Changes in mussel biometry on exposure to metals: implications in estimation of metal bioavailability in ‘Mussel-Watch’ programmes

The occurrence of changes in flesh and shell weights and in other biometric parameters of mussels, Mytilus gallopro¨incialis, has been related to different metal levels found in their soft tissues. The effects of clean and Zn-polluted environments and laboratory experiments where Zn-polluted mussels were exposed to sublethal concentrations of Zn, Cu and Cd were investigated. Zinc-polluted mussel shell weights increased significantly after a 51-day depuration period. Exposure of Zn-polluted mussels to Zn or Cd, however, caused a slightly reduced shell growth in comparison with depurating mussel Cu-exposures not causing any reduction in growth. Apart from metal concentrations, metalrshell weight indices have been used to assess metal bioavailability. Metal concentrations recorded in the soft tissues of depurating mussels increased without a source of ‘extra’ metals, while the Znrshell-weight index was reduced, as expected from a depuration process, Cu and Cdrshell-weight indices remaining constant. Experimental exposure to Zn, Cu and Cd caused augmented values of Zn, Cu and Cdrshell- weight indices, respectively. These different findings were attributed to changes in flesh weight related with gamete spawning. that would produce inconsistent estimates of whole metal concentration in soft tissues. Since changes in the tissue composition and in growth rates do not affect ZnrCu ratios and metalrshell-weight indices, these parameters are proposed as reliable indices of metal bioavailability for ‘Mussel-Watch’ monitoring programmes. The most sensitive parameter is the metalrshell-weight index, which is, therefore, highly recommended to be used in ‘Mussel-Watch’ monitoring programmes in order to determine metal bioavailability in seawaters.