A phylum of microscopic eight-legged invertebrates under 1 mm long, also called water bears. They are known for surviving extreme conditions such as desiccation, cold, and high radiation by entering a dormant, metabolically halted state.
Tardigrades survived 10 days exposed to the Vacuum of Space and cosmic Radiation in a 2007 ESA experiment. Some even endured solar UV; after rehydration on Earth, they revived and laid eggs—the first animal shown to survive such exposure.