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Abstract Tardigrades are renowned for their ability to survive a wide array of environmental stressors. In particular, tardigrades can curl in on themselves while losing a significant proportion of their internal water content to form a structure referred to as a tun. In surviving varying conditions, tardigrades undergo distinct morphological transformations that could indicate different mechanisms of stress sensing and tolerance specific to the stress condition. Methods to effectively distinguish between morphological transformations, including between tuns induced by different stress conditions, are lacking. Herein, an approach for discriminating between tardigrade morphological states is developed and utilized to compare sucrose- and CaCl₂-induced tuns, using the model speciesHypsibius exemplaris. A novel approach of shadow imaging with confocal laser scanning microscopy enabled production of three-dimensional renderings ofHys. exemplarisin various physiological states resulting in volume measurements. Combining these measurements with qualitative morphological analysis using scanning electron microscopy revealed that sucrose- and CaCl₂-induced tuns have distinct morphologies, including differences in the amount of water expelled during tun formation. Further, varying the concentration of the applied stressor did not affect the amount of water lost, pointing towards water expulsion byHys. exemplarisbeing a controlled process that is adapted to the specific stressors. 

Tardigrades can survive remarkable doses of ionizing radiation, up to about 1,000 times the lethal dose for humans. How they do so is incompletely understood. We found that the tardigrade Hypsibius exemplaris suffers DNA damage upon gamma irradiation, but the damage is repaired. We show that this species has a specific and robust response to ionizing radiation: irradiation induces a rapid upregulation of many DNA repair genes. This upregulation is unexpectedly extreme—making some DNA repair transcripts among the most abundant transcripts in the animal. By expressing tardigrade genes in bacteria, we validate that increased expression of some repair genes can suffice to increase radiation tolerance. We show that at least one such gene is important in vivo for tardigrade radiation tolerance. We hypothesize that the tardigrades’ ability to sense ionizing radiation and massively upregulate specific DNA repair pathway genes may represent an evolved solution for maintaining DNA integrity. 

Tardigrades, commonly known as ‘waterbears’, are eight-legged microscopic invertebrates renowned for their ability to withstand extreme stressors, including high osmotic pressure, freezing temperatures, and complete desiccation. Limb retraction and substantial decreases to their internal water stores results in the tun state, greatly increasing their ability to survive. Emergence from the tun state and/or activity regain follows stress removal, where resumption of life cycle occurs as if stasis never occurred. However, the mechanism(s) through which tardigrades initiate tun formation is yet to be uncovered. Herein, we use chemobiosis to demonstrate that tardigrade tun formation is mediated by reactive oxygen species (ROS). We further reveal that tuns are dependent on reversible cysteine oxidation, and that this reversible cysteine oxidation is facilitated by the release of intracellular reactive oxygen species (ROS). We provide the first empirical evidence of chemobiosis and map the initiation and survival of tardigrades via osmobiosis, chemobiosis, and cryobiosis.In vivoelectron paramagnetic spectrometry suggests an intracellular release of reactive oxygen species following stress induction; when this release is quenched through the application of exogenous antioxidants, the tardigrades can no longer survive osmotic stress. Together, this work suggests a conserved dependence of reversible cysteine oxidation across distinct tardigrade cryptobioses.