Microscopic eight-legged animals that live in damp moss and survive vacuum, freezing and radiation by drying into a glass-like suspended state. Active ones die at around 37°C.
A tardigrade is a barrel-shaped animal under a millimetre long, with four pairs of stubby legs ending in claws, a piercing mouthpart for puncturing plant cells, and a lumbering gait that earned it the name water bear. It is an aquatic animal: it needs a film of water around it to move, feed or do anything at all, which is why it lives in moss, lichen, leaf litter, soil and sediments rather than anywhere exotic. In that ordinary state it is not tough. Measured properly, an active tardigrade dies at around 37°C after an hour — roughly the temperature of a warm bath — and a brief acclimation barely helps. Everything the animal is famous for belongs to a different state entirely. When its water is removed slowly enough, a tardigrade withdraws its legs, contracts into a barrel called a tun, and produces proteins that vitrify: as the last water leaves they set into a glass that immobilises everything delicate inside. Metabolism stops. In that state it has survived ten days of vacuum in orbit, temperatures far beyond its living range, doses of radiation that would kill a person many times over, and years of storage as dust. Each of those results is real and each comes with conditions the retelling drops. The orbital animals were dried first and most of them died anyway — of sunlight, not vacuum. The heat tolerance ends sharply at the temperature the internal glass softens. Survival as a tun declines steadily with time, so it is a slow leak rather than a pause button. And the radiation tolerance is probably not for radiation at all, since drying damages DNA in similar ways and tardigrades never encounter radiation in quantity.
Developed record · 82% complete · reviewed 2026-08-11
What this page covers
A phylum of around 1,300 described species of micrometazoans, most under a millimetre long. Their closest relatives are the arthropods and the velvet worms. Anhydrobiosis — surviving desiccation — is well developed in terrestrial and freshwater species and largely absent in marine ones.
Often confused with: Rotifers and nematodes, which are unrelated and which also enter anhydrobiosis — several famous survival results are theirs; Bacteria and archaea, which are genuinely extremophilic in a way tardigrades are not
Quick facts
Size
Mostly 0.1–1.2 mm; visible only under magnification
Active heat limit
Around 37°C for an hour — an ordinary small animal
An active tardigrade is a fragile little animal; the famous one is switched off
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Tardigrades are aquatic micrometazoans requiring a film of water to be metabolically active, living in moss, lichen, leaf litter, soil, freshwater and marine sediments. In the active hydrated state they are not notably tolerant: median lethal temperature for Ramazzottius varieornatus over a one-hour exposure is approximately 37°C, and brief acclimation raises this only modestly. Extreme tolerance is a property of cryptobiosis — a set of distinct reversible ametabolic states, of which anhydrobiosis is the best characterised — entered in response to specific stresses. Cryptobiosis takes several forms with different triggers and mechanisms, and tolerance does not transfer between them.
Who this applies to
tardigrades generally; the thermal figures are from one well-studied species
Studied in
Tardigrada, Ramazzottius varieornatus
You may have heard
“Tardigrades are extremophiles that can survive anything”
Both halves are wrong, and the first is wrong in a way that explains the second. An extremophile *lives* in extreme conditions; a tardigrade lives in a damp piece of moss and *survives* extreme conditions by switching itself off. In its ordinary active life it dies at about the temperature of a warm bath. Every spectacular result in the literature begins by drying the animal out first, and the tun state is not the tardigrade going about its business — it is the tardigrade having stopped.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Direct comparison of active and desiccated animals across temperature exposures, supported by a broad physiological review of the several distinct cryptobiotic states.
How far it can be extended
The active/cryptobiotic distinction is general to anhydrobiotic tardigrades; specific tolerance limits vary considerably between species and are measured in few of them.
Caveats
Thermal limits are measured in a small number of species and vary substantially across the phylum.
Marine tardigrades largely lack anhydrobiosis and are poorly characterised.
Laboratory exposure at constant temperature differs from natural fluctuation.
Still unanswered
How much of the variation in tolerance between tardigrade species reflects different mechanisms rather than different degrees of the same one?
Active tardigrades killed around 37°C, against much higher tolerance in the desiccated state — the measurement that separates the animal from its reputation.
The several distinct forms of cryptobiosis, their triggers and mechanisms, and tardigrade ecology as ordinary aquatic micrometazoans.
