The New York Times

Cornelia Dean

September 7th 2015

The Tardigrade: Practically Invisible, Indestructible ‘Water Bears'

When scientists at the American Museum of Natural History mounted an exhibit about creatures that survive under conditions few others can tolerate, they did not have to go far to find the show’s mascot.

“We just got them from Central Park,” said Mark Siddall, a curator of the show, Life at the Limits. “Scoop up some moss, and you’ll find them.”

He was talking about tardigrades, tiny creatures that live just about everywhere: in moss and lichens, but also in bubbling hot springs, Antarctic ice, deep-sea trenches and Himalayan mountaintops. They have even survived the extreme cold and radiation of outer space.

Typically taupe-ish and somewhat translucent, and a sixteenth of an inch or so long, they are variously described as resembling minuscule hippopotamuses (if hippos had giant snouts and eight legs, each with several claws), mites or, most commonly, bears. Many people call them “water bears” or “bears of the moss.” (The word “tardigrade” is from the Latin for “slow walker” and pronounced TAR-dee-grade.)

Once an object of interest only among zoological specialists, tardigrades now are generating widespread enthusiasm. Admirers have produced artwork and children’s books about them, and have even organized the International Society of Tardigrade Hunters “to advance the study of tardigrade (water bear) biology while engaging and collaborating with the public.”

According to the society, formed this year at the University of North Carolina at Chapel Hill, people can find tardigrades if they gather some lichen or moss, especially on a damp day, put it in a shallow dish of water, and “agitate” it a bit. Debris will settle to the bottom of the dish, and tardigrades will probably be prowling in it.

The museum exhibit, which runs until January, also includes beetles, flowers, corals and other animals with unusual ways of coping with hostile environments. But its entrance is guarded by a 10-foot replica of a tardigrade, seemingly floating overhead. That’s fitting, because the tardigrade, which has a natural life span of about a year, is particularly impressive among the exhibit’s “extremeophiles.”

Confronted with drying, rapid temperature changes, changes in water salinity or other problems, tardigrades can curtail their metabolism to 0.01 percent of normal, entering a kind of suspended animation in which they lose “the vast, vast, vast majority of their body water,” Dr. Siddall said. They curl up into something called a “tun.”

Tuns can be subjected to atmospheric pressure 600 times that of the surface of Earth, and they will bounce right back. They can be chilled to more than 300 degrees Fahrenheit below zero for more than a year, no problem. The European Space Agency once sent tuns into space: Two-thirds survived simultaneous exposure to solar radiation and the vacuum of space.

Without water, “the damaging effects of freezing cannot happen,” Dr. Siddall explained. “It protects against heat because the water inside cannot turn into a gas that expands.” Even radiation needs water to do damage, he said. When cosmic radiation hits water in a cell, it produces a highly reactive form of oxygen that damages cell DNA. The tun doesn’t have this problem.

Tuns have been reconstituted after more than a century and brought back to life as tardigrades, looking not a day older.

Little is known about their evolution, which is too bad because biologists think it must have been interesting. But tardigrade fossils are hard to spot.

For a long time, biologists grouped them with arthropods, other creatures, mostly small, with eight legs. Only recently have tardigrades been given their own phylum, a major taxonomic category.

People who have become transfixed by tardigrades often say they came across a photo or article by chance.

“I just stumbled across it,” said Thomas Gieseke, an artist and illustrator in Merriam, Kan., who created “The Tardigrade Queen,” an acrylic-on-canvas work depicting a tardigrade on a throne, complete with tiara and royal crest, which was shown at the Todd Weiner Gallery in Kansas City, Mo.

“I stumbled on a photograph of one,” he said in a telephone interview. “I was just fascinated.” Though he has never seen a tardigrade in the wild, he said, “it’s just the most resilient creature on the face of the planet.”

“I like their little claws. They look like hands,” he added. “I thought, ‘This thing warrants royalty status.’ ”

Another tardigrade enthusiast, Michael W. Shaw of Richmond, Va., got interested in them more than a decade ago when he was helping his two daughters with school science projects. Though he knew nothing about tardigrades — his degree was in fine arts — he ended up taking microscopes into his daughters’ classes to spread the word about the fascinating creatures.

Later, he made his own contribution to the scientific literature. “I read a paper about tardigrades showing where they were in the U.S., and New Jersey, where we were living at the time, had a zero,” he said.

Mr. Shaw, who was living then in Somerset, decided to visit every one of the 21 counties in New Jersey and sample lichen and tree bark, two microenvironments hospitable to tardigrades. Between 2001 and 2009, he said, “I went to rural and urban sites, parking lots, nature preserves, anywhere. I found them in every county.”

His family thought his obsession was “strange,” he said, but the work, which he completed with the help of Dr. William Miller, a tardigrade expert at Baker University in Kansas, was published in The Journal of the New York Microscopical Society.

Then Vice did a video about Mr. Shaw. Soon new fans were arguing online about whether tardigrades came from outer space (an idea Mr. Shaw does not rule out) and how — or even if — they evolved.

Eventually, the work turned into two books Mr. Shaw has self-published — “Tardigrade Quiz and Fact Book” (Fresh Squeezed Publishing, 2014) and “Tardigrade Science Project Book” (Fresh Squeezed Publishing, 2011). Both discuss tardigrades and explain how young naturalists can gather specimens, make slides and otherwise dive into their snouty, eight-legged world.

Today Mr. Shaw’s tardigrade guides are selling slowly but steadily (typical reader comment: “Love those tardigrades!”), and he has done another guide for microscope hobbyists.

“The good news is you can find them almost anywhere,” according to the Tardigrade Hunters website. The group invites tardigrade hunters to submit their “prized specimens” for examination under the university’s high-powered microscopes.

The samples will not be returned, the society notes, but photographs of particularly interesting specimens may be posted online as the Tardigrade of the Week.

In ordinary life, tardigrades don’t get up to much. Dr. Siddall said that like most animals, they spend their time “hanging out and eating” plants and animals smaller than themselves, and possibly even indulging in cannibalism.

“People often say, ‘What’s their purpose? What’s their role in the universe?’ ” Dr. Siddall said. He has no ready answer. They might be useful for the study of suspended animation. But, he added, “are we going to find a way to put humans into suspended animation? I doubt it.”

Anyway, he said, attributing some kind of larger purpose to the tardigrade is not something a biologist would want to do. Creatures don’t have to have a purpose. “They merely are.”