Tardigrades are half-millimeter invertebrates found in moss, soil, and marine sediments on every continent including Antarctica. They have been to space. They have survived doses of ionizing radiation that would kill a human a thousand times over. They have been revived from dried specimens more than a century old. They are, by most reasonable definitions, the most physically resilient animal known to science.
They are also not indestructible. Understanding exactly what they can and cannot survive reveals something genuinely interesting about how life manages physical stress.
The Tun: What Cryptobiosis Actually Looks Like
When a tardigrade faces desiccation—water slowly evaporating from its environment—it responds by drawing in its eight legs, contracting its body into a barrel shape called a tun, and entering cryptobiosis. The word means "hidden life." Metabolic activity drops to below 0.01% of normal. The tardigrade's water content falls from roughly 85% to around 3%. It is, by every measurable indicator, almost not alive.
In the tun state, the tardigrade can remain dormant for decades. It can be heated to 151°C or cooled to -272°C (near absolute zero). It can be exposed to pressure 6,000 times atmospheric, or to hard vacuum. It can receive doses of ionizing radiation up to 570,000 röntgens—the dose lethal to most animals is around 500. It can tolerate organic solvents and pH extremes that dissolve other tissues.
Add water, and it wakes up. Within hours it is feeding again.
The CAHS Proteins: Biological Glass
For decades, the mechanism of tardigrade desiccation tolerance was unclear. The leading hypothesis involved trehalose—a sugar known to protect other desiccation-tolerant organisms—but tardigrades produce little of it. The breakthrough came in a 2017 paper from the Boothby lab at the University of North Carolina, published in Molecular Cell.
The researchers identified a family of proteins unique to tardigrades: Cytoplasmic Abundant Heat Soluble (CAHS) proteins. These disordered proteins—meaning they have no fixed three-dimensional structure in solution—respond to desiccation by forming a gel-like matrix around cellular structures. As water content drops further, this matrix vitrifies: it becomes an amorphous solid, a biological glass.
The glass immobilizes cellular components, preventing them from aggregating or unfolding in the absence of water. It preserves the architecture of the cell in suspended animation. When rehydrated, the glass liquefies, and the cell reassembles its normal dynamics.
What Tardigrades Cannot Survive
The limits are worth knowing. Tardigrades in the tun state survive extreme temperatures—but tardigrades in their active, hydrated state are killed by prolonged heat above about 37°C. They survive space vacuum only because they enter cryptobiosis in response to desiccation; an active tardigrade in vacuum would die immediately. They survive radiation because desiccation-induced cessation of metabolism removes the mechanism by which radiation does most of its damage (breaking replicating DNA)—but they do still accumulate DNA damage, and specimens exposed to very high doses show reduced fitness after revival.
The tun is not a superpower so much as a very effective pause button.
Research Programs and Open Questions
The Jönsson lab in Sweden has focused extensively on tardigrade radiation biology, particularly the Ramazzottius varieornatus species, which enters cryptobiosis unusually rapidly and is now a model organism for tardigrade genetics. The discovery in 2016 (Hashimoto et al., Nature Communications) that R. varieornatus has a unique nuclear protein called Dsup (Damage Suppressor) that physically shields DNA from radiation damage was a significant finding: Dsup is expressed even in active tardigrades, providing continuous protection rather than only during cryptobiosis.
When Dsup was expressed in human cultured cells, radiation-induced DNA damage dropped by roughly 40%. The implications for biomedical applications—protecting cells from radiation damage during cancer therapy, for instance—are being explored but are still in early stages.
Three Observations
First: tardigrade resilience is not a single adaptation but a suite of independent mechanisms—CAHS glass formation for desiccation, Dsup for radiation, other systems for osmotic and thermal stress. Each evolved separately and they compound in effect.
Second: the glass-forming proteins are disordered proteins, which were long considered functionally uninteresting because they lack stable structure. The tardigrade case is a strong example of why intrinsically disordered proteins are now a significant area of biological research.
Third: the century-old revival claims that appear in popular accounts are almost certainly true—dried specimens in museum collections have been successfully revived after decades—but the limits of long-term viability are not fully characterized. The record is reliably in the decades range, not the centuries range, despite widespread claims to the contrary.
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