Start with where they actually live, because the reputation has completely detached from it. Tardigrades are found in moss, lichen, leaf litter, soil, freshwater sediment and marine sediment — damp, unremarkable places, in enormous numbers. A handful of moss from a wall will usually contain some. They are aquatic animals: without a film of water around them they cannot move, feed, or do anything.
Barrel-shaped body, mostly a tenth of a millimetre to just over a millimetre long
Four pairs of stubby unjointed legs, each ending in claws or adhesive discs
A pair of sharp stylets for puncturing plant, algal or animal cells and sucking out the contents
A cuticle that must be moulted to grow, shed complete with the claws
No respiratory or circulatory system — the body is small enough for gas to diffuse
They are a phylum of their own, around 1,300 described species, most closely related to arthropods and velvet worms. The lumbering gait is what named them: *tardigrada* means slow stepper, and "water bear" is a description of how one walks under a microscope.
Anhydrobiosis is not universal in the phylum. Marine tardigrades — a large fraction of the species — mostly cannot do it at all, because a species that never dries out has no use for the machinery.
Words used here
Micrometazoan
A microscopic multicellular animal. Tardigrades, rotifers and many nematodes are the common ones in moss and soil.
Stylet
A needle-like mouthpart used to pierce a cell wall and drain the contents. Shed and regrown at each moult.
The state everything famous about tardigrades belongs to.
Drying out, it sets into glass — and the glass is why the heat limit is where it is
Well supported
Good evidence backs this, though some details remain open.
On desiccation, tardigrades induce lineage-specific intrinsically disordered proteins that vitrify, forming a glass-like solid within the cell that immobilises cellular structures. Expressing these proteins in yeast and bacteria — organisms lacking desiccation tolerance — confers substantially improved survival, establishing that the proteins are causal rather than merely correlated. Survival of desiccated tardigrades at high temperature falls sharply at temperatures corresponding to the glass transition of the vitrified state, and the transition temperature depends on residual water content.
Who this applies to
anhydrobiotic tardigrades; protein sets differ between lineages
“Tardigrades survive drying out by replacing their water with sugar”
Sugars were the original explanation, borrowed from other anhydrobiotic organisms, and tardigrades turn out to rely mainly on proteins instead. As water leaves, particular tardigrade proteins set into a glass, and everything delicate is held immobile inside it. That also explains a limit that a sugar account does not: glass has a softening point, and desiccated tardigrades stop surviving heat at precisely the temperature theirs softens.
Why we rate it this way, and what the caveats are
Well supportedHigh confidence
The transfer experiment establishes causation rather than association, and the independent thermal work identifies a physical limit that the vitrification account predicts.
How far it can be extended
Vitrification underlies anhydrobiosis across the group, but different tardigrade lineages use partly different protein sets and some rely more on sugars.
Caveats
Improved tolerance conferred on yeast is substantial but far below tardigrade levels, so other components are involved.
Vitrification is characterised largely in vitro; direct observation inside a drying tardigrade is limited.
Glass transition was measured on bulk samples rather than within a single animal.
Still unanswered
What else, besides vitrification, accounts for the gap between engineered yeast tolerance and tardigrade tolerance?
Hengherr et al., 2009 · Physiological and Biochemical Zoology
Survival collapsing at the glass transition temperature, giving the vitrification account a testable physical prediction that held.
When its habitat dries slowly enough, a tardigrade pulls in its legs and head, contracts into a barrel a fraction of its normal size, and stops. This is the tun. Metabolism becomes undetectable. It is not sleep and it is not hibernation, both of which involve a body still running slowly; this is a body that has essentially halted.
How we know
Moving the survival machinery into yeast
What actually protects a tardigrade as it dries out?
Finding which genes switch on during drying is straightforward and shows nothing on its own — a great many things change in a drying animal, and being present is not the same as doing the work. The design that settles it is a transfer. Genes strongly induced by desiccation were identified, and the proteins they encode were then expressed in yeast and in bacteria: organisms that do not survive drying and have no tardigrade machinery of their own. If the proteins are causal, the recipients should survive better. If they are bystanders, nothing should change. The proteins were separately characterised as they dried, to see what they physically become.
