Invertebrates
Tardigrade
Ramazzottius varieornatus · also called water bear, moss piglet
The tardigrade is a microscopic animal that has become famous for one reason: it is extraordinarily hard to kill. Boil it, freeze it near absolute zero, put it in a vacuum, blast it with radiation, or leave it bone-dry for decades, and a fraction of a population will still walk away. It does this not by being tough while active, but by switching itself almost entirely off.
Overview
Tardigrades — “water bears” — are a phylum of their own, with around 1,300 described species. They are between a tenth of a millimetre and just over a millimetre long, with a plump, segmented body, four pairs of clawed legs, and a slow, rolling gait that earned them their name. They live everywhere there is at least a film of water: in the cushions of moss and lichen on walls and trees, in soil and leaf litter, in lakes and the deep ocean, in hot springs, and on Antarctic rock and Himalayan ridges. Most pierce plant or moss cells with needle-like mouthparts and suck out the juices.
Cryptobiosis: switching life off
The tardigrade’s superpower is cryptobiosis — “hidden life.” When its water dries up, the animal draws in its legs, squeezes out almost all of its body water, and curls into a shrunken barrel called a tun. Its metabolism falls to less than a hundredth of a percent of normal — effectively to zero. It floods its cells with protective sugars and with unique disordered proteins that set into a glass, cradling membranes and DNA so they do not shatter. In this paused state the tardigrade is not doing anything that looks like living. It is waiting.
What the tun can survive
A tardigrade in the tun state has been shown to withstand:
- Temperatures from about -272 °C (a degree above absolute zero) to 150 °C
- Pressures of 6,000 atmospheres, six times the deepest ocean
- The vacuum and solar radiation of open space for ten days
- Radiation doses hundreds of times what would kill a person
- Years to decades completely dry
Add water, and over minutes to hours it rehydrates, its metabolism restarts, and it resumes walking, feeding, and breeding.
What tardigrades are not
They are not immortal and not indestructible. An active, hydrated tardigrade is as vulnerable as any other tiny animal — it is eaten by mites, nematodes, and other tardigrades, and it dies within months to about two years. The famous resistance applies only to the dormant tun, it is always a matter of survival rates rather than certainty, and it drops off with longer and harsher exposure. Their real distinction is that they can sit out bad times almost indefinitely.
Why they matter
Tardigrade survival proteins are being studied for preserving vaccines and cells without refrigeration, for protecting crops from drought, and for understanding how life might endure on other worlds. They are also a favourite first find for anyone with a home microscope — a genuinely alien-looking animal living in the moss on the roof.
Status
Tardigrades have not been assessed by the IUCN. As a group they are among the most widely distributed animals on the planet and are in no danger; individual specialised species with tiny ranges could be, but almost none have been evaluated.
Tardigrade: common questions
Can a tardigrade really survive in space?
In its dried "tun" form, yes, briefly. In a 2007 European Space Agency experiment, tardigrades were exposed to the open vacuum of low Earth orbit for 10 days, including the full unfiltered solar ultraviolet radiation, and a fraction survived and went on to lay viable eggs. Vacuum alone they tolerate well; the UV is the harder part. This does not mean tardigrades "live in space" — they were dormant the whole time, and an active, hydrated tardigrade is about as fragile as any other tiny animal.
How does a tardigrade survive drying out?
It performs cryptobiosis. As its surroundings dry, the tardigrade contracts into a compact barrel called a tun, loses up to 95–99% of its body water, and shuts its metabolism down almost completely. It replaces the lost water with protective sugars and, crucially, with its own tardigrade-specific proteins that vitrify — turning the cell interior into a glass-like solid that holds delicate structures in place. In this state it is not really "alive" in any active sense; it is chemically paused. Add water and, over minutes to hours, it swells, restarts, and walks away.
Are tardigrades immortal or indestructible?
No to both. Active tardigrades die easily — from predators, from being crushed, from running out of food. Their extreme resistance only applies in the dormant tun state, and even then survival is a percentage, not a guarantee, and it declines with time and dose. What is true is that as a group they are astonishingly hard to eradicate from a habitat, and that a tardigrade can effectively "skip" years of bad conditions, so its total elapsed lifespan can be very long even though its active life is short.
Where can I find tardigrades?
Almost anywhere damp. The classic method is to soak a clump of moss or lichen from a roof, wall, or tree in water for a few hours, squeeze the water into a dish, and search the sediment under a cheap microscope at 40 to 100 times magnification. You are looking for a translucent, plump, eight-legged animal ambling through the debris. They also live in soil, beach sand, freshwater, and the deep sea, on every continent including Antarctica.