Radiation tolerance in the eutardigrade Richtersius coronifer

Purpose: Tardigrades have a reputation of being extremely tolerant to extreme environmental conditions including tolerance to ionizing radiation while in a desiccated, anhydrobiotic state. However, the evidence for radio-tolerance in tardigrades is based on only one previous report, and there is an obvious need for complementary studies. In this paper we report an investigation on radio-tolerance in desiccated and hydrated specimens of the eutardigrade Richtersius coronifer. Materials and methods: Groups of 30 - 50 tardigrades were exposed to γ-radiation at doses between 1.0 - 9.0 (anhydrobiotic animals) or 0.5 - 5.0 (hydrated animals) kGy and the animals were followed until all were dead. Radiation tolerance of both desiccated and hydrated tardigrades was studied. Results: Both desiccated and hydrated animals irradiated with 0.5 and 1 kGy did not deviate in survival from the control groups. Animals from all exposed groups underwent their moulting and egg production cycle, but at decreasing frequency for doses above 1 kGy. No eggs laid by irradiated animals hatched, while eggs laid by controls did so. Conclusion: Our study suggests that radiation tolerance in tardigrades is not due to biochemical protectants connected with the desiccated state. Rather, cryptobiotic tardigrades may rely on efficient mechanisms of DNA repair, the nature of which is currently unknown.