Experimental determination of stable carbon and nitrogen isotope fractionation between mangrove leaves and crabs

The analysis of benthic food webs in mangrove forests is complicated by the lack of experimental investigations of consumer-diet discrimination factors (Delta N-15 and Delta C-13) for most mangrove invertebrates. This study aimed to determine the fractionation of delta C-13 and delta N-15 between mangrove leaves and Episesarma singaporense and E. versicolor (Sesarmidae) for 90 d in Java, Indonesia. The consumption rates of both species and stable isotope fractionation of leaves from 5 plant species (Acanthus ilicifolius, Aegiceras corniculatum, Avicennia alba, Derris trifoliata, Rhizophora apiculata) were compared. Delta N-15 between R. apiculata leaves and muscle tissue was 5.0 parts per thousand for E. singaporense and 5.4 parts per thousand for E. versicolor after 50 d. delta N-15 of muscle tissue increased significantly after 50 d, most likely due to the internal recycling of nitrogen. delta C-13 did not change during the experiment, and Delta C-13 was 5.1 parts per thousand for E. singaporense and 4.1 parts per thousand for E. versicolor after 90 d. 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.