The Gironde estuary has been shown to be the most Cd-contaminated area In France. Oysters Crassostrea gigas of different size classes were sampled from 3 sites between the mouth and 25 km upstream, to study the relative importance of natural and contamination factors on metal accumulation and metallothionein-like protein (MTLP) induction. The highest metal and MTLP concentrations were observed in the digestive gland. In all organs, the 3 metals were mainly cytosolic. An inverse relationship was

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... generally observed between weight of soft tissues and metals or MTLP concentrations in the whole soft tissues, whereas the influence of weight on metal distribution among cytosolic and insoluble fractions was small. When oysters with similar weight of soft tissues were considered, no influence of salinity at different sites was found for Cu and Zn whereas the lowest soluble and insoluble Cd concentrations were observed at the mouth of the river. Salinity did not influence Cu and Zn distribution among the cytosolic or insoluble fractions whereas cytosolic Cd in gills and digestive gland (%) was higher at the mouth of the estuary than upstream. MTLP and metal concentrations were generally positively correlated except at those sites where metal concentrations were lowest. However when multiple regression analyses were performed, weight appeared also as a major or an important factor to explain vanations of MTLP levels, the prevalence of this natural factor versus contan~ination factors being different at ddferent sites. KEY WORDS. Oyster. Heavy metals. Metallothionein