Welcome to the Wonderful World of Tardigrades (Our World)

By Lambert Strether of Corrente.

For whatever reason, tardigrades came across my timeline, so I thought I’d make my contribution to the zeitgeist. Cute?

(Via) I’d say so. However, the tardigrade (scientific name, Tardigrada) is so prolific of amazing factoids that most writers find it very difficult to get their arms around the topic, and end up writing surveys that seem exhaustive but aren’t, or are listicles like “8 Keen Things About the Water Bear” (tardigrades are prolific of names[1], as they are of everything else). Those writers include me. After giving links for some of the better stories, and then lazily proffering a brief extract about the tardigrade and its amazing talent for survival under extreme conditions, I’ll do my own listicle, but with list items that I don’t think anyone else would consider, so at least there will be some value add, here. I’ll conclude with a disquieting, and recent, factoid about tardigrade survival.

If you want to get a quick handle on tardigrades, here are some links:

Tardigrades return from the dead BBC

Tardigrades — the microscopic, oddly cute toughest animals on Earth — explained Vox

8 Reasons Why We Love Tardigrades Live Science

Absurd Creature of the Week: The Incredible Critter That’s Tough Enough to Survive in Space Wired

Water bear genomes start to reveal hardy critter’s secrets Chemical and Engineering News

Tardigrades have their own website, Tardigrada. They also occupy a place in the popular imagination, as shown by 2017’s Season 21 (really?), Episode 8 of South Park, “Moss Piglets” (their third name):

(Relevant portion ends at 00:59.) Wacky shenanagains!

Now to the extracts. From the BBC, “Tardigrades return from the dead“:

There are 900 known species. Most feed by sucking the juices from moss, lichens and algae. Others are carnivores, and can even prey on other tardigrades.

They are truly ancient. Fossils of tardigrades have been dated to the Cambrian period over 500 million years ago, when the first complex animals were evolving.

Tardigrades, therefore, have survived through the entire Phanerozoic Eon, including the Paleozoic, Mezozoic, and Cenozoic Eras (which includes our own day). Impressive. They have adapted to survive in many conditions. Their most impressive display of adaptability[2] is the tun[3]. From Vox, “Tardigrades — the microscopic, oddly cute toughest animals on Earth — explained“:

When removed from water and dried out, tardigrades can transform into a cellular fortress, tucking in their legs and head, forming a compact pill shape called a “tun.”

In this tun state, the tardigrades produce glycerol (antifreeze), and also secrete trehalose, a simple sugar with remarkable preservation properties. “Trehalose is viewed as a cocoon that traps the biomolecule inside a glassy matrix, like amber-encasing insects,” explains a 2009 paper in Protein Science. When the trehalose crystalizes, the tardigrade becomes mummified in a glass suit of armor.

A more detailed explanation of tuns comes from Rachel Armstrong, Simone Ferracina, and Rolf Hughes in “Liquid Life: On Non-Linear Materiality” (PDF):

At times of environmental stress, tardigrades reduce their metabolic activity to as low as one-hundredth of normal levels. This state of suspended animation is further conserved within a glass-like structure (Stromberg 2012) from which they may be fully revived after a few hours of hydration. This amazing capacity to enter into a vitrified state is conferred by tardigrade-specific intrinsically disordered proteins (TDPs). The ‘glass coffin’-producing capacity of TDPs is so powerful that tardigrades must ‘wear’ protection at all times by having a thin coating of water around their bodies. In their hydrated form, TDPs are jelly-like and lack the typical well-defined three-dimensional structures of most known proteins, but during desiccation, these proteins solidify into a glassy structure, enabling them to survive for decades in a state of cryptobiosis. Glass-coated tardigrades can withstand irradiation, boiling liquids, extreme pressure, environments as cold as −200°C and up to around 150°C, as well as the vacuum of space without any protection (Coghlan 2017a). While ‘extremophiles’ are physically adapted to life in extreme environments, tardigrades are not. They are simply able to weather disaster through a vitrification escape route, by becoming liquid stone.

(I’m not sure a glass suit of armor would be very protective, and glass coffin seems to preclude revivification, which is false. Liquid stone is good, though. Once again, tardigrades inspire prolificity.)

Finally, the most amazing factoid of all. From New Scientist, “‘Water bears’ are first animal to survive space vacuum“:

To further test their hardiness, Ingemar Jönsson of Sweden’s Kristianstad University and colleagues launched two species of dried-up tardigrades from Kazakhstan in September 2007 aboard ESA’s FOTON-M3 mission, which carried a variety of experimental payloads.

After 10 days of exposure to space, the satellite returned to Earth. The tardigrades were retrieved and rehydrated to test how they reacted to the airless conditions in space, as well as ultraviolet radiation from the Sun and charged particles from space called cosmic rays.

The vacuum itself seemed to have little effect on the creatures. But ultraviolet radiation, which can damage cellular material and DNA, did take its toll.

