Diet-tissue isotopic fractionation of the Pacific oyster Crassostrea gigas

Juveniles of the Pacific oyster Crassostrea gigas were reared on a monospecific microalgal diet in order to quantify their C-13 and N-15 diet-tissue fractionations. The weights of these juveniles increased by up to 18-fold within 33 d. The juveniles reached isotopic equilibria with the diet, enabling calculations of fractionation values. The C-13 fractionation for tissues containing lipids ranked as 0.6 parts per thousand (gill lamella) > 0.3 parts per thousand (adductor muscle) >-0.2 parts per thousand (mantle lobe) >-0.9 parts per thousand (whole soft body) >-2.2 parts per thousand (midgut gland), while the N-15 fractionation ranked as 8.7 parts per thousand (adductor muscle) > 6.5 parts per thousand (mantle lobe) > 5.4 parts per thousand (whole soft body) > 5.2 parts per thousand (gill lamella) > 2.3 parts per thousand (midgut gland). Removal of lipids shifted the diet-equilibrated delta C-13 and delta N-15 values in all tissues except the adductor muscle, with resultant increases in the C-13 and N-15 fractionation values. The expected annual mean delta C-13 value for the diet of the oyster in the field is -17.0 parts per thousand, which is an intermediate value among the VC of coastal phytoplankton (-20.2 parts per thousand), epilithon (-20.0 parts per thousand), epipelon (-14.8 parts per thousand), and seaweeds (-14.9 parts per thousand), suggesting that the oyster feeds on a mixture of these micro- and macroalgae. The expected delta N-15 diet value is 3.1 parts per thousand, which is more depleted than values for micro- and macroalgae, suggesting that the N-15 fractionation in the field is smaller than that obtained from the feeding experiment.