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

The Tohoku hynobiid salamanders, Hynobius lichenatus, were chronically irradiated with γ-rays from embryonic to juvenile stages for 450 days. At 490 μGy h−1 or lower dose rates, growth and survival were not significantly affected by irradiation, and any morphological aberrations and histological damages were not observed. At 4600 μGy h−1, growth was severely inhibited, and all the individuals died mostly at the juvenile stage. Chronic LD50 was 42 Gy as a total dose. In the liver, the number of hematopoietic cells was significantly reduced in the living juveniles, and these cells disappeared in the dead juveniles. In the spleen, mature lymphocytes were depleted in the living larvae, and almost all the heamtopoietic cells disappeared in the dead juveniles. These results suggest that this salamander died due to acute radiation syndrome, i.e., hematopoietic damage and subsequent sepsis caused by immune depression. The death would be also attributed to skin damage inducing infection. At 18,000 μGy h−1, morphological aberrations and severe growth inhibition were observed. All the individuals died at the larval stage due to a multiple organ failure. Chronic LD50 was 28 Gy as a total dose. Assuming that chronic LD50 was 42 Gy at lower dose rates than 4600 μGy h−1, a chronic median lethal dose rate could be estimated to be <340 μGy h−1 for the whole life (>14 years). These results suggest that, among guidance dose rates, i.e., 4-400 μGy h−1, proposed by various organisations and research programmes for protection of amphibians and taxonomic groups or ecosystems including amphibians, most of them would protect this salamander but the highest value may not on the whole life scale.