The influences of diet and seawater dissolved inorganic carbon (DIC) on the carbon isotope composition of shell aragonite (Cinched in the Manila clam Ruditapes philippinarum reared under laboratory conditions were investigated. Clams were exposed to two successive negative carbon isotope shifts: a first shift in diet (delta C-13(phytoplankton)) and a second shift, 35 days later, in DIC (delta C-13(DIC)). Both successive shifts induced a decrease in delta C-13(shell). These results are the first to experimentally confirm an incorporation of respired carbon derived from food and carbon from DIC into shell carbonate of adult bivalves. Skeletal delta C-13 responded to changes in the delta C-13 of both diet and DIC in less than 7 days. Consequently, proxies based on delta C-13(shell) may be used with high temporal resolution. Using delta C-13(phycoplankton) as a proxy for the carbon isotope composition of respired carbon (delta C-13(R)) resulted in a rather constant percentage of metabolic carbon (C-M) into the shell carbonate over time (close to 12%). However, an accurate estimation of delta C-13(R) is required in order to precisely estimate the percentage of metabolic carbon incorporated into the shell. Despite the significant incorporation of metabolic carbon into shell carbonate, our experimental results revealed that delta C-13(shell) was highly correlated with delta C-13(DIC) (r(2)=0.77, p<0.0001). Thus it seems that delta C-13(shell) is a promising proxy of large scale variations in delta C-13(DIC) and therefore of salinity in estuarine water. (C) 2010 Elsevier B.V. All rights reserved.