Tardigrades can enter a state known as 'cryptobiosis' in which they are nearly impossible to kill.

© Eye of Science/Science Source

Tardigrades—microscopic eight-legged animals that resemble plump piglets in puffer coats—have been charming and astonishing biologists since they were first discovered in the 1770s. Now they’re starring, as 10-foot models, no less, in the Museum’s new exhibition Life at the Limits.

Zoologist Johann Goeze first dubbed the tiny aquatic animal he saw lumbering around on clawed legs “kleiner Wasserbär”—German for little water bear. A few years later, Italian naturalist Lazzarro Spallanzini named them slow steppers (tardi grada)—and provided the first description of the amazing transformation tardigrades undergo when under environmental stress.

Tardigrades are phenomenally successful organisms, having first appeared more than 600 million years ago. More than 1,000 species can be found all over the world, in sea and fresh water, as well as on land, where they cling to moist moss or lichens. Though they’re common in moderate climes, terrestrial tardigrades are also one of the few animals that thrive in spots that are particularly inhospitable to life, such as Antarctica’s McMurdo Valleys, thought to be the driest and coldest desert on Earth.

To eke out a living in the mosses of Antarctica, and even in more mild places where their habitats are very vulnerable to sudden water loss, tardigrades have evolved a remarkable ability. When conditions turn life-threatening—whether from rapid drying, extreme dips in temperature, or spikes in salinity—they seem to defy death by imitating it. They temporarily wind down their metabolism in a reversible process called cryptobiosis—literally, hidden life.

There’s still much to be learned about the mechanisms by which tardigrades become cryptobiotic when faced with different stressors. The dramatic change they undergo in response to lack of water—anhydrobiosis, first described by Spallanzini in 1776—is still the best understood.

First, the animal curls into itself, tucking its eight limbs and head inside its body. It sheds more than 95 percent of the water in its body, shriveling into a blob, known as a tun for its resemblance to a beer barrel. In the process, the tardigrade produces a sugar that replaces the lost water, protecting internal structures from fatal damage. Metabolic processes dwindle to less than 0.01 percent of normal activity as the animal waits for conditions to improve. Just add water, and these “barrels” transform back into active “bears.”

The famously hardy tardigrade species Milnesium tardigradum survived space.

Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al.

As tuns, tardigrades appear to be lifeless—and close to indestructible. Researchers have exposed tuns to extreme temperatures on either side of the scale, to extreme pressures, and to toxic concentrations of gasses such as carbon monoxide. In all cases, tardigrades have sprung back to life once water was resupplied. In 2007, the European Space Agency even tested tuns in space, sending two species of tardigrades into low Earth orbit on the FOTON-M3 mission. The tuns didn’t disappoint: tardigrades became the first animals to survive the one-two punch of solar radiation and space vacuum.

Tardigrades are far from the only animals that enter cryptobiotic states. But, in part because it’s so extraordinary, the tardigrade’s hardiness has, at times, inspired some tall tales, including legends about their longevity. The total lifespan of tardigrades—that includes pauses to wait out unfavorable conditions—is actually thought to be under a decade.

And while phenomenal, the tardigrade’s resiliency is not absolute: though the majority of tardigrades in the 2007 experiment survived simultaneous exposure to space vacuum and solar radiation, 32 percent didn’t. Most humbling, perhaps: the longest verified uninterrupted lifespan of Milnesium tardigradum, the ultra-resilient tardigrade species that can endure open space? Just 58 days after hatching.

Tardigrade models at the entrance of the Life at the Limits exhibition.

©AMNH/R.Mickens

See 10-foot tardigrades—and models of other amazingly resilient animals—in Life at the Limits, which is open from April 4 through January 3 and is free for Members.

A version of this story originally appeared in the Spring issue of Rotunda, the Member magazine.