Nature Obscura. Kelly Brenner
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It had started that morning when I’d gathered two small clumps of moss from under the maple tree in my front yard. I added some rainwater to a pair of petri dishes, set a clump of moss inside each, and left them to soak on the picnic table while I went about my morning routine. Later that afternoon, I’d retrieved the petri dishes, dumped the water, and then holding each clump of moss over an empty dish, squeezed it. The water poured out; like a wet sponge, the moss held a surprising amount of it. Carefully, I brought the dish back to my office, where the dissecting microscope lives, and spent the next few hours with my neck bent over the scope, searching for a certain tiny creature.
The remaining moss water was a minuscule floating jungle of debris, the brown fluff moving with animals invisible to the naked eye. I first found tiny, transparent, wormlike organisms that propelled themselves quickly, transforming into a compact ball and stretching back out again. I later learned they were rotifers. I also observed a couple of larger nematodes, translucent worms that look like they’re made of glass, their insides shifting about as they wiggled around. There were tiny dots, too small to see as anything but little blurs, swirling in circles, and teeny rice-like shapes sliding across the bottom, jerking away when they bumped into something. But it wasn’t until just before dinnertime when I finally found the object of my quest: a tardigrade.
These microscopic animals are famous for having survived in space; yet to me it seemed improbable that such accomplished animals lived in my yard in the city, even though I knew they could be found here. I desperately wanted to find one, while still half believing it was impossible. But only after I had stared at every movement in the dish, hoping for a tardigrade, there it was finally, almost larger than life, and exactly like I imagined. Once I saw it, I knew there was no mistaking it for anything else. The creature’s slow movement, eight stumpy legs tipped with long, curved claws framing a rotund body, plodding through the water, was very different from that of every other living creature in that dish. The tardigrade is also known as a moss piglet and water bear because of its obvious resemblance to those animals. Tardigrade means “slow walker,” but naming the animal for its plodding movements on the flat, slippery surface of a petri dish is unfair. Putting a tardigrade on such a surface is as unnatural as putting a sloth on the ground, or a human on ice. Placed in its natural home of mosses and lichens, a tardigrade uses its long claws to climb through the miniature plants with ease.
After my initial success, I continued looking for tardigrades around my yard. I collected moss from a dozen places, including on a maple tree, in the lawn, on the driveway, on rocks, the fence, and concrete stairs, and on the roof. To my surprise, the sample with the greatest abundance of tardigrades, the one in which they were a dime a dozen, was the moss from my roof. Not the moss in the grass, or the moss that clung to the tree bark, but the moss on top of our two-story house. I realized there must be thousands, maybe millions, of tardigrades on our roof crawling around in the Bryum argenteum moss above my head every day. Up there, where they are exposed to all the elements the Pacific Northwest can throw at them: hot sun in the summer, ice and occasional snow in the winter, and torrential rain in the autumn. But therein lies the key—rain.
I began keeping a large glass dish containing tardigrades and moss from our rooftop on my desk, where the creatures became my tiny pets. I added a little rainwater every couple of days to keep the dish from drying out in the warm house, and each morning I set it under my microscope and observed the tiny organisms living inside it.
But over one weekend the dish dried out, the loose debris turned into a hard crust, the moss became brown and crispy, and nothing moved. I added rainwater Monday morning and let it sit to see what would happen. A few hours later I poked around in the dish and found a tardigrade, compressed and unmoving. I prodded it with one of my dissecting tools and it moved a little, as though it were just waking up. Very slowly its legs rehydrated, like a balloon being blown up, until it was in a spread-eagle position. Eventually it started moving and righted itself, becoming more active.
Although tardigrades live on land and are often thought of as terrestrial, they are true water beings. Their lives are intrinsically tied to water. Many species live in marine environments and others can be found in freshwater environments, but many are limnoterrestrial, meaning they live in wet, soggy places, like lichens and mosses. The water bear requires a film of water around it for gas exchange, and without that water, it quickly desiccates and turns into a tiny, oval-shaped “tun,” which I observed when the dish dried out. Over the month I watched them, the dish dried out several times, and each time I added water, life soon returned.
It’s this survival ability that has made the tardigrade famous enough to be featured in popular culture, including the movie Ant-Man and the Wasp and the television show Star Trek: Discovery. Article after article rolls out regularly, declaring the tiny tardigrade the ultimate survivor, the animal that will outlast us all, the one that will survive the apocalypse. These claims may sound sensational, but there is more than a grain of truth behind the tardigrade’s survival story. It has survived five mass extinction events, after all; and it’s true that it is very adept at surviving harsh conditions by turning into a tun. But this fame comes from the study of only a handful of the roughly one thousand species of tardigrades, and not all of them have this ability.
Another common refrain is that tardigrades can live a hundred years in this indestructible tun form and then be revived with a little water. This story traces back to Italian biologist Tina Franceschi, who examined an old moss specimen and found tardigrade tuns on it. She rehydrated the sample and several of the tuns expanded, but showed no further sign of life, except one. That lone tardigrade showed some limited, brief movement, indicating there was some life left in it, but it ultimately failed to revive successfully. One hundred years may be a stretch, but tardigrades have been revived with success after seven years, and in 2015, water bears found frozen in moss from Antarctica were revived after thirty years as tuns.
But what exactly is a tun? The short answer is this is what some tardigrades turn into when they encounter conditions they cannot survive in. When the water in the dish on my desk began to evaporate, the tardigrade’s limbs and head retracted, and the water bear folded in on itself, shrinking down to a barrel shape. It was then in cryptobiosis, a state of being where metabolism slows to an undetectable level, and it is, for all intents and purposes, essentially dead. Loss of water and desiccation, called anhydrobiosis, is the most commonly studied type of cryptobiosis, but other conditions can also trigger tun formation. Cryobiosis, for example, is a state triggered by cold temperatures and is what allows tardigrades to survive in the freezing climate of the Arctic and Antarctic. Less well studied is osmobiosis, which has to do with the level of salinity in water; but considering some species live in the tidal zone, a great deal is still unknown. The final type of cryptobiosis is anoxybiosis, an organism’s response to a lack of oxygen. Cryptobiosis is a gradual process, and tardigrades must become sensitized to the changes encountered. But there’s some evidence that water bears retain the memory of these transitions and in future episodes can change into a tun faster.
While in the tun state, tardigrades can survive truly astonishing conditions that would kill nearly every other living creature. Their statistics, like those of a star baseball player, are impressive. They can survive temperatures as low as -459.31 degrees F and as high as 248 degrees F. X-ray levels at 570,000 rads? Piece of cake! Five hundred rads would kill us humans, if you were wondering. Space vacuum? No problem. Tardigrades were the first animals recorded to survive exposure to the vacuum of space as well as solar/galactic radiation simultaneously. Previously, lichens and bacteria were the only proven survivors. However, just because water bears can survive in the vacuum of space doesn’t mean they will. A study of tardigrades taken to space in 2007 found that when exposed to only the vacuum of space, they mostly revived without any problem. But the double whammy of the vacuum of space and solar radiation hit them much harder, and at the most lethal levels, only a few individuals revived. Tardigrades exposed to the vacuum of space subsequently laid eggs that successfully hatched, and eggs directly exposed also hatched at rates the same as those in a control group.
Despite these survival abilities, a tun is still a tun, and although it is alive, it is not living. Tardigrades in a tun phase cannot continue living until they are given their medium back. Without water they cannot feed, mate, or attend to their most important task,