Are tardigrades really immortal?

No — that is a common myth. Tardigrades are extraordinarily tough, but they are not immortal. When they are active and feeding, they live for only a few months and can die like any animal. Their superpower is the dried-out, dormant tun state: in that condition they barely age and can survive incredible extremes, then revive when water returns. So it is fairer to say they can pause life almost indefinitely rather than that they never die. An active, crawling water bear is just as mortal as a snail.

How do they survive things that would kill almost anything else?

When a tardigrade dries out, it pulls in its legs, curls into a barrel-shaped 'tun', and removes almost all the water from its body. Without liquid water, the damaging chemistry of life nearly stops. Scientists have found that tardigrades make special protective molecules — including a kind of glass-like substance and unique proteins — that surround and shield their cells and DNA from damage. Some also produce protein 'shields' that protect their DNA from radiation. Together these tricks let a tun survive heat, deep cold, drought and radiation that would destroy almost any other animal.

Tardigrades: The Toughest Animals on Earth | Nature

Meet the world's toughest animal

What is the toughest animal on Earth? Not the lion, the shark, or the elephant. The champion is so small you need a microscope to see it: the tardigrade, also called the water bear or "moss piglet." Less than a millimetre long, this chubby, eight-legged creature can survive things that would instantly kill almost any other living thing — boiling, freezing, total drought, blasts of radiation, and even the airless vacuum of outer space. Let's find out how.

What is a tardigrade?

Tardigrades are real animals, members of their own group in the animal kingdom. A typical one is 0.1 to 1.2 millimetres long — about the size of a speck of dust or a full stop on this page. Under a microscope they look surprisingly cute: a plump, segmented body with eight stubby legs, each ending in tiny claws, and a slow, lumbering crawl that earned them the nickname "water bears."

They have a mouth that can pierce plant cells, mosses and tiny creatures to suck out their juices, and they need a thin film of water around them to stay active. There are over 1,300 known species, and they have been on Earth for more than 500 million years — long before the dinosaurs.

Living almost everywhere

One reason tardigrades are so successful is that they live nearly everywhere on the planet:

In cushions of moss and lichen on walls, roofs and trees
In soil and leaf litter
In freshwater lakes, ponds and puddles
In the deep sea and on the ocean floor
On the highest mountains and in polar ice

A single clump of damp moss can contain hundreds of them. They thrive especially in mosses and lichens, which is why those make perfect places to hunt for them. (You can read about their favourite homes in Mosses and Ferns and Lichens: Nature's Air Detectives.)

The secret superpower: cryptobiosis

Here is the real magic. When their watery home dries up, tardigrades do something astonishing. They pull in their legs, curl into a tiny barrel shape called a tun, and squeeze almost all the water out of their bodies. In this state, their life processes slow down to almost nothing — they are not quite dead, but barely alive. This survival trick is called cryptobiosis, meaning "hidden life."

A tardigrade in its tun state can wait out terrible conditions for years. Then, when water returns, it rehydrates, unfolds its legs, and crawls away as if nothing happened — sometimes after decades of being completely dried out.

Surviving the impossible

Scientists have tested tardigrades against extreme after extreme, and the dried-out tuns keep surviving:

Heat: brief exposure to temperatures well above boiling water.
Cold: down to near absolute zero, the coldest temperature possible.
Drought: years with almost no water at all.
Pressure: far greater than at the deepest point in the ocean.
Radiation: doses hundreds of times stronger than what would kill a human.
Outer space: in a famous 2007 experiment, dried tardigrades were taken into orbit and exposed to the vacuum and radiation of space outside the spacecraft. When they were brought back and rehydrated, many revived. They are the first animals known to survive open space.

How? In their dried state, the harmful chemistry of life nearly stops. Tardigrades also produce special protective molecules — including unique proteins and a glass-like substance — that surround their cells and DNA, shielding them from damage until water and better conditions return.

Important: not immortal

It is easy to imagine tardigrades are invincible, but that is a myth. When they are active — crawling, eating and reproducing — they are quite fragile and live only a few months. They can be eaten, squashed or simply die of old age. Their amazing toughness only appears in the dried-out tun state. So a water bear isn't a superhero that never dies; it is an animal with an extraordinary "pause button."

Why scientists study them

Tardigrades are not just a curiosity. Researchers want to understand how they protect their cells and DNA so well, because the answers could help us:

Store medicines and vaccines without keeping them cold.
Protect human cells from radiation damage.
Understand whether life could survive on other planets — an idea called astrobiology.

By learning the tardigrade's tricks, scientists might solve problems for medicine and even space travel. This connects to the bigger story of how life adapts to harsh places, explored in Habitats and Adaptation.

Safe activity: go on a water-bear hunt

You can search for real tardigrades with simple, safe steps and a microscope (a school or hobby microscope works).

Collect moss or lichen. With an adult's permission, take a small clump of dry moss or lichen from a wall, roof or tree. Take only a little and leave the rest.
Soak it. Put the moss in a shallow dish and cover it with clean water. Leave it for several hours or overnight. The water wakes up any sleeping tardigrades.
Squeeze and collect. Gently squeeze the soaked moss over the dish so the trapped water drips out. Let the dish settle, then use a dropper to take water from the bottom, where tiny creatures gather.
Look closely. Put a drop on a microscope slide and search at low-to-medium magnification for slow, plump, eight-legged crawlers.
Be patient and gentle. Keep your samples in a fair, comparable way if you test different mosses. Afterwards, wash your hands and return the moss outdoors.

Safety: only handle moss and clean water, never taste your samples, and let an adult help with glass slides. To make your hunt a proper experiment, see What Is a Fair Test? Variables Explained.

Why this matters

Tardigrades remind us that some of the most remarkable life on Earth is invisible to the naked eye and right under our feet — in the moss on a garden wall. By surviving the most extreme conditions imaginable, water bears push the limits of what we thought life could endure, and they may hold secrets that one day help protect human health and explore other worlds.

Tardigrades: The Toughest Animals on Earth

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