Tardigrades: The Toughest Creatures on Earth (And in Space)

Somewhere on your roof, in a gutter, clinging to a patch of moss, there are animals that have been to space and survived. They are smaller than a grain of sand, they look like something out of a science fiction nightmare, and they are almost certainly the toughest living things on the planet. Meet the tardigrade, also known as the water bear, and prepare to have your mind completely blown.

What Exactly Is a Tardigrade?

Tardigrades are microscopic animals, typically measuring between 0.1 and 1.5 millimeters in length. They were first described by German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbar, meaning “little water bear.” The name stuck, and it is easy to see why once you get a look at them under a microscope.

Under magnification, tardigrades look like something that crawled out of an alien world. They have a plump, barrel-shaped body with eight stubby legs, each tipped with tiny claws or suction discs. Their feeding apparatus is a remarkable bit of engineering: a pair of solid piercing stylets made of aragonite, the same calcium carbonate that builds seashells, which puncture a plant cell or prey animal, and a separate hollow buccal tube behind them through which a muscular pharynx sucks out the contents. The stylets are the needles, not the straw. Their face, if you can call it that, is a circular, puckered opening that looks almost comically strange. There are over around 1,500 described species of tardigrade, and they have colonized virtually every habitat on Earth.

You can find tardigrades in the Himalayas, in the deep sea (published marine collections reach around 5,000 meters, which is abyssal rather than true trench depth), in hot springs, in Antarctic ice, and in the moss growing on the wall outside your window. They are everywhere, and they have been here for a very long time.

The Tun State: Nature’s Ultimate Pause Button

The secret to the tardigrade’s legendary toughness is a biological trick called cryptobiosis, and specifically a form of it known as the tun state. When conditions become hostile, a tardigrade does something extraordinary: it pulls in its legs, curls into a tiny ball, and essentially shuts itself down.

The protective work is done primarily by a family of intrinsically disordered proteins called CAHS (cytoplasmic abundant heat-soluble) proteins, which form a gel-like matrix that physically stabilizes cell structures during desiccation. A sugar called trehalose is also present, in tiny amounts, around 0.0019 percent. It is tempting to write that off as negligible, and this article used to. Boothby's later work shows the opposite: trehalose acts as a synergistic cosolute, and CAHS proteins expressed on their own provide little protection without it. Scarce is not the same as unimportant.

A tardigrade in the tun state can remain in suspended animation for a very long time, though the headline numbers come from freezers rather than from dry tuns on a shelf. That distinction gets blurred constantly and is worth keeping straight: under controlled desiccation, survival stays above 80 percent out to about 120 days and then falls off a cliff, down to between 1 and 6 percent at 240 days in two of the four species tested. The famous multi-decade revivals are all cryobiotic. Scientists have revived tardigrades frozen at −20°C for 30 years, though the recovery was nothing like flicking a switch. Of the two adults revived, one showed some movement and then died 20 days later. The survivor took around two weeks to crawl and feed properly. The researchers read those long recovery times as the animals repairing damage accumulated over three decades, which is a more interesting result than waking up unharmed.

Surviving the Vacuum of Space

In 2007, the Russian Foton-M3 mission launched from Baikonur on a Soyuz-U rocket, carrying a European Space Agency payload, and tucked aboard were some very unusual passengers: tardigrades. Researchers exposed desiccated tardigrades to the full vacuum of open space, complete with cosmic radiation and ultraviolet radiation from the sun, for ten days.

When the spacecraft returned to Earth, the scientists checked on their tiny passengers. Many survived vacuum exposure and cosmic radiation. Full solar UV was another matter. A short-term count found three Milnesium tardigradum specimens moving again, but none persisted, and the review of the work states plainly that no animals of either species survived the full UV spectrum. Vacuum and cosmic radiation they handled. Unfiltered sunlight they did not. It was the first confirmed instance of any animal surviving direct exposure to the vacuum of open space, and it sent shockwaves through the scientific community.

The vacuum of space is one of the most hostile environments imaginable: no air pressure, no oxygen, extreme temperature swings, and a constant bombardment of radiation. Tardigrades shrugged it off. They entered the tun state, waited out the ordeal, and came back to life when conditions improved. It is the kind of resilience that makes every other animal on Earth look fragile by comparison.

Radiation? No Problem.

Humans begin suffering fatal radiation damage at around 4 to 5 gray (Gy). Tardigrades have a lethal dose (LD50) of roughly 5,000 Gy, about 1,000 times the human threshold. Interestingly, hydrated, active tardigrades are marginally more radiation-tolerant than tuns (around 5,000 Gy versus 4,400 Gy for tuns), so it is not the tun state that drives this resistance. High doses are still damaging: exposures above roughly 1,000 Gy can sterilize them even when they survive. Even so, their radiation tolerance vastly exceeds that of virtually every other complex organism on Earth.

The reason for this extraordinary resistance was partially unlocked in 2016, when scientists discovered that tardigrades produce a unique protein called Dsup (short for Damage Suppressor). Dsup does not wrap around the DNA, which is how it usually gets described. It binds to chromatin, preferring DNA already packaged around nucleosomes over bare strands, and shields it from the hydroxyl radicals that radiation generates. Part of it closely resembles a domain found in vertebrate HMGN proteins, so it is less an exotic invention than a familiar tool put to an unusual use. Researchers were so impressed that they successfully transferred the Dsup gene into human cells in a lab, where it reduced radiation-induced DNA damage by around 40%. The implications for medicine and space travel are enormous.

