The Variations with Growth in Heavy Metal Contents of Rock Shell

Abstract

In odrder to elucidate how the heavy metal content of rock shell Rapana thomasiana varies with its growth, the contents of 9 kinds of metals, i. e., Cadmium (Cd), Lead (Pb), Copper (Cu), Zinc (Zn), Manganese (Mn), Nickel (Ni), Cobalt (Co), Chromium (Cr), and Asenic (As) in the muscular portion were determined by atomic absorption spectrophotometry and colorimetry after wet digestion. The rock shell, a carnivorous shell, was obtained in Yamaguchi Bay in Yamaguchi Prefecture in June, 1982. The contents of Cd, Co, Cr, and As increased linearly with growth. The regression equations between the metal content y ppm and the shell heigh x mm were as follows: y=3.0×10^-4x+0.03 for Cd, y=4.8×10^-4x+0.12 for Co, y=2.6×10^-4x+0.048 for Cr, and y=5.5×10^-2x-0.096 for As. On the other hand, only Cu content increased exponentially with growth, with a regression equation of y=10^{2.8×10-3x+0.13}. The contents of Pb, Zn, Mn, and Ni were nearly invariable regardless of the growth, although each level varied with metal species. The contents of Cu and As were higher in rock shell than in short-neck clams, a plankton-feeding shell, inhabiting Yamaguchi Bay and its surrounding waters; whereas the contens of Cd, Mn, Ni, Co, and Cr were lower in the former than in the latter. The contents of Pb and Zn in rock shell were as high as those in short-neck clams. These findnngs suggest that the bioaccumulation of heavy metals depends on specific metals rather than the place on food chain web where the animal is located, and the metal contents are not necessarily higher in predator than in the prey.