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What is a tardigrade?

Tardigrade, SEM
Colored scanning electron micrograph (SEM) of a "water bear," or tardigrade. -
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Tardigrades are tiny, cute and virtually indestructible. The microscopic animals are able to survive in a pot of boiling water, at the bottom of a deep-sea trench or even in the cold, dark vacuum of space. In August, an Israeli spacecraft carrying tardigrades as part of a scientific experiment crashed on the moon, and scientists believe they may have survived.

The hundreds of species belonging to the phylum Tardigrada are so hardy that many could be here long after other life on Earth has perished, enduring as long as the sun continues to shine. It's this uncanny ability to endure extreme conditions that has drawn the attention of scientists, who say tardigrades may hold the key to human survival. What we learn from ongoing research on tardigrades could help us stay alive on the operating table or in outer space.

What do tardigrades look like?

Tardigrades have long, plump bodies and eight stubby legs. They're closely related to insects and crustaceans but look a bit like pigs or bears — and are sometimes called "water bears."

"Their proportions are a little bit similar to a bear's, and they're kind of cute — at least some of them are cute to some people," Roger Chang, a Harvard University molecular biologist who studies tardigrades, said.

Most tardigrade species are less than half a millimeter long, around the size of a dust mite. Some species are larger, growing up to 1.5 millimeters, around the size of a grain of sand — big enough to be seen with the naked eye, according to Chang.

Where do tardigrades live?

Tardigrades are semi-aquatic. They can survive in watery as well as terrestrial environments — from oceans and lakes to mountains, forests and sand dunes. They're found all over the world, from frigid Antarctic glaciers to active lava fields. They're most commonly found living in moss.

Most tardigrades eat algae and flowering plants, piercing plant cells and sucking out their contents though their tube-shaped mouths. Some, however, are carnivorous and may eat other tardigrades.

Tardigrades are nature's pioneers, colonizing new, potentially harsh environments, providing food for larger creatures that follow. Scientists say, for instance, that tardigrades may have been among the first animals to leave the ocean and settle on dry land.

Tardigrades pose no threat to humans. Scientists have yet to identify a species of tardigrade that spreads disease.

What's the lifespan of a tardigrade?

Tardigrades typically live for only a few months when fully active. When short on water, they may curl up in a ball, entering the "tun" state — so named because it looks like a large barrel called a "tun."

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In this state, a tardigrade grows a glass-like protective coating and slows its metabolism to 0.01 percent of the usual rate. Chang said a tardigrade could potentially survive for centuries like this, though it wouldn't be much of a life. The tun state looks more like a temporary death than a long hibernation.

"What do you call alive, exactly?" Chang said. "It's kind of a matter of semantics."

Are tardigrades immortal?

In their active state, tardigrades are decidedly mortal. Chang said he has accidentally killed countless tardigrades by starving them or drying them out too fast. Once he inadvertently sent a test tube full of them through an airport security scanner.

"They are actually relatively easy to kill when they're not in this tun state," he said. "They are really just as fragile as most microscopic animals."

As tuns, however, tardigrades can endure radiation, extreme pressure and extreme heat and cold, including temperatures near absolute zero.

Scientists have subjected tardigrades to all manner of insult to test their hardiness. For one study, Japanese researchers froze tardigrades for 30 years before reviving them and watching them reproduce. For another, the European Space Agency sent tardigrades into space to see how they would cope with solar radiation — and a handful actually managed to survive.

How are tardigrades used in scientific research?

German pastor J.A.E. Goeze published the first paper on tardigrades in 1773. Scientists have long studied the creatures to better understand how animals could persist in the most hostile environments. In recent years, researchers have been working to apply what they learned to humans.

Chang and his colleagues hope to imbue humans cells with a tardigrade's ability to shut down temporarily, aiming to develop synthetic proteins like those discharged by water bears as they enter the tun state. These synthetic proteins could be used, for example, to preserve organs needed for transplants, potentially keeping organs viable for longer than is possible by storing them on ice.

Someday it may be possible to use what we learn from tardigrades to aid victims of strokes or heart attack by protecting their vital organs against further damage as they await treatment, or to help technicians working in nuclear power plants to guard against radiation, or conceivably, to help astronauts to survive on long spaceflights.

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