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

Abstract

Microscopic analysis of gut contents performed on three Littoraria species from mangrove forests in Thailand revealed differences in diet among species. Analysis of carbon and nitrogen stable isotopes was used as an alternative way of tracing food sources. Rhizophora leaves, scrapings from both leaf and prop-root surfaces, and local particulate organic matter (POM) were well separated on the basis of their δ13C and δ15N values. In contrast, the three Littoraria species exhibit considerable overlap and scatter in both carbon and nitrogen isotope ratio values, suggesting that the snails are opportunistic feeders sharing similar food resources. The wide range of δ13C values of Littoraria (-17.2% to -26.3%) is consistent with carbon assimilation from multiple sources (epiphytes from leaves and prop roots, suspended POM, and Rhizophora detritus). Littoraria intermedia and L. pallescens, the smallest species, had similar δ13C values, whereas L. scabra was significantly more 13C depleted. A diet of microalgae and cork cells from prop roots could explain this pattern, with L. scabra, being larger, consuming relatively more cork cells. However, only a few of the L. scabra and L. intermedia individuals had δ15N 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 15N-depleted source. Such a source is present on Rhizophora leaf surfaces (δ15N = 0.30 ± 0.05; n = 2). Some very low values of Littoraria δ15N, down to -7%, indicate that some individuals have assimilated a yet unknown, highly 15N-depleted food source or that other unknown fractionation processes are involved.