Effect of radiation on a terrestrial earthworm, Enchytraeus japonensis

Because of their ecological importance, the soil invertebrates, especially earthworms are increasingly used for ecological impact assessment of chemical substances for the terrestrial ecosystem. Earthworm toxicity test guideline was already adopted in OECD, and earthworms are considered to be reference organisms by ICRP. Among earthworms, Enchytraeus japonensis discovered in 1991 by a Japanese investigator has a peculiar reproductive manner. The earthworms reproduce asexually by fragmentation and subsequent regeneration in the normal breeding condition, but develop reproductive organs and achieve sexual reproduction in a particular breeding condition. There is only one report that investigated the effect of radiation on the earthworms. Miyachi et al. reported the induction of sexual reproduction by extremely low dose of beta-irradiation. In the present study, we investigated the effect of gamma-irradiation on the population increased by asexual reproduction. The population of the earthworms 30 days after irradiation at a dose of 20Gy was decreased to 50% of sham control. On the other hand, ³H-thymidine incorporation into DNA from 24hr to 48hr after irradiation decreased to a half of control by irradiation at a dose range of 3-4Gy. It was concluded that the radiosensitivity of the earthworms at the cellular level was slightly lower than that of mammalian cells, but the radiation effect on the population of the earthworms was not easily observed due to the vigorous proliferation of only a few cells survived irradiation.