Tiny, Tough, and Totally Fascinating

It sounds like hyperbole, but it isn’t: Minnesota is home to a group of animals that can go into a state akin to suspended animation and survive temperature extremes, an almost complete lack of water, extreme high or low pressures, toxic gases, even exposure to outer space. Oh, and they’re basically invisible to the unaided eye. Meet the tardigrade, a microscopic animal found all around Minnesota—and practically everywhere else—but especially in lichen, moss, and soil, as well as fully aquatic habitats.

As a rule, microscopic critters don’t get a lot of attention, but you may already have heard of tardigrades by another name: water bears. Tardigrade means “slow-moving” in Latin, but “water bear” is a common nickname and certainly apt: Under a microscope, the tardigrades you’re likely to find in Minnesota are often a bit rotund, have prominent claws, and even seem to amble about, like our more familiar ursines.

For all their fascinating qualities, tardigrades are largely understudied in Minnesota. As of this writing, there are just four scientific records of tardigrades in the state. (Until recently, some states didn’t have any records at all.) According to tardigrade expert Dr. William “Randy” Miller, with some effort, finding two dozen species in the state wouldn’t be difficult, and he noted that there could perhaps be many more, possibly even some species new to science.

Hunting for (Water) Bears. I have a strong interest in—some might say an obsession for—small living things. I spend my springs, summers, and falls outside, often looking for insects and other invertebrates and photographing any that I can find. But by late October and early November, entomological silence strikes, even indoors. I call it SAD season. When COVID-19 hit, everything compounded. In desperation, I decided to try to find a tardigrade.

I first learned of tardigrades via passing references in books and brief clips online, where they are quite popular. Perusing YouTube videos gave me the most basic of equations—find some moss or lichen, add bottled water, then voila: tardigrade. Beyond that, I was on my own.

What the Heck Is a Tardigrade? To put it simply, tardigrades are aquatic microscopic animals. And they’re tiny: They range in size from 0.1 mm to a little over 0.5 mm.

On the tree of life, tardigrades belong to their own branch, the phylum Tardigrada. Within the Tardigrada are two main classes. Eutardigrada have a smooth, or naked, outer covering called a cuticle. Heterotardigrada are covered in armorlike plates.

Estimates suggest tardigrades date back to the Cambrian period, around 500 million years ago. Incredibly, there are even four fossil tardigrades on record—all naturally preserved in amber. The oldest dates to the Cretaceous period, which means tardigrades trundled, invisibly, alongside the dinosaurs (and well before). This also means that tardigrades have endured all five of the mass-extinction events that have occurred on Earth, making them true survivors.

Wait, They’re Aquatic? Though tardigrades are considered aquatic, not all live in constantly wet environments. Some live in saltwater, others in freshwater, but members of a third group live in limno-terrestrial environments, which oscillate between wet and dry. Examples include the tiny spaces in between moss and lichen, sand grains in soil or beach sand, or leaf litter in the forest. 

To be clear, tardigrades aren’t a monolith, and freshwater and marine tardigrade species can’t survive the extreme conditions that make them famous as a group. But limno-terrestrial species can.

To start to understand what life is like for a tardigrade, it’s best to get a piece of lichen or moss and look at it under a stereo microscope. On a hike or a walk, lichen and moss are familiar, if underappreciated, features of everyday life. But under a microscope, moss is a jungle and lichen is an alien planet. Both are replete with nooks and crannies where a host of organisms, including tardigrades, can flourish if water is present. A small section of moss or a lichen-covered branch can harbor dozens, if not hundreds, of tardigrades. A fallen, moss-covered tree? That’s a tardigrade metropolis.

In Search of Expertise. In my first tardigrade hunt, the problem was the particulars. I was working with a toy compound microscope and a dim light source, searching through single droplets of recently warmed up “lichen water.” It took me two weeks of pure persistence to find one.

Once I’d snapped a few quick pictures by holding my iPhone over the eyepiece, I wanted to narrow down my identification, but I couldn’t find any detailed ID guides for tardigrades. So I reached out to some scientists, and a few folks recommended I contact Miller, a tardigrade expert at Baker University in Baldwin City, Kansas. Miller, who passed away last year, was a brilliant, prolific, and, above all, generous scientist.

He and his colleagues recorded many tardigrade species new to science and published a host of papers from around the country, documenting tardigrade populations in places where there were few, if any, records before, including in Minnesota.

Life, But Littler. Though tardigrades are microscopic invertebrates, once you observe them for long enough, their way of life starts to look somewhat familiar. They have symmetrical bodies with a head and four other body segments—each with a pair of legs—and the sturdy cuticle, which they repeatedly molt over time. The eight legs have impressive claws, which vary greatly by genus. The back pair of legs are positioned backward.

Tardigrades have mouthparts for consuming food, a digestive system, muscles, and a nervous system, though they don’t have a circulatory system or lungs. Instead, they absorb oxygen directly from their environment, and their liquid-filled bodies help them maintain sufficient pressure while distributing nutrients.

