In 2007 a European spacecraft carried a payload of tiny animals into orbit, opened the container, and left them exposed to the raw vacuum of space and the unfiltered radiation of the Sun. When the survivors were brought back to Earth and rehydrated, some got up and walked away. A few even laid eggs that hatched. No other animal has done anything comparable. The creatures were tardigrades — better known, thanks to their plump, lumbering gait and stubby legs, as water bears.
Tardigrades are microscopic invertebrates, most under a millimetre long, with eight legs tipped in claws and a curiously endearing appearance. They live almost everywhere there is a film of water: in ocean sediment, in soil, and most famously in the cushions of moss and lichen on walls and rooftops the world over. You have almost certainly walked past millions of them. They are not, in ordinary conditions, especially tough. What makes them nearly indestructible is a trick they perform when conditions turn deadly.
Turning to glass
That trick is called cryptobiosis — hidden life. When their surroundings dry out, tardigrades pull in their legs, expel most of the water from their bodies and shrink into a wrinkled, dormant ball known as a tun. In this state their metabolism falls so low as to be, by most measures, undetectable. They are neither obviously alive nor dead, but suspended. And in the tun state, their tolerances become almost absurd.
A dried tardigrade can survive temperatures near absolute zero, around minus two hundred and seventy degrees Celsius, and can withstand being heated well above the boiling point of water, at least briefly. It can endure pressures many times greater than those in the deepest ocean trench. It can absorb doses of radiation that would kill a human hundreds of times over. It can be left desiccated for years — in some reports, decades — and revive when water returns. And, as the orbital experiment showed, it can survive the vacuum of space.
How? Part of the answer is chemistry. As they dry, some tardigrades produce sugars and, crucially, a set of specialised proteins unique to them — sometimes called tardigrade-specific intrinsically disordered proteins. As the animal dehydrates, these molecules are thought to vitrify: they turn the cell's interior into a kind of biological glass, locking delicate structures such as membranes and DNA in place so they cannot shatter or unravel. Against radiation, tardigrades carry a protein that appears to physically shield their DNA, along with unusually efficient DNA-repair machinery. The revival is not resurrection; it is a body that was engineered to be paused and restarted.
The limits of the water bear
It is worth puncturing one myth cleanly, because it is exactly the sort of thing a quiz will test. Tardigrades are not invulnerable, and they are certainly not indestructible while active. A tardigrade going about its business in a drop of water is a soft, fragile creature, easily killed. All those astonishing feats require the animal first to enter the tun state; a hydrated, metabolising tardigrade enjoys none of that protection. Nor do the extremes they survive occur in their natural lives — no tardigrade needs to endure space or absolute zero. These tolerances are, in a sense, an accidental by-product of a body built to survive one very ordinary hazard: drying out.
That is the real point. The moss on a wall dries and rewets constantly, and an animal that lives there must be able to ride out the droughts. Everything else — the radiation resistance, the space survival, the near-immortality in the freezer — comes free with the ability to turn to glass and wait. Evolution solved a small, common problem and, without meaning to, produced the toughest animals known.
Tardigrades have become an icon of resilience, and even a minor casualty of the space age: in 2019 a privately funded lunar probe, Beresheet, carried dehydrated tardigrades among its cargo and crashed into the Moon, prompting cheerful headlines about the water bears now colonising it. In truth, without water and warmth they would simply stay locked in their tuns, going nowhere. They are not conquering the cosmos. They are doing what they have done for hundreds of millions of years: shrivelling up, turning to glass, and waiting, with infinite patience, for the rain.
Quiz nuggets
- Tardigrades, also called water bears, are microscopic eight-legged animals usually under a millimetre long, common in moss and lichen.
- Their survival trick is cryptobiosis: they expel most of their water and shrink into a dormant "tun" state.
- In 2007, tardigrades became the first animals shown to survive direct exposure to the vacuum of space, in a European orbital experiment.
- As they dry, they use special proteins and sugars thought to "vitrify" the cell interior into a protective biological glass.
- Their extreme tolerances only apply in the dormant tun state; an active, hydrated tardigrade is soft and easily killed.