What happened
Tardigrade-specific intrinsically disordered proteins vitrify on drying, forming a glass-like solid. Expressing them in yeast and bacteria conferred substantially improved desiccation survival on organisms that otherwise have none.
What it shows
That these proteins are doing the protecting rather than accompanying it — the transfer is what turns a correlation into a mechanism. And the mechanism is unexpectedly physical: as water leaves, the proteins set into glass and everything delicate is immobilised inside it rather than collapsing.
What it does not show
The engineered yeast survives far better than ordinary yeast and far worse than a tardigrade, so this is one component among several rather than the whole system. Two tardigrade species were used, and different lineages rely on partly different proteins — some on sugars instead. Vitrification was characterised largely in vitro; watching it happen inside a drying tardigrade remains limited.
The controls — what makes this evidence rather than a story
Yeast and bacteria carrying an empty vector, drying under identical conditions.
Two recipient organisms from different kingdoms, so the effect is not a quirk of one host.
Proteins characterised physically as they dry, independently of the survival assay.
Multiple protein families tested, distinguishing the ones that work from the ones merely induced.
The mechanism was worked out by moving it somewhere it does not belong. Genes strongly switched on during drying were identified, and the proteins they encode expressed in yeast and bacteria — organisms with no desiccation tolerance of their own. Their survival improved substantially, which is what turns a correlation into a mechanism: the proteins are doing the work rather than merely being present while something else does it.
What the proteins do is oddly physical. They are intrinsically disordered — floppy, without a fixed shape — and as the last water leaves they set into a glass. Everything delicate inside the cell is held immobile in it rather than collapsing or crystallising. The animal is not preserved in the sense of being protected from damage; it is held still so that damage has nothing to act on.
Drying has to be slow. A tardigrade dropped into dry air dies: forming a tun takes hours, and the machinery has to be switched on before the water is gone.
Cryptobiosis is also plural, which matters because results do not transfer between its forms. Anhydrobiosis handles drying; cryobiosis handles freezing; anoxybiosis handles a lack of oxygen; osmobiosis handles salt. These have different triggers and different mechanisms, and an animal that survives one is not thereby able to survive another.
Words used here
Tun
The contracted barrel-shaped state a tardigrade forms on drying. Named for a large cask, which is what it looks like.
Anhydrobiosis
Surviving the loss of almost all body water in a reversible ametabolic state. The best-studied form of cryptobiosis.
Vitrification
Setting into a glass rather than crystallising. In a tardigrade, disordered proteins do this as the last water leaves.
Intrinsically disordered protein
A protein with no fixed three-dimensional shape. Unusual, and exactly what is needed to fill space and set as a glass.
Every feat below is real. Every one comes with conditions that the retelling drops.
How we know
The experiment that finds out how tough a tardigrade actually is
Can tardigrades survive boiling water — and what about a warm afternoon?
Nearly all tardigrade experiments start by drying the animals out, which means nearly all tardigrade results describe the tun state rather than the animal. This one tested both. Tardigrades in the active hydrated state and in the desiccated tun state were exposed to a graded range of high temperatures for defined durations, and survival scored after recovery. Two further variables were built in: some active animals were given a gradual acclimation period beforehand, to test whether tolerance can be trained up, and exposure durations were varied, to test whether the limit is a temperature or a temperature-and-time.
What happened
Active tardigrades died at temperatures around the mid-30s Celsius, with a median lethal temperature near 37°C for a one-hour exposure. Acclimation raised this only modestly. Desiccated animals tolerated substantially higher temperatures — and their tolerance also fell as exposure lengthened.
What it shows
The single most useful tardigrade result for a general reader, and the one that never gets reported. An active tardigrade — one going about its life in a film of water on a piece of moss — dies at about the temperature of a warm bath. Everything spectacular about the group belongs to the tun state, and even there the limit gets worse the longer it lasts.
What it does not show
It does not establish limits for the phylum: one species was tested, and tardigrades vary considerably. It does not explain the mechanism of heat damage, only where the boundary sits. And constant-temperature laboratory exposure is not the fluctuating conditions of real moss, which may matter in both directions.
The controls — what makes this evidence rather than a story
Active and desiccated animals run through the same protocol, which is what makes the comparison possible at all.
Unexposed animals from the same culture, so handling and recovery conditions are accounted for.
