To understand the effects of radionuclides on non-human biota and the environment, it is essential to
study the intake and metabolism of radio-isotopes in earthworms which are among the most important
soil organisms, and Eisenia fetida, which were used in this study, are known to be sufficiently sensitive to
chemicals and representative of common earthworms. In this study, we assessed the concentration ratios,
uptake and retention, absorbed dose rate, and distribution of radio-caesium in earthworms. The
concentration ratios of 137Cs (i.e., the concentrations of radio-caesium in earthworms relative to those in
dry soil) were higher early in the culturing period and decreased gradually over the experimental period.
137Cs taken up by E. fetida was cleared rapidly after the worms were cultured in radio-caesium-free soil,
suggesting that the metabolism of radio-caesium in earthworms is very rapid. Autoradiography
demonstrated that the concentration of radio-caesium within the digestive tract was as high as that in
the soil, while radio-caesium in the body tissue was lower than radio-caesium in the soil and was almost
uniformly distributed among earthworm tissues. The highest absorbed dose rate of total exposure to
radio-caesium (137Cs þ 134Cs) was calculated to be 1.9 103 (mGy/day) in the earthworms.