Isotopic trophic-step fractionation of the freshwater clam Corbicula sandai

Abstract

Diet-switch experiments with three different species of microalgae were conducted to estimate diet-tissue isotopic fractionation of carbon (C) and nitrogen (N) isotopes in the freshwater clam Corbicula sandai. The δ13C and δ15N values in the soft tissues of C. sandai changed in each experiment, with little inter-individual deviations, thereby reflecting the switch in diet. Isotope values of the clam reached the asymptotic value at around 40 days after the switch. Equilibrium isotopic signatures, as well as turnovers of carbon and nitrogen in the whole soft tissues, were estimated by exponential decay models. Fractionations for C. sandai varied from 0.1 to 0.7 ‰ for carbon, and from 2.1 to 3.6 ‰ for nitrogen; these values fell within or were close to the range of previously accepted fractionation values (0–1 ‰ for carbon and 3–4 ‰ for nitrogen). The half-life values for carbon in the bivalves (12–22 days) were about twofold longer than those for nitrogen (7–9 days). The specific fractionation values estimated in this study contribute towards improving our understanding of inter-specific trophic relationships and aquatic food webs.