Acclimated and unacclimated active animals, separating fixed tolerance from inducible tolerance.
Multiple exposure durations at each temperature, distinguishing a threshold from a dose.
Begin with the number nobody quotes. Tested properly — active animals and dried animals through the same protocol — an active tardigrade dies at around 37°C after an hour. Acclimation helps a little. That is the animal in its ordinary life, and it is not tough.
The famous claims, and what the experiments actually established
The claim
What was done
What it showed
Survives boiling water
Desiccated tuns heated across a range, with thermal analysis of the internal glass
Survival collapses at the glass transition temperature and falls further with longer exposure. Active animals die near 37°C.
Survives the vacuum of space
Dried tuns flown ten days in low Earth orbit, vacuum and ultraviolet separated
Vacuum alone was nearly survivable. Full solar ultraviolet killed nearly all of them.
Survives radiation that would kill a human
Irradiation of tuns; a tardigrade DNA-binding protein expressed in human cells
Real tolerance, and the protein cut DNA damage in cultured human cells by about 40%. A dish, not an organism.
Can live without water for a century
Rehydration of herbarium moss of documented age
Revival up to about nine years, declining steeply with storage time.
Survives absolute zero, and 150°C
Brief laboratory exposures of tuns at extremes
Brief exposures, of dried animals, with survival falling as duration rises. Not a habitat.
How we know
Ten days in orbit, dried out and unshielded
Can an animal survive direct exposure to space?
The point of the design is that "space" is not one condition. It is vacuum, cold, and unfiltered solar radiation arriving together, and an experiment that exposes animals to all three at once can only report whether they lived — not what nearly killed them. So the exposures were separated. Desiccated tardigrades were flown on a satellite and held for ten days in low Earth orbit in three conditions: vacuum alone, vacuum plus solar ultraviolet filtered to longer wavelengths, and vacuum plus the full unfiltered solar ultraviolet spectrum. Ground controls were kept desiccated in the same tun state, so the cost of being dried out for ten days is subtracted from the cost of being in orbit. Everything was rehydrated on return and survival scored, with survivors watched to see whether they could still reproduce.
What happened
Survival of vacuum alone was high and close to ground controls. Adding full-spectrum solar ultraviolet cut survival drastically, with only a small proportion of one species recovering. Some survivors went on to reproduce.
What it shows
That vacuum is nearly survivable for a tardigrade in the tun state — which is remarkable, since vacuum boils water out of ordinary tissue and a tun has already removed its water. And that the thing which actually kills them in orbit is sunlight, not vacuum or cold.
What it does not show
It does not show that tardigrades survive space in the sense the phrase implies. The animals were desiccated first, and an active tardigrade in vacuum dies immediately — the tun state is a precondition, not a footnote. Under full sunlight nearly all of them died. Ten days in low Earth orbit is also not interstellar space; the radiation environment differs substantially, and nothing here speaks to surviving a journey between worlds.
The controls — what makes this evidence rather than a story
Ground controls held desiccated for the same duration, separating the cost of the tun state from the cost of orbit.
Vacuum and ultraviolet exposure separated rather than combined, so the lethal factor can be identified.
Two ultraviolet wavelength ranges, distinguishing which part of the spectrum does the damage.
Two species, so a result is not a peculiarity of one.
Reproduction checked after revival, since surviving rehydration and being viable are different things.
They survived orbit dried out and shielded from sunlight — and most still died
Well supported
Good evidence backs this, though some details remain open.
Desiccated tardigrades flown for ten days in low Earth orbit survived exposure to vacuum at rates comparable to ground controls held in the same desiccated state. Exposure to vacuum combined with full-spectrum solar ultraviolet radiation reduced survival drastically, with only a small proportion of one species recovering. Some survivors subsequently reproduced. Animals were desiccated before flight; active tardigrades were not tested and would not survive vacuum.
Who this applies to
two species, desiccated, for ten days in low Earth orbit
Studied in
Richtersius coronifer, Milnesium tardigradum
You may have heard
“Tardigrades can survive in space”
True with the conditions restored. The animals were dried into the tun state first — an active tardigrade in vacuum dies at once. Vacuum alone turned out to be nearly survivable, which is genuinely remarkable. Unfiltered sunlight was what killed them, and it killed nearly all of them. Radiation tolerance is also probably not an adaptation to radiation, which tardigrades never meet in quantity; it looks like a side effect of machinery for surviving drying, which damages DNA in similar ways.
