Uptake of waterborne 109Cd(II) in the scallop (Chlamys islandica) studied by in vivo gamma counting and quantitative whole-body autoradiography

Scallops caught in the North coast of the Gulf of St. Lawrence (Quebec, Canada) have been found to contain elevated levels of cadmium (Cd), though this area is far from any point source of pollution. Concentrations observed ranges from 2 to more than 20 mg/kg (w.w.) in the whole animal. This exceeds the European guidelines of 1 mg Cd/kg set for molluscs and have led to the refusal of scallop stocks from the area mentioned above. Relatively little is known about the uptake of Cd by scallops, apart from the fact that they often exhibit high concentrations of this metal in their tissues, especially in the digestive gland. To better understand the fate of Cd in scallop, we have exposed Chlamys islandica (60- 80 mm shell height) to waterborne 109Cd(II) (nominal concentration 37 kBq/L, closed system) for 45 hours. Accumulation of Cd was followed over time by non-destructive in vivo gamma counting (IVGC). At the termination of the exposure period, scallops were then placed in clean running seawater and radioactivity loss was monitored for 35 d. Results from IVGC were used to quantify the kinetics of uptake and elimination. Six of these scallops were sampled at t = 0, 7, and 35 d during the depuration period and used for whole-body autoradiography to precisely determined the distribution of the accumulated 109Cd. Preliminary analysis of data from IVGC showed that scallop rapidly accumulated 109Cd(II) from the water and that elimination was bi-exponential, with 30-35% of the radioactivity being eliminated within 2 d while the remaining 65-70% was eliminated at a slower rate, with half-life values ranging from 20 to 140 d (average 73 ± 52 d). Whole-body autoradio-graphy shows that 109Cd was distributed in all tissues of the scallop, the highest concentration being found in the kidney and the lowest in the muscle. Actually, the concentration of the radiolabel in the kidney is some 10 times higher than in any other tissue. Despite of its small size (ca. 1% of soft tissue weight), this organ contains 40 to 60 % of the 109Cd body burden. Our results indicate that the scallop Chlamys islandica readily accumulates cadmium from seawater. However, the distribution observed by whole-body autoradiography does not correspond to the one observed for wild scallops, since in the latter the hepatopancreas was the main accumulation site, containing 80 to 90 % of the Cd body burden (C. Rouleau, unpublished data). This may indicate that the main accumulation route of cadmium in wild scallop is not direct uptake through water. Further work is planned to look at the uptake of this metal via the phytoplanctonic food of scallop. The results of this study on the accumulation of Cd via water and food will provide a better understanding of the fate of this metal in scallop. Data collected will be used to build a predictive and quantitative accumulation model that will be useful to assess Cd contamination of scallops in pristine and human-impacted areas