Boil them, deep-freeze them, crush them, dry them out or blast them into space: tardigrades will survive it all and come back for more.

 

If you go into outer space without protection, you'll die.

The lack of pressure would force the air in your lungs to rush out. Gases dissolved in your body fluids would expand, pushing the skin apart and forcing it to inflate like a balloon. Your eardrums and capillaries would rupture, and your blood would start to bubble and boil. Even if you survived all that, ionising radiation would rip apart the DNA in your cells. Mercifully, you would be unconscious in 15 seconds.

 

How do these seemingly insignificant creatures survive in such extreme conditions?

 

But one group of animals can survive this: tiny creatures called tardigrades about 1mm long. In 2007, thousands of tardigrades were attached to a satellite and blasted into space. After the satellite had returned to Earth, scientists examined them and found that many of them had survived. Some of the females had even laid eggs in space, and the newly-hatched young were healthy.
It's not just the harsh environs of outer space that tardigrades can survive in. The little critters seem adept at living in some of the harshest regions of Earth. They have been discovered 5546m (18,196ft) up a mountain in the Himalayas, in Japanese hot springs, at the bottom of the ocean and in Antarctica. They can withstand huge amounts of radiation, being heated to 150 °C, and being frozen almost to absolute zero.
How do these seemingly insignificant creatures survive in such extreme conditions, and why have they evolved these superpowers? It turns out that tardigrades have a host of tricks up their sleeves, which would put most organisms to shame.

Tardigrades, at first glance, are intimidating. They have podgy faces with folds of flesh, a bit like a Doctor Who monster. They have eight legs, with ferocious claws resembling those of great bears. Their mouth is also a serious weapon, with dagger-like teeth that can spear prey.

 

Fossils of tardigrades have been dated to the Cambrian period over 500 million years ago.

 

But there's no need to worry. Tardigrades are one of nature's smallest animals. They are never more than 1.5 mm long, and can only be seen with a microscope. They are commonly known as "water bears".

There are 900 known species. Most feed by sucking the juices from moss, lichens and algae. Others are carnivores, and can even prey on other tardigrades.

They are truly ancient. Fossils of tardigrades have been dated to the Cambrian period over 500 million years ago, when the first complex animals were evolving. And ever since they were discovered, it has been clear that they are special.

 

Tardigrada macrobiotus

 

Tardigrades were discovered in 1773 by a German pastor named Johann August Ephraim Goeze. Three years later, the Italian clergyman and scientist Lazzaro Spallanzani discovered that they had superpowers.

Spallanzani added water to sediment from a rain gutter, and looked under a microscope. He found hundreds of little bear-shaped creatures swimming around. In his book "Opuscoli di Fisica Animale, e Vegetabile", he named them "il Tardigrado", meaning "slow-stepper", because they moved so slowly.

 

In 1995, dried tardigrades were brought back to life after 8 years.

 

In truth, this wasn't a first. Back in 1702, the Dutch scientist Anton van Leeuwenhoek sent a letter to the Royal Society in London, entitled "On certain animalcules found in the sediment in gutters on the roofs of houses". He took dry, apparently lifeless dust from a gutter and added water. Using a microscope of his own devising, Leeuwenhoek found that within an hour many small "animalcules" became active, and began swimming and crawling around.
These animals were rotifers, tiny aquatic creatures that look like they have wheels on their heads. They could seemingly survive months without water.
However, tardigrades may be able to survive without it for decades. In 1948, the Italian zoologist Tina Franceschi claimed that tardigrades found in dried moss from museum samples over 120 years old could be reanimated. After rehydrating a tardigrade, she observed one of its front legs moving.
This finding has never been replicated. But it does not seem impossible. In 1995, dried tardigrades were brought back to life after 8 years.

 

The tardigrade Hypsibius dujardini

 

For most animals, life without water is completely impossible.

"When a typical cell dries out its membranes rupture and leak, and its proteins unfold and aggregate together, making them useless," says extremophile researcher Thomas Boothby  of the University of North Carolina in Chapel Hill. "DNA will also start to fragment the longer it is dry."

 

The tardigrade curls up into a dry husk.

 

Somehow tardigrades avoid all this. "Since water bears can survive drying, they must have tricks for preventing or fixing the damage that cells like ours would die from," says Boothby.

How do they do it? One of the key discoveries came in 1922, courtesy of a German scientist named H. Baumann. He found that when a tardigrade dries out it retracts its head and its eight legs. It then enters a deep state of suspended animation that closely resembles death.

Shedding almost all the water in its body, the tardigrade curls up into a dry husk. Baumann called this a "Tönnchenform", but it is now commonly known as a "tun". Its metabolism slows to 0.01% of the normal rate. It can stay in this state for decades, only reanimating when it comes into contact with water.

 

Paramacrobiotus richtersi in its tun state

 

Besides tardigrades, some nematode worms, yeast and bacteria can also survive desiccation. They do this by making a lot of a particular sugar called trehalose. This sugar forms a glass-like state inside their cells that stabilises key components, such as proteins and membranes, which would otherwise be destroyed.

