A tardigrade dropped into liquid helium at -272°C, boiled in a beaker, irradiated with a dose that would kill a human a thousand times over, or exposed to the raw vacuum of low Earth orbit will, in most cases, do the same thing: pull its eight legs inward, fold its cuticle into a wrinkled barrel about a quarter of a millimetre long, and wait. Drop water on it hours or decades later and the legs unfurl. The animal walks away.
The barrel is called a tun. The waiting is called anhydrobiosis, a reversible metabolic shutdown that researchers describe as a near-complete pause on being alive. And the substance doing most of the structural work inside the tun appears to be a glassy matrix of disordered proteins and sugars that takes over the jobs water used to perform. It holds membranes apart, keeps proteins from collapsing into useless tangles, and shields DNA from the chemical havoc of dehydration.
The animal that should not exist
Tardigrades are about half a millimetre long, eight-legged, related to arthropods, and almost comically chubby under a microscope. They’ve been documented in soil, moss and rain gutters across Denmark, where environmental DNA surveys consistently reveal many tardigrade sequences that don’t match known species.
They live on Mount Everest. They live in deep-sea trenches. Thousands of them have been aboard the International Space Station, and a payload of them was spilled across the lunar surface in 2019 when the Israeli Beresheet lander crashed.
None of those environments is where a tardigrade prefers to be. Given the choice, the animal wants a damp film of water on a piece of lichen. The extreme-survival reputation comes from what happens when the water runs out.
What happens inside the tun
When a tardigrade dries, it contracts. Its musculature actively folds the cuticle inward. Volume drops. Surface area drops. Water loss slows to a crawl. Inside the cell, something stranger happens.
In a hydrated tardigrade, two families of proteins called CAHS (Cytoplasmic Abundant Heat Soluble) and SAHS (Secretory Abundant Heat Soluble) float around with no fixed structure. They are intrinsically disordered: floppy, shapeless, doing little. As the cell dries, they snap into amphiphilic α-helices and assemble into a kind of internal scaffolding. Genomic work on the species Ramazzottius varieornatus has identified six families of desiccation-related proteins, with CAHS the largest, and shown that this species keeps the protective machinery on standby rather than waiting for stress to switch the genes on.
That scaffolding, together with sugars including trehalose in many tardigrade lineages, replaces water’s hydrogen bonds. Membranes that would otherwise fuse stay apart. Proteins that would aggregate stay folded. The cell’s interior solidifies into something with the optical and mechanical properties of glass: rigid, transparent, chemically inert. Biochemists call it vitrification. The animal’s metabolism, by every measurable indicator, stops.
Two species, two strategies
Not every tardigrade does this the same way. Comparative genomics of Hypsibius dujardini and Ramazzottius varieornatus has shown two distinct approaches. H. dujardini needs a long preconditioning period, during which it ramps up expression of hundreds of genes before it can survive drying. R. varieornatus just dries. Its protective proteins are already in place.
That second strategy is the one that makes the headlines, because it means the animal can be hit by a sudden desiccation event with no warning and still survive. It is also the reason R. varieornatus is the species most often used in radiation experiments, vacuum experiments, and the genetic studies that have isolated the Dsup protein.
The protein that hugs your chromosomes
Dsup, short for damage suppressor, is a tardigrade-specific protein that binds directly to nucleosomes, the spools of DNA that make up chromatin. When a cell is dehydrated or irradiated, hydroxyl radicals form and cut through DNA strands. Dsup sits on the chromatin like a physical shield and absorbs those radicals before they reach the genetic material.
The conserved domain that lets Dsup latch onto nucleosomes looks structurally similar to vertebrate nucleosome-binding motifs, which is why labs working on radiation therapy and long-duration spaceflight have spent the last decade trying to splice it into human cells. The results so far are partial. Some research suggests that human cells expressing Dsup may show reduced DNA damage under X-rays. They do not become tardigrades.
