Tardigrade

Water bears live all over the world.

They live in wet moss and in ponds. Some live in the sea. They might even live in moss near your home!

Tardigrades live all over the world. Many live in wet moss and lichens. Others live in sand, in fresh water such as ponds, and in the sea.

Moss often dries out in the sun. When that happens, tardigrades dry out too and wait, sometimes for years, until water brings them back to life. While they are dried up, the wind and water can carry them to new places.

Britannica lists damp moss, flowering plants, sand, fresh water and the sea among tardigrade habitats; the group occurs worldwide, which Britannica links to their ability to withstand extreme conditions. Many live on wet mosses and lichens, homes that often dry out; tardigrades can revive after years of dryness (Cell Press, 2008).

In the dried tun state they are easily carried by wind and water. The map on this page shows where tardigrades as a group are found, not a mapped range for Milnesium tardigradum alone.

Most water bears eat tiny bits of plants.

The water bear in our photo is a hunter. It catches even tinier animals and slurps them up.

Most tardigrades are plant-eaters. They poke a hole in a single plant cell with sharp, spear-like parts near the mouth, called stylets, then suck out the juice inside, like drinking through a straw.

Milnesium water bears are among the few hunters. In a lab study in Japan, a scientist raised Milnesium on rotifers, another kind of tiny animal that lives in water, and watched them feed and grow every day.

According to Britannica, most herbivorous tardigrades feed by piercing individual plant cells with their stylets, spear-like structures near the mouth, and sucking out the contents; a few species are predatory carnivores.

Milnesium tardigradum is one of those predators. In a 2003 rearing study, Atsushi Suzuki kept a Japanese laboratory strain then identified as M. tardigradum (reidentified in 2019 as M. inceptum) in water on agar plates at 25 °C and offered the rotifer Lecane inermis as food, a setup that allowed its feeding and growth to be watched day by day under the microscope.

When its home dries up, a water bear curls up into a tiny ball.

It stays very still for a long time. When water comes back, it wakes up again!

When their home dries out, tardigrades dry up and shrink into a little barrel called a tun. Their metabolism, the body's chemistry that uses energy, slows to as little as 0.01 percent of normal. They can stay like this for years, even decades.

Add water, and within about half an hour the water bear can become active again. In this dried-up state tardigrades have survived amazing tests, including freezing cold in a lab and even a trip into space. But they are not unbreakable: a long soak in very hot water can kill them.

Tardigrades survive desiccation by entering cryptobiosis: the body dries out into a 'tun' and metabolism may fall to about 0.01 percent of normal (Britannica). Tuns can persist for years or decades and, per the University of Stuttgart, can return to activity within half an hour of rewetting.

Britannica reports specimens reviving after eight days in a vacuum, three days in helium and several hours at −272 °C, and 60 percent reviving after 21 months at −190 °C. The tun is not indestructible, though: one study found that an hour in water at 82.7 °C can kill tuns. How tardigrades protect and repair their cells under such stress is still being worked out.

Scientists sent dried-up water bears into space!

They went around the Earth for ten days. Back home, many woke up and even laid eggs.

In September 2007, scientists sent dried-up tardigrades into space on a European spacecraft called FOTON-M3. For ten days the water bears circled the Earth about 270 kilometers up, outside the spacecraft, with no air around them.

Back on Earth, the scientists added water. Most of the tardigrades had survived the airless vacuum of space. Some even survived the Sun's strong ultraviolet light, which is more than 1,000 times stronger up there. The survivors ate, laid eggs, and their babies were normal.

Jönsson, Rabbow, Schill, Harms-Ringdahl and Rettberg (Current Biology, 2008) flew dried Richtersius coronifer and Milnesium from the Stuttgart laboratory culture (then called M. tardigradum, since 2019 M. inceptum) on ESA's FOTON-M3 mission in September 2007, in the BIOPAN-6 module outside the capsule, at about 270 kilometers for ten days.

Both species survived space vacuum without significant losses compared with controls. Adding solar radiation cut survival sharply: with the full UV spectrum plus cosmic radiation, just over 2 percent of the Milnesium survived (University of Stuttgart), while animals exposed to vacuum with only UV-A and UV-B did much better. Survivors fed and laid eggs that developed normally. The authors called their recovery from such UV doses a mystery.