Why we rate it this way, and what the caveats are
Well supportedHigh confidence
A controlled orbital experiment with matched ground controls and separated exposure conditions, so the contribution of vacuum and of ultraviolet can be told apart.
Caveats
Only desiccated animals were tested; the tun state is a precondition rather than a detail.
Ten days in low Earth orbit is not interstellar space, and the radiation environment differs substantially.
Survival under full solar ultraviolet was very low, which the popular summary omits.
Still unanswered
Is radiation tolerance in tardigrades an incidental consequence of desiccation tolerance, given that they never encounter significant radiation in nature?
The DNA-protective protein behind part of the radiation tolerance, and the reason it is best read as a side effect of desiccation tolerance.
The space experiment is the best example of why conditions matter, because it was designed to separate them. Vacuum, cold and unfiltered sunlight arrive together in orbit, and an experiment that applies all three at once can only report whether the animals lived. This one applied them separately — and found that vacuum alone was nearly survivable while sunlight was lethal to almost all of them. "Tardigrades survive space" is true of dried animals shielded from ultraviolet. The qualifiers are the finding, not the small print.
How we know
Putting a tardigrade protein into a human cell
Why do tardigrades tolerate radiation, and is the mechanism transferable?
A genome tells you which proteins are unique to an animal and nothing about what they do. The test was to take one candidate — a protein found associated with tardigrade DNA and present in no other lineage — and put it somewhere it has no business being: cultured human cells. Those cells were then irradiated alongside untransfected controls, and DNA damage measured directly rather than inferred from survival. If the protein shields DNA, damage should fall; if it does something else, it should not.
What happened
Human cells expressing the tardigrade protein Dsup suffered around 40% less X-ray-induced DNA damage and tolerated irradiation better. The tardigrade genome also showed expansions in antioxidant and DNA-repair gene families.
What it shows
That part of tardigrade radiation tolerance is physical shielding of the DNA molecule by a protein sitting on it, and that the mechanism is portable — it works in cells that never evolved it.
What it does not show
It is a result in a dish. Reduced DNA damage in cultured cells is not protection of an organism, and nothing here is a therapy or close to one. Dsup is not found across all tardigrades, so it is not the group's explanation. And the tolerance itself is almost certainly not an adaptation to radiation, which tardigrades never encounter in quantity — drying damages DNA in similar ways, so this looks like a side effect of surviving desiccation.
The controls — what makes this evidence rather than a story
Untransfected human cells irradiated identically, giving the baseline damage level.
DNA damage measured directly rather than inferred from cell survival.
A dose range rather than a single exposure.
Genome assembly independently checked for contamination, which had produced a false horizontal-transfer claim in an earlier assembly of a related species.
A tardigrade protein physically shields DNA, and it works in human cells too
Well supported
Good evidence backs this, though some details remain open.
Genome sequencing of Ramazzottius varieornatus identified a tardigrade-unique DNA-associating protein, Dsup, which reduces X-ray-induced DNA damage. Human cultured cells expressing Dsup suffered approximately 40% less DNA damage and showed improved tolerance of irradiation. The genome also showed expansions in antioxidant and DNA-repair gene families. An earlier report that this lineage had acquired extensive foreign DNA by horizontal transfer was shown to reflect contamination.
Who this applies to
one tardigrade species, with the protein tested in human cell culture
Studied in
Ramazzottius varieornatus, Homo sapiens
You may have heard
“Tardigrades are radiation-proof, and their genes could make humans radiation-proof”
The measured effect is about 40% less DNA damage in cultured human cells — a real, reproducible result in a dish, and a long way from a shield or a therapy. Tardigrades are also not radiation-proof: they tolerate doses that would kill us, and they die at higher ones. And they almost certainly did not evolve this for radiation, which they never meet in quantity. Drying damages DNA in similar ways, so the most likely story is that radiation tolerance is a side effect of surviving desiccation.
Why we rate it this way, and what the caveats are
Well supportedModerate confidence
The cell-culture result is a direct measurement with controls, and the effect size is modest and clearly stated. It is a result in a dish, and Dsup is not universal across tardigrades.
