Digestive strategies in two species of leaf-eating land crabs (Brachyura: Gecarcinidae) in a rain forest.

Two species of herbivorous land crabs from Christmas Island, Cardisoma hirtipes and Gecarcoidea natalis, overlap in both diet and distribution. This study compared the dietary preferences and digestive capabilities of these two species on a diet of leaf litter to establish the digestive strategies each adopts and the likely degree of competition for food. C. hirtipes preferred green to yellow or brown leaves of Ficus macrophylla in short-term food-choice experiments. Brown leaves were least favoured. G. natalis showed no preference for the different leaf types and in the field ate chiefly brown and decomposing leaf litter. When fed green leaves, C. hirtipes had a low food intake (4.5+/-0.36 g kg-1 d-1) and a short retention time for food, and the readily digestible components of the diet constituted greater than 84% of the dry matter assimilated. When fed brown leaves, the intake was increased 3.3 times, but retention time remained short, and assimilation coefficients for all nutrients were low. The readily digestible fraction of the diet made the chief contribution to dry matter assimilation (69%), and hemicellulose (19%) and cellulose (21%) were also significantly used. This pattern of food intake and assimilation contrasts with that for G. natalis, which had a low intake of brown leaves and a longer retention time associated with higher nutrient assimilation, particularly of complex polysaccharides. It is suggested that through their feeding preferences and habits, these two sympatric species use opposite ends of the leaf litter quality spectrum on Christmas Island.