The analysis of benthic food webs in mangrove forests is complicated by the lack of experimental investigations of consumer-diet discrimination factors (Δ 15 N and Δ 13 C) for most mangrove invertebrates. This study aimed to determine the fractionation of δ Leaves of D. trifoliata and R. apiculata were preferentially consumed, indicating their higher nutritive value. We concluded that (1) the discrimination values for mangrove crabs feeding on a leaf diet are much higher than previously assumed, which can most likely be explained by the selective assimilation of isotopic heavy carbon compounds and by metabolic cycling of non-essential amino acids; (2) the frequently used average discrimination factors are inappropriate for the study of benthic food webs in mangrove forests with a high biomass of leaf-eating crabs; and (3) E. singaporense and E. versicolor can meet their nitrogen demand by assimilating nitrogen from R. apiculata leaves and by using internal reserves for at least 50 d.