Radiation tolerance and bystander effects in the eutardigrade species Hypsibius dujardini (Parachaela: Hypsibiidae)

Some tardigrade species can tolerate high radiation doses. Radiation‐induced bystander effects – damage in unirradiated cells associated with irradiated cells – have yet to be reported in tardigrades. We investigated radiation tolerance and radiation‐induced bystander effects on long‐term survival in the eutardigrade species Hypsibius dujardini . To study direct radiation effects, groups were irradiated with gamma radiation at levels of 3 and 5  kG y. To study radiation‐induced bystander effects, unirradiated groups were exposed to a single irradiated (3 or 5 kG y) individual. Survivorship was monitored every 2 days until all individuals had died. Direct radiation decreased survivorship in a dose‐dependent manner. Survivorship in bystander groups differed significantly from survivorship in groups irradiated directly with 5 kG y but insignificantly from survivorship in groups irradiated directly with 3 kG y, suggesting that radiation‐induced bystander effects had saturated. The eutardigrade species H. dujardini can tolerate high radiation doses, with radiation‐induced bystander effects that manifest as a threshold response.