What Are Tardigrades? The Tiny ‘Water Bears’ That Can Survive Space

Under a microscope, a tardigrade looks almost cute: a plump, transparent body, a round head and eight stubby legs tipped with tiny claws, lumbering through a drop of water like a miniature bear. Yet this little creature has a reputation as the toughest animal on Earth. It has survived boiling, freezing, crushing pressures and even a trip into space. How?

Meet the water bear

Tardigrades were first described in 1773 by German pastor Johann August Ephraim Goeze, who called them kleiner Wasserbär, “little water bear”. The scientific name, from Latin, means “slow stepper”.

They are true animals, with a brain, muscles, a gut and eggs, packed into a body usually 0.1 to 1 millimetre long. More than 1,300 species are known, and they live almost everywhere: in moss and lichen on walls and trees, in soil, in lakes, in the deep sea, on Himalayan mountains and in Antarctica. Most feed by piercing plant cells, algae or tiny animals with sharp mouthparts and sucking out the contents.

The tun: surviving by drying out

Many tardigrades live in places that regularly dry out, such as a cushion of moss on a sunny wall. Their secret is a state called cryptobiosis, “hidden life”.

When water starts to disappear, a tardigrade pulls in its legs and head and shrivels into a tiny dried barrel called a tun. It loses up to 97% of its body water, and its metabolism slows to less than 0.01% of normal, so low that it is barely detectable. In this state it can wait for years. When water returns, it rehydrates and starts walking again within minutes to hours.

How do its cells survive drying out, when ours would be destroyed? In 2017, a team led by Thomas Boothby found that tardigrades produce special proteins, called tardigrade-specific intrinsically disordered proteins. As the animal dries, these floppy proteins form a glass-like solid inside the cells, which locks other molecules in place and prevents them from unfolding or clumping together. Some tardigrades also use a sugar called trehalose for similar protection.

Survival records

Space. In 2007, a team led by K. Ingemar Jönsson sent dried tardigrades into orbit on the European FOTON-M3 satellite and exposed them directly to space. Many survived the vacuum and cosmic radiation. Those also exposed to the full ultraviolet light of the Sun fared much worse, but a few still survived, and some later laid eggs that hatched.

Radiation. In 2016, Takuma Hashimoto and colleagues sequenced the genome of a tough tardigrade species and found a unique protein they named Dsup, short for “damage suppressor”. It binds to DNA and helps shield it. When the researchers put the gene into human cells grown in the laboratory, those cells suffered much less DNA damage from X-rays.

Freezing for decades. In 2016, Japanese researchers revived tardigrades from a sample of Antarctic moss that had been frozen for about 30 years. One later laid eggs that hatched.

Not quite indestructible

Tardigrades are often called “indestructible”, but that is an exaggeration. Their extraordinary toughness applies mostly to the dried tun state. When active, they are far more fragile. Studies have shown that many active tardigrades die after long exposure to temperatures not far above human body temperature, which raises concerns about how they will cope with a warming climate. And they cannot survive being dried out over and over indefinitely.

Why scientists care

Understanding tardigrades could have practical benefits:

• Their protective proteins might help preserve medicines and vaccines without refrigeration, a big advantage in hot regions.
• Proteins like Dsup inspire research into protecting human cells from radiation, for example during cancer treatment or long space missions.
• They help scientists think about whether life could survive on other planets or travel between them.

In 2019, an Israeli lunar lander carrying dried tardigrades crashed on the Moon. Scientists think they could not survive the impact, but the story shows how much these tiny animals capture our imagination.

In short

• Tardigrades are microscopic eight-legged animals, usually under 1 mm long.
• They live almost everywhere, from moss to the deep sea; over 1,300 species are known.
• When dried, they become a tun, a state of almost suspended life called cryptobiosis.
• Special proteins form a protective glass in their cells, and Dsup shields their DNA.
• As tuns, they have survived space, extreme cold and intense radiation.
• They are not indestructible: active tardigrades are much more fragile.