more abundant in forest sites than in either type of agricultural site. However, there was no difference in biomass among habitat types, because individual crabs were significantly smaller on average in forest sites. This was due to the large numbers of small juvenile crabs re-corded in forest sites, whereas small juveniles were almost absent from all agricultural sites. Although numeri-cally unimportant relative to other macroinvertebrates, crabs accounted for at least 70 % of total macroinverte-brate biomass from forest and shaded agricultural sites, and averaged around 40 % in open agricultural sites. It is possible that crab reproduction occurs mainly or exclusively in forested areas, which would therefore act as a re-cruitment source for populations farther downstream in agricultural areas. Key words: Shredders, East Africa, detritus processing, biomass estimates, rivers, crustaceans. the size and species composition of crab populations may have important bottom-up implications for the river catchment ecosystem. Crabs play an important role in human disease transmission, as second inter-mediate hosts for the vectors of paragonimiasis (e.g