Effects on the middle adult stage of the Tohoku hynobiid salamander, Hynobius lichenatus, chronically γ-irradiated from the embryonic stage

Tohoku hynobiid salamanders, Hynobius lichenatus (Amphibia, Caudata), were chronically irradiated with ¹³⁷Cs γ-rays from the embryonic to the middle adult stages for over 3000 days, and the effects on survival, somatic growth (body weight increase), sexual maturation, and histological characteristics were examined. This article reports the effects observed in the middle adult stage (days 1955–3039). Irradiation at a dose rate of 34 μGy h⁻¹ had no effect on any endpoint. At 150 or 500 μGy h⁻¹, secondary sexual characteristics (tubercle at the anterior angle of the cloacal vent for males and ovisac development for females) were not observed. Gonads were either severely atrophied or unidentified by histological examination. At some peripheral liver sites, the number of haematopoietic cells was significantly reduced. Renal damage was also observed. At 500 μGy h⁻¹, somatic growth was significantly inhibited, and 74 % of individuals died due to multiorgan failure. No tumours were observed in the dead salamanders. A dose rate-response relationship obtained in this study suggests that the derived consideration reference level of the International Commission on Radiological Protection for the Reference Frog (i.e., 40–400 μGy h⁻¹) is applicable for the protection of H. lichenatus, and that this salamander may not have been adversely affected even in the most severely contaminated area in Fukushima, where the highest dose rate to salamanders was estimated to be 47 μGy h⁻¹ in April 2012.