TL;DR they have several genes that are very good at repairing damage from radiation and they are expressed at high rates after damage.
HOwever I think the growing consensus in the tardigrade field is that the radiation resistance is mostly an unexpected byproduct of evolving to survive lower-moisture conditions, which are much more common than huge radiation bursts.
If there is nuclear war, if anything survives, it will be bacteria, algae and tardigrades, and then the cycle will begin again, but with tardigrade overlords.
TL;DR they have several genes that are very good at repairing damage from radiation and they are expressed at high rates after damage.
HOwever I think the growing consensus in the tardigrade field is that the radiation resistance is mostly an unexpected byproduct of evolving to survive lower-moisture conditions, which are much more common than huge radiation bursts.
If there is nuclear war, if anything survives, it will be bacteria, algae and tardigrades, and then the cycle will begin again, but with tardigrade overlords.