What is a tardigrade? The tiny animal that survives anything

A tardigrade is a tiny animal, usually less than half a millimeter long, with eight stubby legs and a body like a miniature bear, which is why it is also called a water bear. Tardigrades live almost everywhere there is a film of water, from garden moss to the deep sea. Their fame comes from one trick: they can dry out into a dormant state and survive conditions that would kill nearly anything else.

In short

• Tardigrades are real animals, with a brain, gut and muscles, related to insects and other arthropods.
• Their toughness belongs almost entirely to the dried-out "tun" state, in which metabolism nearly stops.
• They are hardy, not indestructible: active tardigrades die from heat that people survive, and none live forever.

What does a tardigrade look like, and where does it live?

A tardigrade has a short, segmented body, four pairs of legs ending in claws, and a mouth with a tube and two sharp stylets for piercing plant cells or small prey. Most are under half a millimeter long, and the full range runs from about 0.05 to 1.2 millimeters, according to the Microbial Life project at Carleton College. That is about the size of the period at the end of this sentence, small enough to need a microscope to see clearly but a complete animal all the same.

They were first described in 1773 by the German pastor and naturalist Johann August Ephraim Goeze, who called them "little water bears." Three years later the Italian biologist Lazzaro Spallanzani named the group Tardigrada, "slow steppers." About 1,500 species have been described since, living in moss and lichen, leaf litter, soil, lakes and ocean sediments, from mountaintops to Antarctica. Collect a damp clump of moss from a wall, soak it in water and look under a microscope, and you have a good chance of finding one.

Tardigrades are not microbes. They are animals with nerves, muscles, a digestive system and eggs, and their closest living relatives are arthropods, the group that includes insects and crabs, and velvet worms. Their ability to survive drying is usually explained by evolution in thin films of water that often dry up: an animal that could pause and wait for the next rain outlived one that couldn't.

How do tardigrades survive drying out?

The key to the tardigrade's toughness is a reversible state called cryptobiosis, "hidden life," in which the animal shuts down almost completely. The best-studied kind is anhydrobiosis, survival without water. It unfolds in steps:

1. The water starts to go. As its moss dries, the tardigrade senses the loss and has to dry slowly. A tardigrade dried too fast usually dies.
2. It curls into a tun. It pulls in its head and legs and shrinks into a dry, barrel-shaped ball called a tun, which loses less water through its surface.
3. It protects its cells. As water leaves, it relies on protective molecules. Some species make the sugar trehalose, and tardigrades make unusual proteins that are believed to set into a glass-like gel, holding cell parts in place so they don't fold up or stick together.
4. It waits. In the tun, metabolism drops to less than 0.01% of normal. There is no eating, moving or aging in any normal sense, for years if need be.
5. It wakes up. Add water and the tun swells, and within minutes to hours the animal walks away, if the damage it built up while dormant isn't too great.

The same shutdown helps with freezing. Tardigrades can also survive cold by tolerating ice, and a separate trick, forming a tough cyst, lets some species ride out bad seasons.

What can tardigrades actually survive?

The claims about tardigrades are often true but stripped of their conditions. Almost every famous result applies to dried tuns, often for a short time, and not to all species. The table sets out what experiments have found.

Condition	What happened	The catch
Outer space	Dried tardigrades spent about 10 days exposed in orbit in 2007; most survived vacuum and cosmic radiation, and a few survived unfiltered sunlight too (Jönsson and colleagues, 2008)	Tuns only; solar ultraviolet killed most of them
Radiation	Survive X-ray doses about 1,000 times what would kill a person, according to the Carleton College summary	Tolerance varies widely between species and states
Freezing for decades	Two animals and an egg revived from Antarctic moss stored at −20 °C for over 30 years (National Institute of Polar Research, 2016)	One revived adult died after 20 days; the other, and the young from the egg, went on to breed
Heat, dried	Half died after 1 hour at 82.7 °C, or after 24 hours at 63.1 °C	Dried state only
Heat, active	Half died after 24 hours at 37.1 °C (Neves and colleagues, 2020)	A warm summer day can be lethal
Being shot at a target	Survived impacts up to about 0.9 km/s, a shock of 1.14 gigapascals (Traspas and Burchell, 2021)	Faster impacts killed them

The heat numbers are the clearest correction. An active tardigrade of that species dies at a temperature close to that of the human body, and the researchers note Denmark's record high, 36.4 °C, is barely lower. Their toughness is a way to wait out bad conditions, not to live in them. That's why biologists call them extremotolerant rather than extremophiles, the microbes that grow and thrive in boiling springs or brine.

Did tardigrades survive on the Moon?

Probably not, though no one can check. In April 2019, the Israeli lander Beresheet crashed on the Moon carrying a small archive that included dried tardigrades. The 2021 impact experiments at the University of Kent found that tuns survive collisions up to about 0.9 kilometers a second, a shock of about 1 gigapascal, but not harder ones. A spacecraft crashing into rock is a far more violent blow than the tests delivered, so the shock was probably past that limit, and even a survivor would have no liquid water to wake up in.

The same experiments bear on an old question: could life ride on rocks blasted between worlds? The researchers worked out that tardigrades riding on rock blasted off Earth would not survive the average landing on the Moon, though about 40% of such rock comes down slowly enough, and that the same holds for rock from Mars landing on its moon Phobos. Microbes may be a different matter. The question of whether life exists elsewhere, and why none has been found, is at the heart of the Fermi paradox.

What can scientists learn from tardigrades?

Their biology has practical interest. In 2016, Japanese researchers reading the DNA of the tough species Ramazzottius varieornatus found a protein unique to tardigrades that binds to DNA and shields it. They named it Dsup, for damage suppressor. Human cells grown in the lab with the tardigrade gene added suffered about 40% less X-ray damage to their DNA than normal cells.

That doesn't mean tardigrade genes could make people radiation-proof. It was one protein tested in cells in a dish, and tardigrade survival depends on many molecules working together. The more immediate interest is in drying and storage: if the way tardigrades protect their proteins can be copied, vaccines and medicines might be kept without refrigeration. Researchers are testing that, and nothing based on it is in routine use yet.

Tardigrades are a good example of how science corrects a popular story. The headline "the animal that survives anything" is built from real experiments. Read with their conditions attached, they describe something more interesting: an ordinary-looking animal that has evolved an extraordinary way to pause.

Check yourself

• Without looking back, explain what a tun is and why the tardigrade's toughness depends on it.
• Why are tardigrades called extremotolerant rather than extremophiles?
• What did the space and impact experiments each find, and what were their limits?

Sources

• Tardigrades (water bears) — Microbial Life, Carleton College
• Tardigrades survive exposure to space in low Earth orbit — Jönsson and colleagues, Current Biology, 2008
• Recovery and reproduction of an Antarctic tardigrade frozen for over 30 years — National Institute of Polar Research, Japan, 2016
• Thermotolerance experiments on active and desiccated states of Ramazzottius varieornatus — Neves and colleagues, Scientific Reports, 2020
• The survival of tardigrades in impacts — Traspas and Burchell, Astrobiology, University of Kent, 2021
• Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells — Hashimoto and colleagues, Nature Communications, 2016