A tardigrade is about half a millimetre long. It has eight legs, each ending in claws, and a face that under a microscope looks like a stubby, slightly sleepy bear. There are more than a thousand known species, and they live almost everywhere: on moss, in soil, at the bottom of the sea, on the tops of mountains, in the gutter of your roof. They are, by most measures, unremarkable animals.
What makes them remarkable is what they do when conditions turn bad.
When a tardigrade dries out, it does not die in the ordinary sense. It pulls in its legs, sheds almost all the water from its body — down to as little as one per cent of normal — and rolls into a compact ball called a tun. In that state its metabolism drops to something unmeasurable. It has no detectable heartbeat, no breathing, no measurable chemical activity. It can stay that way for years. When water returns, the animal rehydrates and walks away, apparently unharmed.
This is called cryptobiosis — hidden life — and it is the reason tardigrades have become the most famous unkillable animal on the planet. The list of things a tun can survive is genuinely strange. Laboratory work has shown tardigrades surviving temperatures near absolute zero and brief exposure to well over 100 degrees Celsius, pressures several times greater than the deepest ocean trench, and doses of radiation hundreds of times higher than what kills a human being. In 2007, the European Space Agency flew dehydrated tardigrades on the FOTON-M3 mission and opened the hatch, exposing them directly to the vacuum and ultraviolet radiation of low Earth orbit. When the samples came back and were rehydrated, some of the animals revived and went on to reproduce.
It is worth being precise about one thing, because the popular version of this story usually gets it wrong. Tardigrades are not extremophiles in the sense that they seek out extreme places. They are ordinary, damp-loving animals that happen to have a stress response that is unusually effective. The tun is not a superpower they use all the time. It is what they do when the moss dries out, which is a thing that happens to moss constantly.
How the tun works is still being worked out, but the outline is now fairly clear. One mechanism is a set of tardigrade-specific proteins — CAHS and SAHS — that, when the cell loses water, turn the inside of the cell into something closer to a glass than a liquid. The technical word is vitrification. By gelling the cytoplasm, these proteins stop the delicate machinery of the cell from collapsing and fusing as it dries. When water comes back, the gel dissolves and the cell resumes.
The other mechanism is repair. In 2016, a team led by Takekazu Kunieda reported a protein they named Dsup — damage suppressor — found in the species Ramazzottius varieornatus. When human cells growing in a dish were engineered to make Dsup, they showed roughly half the DNA damage of normal cells after being hit with X-rays. Dsup appears to work by physically associating with DNA and shielding it. It is not a tardigrade-specific magic trick that makes the animal invulnerable; it is damage reduction, and it buys the cell enough time to repair what got broken.
There is a good reason to keep the caveats in view. The famous radiation numbers come from experiments on a few species under specific conditions, and other tardigrade species are far less tolerant. Cryptobiosis is not immortality. A tun can be destroyed — by fire, by being eaten, by being ground into dust. And an animal that has stopped its metabolism is not, for practical purposes, doing anything at all. It is a very well-built pause button, not a very well-built organism.
The most memorable consequence of all this happened by accident. In 2019, an Israeli lunar lander called Beresheet crashed on the Moon while carrying a payload that included thousands of dehydrated tardigrades. Reports at the time noted that a tun can survive vacuum and extreme cold, which raised a genuinely odd question: are there now living animals on the Moon, waiting for water that will never come? The answer is that nobody knows. They may be intact. They may have been vaporised in the impact. They are, either way, the only animals humans have ever accidentally left on another world.
That fact says less about tardigrades than about us. We built a machine designed to record human knowledge for a very long time, and the only thing on it that might still be alive is a half-millimetre animal from a patch of moss. Scale, it turns out, is not the same thing as durability.
https://3026alive.com