Tardigrades eat using what is known as a buccal-pharyngeal apparatus that works a bit like a vacuum. The tardigrade hoovers up food into its mouth, and then it’s sucked into the buccal tube and into the rest of the digestive system. The animals also have a pair of piercing structures called stylets that jut out and help them puncture food.

Many tardigrade species are herbivores, subsisting on fungi or algae. A few are outright predators, consuming other microscopic critters—or even other tardigrades. And some, like our black bears, are true omnivores.

In terms of tardigrade reproduction, it’s complicated. Some tardigrade species reproduce sexually, but many can reproduce asexually via parthogenesis, which is a bit like natural cloning. Still other tardigrades are hermaphroditic, possessing both sex organs and the ability to reproduce independently or by interacting with others.

However they reproduce, tardigrades deposit eggs, which are tiny, ornate, and beautiful. Eggs are laid singly or in groups in the substrate where tardigrades are found or, in some cases, in the female’s shed cuticle. This means that it’s not rare for microscope-wielding observers to find tardigrade eggs, and it’s possible to watch them develop and then for young ones to emerge.

A Generous Mentor. In addition to the myriad papers Miller published, he taught legions of students—many of whom made discoveries alongside him—and he even took the time to mentor hosts of enthusiastic amateurs, including high school science students, leading them through the process of collecting tardigrades, mounting them on slides, and compiling data. Then he’d guide them through the publishing process. Sometimes a single new paper would double a state’s overall tardigrade total.

Miller was just as generous to me: He offered expert advice, shared scientific papers about tardigrades, and answered my flood of questions as I navigated this new microscopic world.

Entering the Tun State. Tardigrades are famous in large part because of their ability to survive and react to changing environmental conditions, such as loss of water or low oxygen levels.

One of the primary ways that tardigrades survive extreme conditions is by entering a mode called the tun state. When this occurs, the tardigrade’s limbs retract and it dehydrates and shrivels up into what looks, under the microscope, a bit like a dried-up bean. In this state, also known as cryptobiosis, their biological processes slow down to next to nothing. There are several “flavors” of cryptobiosis that occur when a tardigrade is faced with differing adverse environmental conditions: anhydrobiosis (water loss), cryobiosis (freezing), and osmobiosis (extreme salinity or other chemical changes). All of these varieties of cryptobiosis produce a tun, but their mechanisms and recovery processes vary.

Crucially, however, these processes are reversible. In anhydrobiosis, for example, once the tardigrade is rehydrated for a short period—usually something like four to eight hours—it wakes up and continues as if the intermission, whether a day or a week, never happened. In an extreme case, a tardigrade revived from a tun state after two decades.

As if that weren’t enough, tardigrades have another trick or two up their sleeves: When faced with low oxygen levels (anoxybiosis), they puff up and stop moving until oxygen levels improve. In another strategy, called encystment, some effectively retreat and wrap themselves in multiple layers of their cuticle to survive.

A Bit of Immortality. In one of my first email exchanges with Miller, he indicated that one of my early findings wasn’t represented in the scanty published scientific literature for Minnesota. This doesn’t mean the species was new to science, simply that it hadn’t been recorded in the state yet. He said such findings were likely publishable and that published authors are often referenced in later publications, providing what he described as “a bit of immortality.”

Miller and I began drafting a paper about Minnesota’s tardigrades, but he passed away before we could publish it. Appropriately enough, Miller was a posthumous coauthor in a recently published paper that identified the first tardigrade to the species level in Minnesota, so his name will forever be linked to Minnesota tardigrades.

How the Tun State Works. Professional and amateur tardigrade hunters alike have long wondered how the tun state works and especially how tardigrades can survive such harsh conditions, especially extreme desiccation (anhydrobiosis). The process behind anhydrobiosis involves chemistry, according to an insightful 2024 paper in the journal PLOS One.

According to the study, which exposed tardigrades to different chemical stressors, tardigrades enter the tun state when they sense that conditions are deteriorating. (How exactly that occurs remains a mystery.) During their transformation, an amino acid, cysteine, is oxidized. The tardigrade quickly begins to dehydrate, and it survives thanks to protective proteins that prevent it from dying of desiccation. Once in the tun state, the tardigrade is well protected from everything from temperature extremes and a lack of oxygen to high levels of radiation and the vacuum of space.

Rugged But Not Indestructible. Despite their resilience, tardigrades aren’t immortal. While they are certainly tough in their tun state, outside of it they die as easily as anything else. For example, as they trundle about, tardigrades can be consumed by nematodes, rotifers, or even each other. And of course, like anything else, they can die of old age. For a tardigrade, that’s usually a few months up to a year or two, not counting any time spent in the tun state.

And even their tun state isn’t foolproof. As a general rule, the longer a tardigrade stays a tun, the less likely it is to revive, especially if it was subjected to extreme conditions. That is, the tun state is an evolutionary adaptation to survive, not a get out of jail free card from mortality.

