The clam R. decussatus is a suspension-feeding bivalve mollusc widely distributed in European and Mediterranean coastal waters. Due to its economic importance it is extensively cultured in many countries, and particularly in Portugal where its production represents > 80 % of total shellfish production. The selection of the clam R. decussatus as a bioindicator was a consequence of its economic importance and the need to use a filter-feeding bivalve to evaluate environmental changes of metal concentrations [1].Like many other bivalves, this species resides in sediments and accumulates metal concentrations reflecting gradients of metal contamination in the surrounding environment [2,3,4]. This suggests that its tissues have mechanisms, related to its filter-feeding habit, that inhibit the toxic effects of these contaminants.Like many other molluscs R. decussatus has evolved a number of subcellular systems for accumulation, regulation and immobilization of metals. These include, among others, the binding of essential and pollutant metals to soluble ligands such as MT. These two-domain molecules are low molecular, heat-stable proteins of non-enzymatic nature that occur mainly in the cytoplasm. They have strong affinity to class B metal cations which enable them to be differentiated from most of the other proteins [5]. The induction of these proteins has, therefore, been proposed as a specific indicator and possible "early warning marker" for the detection of detrimental effects caused by exposure to excess of essential and pollutant metals.MT induction has been detected in the whole soft tissues, gills, digestive gland and remaining tissues of the clam R. decussatus after exposure to essential and toxic metals [4,6] and this paper represents an overview of MT function in this bivalve species.
MT induction in laboratory experimentsSeveral laboratory experiments have been carried out to assess MT induction in R. decussatus exposed to sublethal concentrations of cadmium (100 and 400 µg/l), copper (75 µg/l) and lindane (34.5 µg/l) [4,7,6,8].Chromatographic elution profiles of the heat-treated cytosol of the whole soft tissues of unexposed clams revealed that cadmium in this pool (> 80 % of total Cd) was distributed between two cadmium-binding proteins of a molecular weight of around 10,000 and 20,000 Da, respectively, that increased with Cd accumulation. Similarly an increase in the -SH containing proteins (by 70 % compared with unexposed clams), based on differential pulse polarography Similarly the amino acid sequence of the β domain of the MT of the digestive gland of the clam also shows some degree of similarity with the similar MT sequence from mussels and oysters. It is, therefore suggested that there is some degree of similarity in the MT structure among these species.