A tardigrade dropped into liquid helium at -272°C, boiled in a beaker, irradiated with a dose that will kill a human a thousand occasions over, or uncovered to the uncooked vacuum of low Earth orbit will, generally, do the identical factor: pull its eight legs inward, fold its cuticle right into a wrinkled barrel a few quarter of a millimetre lengthy, and wait. Drop water on it hours or many years later and the legs unfurl. The animal walks away.
The barrel known as a tun. The ready known as anhydrobiosis, a reversible metabolic shutdown that researchers describe as a near-complete pause on being alive. And the substance doing many of the structural work contained in the tun seems to be a glassy matrix of disordered proteins and sugars that takes over the roles water used to carry out. It holds membranes aside, retains proteins from collapsing into ineffective tangles, and shields DNA from the chemical havoc of dehydration.
The animal that ought to not exist
Tardigrades are about half a millimetre lengthy, eight-legged, associated to arthropods, and nearly comically chubby beneath a microscope. They’ve been documented in soil, moss and rain gutters throughout Denmark, the place environmental DNA surveys constantly reveal many tardigrade sequences that don’t match recognized species.
They dwell on Mount Everest. They dwell in deep-sea trenches. 1000’s of them have been aboard the Worldwide Area Station, and a payload of them was spilled throughout the lunar floor in 2019 when the Israeli Beresheet lander crashed.
None of these environments is the place a tardigrade prefers to be. Given the selection, the animal needs a humid movie of water on a bit of lichen. The acute-survival status comes from what occurs when the water runs out.
What occurs contained in the tun
When a tardigrade dries, it contracts. Its musculature actively folds the cuticle inward. Quantity drops. Floor space drops. Water loss slows to a crawl. Contained in the cell, one thing stranger occurs.
In a hydrated tardigrade, two households of proteins referred to as CAHS (Cytoplasmic Ample Warmth Soluble) and SAHS (Secretory Ample Warmth Soluble) float round with no fastened construction. They’re intrinsically disordered: floppy, shapeless, doing little. Because the cell dries, they snap into amphiphilic α-helices and assemble right into a sort of inside scaffolding. Genomic work on the species Ramazzottius varieornatus has recognized six households of desiccation-related proteins, with CAHS the biggest, and proven that this species retains the protecting equipment on standby quite than ready for stress to change the genes on.
That scaffolding, along with sugars together with trehalose in lots of tardigrade lineages, replaces water’s hydrogen bonds. Membranes that will in any other case fuse keep aside. Proteins that will combination keep folded. The cell’s inside solidifies into one thing with the optical and mechanical properties of glass: inflexible, clear, chemically inert. Biochemists name it vitrification. The animal’s metabolism, by each measurable indicator, stops.
Two species, two methods
Not each tardigrade does this the identical means. Comparative genomics of Hypsibius dujardini and Ramazzottius varieornatus has proven two distinct approaches. H. dujardini wants a protracted preconditioning interval, throughout which it ramps up expression of a whole lot of genes earlier than it might probably survive drying. R. varieornatus simply dries. Its protecting proteins are already in place.
That second technique is the one which makes the headlines, as a result of it means the animal will be hit by a sudden desiccation occasion with no warning and nonetheless survive. Additionally it is the rationale R. varieornatus is the species most frequently utilized in radiation experiments, vacuum experiments, and the genetic research which have remoted the Dsup protein.
The protein that hugs your chromosomes
Dsup, quick for injury suppressor, is a tardigrade-specific protein that binds on to nucleosomes, the spools of DNA that make up chromatin. When a cell is dehydrated or irradiated, hydroxyl radicals type and reduce via DNA strands. Dsup sits on the chromatin like a bodily protect and absorbs these radicals earlier than they attain the genetic materials.
The conserved area that lets Dsup latch onto nucleosomes seems structurally just like vertebrate nucleosome-binding motifs, which is why labs engaged on radiation remedy and long-duration spaceflight have spent the final decade making an attempt to splice it into human cells. The outcomes thus far are partial. Some analysis means that human cells expressing Dsup might present diminished DNA injury beneath X-rays. They don’t turn into tardigrades.
