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.)