 

Tardigrades might have unique tricks for surviving desiccation.

 

Trehalose can also wrap itself around any remaining water molecules, stopping them from rapidly expanding if the temperature rises. Rapidly expanding water molecules are dangerous because they can rupture cells, which can be fatal.

You might expect that tardigrades would use this trick to survive drying, but according to Boothby, only some species seem to make trehalose. "Some species do not appear to contain trehalose, or make it at such low levels that the sugar is undetectable," he says.

"This suggests that tardigrades might have unique tricks for surviving desiccation," says Boothby. "We know that, as they start to dry out, tardigrades make protectants that allow them to survive becoming completely dry. But what exactly these protectants are is still a mystery."

 

Tardigrades first evolved in water

 

When tardigrades start to dry out, they seem to make a lot of antioxidants. These are chemicals, like vitamins C and E, that soak up dangerously reactive chemicals. This may mop up harmful chemicals in the tardigrades' cells.

 

The tun state is key to tardigrades' ability to cope with being dried out.

 

Tardigrades face a particular threat from "reactive oxygen species". These substances are produced as by-products of normal cell function, but can break down the main components of a cell, including its DNA. Animals exposed to environmental stress often have lots of them floating around.

The antioxidants may explain one of tardigrades' neatest abilities. If a tardigrade stays in its dry tun state for a long time, its DNA gets damaged. But after it reawakens it is able to quickly fix it.

It's clear that the tun state is key to tardigrades' ability to cope with being dried out. But long before Baumann discovered it, tardigrades had revealed other superpowers.

 

Tardigrades can tolerate blazing heat 

 

For starters, they seem not to care what temperature it is. In 1842 a French scientist named Doyère showed that a tardigrade in its tun state could survive being heated to temperatures of 125 °C for several minutes. In the 1920s, a Benedictine friar named Gilbert Franz Rahm brought tardigrades back to life after heating them to 151 °C for 15 minutes.

Rahm also tested them in the cold. He immersed them in liquid air at -200 °C for 21 months, in liquid nitrogen at -253 °C for 26 hours, and in liquid helium at -272 °C for 8 hours. Afterwards the tardigrades sprang back to life as soon as they came into contact with water.

We now know that some tardigrades can tolerate being frozen to -272.8 °C, just above absolute zero. To put that into perspective, the lowest temperature ever recorded on Earth was a balmy -89.2 °C in central Antarctica in 1983. The tardigrades coped with a profound chill that does not occur naturally and must be created in the lab, at which atoms come to a virtual standstill.

 

Sub-zero temperatures of Antarctica? No problem

 

The biggest hazard tardigrades face in the cold is ice. If ice crystals form inside their cells, they can tear apart crucial molecules like DNA.

 

Tardigrades can actually tolerate ice forming within their cells.

 

Some animals, including some fish, make antifreeze proteins that lower the freezing point of their cells, ensuring that ice doesn't form. But these proteins haven't been found in tardigrades.

Instead it seems tardigrades can actually tolerate ice forming within their cells. Either they can protect themselves from the damage caused by ice crystals, or they can repair it.

Tardigrades may produce chemicals called ice nucleating agents. These encourage ice crystals to form outside their cells rather than inside, protecting the vital molecules. Trehalose sugar may also protect those that produce it, as it prevents the formation of large ice crystals that would perforate the cell membranes.

 

Some bacteria can live in hot springs

 

The most heat-tolerant organisms known are bacteria that live around the edges of hydrothermal vents in the deep sea. They can still grow at 122 °C. If Rahm is to be believed, tardigrades can survive even higher temperatures.

Many animals that have evolved to live in hot places, like hot springs and scorching deserts, produce chemicals called heat shock proteins. These act as chaperones for proteins inside cells, helping them keep their shape. They also repair heat-damaged proteins.

 

Tardigrades can even survive high doses of radiation

 

That's all well and good, but there is no conclusive evidence that tardigrades produce these chemicals. Factor in the other things they can survive, and the picture becomes even more baffling.

In 1964, scientists exposed tardigrades to lethal doses of X-rays and found that they could survive.  Later experiments showed they can also cope with excessive amounts of alpha, gamma and ultraviolet radiation – even if they're not in the tun state.

Radiation was one of the biggest threats facing the tardigrades sent into space in 2007. Those exposed to higher levels of radiation fared worse than those protected, but the mortality rate was not 100%.

 

Tardigrades spend most of their lives on moss and lichen

 

They can also cope with extreme pressure that would squash most animals flat, according to a study published in 1998 by Kunihiro Seki and Masato Toyoshima of Kanagawa University in Hiratsuka, Japan. They found that tardigrades in the tun state could survive a pressure of 600 megapascals (MPa).

 

At these crushing pressures, proteins and DNA are ripped apart.