A trick at least 250 million years old
Only four tardigrade fossils have ever been found, all in amber. In 2024, Marc Mapalo of Harvard’s Museum of Comparative Zoology and colleagues re-examined two specimens trapped inside a single Canadian amber pebble dated between 84 and 72 million years old. Using high-contrast microscopy on the claws, the most informative anatomical feature in tardigrade taxonomy, they identified a new genus and species, Aerobius dactylus, and reclassified the previously described Beorn leggi.
Plotting those species on a recalibrated family tree, the team calculated that the two main tardigrade lineages capable of cryptobiosis diverged during the Carboniferous period, between 359 and 299 million years ago. That places the evolution of the suspended-animation trick before the Permian extinction. The Great Dying, 252 million years ago, erased 96 percent of marine species and 70 percent of life on land.
Tardigrades walked through it. Or, more accurately, they curled into tuns and waited it out.
How long can the wait be?
The honest answer is that nobody knows the upper limit. Tardigrades have been revived after decades of cryptobiosis. The Antarctic moss-dwelling species have been revived after extended periods in frozen moss samples.
Cryptobiosis is not unique to tardigrades. Certain nematodes, rotifers, and brine shrimp do versions of it. In 2023, researchers radiocarbon-dated permafrost sediments containing nematodes that had been revived after 46,000 years of frozen dormancy. The mechanisms differ, but the principle is the same: drop the water, vitrify the interior, halt the chemistry, wait for conditions to change.
The limits of indestructibility
The reputation needs caveats. Tardigrades in their active, hydrated state are about as fragile as any other microscopic invertebrate. A squeeze with tweezers kills them, a pH swing kills them, a hungry rotifer eats them. The extreme tolerances apply only to the tun.
Even in tun form, mortality is real. The 2007 FOTON-M3 experiment that exposed tardigrades to the vacuum of low Earth orbit found that most survived vacuum alone, but combined exposure to vacuum and unfiltered solar UV killed the great majority. Boiling water kills most tardigrades that have not been allowed to dry first. The trick is the glass, and the glass only forms during a controlled descent into dormancy.
That is also why recent work on the animals has had to invent new ways to study them without breaking them. In April 2025, researchers reported a technique for tattooing tardigrades with patterns of biocompatible ink at the micrometre scale, applied while the animal was in its tun state, a method that may eventually let researchers track individual specimens through repeated cycles of drying and rewetting.
Why anyone other than a biologist cares
The biomedicine and pharmaceutical applications are the obvious draw. If CAHS proteins can vitrify a tardigrade’s cellular interior, perhaps they can vitrify a vial of vaccine without refrigeration. Trial work on dry-stabilised blood products and biologics using tardigrade-derived proteins is underway in several labs, with mixed but encouraging results. Agricultural researchers are testing whether the same proteins, expressed in crop plants, might let those crops survive drought events that currently destroy them.
Space agencies are interested for different reasons. A spacecraft carrying biological cargo across years of interplanetary transit needs a way to keep that cargo intact through radiation and temperature swings. A tardigrade in a tun would be, in principle, the ideal passenger. The Beresheet payload was an early and somewhat reckless test of the idea.
The thing on the moss
None of this requires a trip to the Himalayas or the ISS. Tardigrades live in the moss on roof tiles, in the lichen on park benches, in the leaf litter behind a garden shed. A patch of dry moss scraped into a dish of water and viewed under a 40x microscope will almost always produce one, walking with the rolling, deliberate gait that earned the animals their German name, Bärtierchen, little bear.
The numbers behind the revival are what hold up. Lineages capable of cryptobiosis trace back 359 to 299 million years. Nematodes have been pulled out of 46,000-year-old permafrost. Tardigrade tuns have endured doses of ionising radiation roughly a thousand times the human lethal limit, survived the vacuum of low Earth orbit on FOTON-M3 in 2007, and tolerated temperatures from near absolute zero up to short exposures above 150°C.
Rehydration itself is fast. Add water to a dry tun and the cuticle unfolds within minutes to hours. CAHS scaffolds dissolve, trehalose redistributes, membranes reseat, and metabolism resumes from the point at which it stopped. The interval in between, whether ten minutes or ten years, leaves no measurable trace on the animal’s physiology.
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