Geographical and seasonal variations in the relationship between trace metal content and body weight in Mytilus edulis

The relationships between trace metal content (Cd, Cu, Fe, Mn, Ni and Zn) and body weight of Mytilus edulis L. show that smaller mussels are richer in trace metal than larger ones. According to the metal, the season and the locality examined, 3 types of relationships were obtained: (1) straight lines over the whole size range of mussels; (2) curves with steeper slopes in larger mussels and (3) curves with lowering slopes in larger mussels. For most metals the slopes changed from one locality to another. A detailed analysis of the relationship between Cd content and body weight showed a gradual decrease in metal content until maturity was reached, thereafter the relationship varied significantly with the season, and 2 different types of curves were obtained. Thus, our results contradict the idea that slopes are uniform for a given metal and species. Multivariate analyses showed that weight accounted for most of the variance in metal content while age and gonad maturation explained little of the variance observed. We propose the metabolic changes associated with sexual maturity and season continually change the relative importance of the circulating and storage compartments of metals and may be responsible for the variations of slope observed.