Effects of chronic γ-irradiation on growth and sexual maturation of the Tohoku hynobiid salamander, Hynobius lichenatus

Highlights: • Hynobiid salamanders were chronically γ-irradiated from embryonic to adult stages. • At 33 μGy h⁻¹, growth and sexual maturation were not adversely affected. • At 150 and 510 μGy h⁻¹, growth and sexual maturation were inhibited. • The ICRP's DCRL for Reference Frog is applicable to this salamander. • It is not likely that radiation adversely affected this salamander in Fukushima. - Abstract: There are still considerable gaps in knowledge regarding the biological effects of chronic ionising radiation exposure in amphibians. To fill these gaps, Tohoku hynobiid salamanders, Hynobius lichenatus (Amphibia, Caudata), were chronically irradiated with ¹³⁷Cs γ-rays from embryonic to adult stages over 1954 days, and the effects on their growth and sexual maturation were examined under laboratory conditions. Irradiation at a dose rate of 33 μGy h⁻¹ had some stimulatory effects on growth (body weight increase) of H. lichenatus, while growth was temporarily or permanently suppressed at 150 or 510 μGy h⁻¹, respectively. On day 1802, secondary sexual characteristics (a tubercle at the anterior angle of the cloacal vent for males and ovisac development for females) were observed in 91% of the salamanders irradiated at 33 μGy h⁻¹, and in a similar percentage of non-irradiated controls. At 150 and 510 μGy h⁻¹, secondary sexual characteristics were not observed in any individuals. These results suggest that the derived consideration reference level (DCRL) of the International Commission on Radiological Protection (ICRP) for Reference Frog, i.e. 40–400 μGy h⁻¹, is applicable for the protection of H. lichenatus, and that growth and sexual maturation of 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 50 μGy h⁻¹. However, observations in the contaminated area are required to confirm this conclusion, considering the possible confounding factors which may make this salamander more sensitive to radiation in the natural environment than under laboratory conditions.