Diet in mangrove snails: Preliminary data on gut contents and stable isotope analysis

Microscopic analysis of gut contents performed on three Littoraria species from mangrove forests in Thailand revealed differences in diet among specie s. Analysis of carbon and nitrogen stable isotopes was used as an alternati ve way of tracing food sources. Rhizophora leaves, scrapings from both leaf and prop-root surfaces, and local particulate organic matter (POM) were we ll separated on the basis of their delta C-13 and delta N-15 values. In con trast, the three Littoraria species exhibit considerable overlap and scatte r in both carbon and nitrogen isotope ratio values, suggesting that the sna ils are opportunistic feeders sharing similar food resources. The wide rang e of delta C-13 values of Littoraria (-17.2 parts per thousand to -26.3 par ts per thousand) is consistent with carbon assimilation from multiple sourc es (epiphytes from leaves and prop roots, suspended POM, and Rhizophora det ritus). Littoraria intermedia and L. pallescens, the smallest species, had similar VC values, whereas L. scabra was significantly more C-13 depleted. A diet of microalgae and cork cells from prop roots could explain this patt ern, with L. scabra, being larger, consuming relatively more cork cells. Ho wever, only a few of the L. scabra and L. intermedia individuals had delta N-15 values consistent with such a diet, and the remaining L. scabra and L. intermedia and all L pallescens individuals were too depleted, indicating that these individuals must derive a significant amount of their food from a strongly N-15-depleted source. Such a source is present on Rhizophora lea f surfaces (delta N-15 = 0.30 +/- 0.05; n = 2). Some very low values of Lit toraria delta N-15, down to -7 parts per thousand, indicate that some indiv iduals have assimilated a yet unknown, highly N-15-depleted food source or that other unknown fractionation processes are involved.