Boiling, Freezing, and Crushing: Still Not Enough

Radiation and the vacuum of space are just the beginning of what tardigrades can handle. Their tolerance for temperature extremes is equally jaw-dropping. Tardigrades can survive temperatures as low as -272°C, just one degree above absolute zero, the coldest temperature theoretically possible in the universe. On the other end of the scale, heat is actually closer to an Achilles heel. Research by Neves et al. (2020, Scientific Reports) found that active tardigrades have a median lethal temperature of just 37.1°C, barely above a warm summer day. Tuns fare better, but are still vulnerable: one hour at 82.7°C, or 24 hours at 63.1°C, proves lethal to half the population. Impressive by any measure, but far from boiling-proof.

Pressure is no obstacle either. Tardigrades have been subjected to pressures six times greater than those found at the bottom of the Mariana Trench, the deepest point in any ocean on Earth. They survived. They have also been frozen solid for 30 years and successfully revived after thawing. Scientists in Japan revived tardigrades in 2014 that had been frozen since 1983, publishing the result in 2016. The animals did reproduce, but calling it normal would be generous: the surviving adult needed about two weeks before it could crawl and feed, laid 19 eggs of which 14 hatched, and its first egg took 19 days to develop against a median of 9.5. Only the animal hatched from a revived egg developed without visible problems.

It is worth pausing to appreciate just how absurd this is. These animals are small enough that you need a microscope to see them properly, and they can survive conditions that would destroy almost any other living thing on Earth.

Survivors of Every Mass Extinction

Life on Earth has been nearly wiped out five times by catastrophic events: asteroid impacts, volcanic eruptions, and dramatic shifts in the planet’s climate and atmosphere. Scientists believe tardigrades almost certainly survived all five of these mass extinctions, but the fossil record offers only indirect support. Only a handful of body fossils are known, and the oldest unambiguously modern-looking specimens date to the Cretaceous. The conclusion rests on their extraordinary physiological resilience and the long continuity of their lineage, rather than on direct fossil documentation of each extinction.

They are old, though the figures get quoted with more confidence than they deserve. The best current molecular estimate puts crown-group Tardigrada at a mean of about 499 million years, with a 95 percent interval running all the way from 614 to 380 million. Older studies pushed it back to 627 to 691 million. The flat "at least 530 million years" you will see everywhere, including in earlier versions of this article, is not any published point estimate; it is the middle of a wide and contested range presented as a floor.

Scientists do think tardigrades could survive extinction-level events that would end complex life as we know it. The modeling behind that is worth reading correctly, because it is usually garbled. Sloan and colleagues defined total sterilization as boiling the oceans, not stripping the atmosphere, and their conclusion was that the cosmic threats are effectively negligible: a gamma-ray burst would have to go off within about 40 light years, and a supernova within 0.04 parsecs. What that leaves is stellar evolution. The sun expanding into a red giant is not one option among several. It is the endpoint.

The Beresheet Lunar Crash: Tardigrades on the Moon

In April 2019, an Israeli spacecraft called Beresheet attempted to land on the moon. It did not go as planned. The lander crashed into the lunar surface at high speed, scattering its payload across the moon’s Sea of Serenity.

Among that payload was a small container of dehydrated tardigrades, sent along by the Arch Mission Foundation as part of a biological archive. The crash almost certainly destroyed both the container and its passengers. Research by Traspas and Burchell (2021, Astrobiology) established that tardigrade tuns do not survive impacts above roughly 900 m/s or pressures above 1.14 gigapascals. Applying that to Beresheet is an inference rather than something the paper does; it never mentions the mission. On the numbers it is closer than the confident version suggests. Beresheet was traveling around 833 m/s at 150 meters altitude, which is under the lethal velocity threshold, and one analysis concludes the tardigrades might have survived the impact itself. The stronger argument is the one Burchell made to reporters afterwards: a metal spacecraft frame hitting rock generates shock pressures far beyond 1.14 GPa, whatever the velocity was. They probably did not survive. Probably is the honest word.

Surprising Facts You Probably Did Not Know

For all their fame as indestructible survivors, tardigrades have a lot of other fascinating qualities that often get overlooked. Here are some of the best ones.

Tardigrades have a brain and a nervous system. They are not simple blobs of cells. They have a distinct head with sensory organs, a central nervous system, and the ability to respond to their environment in complex ways. Some species are active hunters. Certain tardigrade species eat other tardigrades, making them cannibalistic predators in the microscopic world.

Reproduction in tardigrades is surprisingly flexible. Many species can reproduce sexually, with males and females, while others reproduce asexually through parthenogenesis, where females produce eggs that develop without fertilization. Some species do both, depending on conditions. Females of some species even shed their cuticle (outer skin) and lay eggs directly inside it, using their own molted skin as a protective egg case.

The Toughest Animal Alive

Tardigrades are a reminder that life on Earth is far stranger and more resilient than we tend to imagine. Here is an animal you could fit dozens of on the head of a pin, an animal most people have never thought twice about, and yet it has colonized nearly every environment on the planet, been fired into space and come back alive, and may currently be scattered across the surface of the moon. It has almost certainly come through mass extinctions too, though it is worth noting we cannot actually show that: the tardigrade fossil record is four bodies in amber, none older than the Turonian, roughly 90 million years ago. For an animal this durable, the evidence of its endurance is remarkably thin.

Surviving a mass extinction is a different trick from surviving a vacuum, and the animals that managed it did not do it by holding still. Crocodiles came through the asteroid that killed the dinosaurs, and the reason is not that they stayed unchanged for 200 million years.

The next time you walk past a patch of moss or a damp piece of bark, take a moment to appreciate what might be living there. Tiny, stubby-legged, utterly bizarre, and completely indestructible, the tardigrade is proof that you do not need to be big, fast, or fierce to be the greatest survivor in the history of life on Earth. You just need to be a water bear.

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