Temporal variations of metallothionein and metal concentrations in the digestive gland of oysters (Crassostrea gigas) from a clean and a metal-rich site.

The concentrations of metallothionein (MT) in bivalves, a potential biomarker of metal pollution, are variable according to specific organs, the highest concentrations being encountered in the digestive glands of oysters. Thus, the present study has been focussed on this organ with a view to validate the use of MT as a biomarker in the field, the temporal changes of metal and metallothionein concentrations have been examined from March to October 1997 in the digestive gland of resident oysters from a clean site (Bay of Bourgneuf, France) and a metal-rich site, the Gironde estuary which has been shown as the most Cd-contaminated marine area in France but is also enriched with Cu and Zn. Moreover, oysters from the clean site have been translocated to the Gironde estuary over the same period. Taking into account all the samples collected over the 7 months of the study, MT concentrations in the digestive gland were positively correlated with weight whereas metal levels were negatively correlated with weight. However, considering monthly samples including specimens from both sites (resident or translocated oysters), a positive correlation was shown between MT and metal concentrations in autumn (September and October) but not in spring and summer. These findings limit the interest of using the digestive gland of oysters as the preferred tissue for the determination of MT concentration as a biomarker.The alternative use of gills should be considered.