Caveats
A cell-culture result, not an organism-level or clinical one.
Dsup is not found across all tardigrades, so it is not the whole explanation for the group.
Reduced DNA damage is not the same as protection from the physiological effects of radiation exposure.
Still unanswered
Does Dsup confer meaningful protection in a whole organism rather than in cultured cells?
Establishes the wider pattern of tardigrade-unique protective proteins that transfer function to other organisms.
The radiation story has a twist worth knowing. Tardigrades never encounter significant radiation anywhere they live, so tolerance cannot have evolved for it — natural selection does not prepare animals for things that never happen. The likely explanation is that drying damages DNA in ways that resemble radiation damage, so machinery built for one incidentally handles the other. A side effect, not a purpose.
Words used here
Glass transition
The temperature at which a glass softens. In a desiccated tardigrade it sets the upper limit of heat survival, which is why that limit is physical rather than biological.
Dsup
"Damage suppressor": a tardigrade protein that binds DNA and physically shields it. Found in some tardigrades, not all.
Years dried out, credibly. A century traces to a leg twitching in a museum drawer.
Well supported
Good evidence backs this, though some details remain open.
Controlled rehydration of dated herbarium moss samples yields revival from material up to approximately nine years old, with survival declining steeply as storage time increases and no revival from substantially older material. The frequently cited figure of survival exceeding a century traces to a single unreliable report of movement observed in a museum specimen, which was not a demonstration of revival and was not presented as one. Long-term anhydrobiotic survival is therefore progressive decay rather than suspended animation.
Who this applies to
anhydrobiotic micrometazoans in dried moss, from dated collections
Studied in
Tardigrada, Rotifera, Nematoda
You may have heard
“Tardigrades can survive dried out for over a century”
The figure descends from somebody observing a leg move in a museum specimen — which is not revival, and was not reported as revival. Tested against material of documented age, credible survival runs to years and in some cases approaching a decade, which is extraordinary for a millimetre-long animal spending it as dust. The more interesting finding underneath is that survival *declines* with time in the tun: it is not a pause button but a very slow leak.
Why we rate it this way, and what the caveats are
Well supportedModerate confidence
Herbarium material anchors survival times to a documented date rather than an estimate, which is the right design. Absence of revival cannot fully distinguish death from failure to rehydrate under the conditions used, and later work has extended credible times somewhat.
How far it can be extended
The same pattern of declining survival with storage time was found across tardigrades, rotifers and nematodes in the same samples.
Caveats
Herbarium storage conditions were not controlled for this purpose and vary between samples.
Failure to revive cannot be fully distinguished from failure to rehydrate under the conditions used.
Species identity in old moss samples is frequently uncertain.
Still unanswered
What accumulates during the tun state to make revival progressively less likely — oxidative damage, protein degradation, or something else?
Traces the century figure to its origin and shows it was never a report of revival.
How we know
Rehydrating moss with a date on it
How long can a tardigrade really survive dried out?
The obstacle here is not biology but bookkeeping. Claims of very long survival rest on samples whose age is remembered rather than recorded, and a sample of uncertain age can support any number. Herbarium collections solve it: dried moss specimens carry a collection date written at the time by somebody with no interest in tardigrades. Samples of documented age were rehydrated under controlled conditions and examined for revived tardigrades, rotifers and nematodes, giving survival times anchored to a date rather than to an estimate.
What happened
Revival was obtained from material up to around nine years old, with survival declining steeply as storage time increased. No revival came from substantially older samples. Rotifers and nematodes showed the same pattern.
What it shows
That anhydrobiotic survival runs to years — a long time for a millimetre-long animal to spend as dust — and that it declines progressively rather than persisting. The tun is not a pause button but a very slow leak, and the question is how much has gone.
What it does not show
It cannot fully separate death from failure to rehydrate under the conditions used, which is the standing weakness of any negative revival result. Herbarium storage was never controlled for this purpose and varies between samples. Species identity in old moss is often uncertain, and later work has extended credible survival times somewhat.
The controls — what makes this evidence rather than a story
Collection dates documented independently and long before the experiment, by collectors with no stake in the result.
Rehydration performed under controlled and identical conditions across samples.
Recently collected moss processed the same way, confirming the method recovers live animals when they are present.
