Practically invisible, indestructible bears

“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.”)

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 they have even organised the International Society of Tardigrade Hunters “to advance the study of tardigrade 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 per cent of normal, entering a kind of suspended animation in which they lose “the vast, vast, vast majority of their body water,” Mark 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.

Hot & cold
Without water, “the damaging effects of freezing cannot happen,” Mark 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.

Mark 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?'” Mark 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.”