 

This is beyond anything they might encounter in nature. The deepest part of the sea is the Challenger Deep in the Mariana Trench in the Pacific Ocean, which goes down 10,994 m. There, the water pressure is around 100 MPa. Somehow the tardigrades survived six times that.

At these crushing pressures, proteins and DNA are ripped apart. Cell membranes, which are composed of fat, become solid like butter in a fridge. Most microorganisms stop metabolising at 30 MPa, and bacteria can't survive much beyond 300 MPa.

The sheer variety of stressors that tardigrades can survive is almost dizzying. But maybe the explanation is surprisingly simple.

 

Hypsibius dujardini with eggs

 

Extreme heat and cold, radiation and high pressures all have one thing in common: they damage DNA and other bits of the tardigrades' cells. Heat and cold both cause proteins to unfold, stick together and stop working. Radiation tears up DNA and other crucial molecules. High pressures solidify the fatty membranes around cells.

So if all the stressors cause similar problems, maybe the tardigrades only need a handful of tricks to survive them. "Nobody knows for sure," says Boothby. But "there are certainly some good reasons to think that overlapping strategies might be used to cope with some of these extremes."

 

Freezing a tardigrade and drying it out both cause the same problem.

 

For instance, being dried out and being exposed to radiation both damage the tardigrades' DNA. "So it would make sense that tardigrades response to these two conditions in a similar way," says Boothby: by making antioxidants and repairing the damaged DNA.

If that's true, tardigrades' resistance to radiation is a happy accident: a side-effect of their adaptation to sudden drought. Similarly, freezing a tardigrade and drying it out both cause the same problem: not enough liquid water in the animal's cells.

Oddly enough the tun state, their most famous trick, is also the least versatile. "Tardigrades can survive freezing, radiation, and low-oxygen conditions without forming a tun," says Boothby. "So the tun state is probably a specific adaptation for dealing with or slowing water loss." However it does also allow them to survive extreme pressure.

 

Computer-generated image of a tardigrade

 

This idea, that tardigrades are only using one or two survival tricks, might help explain the other big question about them: why do they bother?

 

They have evolved to cope with environments so extreme, they don't even exist on Earth.

 

Unlike bacteria that live in boiling hot springs or other extreme sites, most tardigrades live in relatively unremarkable places. They tend to live in or near water, and there's nothing a tardigrade likes more than a good chunk of moss and lichen. Their lives aren't even that exciting: while most creatures their size dart about frantically, tardigrades are sluggish.

Yet despite their rather tedious lifestyle, they have evolved to cope with environments so extreme, they don't even exist on Earth.

Or rather, some of them have. The oldest and most primitive group of tardigrades, the Arthrotardigrada, cannot survive extreme conditions or suspend their metabolism. These more vulnerable creatures offer a clue to why the other tardigrades got so tough.

 

Not all tardigrades are amazing survivors

 

Arthrotardigrada only live in the ocean. It's only land-dwelling and fresh-water species that have the extreme survival skills. That suggests leaving the ocean was the key.

 

Today they can be found in some of the driest places on Earth.

 

"One reason that marine tardigrades aren't as good at surviving extremes is that they just don't need to be," says Boothby. "Oceans are so big that they don't undergo rapid changes in temperature or salinity, and they certainly don't dry up overnight."

By contrast, the land is dangerously changeable. Tardigrades need a thin layer of water around their bodies to breath, eat, mate and move around. But in many parts of the land, drought is a risk. "The tardigrades that live in these places need to be able to cope when their environments suddenly change," says Boothby.

So it makes sense that land-dwelling tardigrades would evolve a way to survive suddenly being dried out. It was a matter of survival. What's more, once they had it, the land tardigrades could exploit new habitats. Today they can be found in some of the driest places on Earth, where other animals cannot survive.

 

A tardigrade (Macrobiotus sapiens)

 

But this idea just raises another question. If being able to survive drying out is so useful for land animals, why don't they all do it? Why didn't frogs, earthworms and humans evolve the same ability?

Similarly, why can't other animals survive the heat, cold and radiation that tardigrades can? Perhaps the question isn't why tardigrades are so tough, but why other animals are so vulnerable.

 

Going into the tun state is a risky decision.

 

"There are probably several reasons why more animals and plants haven't evolved the tardigrades' abilities," says Boothby. "Many animals probably just don't need to. They either don't live in environments that can quickly dry out, or they can develop ways of avoiding drying out, like the camel."

But beyond that, there are surely costs to the tardigrades' abilities – costs that other animals have avoided paying. In particular, going into the tun state is a risky decision.

"When a tardigrade completely dries out, it becomes inactive and is unable to actively avoid dangers in its surroundings," says Boothby. An inactive tardigrade might not die of thirst, but it could get eaten. "We know that a lot of desiccation-tolerant organisms have to make xenoprotectants: molecules that keep bacteria and fungi from basically eating them while they are in their inactive state."

It may be that becoming as tough as a tardigrade wouldn’t pay off for other animals. But it has worked for them. They are 500 million years old and live all over the planet, so they aren't going anywhere.