How Tardigrades Survive Almost Anything

Somewhere in the moss on a wall near you, there are probably tardigrades. They are tiny, eight-legged animals, usually less than half a millimetre long, and they look a bit like a chubby bear wearing a vacuum-cleaner hose for a face. They are also some of the toughest animals ever studied.

Water bears#

Tardigrades were first described in 1773 by the German pastor Johann August Ephraim Goeze, who called them kleiner Wasserbär, "little water bear". A few years later the Italian scientist Lazzaro Spallanzani gave them the name tardigrade, meaning "slow stepper", because of the way they lumber along.

Today we know of more than 1,300 species. They live almost everywhere there is a film of water: moss and lichen, soil, leaf litter, ponds, the deep sea, and glaciers from the Himalayas to Antarctica.

The trick: stop living, for a while#

A tardigrade isn't tough because it enjoys extreme conditions. When things get bad, it does something much cleverer: it switches itself off.

As its surroundings dry out, a tardigrade pulls in its legs and head and curls up into a dry little barrel called a tun. In this state it loses almost all of its body water, down to just a few per cent, and its metabolism drops to below 0.01% of normal, so low that it is hard to measure at all. This state is called cryptobiosis, literally "hidden life".

A tun can stay like that for years. Add water, and within minutes to hours the tardigrade swells back up, unfolds its legs and walks off as if nothing happened.

How it protects itself#

Drying out would kill most animals, because their proteins and membranes collapse without water to hold them in shape. Tardigrades have several tools to stop that.

• Glass-like proteins. Some tardigrade proteins, known as CAHS proteins, form a gel and then a glassy solid as the animal dries. This locks the cell's contents in place, like insects in amber, until water returns.
• Sugar. Some species also build up trehalose, a sugar that can stand in for water around delicate molecules.
• A shield for DNA. In 2016, researchers found a protein in one species, Ramazzottius varieornatus, that binds to DNA and protects it. They named it Dsup, for "damage suppressor". When human cells in a lab were engineered to make Dsup, they suffered roughly 40% less DNA damage from X-rays.

What they've survived#

In their tun state, tardigrades have survived, at least for a while:

• Temperatures close to absolute zero, and well above the boiling point of water for short periods.
• Pressures several times greater than at the bottom of the deepest ocean trench.
• Radiation doses hundreds of times higher than would kill a human.
• Outer space. In 2007, tardigrades flew on the European FOTON-M3 mission and were exposed to the vacuum of space for ten days. Many survived, and some even laid eggs that hatched afterwards.
• Decades on ice. In 2016, Japanese scientists revived two tardigrades from a sample of Antarctic moss that had been frozen since 1983, more than 30 years earlier. One of them went on to lay eggs.

Not indestructible#

It's easy to get carried away. Tardigrades endure extreme conditions; they don't thrive in them. An active, hydrated tardigrade is far more fragile than a tun, and even tuns eventually die if the stress is strong enough or lasts long enough. Long exposure to moderate heat, for example, kills them well before any famous lab record would suggest.

Their real superpower is patience: switch off, wait it out, and wake up when things get better.

In short#

• Tardigrades are tiny, eight-legged animals found almost everywhere there is water.
• When conditions turn bad, they dry out into a tun and enter cryptobiosis, with metabolism almost at zero.
• Special proteins turn their cells to glass and shield their DNA while they wait.
• They have survived freezing, heat, radiation, crushing pressure and open space.