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 mouth is a remarkable structure called a stylet, a hollow, needle-like tube they use to pierce plant cells or prey and suck out the contents. 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 deep ocean trenches, 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, but research by Boothby (2017) and Yagi-Utsumi (2021) has established it as a minor cosolute rather than the main protective agent.

A tardigrade in the tun state can remain in this suspended animation for decadesScientists have successfully revived tardigrades that had been frozen at −20°C for 30 years. They simply added water, and the animals woke up, stretched their stubby legs, and went about their business as if nothing had happened. It is one of the most astonishing biological phenomena ever documented.

Surviving the Vacuum of Space

In 2007, the European Space Agency launched the FOTON-M3 mission, 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 proved far more lethal: only three specimens of Milnesium tardigradum survived unfiltered solar radiation. The survivors went on to reproduce normally after rehydration back on Earth — not during the mission itself. 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 4–5 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). This protein physically wraps around the tardigrade’s DNA like a protective shield, preventing radiation from breaking the strands apart. 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 2016 that had been frozen since 1983, and the animals went on to reproduce normally.

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. Each of these mass extinctionsScientists believe tardigrades almost certainly survived all five — 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 have been on Earth for at least 530 million years, predating the dinosaurs by hundreds of millions of years. Scientists believe that tardigrades could survive most extinction-level events that would end complex life as we know it. The only scenarios that might finally do them in are truly cosmic in scale: the eventual expansion of the sun into a red giant, or a nearby gamma-ray burst powerful enough to strip away Earth’s atmosphere entirely.

In other words, tardigrades will almost certainly outlast us. They will probably outlast most things. When the last humans are gone and the last elephants and the last whales have vanished, there is a very good chance that tardigrades will still be out there, curled up in their tun states, waiting for things to improve.

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. Beresheet's impact exceeded both thresholds. The tardigrades almost certainly did not survive.

The episode was widely reported as evidence that tardigrades could colonize other worlds, but the impact physics do not support that conclusion. It does highlight a genuine concern in planetary protection: sending uncontained biological material to other bodies in the solar system carries real risks, even when the organisms are assumed to be non-viable. The story of Beresheet's tardigrades is as much a cautionary tale as a wonder story.

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.

Finally, despite their reputation as nearly indestructible, tardigrades are not immortal. Under normal, comfortable conditions, most species live for only a few months to a few years. Their extraordinary survival abilities are emergency measures, not their everyday mode of existence. In the right environment, with plenty of moisture and food, they are just small, strange, rather charming little animals going about their lives.

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 conquered every environment on the planet, survived every catastrophe in Earth's history, traveled to space, and may currently be sitting on the surface of the moon.

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.