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Oysters can accumulate cadmium (Cd) in very high concentrations in their tissues and there is now increasing concern for the seafood safety of farmed oysters worldwide. Bioaccumulation and biokinetics of Cd (dietary assimilation, uptake from the dissolved phase, and efflux) were mea- sured in different populations of Pacific oyster Crassostrea gigas collected along the west coast of the USA in 2 different seasons. Triploid juvenile oysters were first transplanted to 5 Washington sites and 1 Oregon site for 3 mo, then transported back to the laboratory for tissue Cd measurements and bio- kinetic experiments. A second group of seed oysters was collected from California and Washington for additional analysis. Oysters from all sites had Cd concentrations lower than the food standard limit and there was spatial difference in tissue Cd concentrations. A significant fraction of Cd was associ- ated with metallothionein-like proteins, implying that the oysters can detoxify Cd by induction of metallothionein. The difference in Cd dietary assimilation (using diatoms as a food source) and efflux in oysters transplanted to different locations was not significant. The uptake rate constants for Cd from the dissolved phase, however, differed markedly among the transplanted populations, and appeared to be affected by the oysters' clearance rate, depending on the population. This uptake rate was also negatively affected by the size of oysters. Dietary assimilation of Cd was comparable at diatom food concentrations below 2 to 5 mg l -1 , and was lower with ingested sediments than with diatoms. Measurements of Cd kinetics in different populations of Pacific oysters under a variety of environmental conditions provide an insight into the Cd accumulation in this commercially important species.

Dietary toxicity of metals on fish is often studied using commercial pellet food, and there is a lack of investigation on the toxicity of metals that are biologically incorporated into the natural food from the aquatic environment. In this study, we investigated the toxicity of dietborne Cd from the oligochaete Lumbriculus variegatus to the rainbow trout Oncorhynchus mykiss. The oligochaete worms were exposed to waterborne Cd (0.1, 5, 20, and 200 microgL(-1)) for 1 week and the fish were fed this food exclusively (daily ration=3.5% body wet weight) for 1 month. Cd concentrations in the worms averaged 0.1, 0.6, 2.2, and 30.3 microgg(-1) wet weight respectively, whereas the whole fish accumulated 0.002, 0.005, 0.019, and 0.387 microg Cd g(-1) wet weight respectively, after feeding upon control or Cd-contaminated worms for 4 weeks. Highest concentrations of Cd were retained in the gut, followed by the kidney and liver of the fish, with the latter two increasing over time; however, gut tissue accounted for >80% of whole body Cd burdens at all times. The trophic transfer efficiency of Cd was low (0.9-6.4%) although higher than in previous studies using Cd-spiked commercial diets, and was only weakly correlated to the internal Cd storage in the worms. The level of Cd in the contaminated worms did not affect Cd trophic transfer efficiency, but was reduced over the dietary exposure period. Dietborne Cd did not interfere with whole body Ca uptake from the water or alter plasma [Ca], but reduced growth by 50% in the trout exposed to the highest Cd dose. Cd stored in the metallothionein-like proteins of the fish gut tissue increased while that in the heat-denaturable proteins was reduced, suggesting detoxification over time. This study suggests a higher bioavailability and toxicity of Cd from the natural diets than from the commercial diets used in previous studies.

There is increasing evidence to indicate that metal pre-exposure may mediate the subsequent metal uptake from the dietary or dissolved phase by marine invertebrates. In this study, we examined the detoxification and the effects of Ag, Cd, or Zn pre-exposure on the metal uptake of the clam Ruditapes philippinarum. The relationships between metal body burden and metal uptake of Ag, Cd, Hg, and Zn were also concurrently examined. Radiotracers 110m Ag, 109 Cd, 203 Hg, and 65 Zn were used to chase the metal uptake in the clams. The clams accumulated Ag and Cd over time, but there was not a significant Zn accumulation. Metallothionein played a role in the detoxification of Ag and Cd, but the insoluble fraction was also the detoxification pool for Cd and Zn. Generally, metal pre-exposure did not greatly affect metal uptake. Ag pre-exposure had no effect on dietary Ag uptake. Cd and Zn pre-exposure increased Zn dietary uptake by 10%. After 2 wk Ag exposure and 3 wk of Cd or Zn exposure, Cd and Hg dissolved uptake was reduced by 2 to 6 times. However, these effects mainly appeared during the earlier part of the exposure period (i.e. up to Weeks 2 or 3). The effects of metal exposure on uptake of dissolved Cd and Hg were related to the coupling relationships of dissolved metal uptake between the 2 metals and other metals (Ag and Cd or Hg; Cd and Hg; Zn and Hg). The effects on dietary and dissolved uptake of all metals were independent of Ag, Cd or Zn body burden; thus, metal body burdens may not necessarily indicate the potential metal uptake of marine clams in the environment. Our data have important implications for kinetic modeling predicting metal uptake by marine clams.

