The animal that can switch life off

Scoop up a clump of damp moss from a wall or a tree trunk, soak it in water and look at it under a microscope. With some patience you may see something lumbering through the green: a plump, eight-legged animal with claws, moving like a tiny bear. This is a tardigrade, and it is one of the toughest animals known.

Small, plain and everywhere

Tardigrades are not rare and they are not exotic. Encyclopaedia Britannica describes them as a group of more than 1,100 species of free-living, tiny invertebrates. Most are about one millimetre long or less — some are barely visible to the naked eye.

They live in damp moss, on flowering plants, in sand, in fresh water and in the sea. Wherever there is a film of water, there may be tardigrades. Their nicknames — water bears and moss piglets — come from their chubby bodies and slow, rolling walk.

Their bodies are simple to describe. There are four fused segments, and each has a pair of short, stout limbs without joints, usually tipped with several sharp claws. Scientists consider tardigrades to be close relatives of the arthropods, the group that includes insects, spiders and crabs.

The problem with living in moss

A clump of moss is a strange home. After rain it is a small wetland. A few sunny days later, it can be bone dry. For an animal that needs a film of water around it to move, feed and breathe, that is a crisis that can arrive again and again.

For many small animals, that would be the end. Tardigrades have a different answer: they stop.

The tun

When their surroundings dry out, tardigrades pull in their legs, lose most of their body water and shrivel into a compact, barrel-shaped form. This is called the "tun" state, a kind of suspended animation. According to Britannica, a tardigrade’s metabolism in this state may fall to as little as 0.01 percent of its normal rate.

In that condition the animal can wait. Britannica notes that tardigrades can survive as tuns for years, or even decades, until the dry period ends. When water returns, they take it up, unfold and walk on as if nothing happened.

The tun has a second advantage. It is light and dry, so it is easily carried by wind and water. Being blown away from home can be how a tardigrade reaches a new patch of moss on the other side of a field.

Tested to the extreme

Because tuns are so resistant, scientists have tested just how much they can take. Britannica describes one series of experiments in which specimens were kept for eight days in a vacuum, then in helium gas for three days, and then exposed to −272 °C — just above absolute zero — for several hours. They revived when they were returned to room temperature.

In another experiment, 60 percent of tardigrades kept for 21 months in liquid air at −190 °C also came back to life.

It is important to read these results correctly. Tardigrades do not thrive in a freezer or in a vacuum; they survive such conditions in their dormant, dried-out state, by not living actively at all. Their everyday life still depends on water, and an active tardigrade is far more vulnerable than a tun.

Why scientists care

An animal that can dry out completely, wait for years and then resume its life raises big questions. How do its cells avoid damage when almost all their water is gone? How are its proteins and membranes protected? That is why tardigrades have become a favourite subject for researchers who study how life copes with extreme stress.

Why it matters

The tardigrade is a reminder that some of the most impressive animals on Earth are also the easiest to overlook. You do not need to travel to the deep sea to meet one. A clump of moss and a microscope are enough — and in that moss, an animal smaller than a full stop may be waiting out the dry weather, as its relatives have done for a very long time.