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We examined ␦ 13 C values of shallow and deep-water scallop shells as well as ␦ 13 C of dissolved inorganic carbon (DIC) from the Bay of Brest in western Brittany. Time series of shell calcite ␦ 13 C do not reflect seasonal variation in seawater ␦ 13 C, but rather show a consistent pattern of decreasing ␦ 13 C with age, suggesting a metabolic effect rather than direct environmental control. This ␦ 13 C trend reflects an increasing contribution of metabolic CO 2 to skeletal carbonate throughout ontogeny, although this respired CO 2 does not seem to be the major source of skeletal carbon (contribution of only 10% over the first year of life). We propose a model of this effect that depends on the availability of metabolic carbon relative to the carbon requirements for calcification. A ratio of "respired to precipitated carbon" is calculated, and represents the amount of metabolic carbon available for calcification. Interestingly, this ratio increases throughout ontogeny suggesting a real increase of metabolic carbon utilization into the skeleton relative to carbon from seawater DIC. This ratio allows us to separate two different populations of Pecten maximus of different water depth. It is therefore suggested that shell ␦ 13 C might be used to track differences in the metabolic activity of scallops from different populations.

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