We examined the toxicity of Cd, provided in a natural diet and at an environmentally relevant concentration ( approximately 12microgg(-1) dry wt.), to the juvenile rainbow trout (Oncorhynchus mykiss). In addition, we tested the protection by elevated dietary Ca against both the accumulation and toxicity of dietary Cd from this natural diet (background Ca approximately 1mgg(-1) dry wt.). Food pellets were made from blackworms (Lumbriculus variegatus), and spiked with Cd and either no additional Ca or elevated ( approximately 60mgg(-1) dry wt.) concentrations for each of the treatment diets. Survival was unaffected for trout fed diet with 12microgg(-1) dry wt. Cd for a month, but growth was potentially reduced. Tissue burden analysis revealed that the stomach, liver and kidney accumulated the most Cd, with concentrations progressively increasing in the liver and kidney over the whole exposure period. Cd concentrations in the plasma and red blood cells were unaffected by the different treatments, but subcellular fractionation analysis indicated that a higher concentration of Cd was associated with the metal-sensitive fractions of red blood cells of the fish that were exposed to the dietborne Cd. Dietary Cd exposure also caused potential toxicity to cells of the stomach in that they bound more Cd to heat-denaturable proteins. However, detoxification appeared to take place in the Cd-exposed fish because more Cd was bound to metallothionein-like proteins by week 4 of exposure. Elevated Ca in the Cd diet generally protected against accumulation and toxicity of dietborne Cd. The protection against Cd accumulation was almost complete at the gills, robust in the stomach and whole body (or =50% reductions), but not significant in the liver, kidney, carcass, plasma, or red blood cells. Elevated dietary Ca also reduced Cd accumulation in the organelles of the fish stomach and red blood cells. In addition, dietborne Ca not only reduced the uptake of Cd by the cells, but also altered how the cells handled Cd intracellularly. In general, our results have demonstrated the need to use diets with natural compositions for dietary toxicity studies.

This study investigated the kinetics of Cd bioaccumulation, detoxification, subcellular distribution, and efflux in the nereid polychaete Perinereis aibuhitensis after Cd pre-exposure. Cd pre-exposure increased the Cd body burden in the worms, but did not affect the overall Cd uptake and efflux rates and metallothionein-like protein (MTLP) concentrations. During short-term exposure to dissolved Cd, Cd in the cytosolic fraction increased after Cd pre-exposure, and this fraction also increased during the Cd efflux period, indicating that the insoluble fraction of Cd was presumably lost at a faster rate than the loss of cytosolic Cd. Even though the MTLP concentration remained comparable after Cd pre-exposure, both the MTLP synthesis rate and the degradation rate increased, thus leading to a high MTLP turnover in the Cd-exposed worms. However, Cd uptake and efflux into different protein size fractions did not follow the patterns of MTLP synthesis and degradation, strongly suggesting that Cd kinetics is decoupled from the MTLP kinetics in the worms. Our study adds to an increasing body of evidence on the complicated relationship between metal biokinetics and MTLP kinetics in different groups of marine invertebrates which have strong contrasts in their metal handling strategies.

Bioaccumulation and toxicity of Cd were investigated in a marine predatory whelk, Thais clavigera, after being fed with the rock oyster, Saccostrea cucullata, or the herbivorous snail, Monodonta labio, for up to four weeks. The oysters and snails had different subcellular Cd distributions and concentrations in their bodies given their different metal-handling strategies and were exposed to dissolved Cd for two weeks before being fed to the whelks. After four weeks of dietary exposure, the Cd body concentrations in T. clavigera increased from 3.1 microg/g to between 22.9 and 41.8 microg/g and to between 22.7 and 24.1 microg/g when they were fed with oyster and snail prey, respectively. An increasing proportion of Cd was found to be distributed in the metallothionein (MT)-like proteins and organelle fractions, whereas the relative distribution in the metal-rich granules fraction decreased when the whelks were fed Cd-exposed prey. At the highest Cd dosage, more Cd was distributed in the pool of metal-rich granules when the whelks were fed the oysters than when they were fed the snails. Among all the biomarkers measured (MT induction, condition index, lipid peroxidation, and total energy reserve including carbohydrate, lipid, and protein), only MT showed a significant difference from the control treatments, and MT was the most sensitive biomarker for dietary Cd exposure. No toxicity was found in the whelks fed different Cd-exposed prey as revealed by various biomarkers at the different biological levels. Our results imply that metal fractionation in prey can alter the subsequent subcellular metal distribution in predators and that dietary Cd toxicity to the whelks was low, even when the accumulated Cd body concentrations were high.

