Short-term changes in metal concentration in the cultivated pacific oyster, Crassostrea gigas thunberg, and the implications for food standards

Abstract

Two experiments, one of 48 h and one of 100 h duration, were performed with Pacific oysters to ascertain changes in metal concentration after flow-through treatments with seawater, filtered seawater and filtered, diluted seawater. Oysters that were kept in air and had not defaecated were used as controls. To achieve osmotic balance with salinity of 15 × 10^-3, the water of lower salinity was taken up by the oysters, thus reducing the wet weight concentrations of the metals. Mean zinc concentration of oysters in the filtered seawater was 1044 mg kg^-1 compared with 725 mg kg^-1 in filtered diluted seawater in the 48-h experiment and 1122 mg kg^-1 and 860 mg kg^-1, respectively, in the 100-h experiment. Iron was the only metal that appeared to be lost in faeces and pseudofaeces. Particulates adsorbed in the gills may have increased the variability of the metal concentrations, particularly iron. Copper, however, was taken up by oysters in low-salinity water with particulates removed. It is recommended that food standards for metal concentrations in oysters be given on a dry weight basis because of their inherent lower variability and because of the possibility of 'manipulating' metal concentrations based on wet weight.