A trick a minimum of 250 million years previous
Solely 4 tardigrade fossils have ever been discovered, 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 previous. Utilizing high-contrast microscopy on the claws, essentially the most informative anatomical function in tardigrade taxonomy, they recognized a brand new genus and species, Aerobius dactylus, and reclassified the beforehand described Beorn leggi.
Plotting these species on a recalibrated household tree, the group calculated that the 2 fundamental tardigrade lineages able to cryptobiosis diverged through the Carboniferous interval, between 359 and 299 million years in the past. That locations the evolution of the suspended-animation trick earlier than the Permian extinction. The Nice Dying, 252 million years in the past, erased 96 p.c of marine species and 70 p.c of life on land.
Tardigrades walked via it. Or, extra precisely, they curled into tuns and waited it out.
How lengthy can the wait be?
The trustworthy reply is that no one is aware of the higher restrict. Tardigrades have been revived after many years of cryptobiosis. The Antarctic moss-dwelling species have been revived after prolonged durations in frozen moss samples.
Cryptobiosis isn’t distinctive to tardigrades. Sure nematodes, rotifers, and brine shrimp do variations 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, however the precept is identical: drop the water, vitrify the inside, halt the chemistry, look ahead to situations to alter.
The boundaries of indestructibility
The status wants caveats. Tardigrades of their energetic, hydrated state are about as fragile as another microscopic invertebrate. A squeeze with tweezers kills them, a pH swing kills them, a hungry rotifer eats them. The acute tolerances apply solely to the tun.
Even in tun type, mortality is actual. The 2007 FOTON-M3 experiment that uncovered tardigrades to the vacuum of low Earth orbit discovered that the majority survived vacuum alone, however mixed publicity to hoover and unfiltered photo voltaic UV killed the good majority. Boiling water kills most tardigrades that haven’t been allowed to dry first. The trick is the glass, and the glass solely kinds throughout a managed descent into dormancy.
That can be why current work on the animals has needed to invent new methods to review them with out breaking them. In April 2025, researchers reported a way for tattooing tardigrades with patterns of biocompatible ink on the micrometre scale, utilized whereas the animal was in its tun state, a way which will finally let researchers monitor particular person specimens via repeated cycles of drying and rewetting.
Why anybody aside from a biologist cares
The biomedicine and pharmaceutical purposes are the apparent draw. If CAHS proteins can vitrify a tardigrade’s mobile inside, maybe they will vitrify a vial of vaccine with out refrigeration. Trial work on dry-stabilised blood merchandise and biologics utilizing tardigrade-derived proteins is underway in a number of labs, with combined however encouraging outcomes. Agricultural researchers are testing whether or not the identical proteins, expressed in crop crops, may let these crops survive drought occasions that presently destroy them.
Area companies have an interest for various causes. A spacecraft carrying organic cargo throughout years of interplanetary transit wants a option to preserve that cargo intact via radiation and temperature swings. A tardigrade in a tun could be, in precept, the perfect passenger. The Beresheet payload was an early and considerably reckless take a look at of the thought.
The factor on the moss
None of this requires a visit to the Himalayas or the ISS. Tardigrades dwell within the moss on roof tiles, within the lichen on park benches, within the leaf litter behind a backyard shed. A patch of dry moss scraped right into a dish of water and seen beneath a 40x microscope will nearly at all times produce one, strolling with the rolling, deliberate gait that earned the animals their German identify, Bärtierchen, little bear.
The numbers behind the revival are what maintain up. Lineages able to cryptobiosis hint again 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 occasions the human deadly restrict, survived the vacuum of low Earth orbit on FOTON-M3 in 2007, and tolerated temperatures from close to absolute zero as much as quick exposures above 150°C.
Rehydration itself is quick. Add water to a dry tun and the cuticle unfolds inside minutes to hours. CAHS scaffolds dissolve, trehalose redistributes, membranes reseat, and metabolism resumes from the purpose at which it stopped. The interval in between, whether or not ten minutes or ten years, leaves no measurable hint on the animal’s physiology.
