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, laying eggs. And even this they do in a most interesting way.
Some tardigrades will lay eggs on moss or in water and leave them, but others will cleverly lay them as they shed their cuticle, or skin, leaving the eggs inside a protective shell until they hatch. During my time searching through these microscopic worlds, I found many empty cuticles, also called exuvium, but had less success finding any eggs.
But in Dr. Jenny Tenlen’s lab at Seattle Pacific University, tardigrade eggs are easy to find. This isn’t Jenny’s first time teaching in Seattle; before earning her PhD she taught various sciences at a local high school. After time spent at the University of North Carolina–Chapel Hill as a postdoctoral research associate and teaching fellow, she returned to Seattle as an associate professor of biology. Now she teaches undergrad students biology and also studies tardigrades to learn more about germline development. This has to do with the development of cell lineage that leads to reproductive cells. Studying this cell development in mice and other traditional lab-study organisms is challenging because it happens within a few days of conception, and even with dissection, it’s hard to see. But tardigrades—which are small and transparent, allowing their egg development to be easily observed under the microscope—are the ideal study animals for Dr. Tenlen’s work.
When I visited Jenny’s lab, she placed a dish of tardigrades under the microscope so I could see her tiny water bears. My view was filled with them, and there, right in the center, was a cuticle filled with large, round eggs.
Jenny told me that despite their ability to survive in space or when exposed to radiation, tardigrades generally make lousy lab subjects. Many species of tardigrades have been tried in labs, but they survive no longer than one or two generations before dying out. Jenny was impressed I had managed to keep my tardigrades alive for a full month in a water-filled glass dish on my desk.
Still, one species has been grown in labs for three decades now, started by a hobbyist who now sends specimens to researchers. This species, Hypsibius exemplaris, is not the one that was sent to space, and Jenny calls it “wimpy” because it doesn’t form tuns well. But whether that’s because it’s a temperate species or has lived so long in labs, she doesn’t know. It’s a cosmopolitan species that can be found around the world, including the Pacific Northwest.
Dr. Tenlen’s lab is one of just a handful around the world that studies tardigrades. She said she could count the number in the United States on two hands; but there are other labs in Poland, Germany, Japan, and a few other countries. Many of those labs, however, aren’t solely focused on tardigrades. As a result, little is known about the creatures’ life histories. Typically, science funding is directed to studies of organisms that are either beneficial or harmful to humans, and tardigrades have very little influence on human life. Now that their survival skills have become renowned, most of the funding for studies involving them goes toward studying their stress responses. Only recently have tardigrades been used in studies on human health with an interest in cryopreservation of human cells.
During our conversation, Jenny rattled off a list of things we still don’t know about water bears, including basic facts about their life history. How long do they live? We don’t know because it’s often impossible to determine how much time they’ve spent as tuns. Tardigrades are hard to study. Many species don’t survive long in labs, and they also live accelerated lives—an egg transforms into a sexually mature adult in only two weeks. We also don’t have complete information about where they live. Some habitats have been explored, but we don’t have a full picture, nor do we have a good grasp on population densities in different habitats.
Like many invertebrates, tardigrades molt out of their skin as they grow. But they don’t only molt their skin; they also shed their mouthpart, called a stylet. Why do they do this? And how do they have the ability to regrow an entirely new mouth on a weekly basis? When they lay eggs, it’s usually between one and ten, but why does the number vary? Is it environmental or caloric? All of these questions just go to show that despite becoming famous, the tardigrade is still very much an animal of mystery. And even when we think we do know something, we can be thrown a curveball once in a while.
At the time of my visit, Dr. Tenlen had recently finished a protocol and was awaiting her paper’s publication. But there was just one little problem. Scientists at a lab in Poland had their own paper in the pipeline regarding Dr. Tenlen’s lab species, which at that time was called Hypsibius dujardini. The question they were asking was this: Was Hypsibius dujardini actually Hypsibius dujardini? There may have been a translation error from the original French species description to Italian, causing a potential breakdown in the species identification. It was possible the Hypsibius dujardini species currently in labs around the world could actually have been an entirely different species. Such a simple question of identity, but with huge implications. Eventually, the lab in Poland found that, based on some anatomical differences, Hypsibius dujardini is actually Hypsibius exemplaris.
