The Mosses at Our Feet

Like any national park, Great Smoky Mountains in North Carolina and Tennessee boasts its own set of iconic animals: black bears trundling through the underbrush, red-tailed hawks soaring above the park’s verdant vistas, and white-tailed deer grazing nonchalantly in Cades Cove. Now it’s time to add another species to the list: tardigrades, a tiny but remarkable water-loving creature.

Most of the 9 million annual visitors to the Smokies have never heard of tardigrades. Those who have would be hard-pressed to see them without a microscope—the eight-legged, barrel-shaped panarthropods are only about a half-millimeter long—but they are among the park’s most extraordinary inhabitants, capable of surviving for decades in a suspended state of almost total dehydration. And they are finally getting some of the attention previously reserved for higher-profile creatures, thanks to the park’s All Taxa Biodiversity Inventory (ATBI), an ambitious effort to count and catalog every species of flora and fauna living in the park.

Paul Bartels, a biology professor at Warren Wilson College near Asheville, North Carolina, began leading the project’s tardigrade research in 2000 with help from more than 50 students. Before the ATBI, there was only one published record of tardigrades, and only three species had been identified in the park. Now, they can list 81 species—15 of which were previously unknown to science.

But that’s not all. In the past decade, Bartels and his team have collected and sorted about 15,000 specimens from the park, mostly from samples of moss, lichens, and other likely tardigrade habitats that they scooped into paper bags and put on a shelf to await further analysis. They also uncovered some fascinating traits: tardigrades’ range and endurance. Gravitating to wet places like mosses and beach sand, they have been found 20,000 feet high in the Himalayas and 13,000 feet under water. They can withstand temperature extremes from near absolute zero to 300 degrees Fahrenheit. A 2008 experiment showed they could even survive exposure to the vacuum of outer space.

Because of their rounded bodies and awkward, ambling gait, they are colloquially known as “water bears” or “moss piglets.” (Their formal name comes from tardigrada, Italian for “slow walker.”) Tardigrades acquire much-needed fluids either by piercing the walls of plant cells and smaller organisms with tiny fang-like probes and sucking out the contents with their tubular mouths, or even swallowing microscopic animals whole.

But if fluid sources dry up or disappear, tardigrades respond with a tactic straight out of science fiction: They shut down in a process known as cryptobiosis, a sort of suspended animation that allows organisms to wait out inhospitable conditions. Curling up into a tiny ball and filling their cells with trehalose, a protective synthesized sugar, tardigrades can reduce their metabolic activity by about 99.99 percent. When they next encounter moisture, they can come back to life in just a few hours; tardigrades can survive in this cryptobiotic state for a few decades.

All of this interest in the Smokies’ tardigrade population probably wouldn’t exist had it not been for the ATBI, where scientists aren’t just counting species but trying to understand how they relate to one another, too.

“The problem that parks and natural areas face all over the world is that we only know a small fraction of the species that occur in each one,” says Keith Langdon, who was in charge of inventory and monitoring in the Smokies before his recent retirement. “In places where we have a mandate to protect everything, we need to know what’s there.”

The benefits of such an inventory seem obvious for endangered species, which, unfortunately, are common in the Smokies: Indiana bats, now threatened by white-nose syndrome; hemlock trees, under attack by sap-sucking woolly adelgids; and the spruce-fir moss spider, whose habitat is vanishing as the park’s Fraser firs fall prey to pests. Keeping track of those populations is a vital part of the park’s conservation mission. But why go to so much trouble to study a nearly invisible resident like the tardigrade, whose hardiness makes it an unlikely candidate for extinction?

Ernest Bernard, a nematologist from the University of Tennessee who studies small, insect-like arthropods called springtails, says it’s because micro-organisms are probably important in ways we don’t yet understand.

“We know a lot about the big, conspicuous species,” says Bernard. “We know very little about the small, inconspicuous ones, and yet these species probably have, as a whole, important functions in the maintenance of healthy ecosystems.”

To begin to understand those functions, Bartels and his students have developed a key to all the tardigrade species identified so far in the Smokies—now posted online for other researchers. That will allow tracking of the populations over time, as the park’s habitats change.

Bartels and his students have essentially donated their work to the inventory project, which operates on a meager budget, but Bartels says the pay-off has been significant in other ways. Thirteen of his students pursued senior projects related to their tardigrade research, and a few have been inspired to do graduate work in the same field. “It’s eye-opening for students to realize that we don’t know everything and that there’s a lot left to be discovered,” he says. Even in the mosses at our feet.