Uses of subcellular metal distribution in prey to predict metal bioaccumulation and internal exposure in a predator.

In the present study, rock oysters (Saccostrea cucullata) were first exposed to cadmium and zinc for two weeks to modify their subcellular metal partitionings. The relationship between subcellular metal (Cd and Zn) partitioning in the oysters and metal bioaccumulation and fractionation in predatory gastropods (Thais clavigera) was then examined by feeding to the predator oysters that were preexposed to metal for two to four weeks. We also investigated the relationship between the PAM in the oysters and the biochemical biomarkers in the gastropods. Thais clavigera accumulated Cd effectively from their prey, but no correlation was found between the Cd body concentrations in T. clavigera and the internal metal partitioning in the prey. A significant positive correlation was found between the Cd in the trophically available metal (TAM) fraction of oysters and the Cd in the metal-sensitive fraction of T. clavigera and between the Cd in the TAM fraction of oysters and the metallothionein induction in whelks. Zinc was highly regulated by both S. cucullata and T. clavigera, and their Zn body concentrations remained constant throughout the exposure period. No relationship between Zn bioaccumulation and any of the subcellular fractions was found. The present study may lead to a better understanding of the dietary metal exposure mechanism.

Transfer and efflux of cadmium and silver in marine snails and fish fed pre-exposed mussel prey.

Subcellular metal distribution may play an important role in the bioaccumulation and trophic transfer of metals in marine food chains. In the present study, we preexposed the green mussel Perna viridis to Ag and Cd and quantified their trophic transfer efficiencies to two predators (whelks [Thais clavigera] and fish [Terapon jarbua]). For the mussels, more Ag was distributed in the metal-rich granule (MRG) fraction following Ag exposure, and more Cd was distributed in the metallothionein-like protein following Cd exposure. In addition, Cd was mainly bound with the proteins having a molecular size of approximately 20 kDa. After being fed with metal-exposed mussels, the assimilation efficiencies of Ag decreased significantly (from 77 to 29-60% in whelks and from 9 to 2% in fish) with an increasing percentage of Ag deposited in the MRG fraction of the prey. In contrast, the assimilation efficiencies of Cd remained comparable (81-85% in whelks and 6-8% in fish), because its partitioning in the soluble fraction of different treatments of the prey was similar. The efflux of Ag and Cd in the two predators was comparable after feeding on preexposed mussel prey. Our results imply that the subcellular distribution of metals in prey may affect the dietary assimilation of metals in predators, but such influence is clearly metal-specific. The present study may lead to a better understanding of metal trophic transfer in different marine food chains.

Subcellular cadmium distribution, accumulation, and toxicity in a predatory gastropod, thais Clavigera, fed different prey.

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.