Diet–tissue isotopic fractionation of the Pacific oyster Crassostrea gigas

Abstract

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