The contribution of metal/shell-weight index in target-tissues to metal body burden in sentinel marine molluscs. 2. Mytilus galloprovincialis

The tissue distribution of Zn in mussels collected from a Zn-polluted site and the changes in the pattern of distribution for Zn and Cu in response to experimental exposures to Cu, Zn and Cd, as well as the tissue distribution of Cd have been studied. The organs of mussels were accurately separated avoiding mixtures except the kidney area that was included within the remainder tissues. Mussels exposed to intermediate dose (ID)-Cd showed the highest Cd burden first in gills and then more or less equally distributed between the various tissues examined with a dominance of the digestive gland. Mussels exposed to high dose (HD)-Cd, showed Cd equally distributed through all the organs at both exposure times. Below 0.8 μg Cd/l seawater, the gills of Zn-polluted mussels were able to either protect against Cd entry or mobilise uptaken Cd towards other tissues. On the other hand, Zn- and Cu-burdens did not rise in gill tissue, although in depurating mussels Zn-levels were higher than in any other exposure group. Similar conclusions were obtained for the digestive gland. However, the accumulative capacity of the digestive gland tissue for Cd was not exhausted and Cu burdens were depressed on exposure to Cd. Interactions between metals in the various organs studied could be explained by redistribution of haemocytes or brown cells and by enhanced release of metal-bearing digestive lysosomes in response to metal exposure. It is suggested that (a) the pattern of metal accumulation and tissue distribution as well as a part of the interactions between Cu, Zn and Cd cannot be explained solely by the availability of metallothioneins in specific organs; and (b) the specific cell type in which these ligands occur as well as the relative abundance and distribution of these cells are relevant factors controlling metal burdens in organs of mussels. Therefore, in addition to the chemical and the biochemical approaches, investigations on tissue sections are required to better understand these phenomena. Meanwhile, the digestive gland of mussels could be used as the target organ to monitor metal pollution, including essential and non-essential metals, on the basis of metal/shell-weight indices.