Three animal groups scored in the same samples, so a result is not specific to tardigrades.
The claim that tardigrades survive dried out for over a century traces to one report of movement observed in a museum specimen. That was not revival, and was never presented as revival — a leg twitching in a rehydrated fragment is not an animal resuming its life.
Testing it properly requires samples whose age is documented rather than remembered, and herbarium collections provide exactly that: dried moss with a collection date written at the time by somebody with no interest in tardigrades. Rehydrating dated material yields revival up to around nine years, with survival falling steeply as storage lengthens.
The decline is the part worth carrying, because it changes what the tun is. If it were true suspension, survival would be indefinite; it is not. Something accumulates or degrades, slowly, and the probability of coming back falls with every year. A very slow leak rather than a pause button.
Active lifespan is unremarkable: a few months to a couple of years depending on species, spent eating, moulting and reproducing. Some species reproduce sexually, others parthenogenetically, and eggs are often laid inside the shed cuticle. Time spent as a tun does not count against the lifespan, which is the sense in which the animal can be said to have paused — and the only sense.
Tardigrades are eutelic in most species: the adult has a fixed number of cells, and growth happens by cells getting larger rather than by adding more.
Words used here
Eutely
Having a fixed number of body cells as an adult. Growth is by cell enlargement. Shared with rotifers and nematodes.
Parthenogenesis
Reproduction from unfertilised eggs. Common in tardigrades, and one reason a single individual can found a population.
The practical interest in tardigrades is not the animal. It is the possibility of borrowing what it does — specifically, drying biological material and bringing it back.
Vaccines and blood products that currently require refrigeration would reach far more people if they could be dried, shipped warm and rehydrated. That is exactly what the vitrifying proteins do for a tardigrade, and the transfer experiments showed the effect is portable: yeast and bacteria engineered to make these proteins survived drying that would otherwise kill them.
The honest limit is the gap. Engineered yeast survives far better than ordinary yeast and far worse than a tardigrade, so the proteins are one component of a system rather than the whole of it. Nothing here is close to a product, and the DNA-shielding work is a result in cultured cells rather than in an organism.
One thing tardigrades are not is a plausible route to life between planets. That idea attaches itself to every extreme-survival result, and the orbital experiment cuts against it: unfiltered sunlight killed nearly all of them in ten days, at a distance from the sun where an interplanetary journey would only begin.
What accumulates during the tun state to make revival less likely over time?
Why it matters: Survival declines steadily with years spent dried, which means the state is not truly suspended. Whatever is degrading is the actual limit on anhydrobiosis, and nobody has identified it.
What would settle it: Molecular characterisation of tuns across a storage time series, comparing those that revive with those that do not.
What accounts for the gap between engineered yeast and a real tardigrade?
Why it matters: Transferred proteins confer substantial tolerance and nothing like the full amount, so several components are missing — and knowing which ones is the difference between an interesting result and a usable technology.
What would settle it: Systematically adding candidate components to the engineered organisms and measuring how far each closes the gap.
Is radiation tolerance genuinely a by-product of desiccation tolerance?
Why it matters: It is the standard explanation and it is an inference from ecology rather than a demonstration. If wrong, tardigrades are protecting themselves against something nobody has identified.
What would settle it: Comparing radiation tolerance across tardigrade species that differ in anhydrobiotic capacity, including marine species that largely lack it.
How many tardigrade species are there?
Why it matters: Around 1,300 are described, and almost all physiology comes from a handful. Tolerance varies enormously between them, so general claims about the phylum rest on a very small sample.
What would settle it: Broad molecular surveying, which is beginning to show that morphologically identical populations are frequently distinct species.
NatureHQ publishes its own gaps. This record is at 82% completeness against what we would call a finished subject.
Marine tardigrades are a large fraction of described species, mostly lack anhydrobiosis, and are barely covered here because they are barely studied.
Tardigrade phylogeny and its relationship to arthropods and velvet worms is stated rather than explained.
Cryobiosis, anoxybiosis and osmobiosis are named and distinguished from anhydrobiosis but not treated in their own right.
Almost all physiology in this record comes from three or four laboratory species out of around 1,300 described, and generalising to the phylum is not safe.
Last reviewed 2026-08-11 · 5 claims · 49 search questions answered on this page