For example, when tardigrade tuns aboard the International Space Station were exposed directly to the vacuum of space, they basically shrugged it off, springing back to life once rehydrated. In the clever experiment, other groups of tardigrades were exposed to the vacuum of space and varying ranges of radiation, including one group exposed directly to the full radiation and vacuum of outer space. Those exposed to full radiation didn’t fare as well, with nearly all dying, but incredibly, a few rugged specimens survived.

A Very Fine Stick. To make good on Miller’s mentorship, I’m now at work on a new tardigrade project with a simple premise—How Many Tardigrades on That Stick? When I’m out in the woods or the backyard with my kids, I’ll point out lichen-covered sticks and often say, “Now that’s a very fine stick.”

After finding an especially fine stick in my backyard, about 16 inches long, I launched into the new project. After dividing it into four sections, I hydrated all of its lichen over the course of a winter and searched through it, catching every tardigrade I found in the process—about 80 in all. Now, I’m slowly identifying them, working with with a fellow tardigrade enthusiast. I plan to attempt to publish my findings, with Miller as a posthumous coauthor. I also hope to publish our initial work together.

Why Study Tardigrades? In science, discoveries piggyback on one another. If one asked the average person if studying Gila monster venom was worthwhile, especially with tax dollars, they might scoff. But Gila monster venom contains a hormone that mimics one found in humans: glucagon-like peptide-1, aka GLP-1. You may have heard that last phrase, GLP-1, before. Over time, basic research on Gila monsters has directly led to a drug that treats diabetes in humans, followed by breakthrough anti-obesity drugs such as Ozempic.

Tardigrades, of course, are no Gila monsters. They’re microscopic. So why study them? The simple answer is: There’s a lot to learn. And discoveries in science are cumulative. A finding in one area may lead to breakthroughs in a different field.

Tardigrades offer something else: Because they are so understudied, they give enthusiastic nonspecialists a chance to do some real science. With basic lab equipment, enthusiasm, and, perhaps most importantly, a generous mentor, like Miller—community scientists can help put their local tardigrades, including Minnesota’s, on the map.

Gotta Catch 'Em All 
How to find tardigrades

Supplies for preparation:
Stereo, or dissecting, microscope
Distilled water (not tap water)
Petri dishes/pipettes
Various moss/lichen samples
Brown paper lunch bags

Supplies for observation:
Compound microscope
Microscope slides and cover slips
Irwin loop (for catching them)

Be Safe!
Keep basic lab safety considerations in mind (wash hands, don’t eat or drink near your workspace, and clean all gear appropriately or dispose of it in the trash). Tardigrades and used lichen/moss can be deposited in a “tardigrade city” garden spot in your yard.

Step 1: Soak Your Sample. Using a knife, carefully gather some moss and/or lichen samples, about enough to fit in the palm of your hand. Put each sample in its own bag and label the rough location/date. Then, about eight hours before you’re ready to look for tardigrades, put one sample of the moss/lichen in a Petri dish and add about 20 mL of distilled water.

Step 2: Enter a New World. Come back to your Petri dish after eight hours (or at least four) have passed, remove the biggest pieces of lichen or moss, and put your Petri dish under the stereo microscope under the lowest setting (usually 15x). Look around. You’ll probably be overwhelmed: The microworld is both unfamiliar—quartz grains look like gemstones—and oddly enough, seemingly expansive. (A Petri dish doesn’t feel very small when you scan all of it at 15x or 30x!) Your scene will also be messy. Moss and lichen aren’t tidy places; they are full of dirt, plant bits, bacteria, and lots of other living things.

When you first look around, start out at 15x magnification and focus the microscope on the bottom of the Petri dish. Then watch for movement. It’ll be hard to make out a lot of detail, but once you see movement, switch to the higher magnification. This will help you get a closer look. If you see microorganisms but no movement, they may need more time to awaken.

Step 3: Meet the Residents. At 30x, you’ll be able to see tardigrades and some of their more common microscopic neighbors. Here’s a brief field guide to creatures you might spot:

Tardigrades (Phylum Tardigrada)

• have eight legs, with obvious claws

• move using their legs and claws

Nematodes (Phylum Nematoda)

• look like worms

• wiggly, often thrashing about 

• don’t have legs

Bdelloid Rotifers (Phylum Rotifera)

• tube-shaped

• move by stretching themselves out and then expanding forward

Ciliates (Protists)

• tiny

• often seen “driving around” in the background

Step 4: The Tardigrade Rodeo. To get a close look at a tardigrade, you’ll need to view it under a compound microscope. For that, you’ll need to catch one. I call this the tardigrade rodeo. The tool of the trade for this is an Irwin loop, a tiny, specially made loop that has a stainless steel “lasso” on the end. While looking through the stereoscope at the tardigrade, you maneuver the loop to catch it (this takes a lot of practice), and once you think you have it, you transfer it to a waiting slide with a drop of water on it.

Place the slide (without a coverslip) under the compound scope at the lowest magnification. You’ll need to scan for the tardigrade, and if you don’t find it, it either sank back into the water or you dropped it on your workstation. Even at the lower magnifications on a compound scope, you can see a good deal of detail. After observing your finds, return them to the Petri dish.