We examined the dynamics of subcellular distribution of metals (Cd, Ag, and Zn) in the marine green mussel Perna viridis by partitioning the metals into the insoluble fraction (IF), heat-sensitive proteins (HSP), and metallothionein-like proteins (MTLP) during metal uptake and elimination. Variations in metal uptake and elimination then were correlated with the subcellular distributions of these metals. The IF and HSP were the first ligands to bind with the metals during the dissolved exposure, and more metals were found in the HSP when the metal influx rate was higher. However, to minimize toxicity, metals were redistributed from HSP to MTLP afterwards. The subcellular distribution of metals was dependent of the exposure route in the mussels. During dietary metal exposure, the metals attained equilibrium before they were assimilated and the metal assimilation efficiency was independent of the metal partitioning in different subcellular fractions. During the efflux, metals in the soluble fraction mediated depuration, whereas metals in the insoluble fraction acted as a final storage pool. Redistribution also may occur between the metal-sensitive and inactive pools without significant depuration as a secondary protective mechanism. We further demonstrated that the higher efflux rate of Ag and Cd was related to a higher partitioning in the MTLP and a lower partitioning in the IF. Our study shows that subcellular pools other than MTLP were involved in immediate metal handling in the bivalves. The wide dynamics of subcellular metal distribution suggests that the relevance of individual subcellular fractions is dependent on the exposure pathway.

Metal interaction is vital for assessing the use of aquatic organisms in monitoring metal contamination. The present study examined the interactions between Ag and Cd accumulation and between Ag and Cu accumulation in the green mussel (Perna viridis). Accumulation of Ag and Cd in the whole tissue of green mussels exposed to 5 microg/L of Ag and 20 microg/L of Cd for two weeks was independent; however, interaction was observed at the subcellular level. Approximately 25% of Ag shifted from the insoluble fraction (IF) to the metallothionein-like protein in the presence of Cd, which probably resulted from the competition of Cd on IF in the Ag-Cd coexposure. On the other hand, coexposure of the mussels to Ag (5 microg/L) and Cu (30 microg/L) for two weeks increased the Ag and Cu concentrations in the tissue synergistically (two- to fivefold), but Ag and Cu subcellular distributions were similar in the coexposed and the singly exposed mussels. Exposure to Ag alone increased the dietary uptake of Ag by 30%, but the effect was reduced in the presence of Cd. No interaction, however, was observed between uptake rates of metals from the dissolved phase. To conclude, a significant interaction was observed for total Ag and Cu accumulation, but not for total Ag and Cd accumulation, in the mussels. Metal interaction is more likely to be observed at the subcellular level than at the whole-tissue level.

This study attempted to quantify differences in Cd biokinetics from two populations of green mussels (Perna viridis) from two sites (eastern and western) in Hong Kong with contrasting hydrological conditions. Body Cd concentrations were modeled using a simple biokinetic model coupled with measurements of dissolved Cd concentrations at each site. Mussels collected from the western site had three to six times higher Cd tissue concentration than the eastern population collected during two seasons (summer wet and winter dry), but the salinity was only lower in the western site during the summer. More Cd was distributed in the metallothionein-like and heat-sensitive proteins in the western population than the eastern population, and Cd predominantly was distributed in the insoluble fraction during summer. The Cd uptake rate constant from the dissolved phase was higher in the western population during summer due to a much lower salinity, but was comparable during winter. Dietary uptake of Cd was similar in both populations, and assimilation was lower from ingested radiolabeled seston than from diatoms. Efflux of Cd remained comparable between the two populations from two seasons (0.02-0.03 /d). Kinetic modeling demonstrates that the faster influx of Cd from aqueous phase caused the higher body Cd concentrations in the western population. The predicted Cd concentrations in mussels were comparable to those observed in the field. Our study highlights differences in Cd accumulation kinetics in different populations of mussels likely caused by the different physical environments.