Distribution and abundance of freshwater crabs (Potamonautes spp.) in rivers draining Mt Kenya, East Africa

Abstract

Little is known about population densities of freshwater crabs, or their ecological importance, in African 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 recorded in forest sites, whereas small juveniles were almost absent from all agricultural sites. Although numerically 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 recruitment source for populations farther downstream in agricultural areas.