FEEDING HABITS OF NON-OCYPODID CRABS FROM TWO MANGROVE FORESTS IN KENYA

The stomachs of 194 crabs belonging to 11 species, commonly inhabiting mangrove areas, were collected in two Kenyan mangrove forests, in Gazi Bayand Mida Creek, and their contents were analyzed. Both the stomach fullness (a subjective estimation) and the composition (the plant:animal matter ratio) of the food ingested were recorded. Results show that few species are either strictly herbivorous (Sesarma ortmanni and Selatium elongatum) or strictly carnivorous (Eurycarcinus natalensis), while most of the species are non-specialized and normally eat a wide range of plant items, mangrove leaf litter and algae, and animal prey, mostly bivalves, gastropods, anomurans and other brachyurans. Crab stomachs with a higher percentage of mangrove leaf litter and algae were significantly fuller than those with animal items. Most of the crabs display an omnivore feeding habit and feed on a wide range of items. Among the Sesarminae, the most herbivorous group, the only species showing a significant degree of specialization is Sesarma leptosoma, which is able to reach the mangrove canopy to feed on fresh leaves. Among the macrofauna commonly colonizing mangrove forests, brachyuran crabs are one of the most important taxa with regard to both the number of species and the total biomass (Macintosh, 1988). For this reason their feeding habits and the position each species occupies in the food web are of great importance for the understanding of the ecology of the mangrove ecosystem. Many species of mangrove crabs are known to be herbivorous. Chiromanthes onychophorum de Man in Malaysia (Malley, 1978), Neosarmatium meinerti de Man (=Sesarma meinerti, re-diagnosed by Davie (1994) as the Neosarmatium genus) in South Africa (Emmerson and McGwynne, 1992) and Kenya (Dahdouh-Guebas et al., 1997), and Sesarma smithii H. Milne Edwards (Giddins et al., 1986), S. messa Campbell (Robertson, 1986; Micheli, 1993) and S. erythrodactyla Hess (Camilleri, 1989) in Australia, can consume a great amount of leaf litter of various mangrove species. These crabs probably play an important role in the process of leaf degradation, making mangrove leaves more rapidly available to meiofauna. Ocypodid species can feed on mangrove pneumatophores, eat bark and macro-algae (Wada and Wowor, 1989). The herbivorous crabs, which are strongly linked to roots and mangrove leaves, some predators, such as Scylla serrata (Forskal) (Hill, 1976) and Goniopsis cruenata (Latreille) (Von Hagen, 1977), and other species of Grapsidae have been reported to feed mostly on plant matter in situ, but also on dead crabs and fishes in captivity (VonHagen, 1977). The natural diet of Aratus pisonii (H. Milne Edwards) includes fresh Rhizophora mangle L. leaves as well as caterpillars, crickets and beetles (Beever et al., 1979). Little is known about the feeding habits of most crabs colonizing the mangrove habitats of the Kenyan coast, with the exception of some Uca species, which are known to be micro-algal feeders (Icely and Jones, 1978), and N. meinerti and Cardisoma carnifex (Herbst) (Micheli et al., 1991), both important consumers of fallen leaves.