Tardigrades form tuns to survive conditions that would otherwise kill them. When their environment dries out, freezes, or becomes chemically hostile, these microscopic animals contract their bodies into compact, barrel-shaped balls and shut down nearly all biological activity. Their metabolism drops to 0.01% of normal levels, or becomes entirely undetectable, and their body water content falls below 1%. In this suspended state, tardigrades can endure extremes that no active animal could withstand.
What Triggers Tun Formation
The most common natural trigger is desiccation. As the moss or lichen a tardigrade lives in dries out, the animal loses water and begins contracting. But drying isn’t the only route. Tardigrades also form tuns in response to freezing temperatures, high salt concentrations, and chemical stress like oxidative damage. The process is fundamentally a stress response: when the environment becomes dangerous enough, the tardigrade curls up and shuts down rather than trying to power through.
Recent research published in PLOS One demonstrated just how precisely this response scales with threat level. When tardigrades were exposed to hydrogen peroxide (a source of oxidative stress), higher concentrations triggered faster tun formation. At the lowest dose tested, it took at least four hours for 10% of the animals to enter the tun state. At the highest dose, 10% had already formed tuns within 90 minutes. The same pattern held for salt stress and sugar solutions that pull water out of cells through osmosis. In every case, a bigger threat meant a faster response.
There’s a catch, though. If the stress hits too hard and too fast, tardigrades die before they can finish forming a tun. At the highest salt concentrations tested, nearly half the animals died outright because they couldn’t complete the transformation in time. The tun isn’t instantaneous protection; it’s a process that takes minutes to hours, and the animal is vulnerable during that window.
The Chemistry Behind the Switch
Scientists have found that tun formation depends on a specific chemical reaction inside tardigrade cells: the reversible modification of a building block in their proteins called cysteine. When the environment becomes stressful, reactive molecules (like hydrogen peroxide) oxidize these cysteine residues, changing the shape and behavior of the proteins they’re part of. This acts as a molecular switch, telling the body to begin shutting down. The “reversible” part is key. Once conditions improve, the reaction can be undone, allowing the tardigrade to wake back up.
This mechanism explains why such different stressors, from drying to chemical exposure to osmotic pressure, all produce the same tun response. They all generate oxidative stress inside the cell, which flips the same molecular switch.
What Happens Inside the Tun
When a tardigrade enters the tun state, it retracts its eight legs and head inward while its outer cuticle folds and compresses. The result is a tiny, roughly spherical ball that minimizes the surface area exposed to the environment. But the real survival strategy is happening at the molecular level.
Tardigrades produce a family of proteins found in no other known animal, called tardigrade-specific intrinsically disordered proteins. Unlike most proteins, which fold into rigid shapes, these remain loose and flexible in watery conditions. As the tardigrade dries out, however, they undergo a remarkable transformation: they solidify into a glass-like state, a process called vitrification. Think of it like the difference between ice (crystalline) and glass (solid but non-crystalline). This glassy matrix fills the spaces where water used to be, trapping delicate cell components in an amorphous shell that prevents them from breaking apart, clumping together, or losing their structure.
Many other organisms that survive drying, like certain plants and yeast, use a sugar called trehalose to achieve the same glass-like protection. Tardigrades have essentially replaced that sugar strategy with their own unique proteins, which are abundantly produced and appear to be both necessary and sufficient for surviving desiccation. Experiments show that tardigrades survive reliably up to the temperature at which this glassy state breaks down, but not beyond it, confirming that vitrification is doing the heavy lifting.
How DNA Stays Intact
Extreme drying and radiation shatter DNA. Tardigrades have a solution for that too: a protein called Damage Suppressor, or Dsup, that physically associates with DNA and acts as a kind of shield. When researchers engineered human cells to produce Dsup, those cells showed roughly half the DNA breakage after radiation exposure compared to normal human cells. Critically, this protection was measured immediately after irradiation, before any repair mechanisms could kick in, meaning Dsup prevents breaks rather than fixing them after the fact. Scientists believe it works as a flexible physical barrier that absorbs or deflects damage rather than functioning like a traditional repair enzyme.
What Tuns Can Survive
The tun state gives tardigrades tolerance that borders on absurd. They can survive temperatures near absolute zero and well above the boiling point of water. They withstand pressures up to about 600 times atmospheric pressure under static loading. In impact tests, tuns survived being fired at speeds up to 0.728 kilometers per second (roughly 1,600 miles per hour), corresponding to shock pressures of about 0.86 gigapascals. Above 0.9 km/s, survival dropped to zero, establishing a clear upper boundary for impact tolerance.
They’ve also survived the vacuum of space and intense cosmic radiation in low Earth orbit, though radiation exposure during extended space trips does take a toll. After two years in a dehydrated state exposed to cosmic radiation, none of the tested tardigrades revived.
How Long Tuns Last
Tardigrades can remain in the tun state for years, but not indefinitely. In a carefully controlled long-term experiment, tardigrades were dried in lichen samples and stored at room temperature. One species survived up to 1,604 days (about four and a half years), though survival rates declined steadily from 91% at the start to nearly zero by the end. Another species lasted up to 1,085 days before survival hit the same wall.
Anecdotal records push the limits further. One species was reported to survive almost seven years as a tun. More striking, researchers have documented successful revival after 12 and 15 years for two different species. The longest documented case involved a species that revived after approximately 20 years of dehydrated storage, though most of the longest records come from single-specimen observations rather than rigorous laboratory experiments. The practical reality is that survival probability drops with time. A tun isn’t a permanent pause button; it’s borrowed time, and the clock is always running.
Waking Up From the Tun
Rehydration reverses the process, but recovery isn’t instant. When water returns, the glassy protein matrix dissolves, cells reabsorb water, and the tardigrade gradually unfolds its legs and begins moving again. Researchers typically evaluate recovery by watching for coordinated crawling movements, and they check at intervals of 2, 6, 24, and 48 hours after adding water.
The first hours are the most variable. At the two-hour mark, there are significant differences in how many animals are active depending on their age and how many times they’ve been dried before. Younger tardigrades bounce back faster than older ones, and animals that have gone through fewer drying cycles recover more readily. By 48 hours, though, most of these differences even out, and the majority of survivors are moving normally again. Repeated or prolonged episodes of desiccation do take a cumulative toll, particularly on older individuals, suggesting that while tun formation is a powerful survival tool, it isn’t without cost to the animal.