In one of the two species tested, 68% of specimens that were shielded from higher-energy radiation from the Sun were revived within 30 minutes of being rehydrated. Many of these tardigrades went on to lay eggs that successfully hatched.

But only a handful of animals survived full exposure to the Sun’s UV light, which is more than 1000 times stronger in space than on the Earth’s surface.

Before this experiment, only lichen and bacteria were known to be able to survive exposure to the combination of vacuum and space radiation.

“No animal has survived open space before,” says developmental biologist Bob Goldstein of the University of North Carolina at Chapel Hill, who was not affiliated with the study. “The finding that animals survived rehydration after 10 days in open space – and then produced viable embryos as well – is really remarkable.”

It is! And now to our listicle.

(1) Tardigrades Don’t Seem to be Good for Anything

From Farmers Weekly, “What’s in a handful of soil?“:

Scientists have studied tardigrades for more than two centuries, but still have not found any specific medical, commercial or environmental uses for these creatures.

From Kansas School Naturalist:

In the 200 years since the waterbear was first described, we have not identified any specific medical, commercial, or environmental effect of tardigrades.

From the Global Soil Biodiversity Atlas (PDF):

Although their ecological role has not yet been fully evaluated, recent studies suggest they could have a regulatory function for plant-parasitic nematode populations when predatory nematodes have disappeared, due to predation pressure and/or unfavourable environmental conditions.

Unsurprisingly, tardigrades eat things. And from Wikipedia, they are also eaten:

Tardigrades work as pioneer species by inhabiting new developing environments. This movement attracts other invertebrates to populate that space, while also attracting predators

The last two, especially, seem a little thin. As you can tell from the sourcing, I originally thought I would write a post about tardigrades and the soil: Maybe how many tons of tardigrades there are in the biomass, maybe two plots compared, one with tardigrades, one not, but if any such material exists, I couldn’t find it.

It’s remarkable that such a prolific creature isn’t good for anything! [4]

(2) Tardigrades Lost Out to C. Elegans as a Model Organism

From the American Scientists, “Tardigrades“:

At one time water bears were candidates to be the main model organism for studies of development. That role is now held most prominently by the roundworm Caenorhabditis elegans, the object of study for the many distinguished researchers following in the trail opened by Nobel Prize laureate Sydney Brenner, who began working on C. elegans in 1974. Water bears offer the same virtues that have made C. elegans so valuable for developmental studies: physiological simplicity, a fast breeding cycle and a precise, highly patterned development plan. Some species may, like C. elegans, be eutelic, meaning that the organisms retain the same number of cells through their development. Tardigrades have somewhere over 1,000 cells. I and others use water bears as a model educational organism to teach a wide range of principles in life science.

That’s unfortunate, using aesthetics as a hermeneutic. If tardigrades aren’t good for anything, roundworms really aren’t good for anything. Maybe — farfetching freely — we would all have glass armor by now if tardigrades had been the object of study by thousands of scientists!

(3) Tardigrades Can Be Found in Your Backyard

From Microcosmos, “How to Find Tardigrades (Water Bears) in Your Own Backyard.” I’m just going to give the list of materials, so you can get the flavor of the project. The documentation is pleasingly detailed, and illustrated with photographs of every step:

Here are the materials you will need:

• Paper bags
• Razor blade or pocket knife (optional)
• Pen or Marker
• GPS-enabled device (optional) – for coordinates
• Bag or backpack (optional) – to hold samples
• Paper towels (optional)
• 2 oz plastic cup (any small cup or container will do, but I prefer this size)
• Pipette or eye-dropper (optional)
• Spring water (tardigrades prefer this, but any water should be fine for short-term use)
• Foldscope

(The last item, the “foldscope,” “is an ultra-affordable field microscope, that you build from common materials such as paper. It is designed to be produced affordably, to be durable, and to give optical quality similar to conventional research microscopes.” That is extremely neat.)

This project strikes me as useful to the hobbyist, the citizen scientist, or the teacher.[5]

(4) Tardigrades Are Missing Hox Genes

Tardigrades are cute, but what’s with the body plan? From Current Biology, “The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region“:

Based on our analysis, we conclude that tardigrades have lost a large intermediate region of the body axis—a region corresponding to the entire thorax and most of the abdomen of insects—and that they have lost the Hox genes that originally specified this region. Our data suggest that nearly the entire tardigrade body axis is homologous to just the head region of arthropods. Based on our results, we reconstruct a last common ancestor of Panarthropoda that had a relatively elongate body plan like most arthropods and onychophorans, rather than a compact, tardigrade-like body plan. These results demonstrate that the body plan of an animal phylum can originate by the loss of a large part of the body.

How does such a thing happen? Has it happened elsewhere? (No: “[A] compact body plan is a tardigrade novelty.”) If you read this in a science fiction story you’d find it hard to believe!

(5) Tardigrades Have DNA Protected by Electric Shielding

From New Scientist, “Secret to tardigrades’ toughness revealed by supercomputer simulation“:

The most resilient animal known to science – the tardigrade – is yielding its secrets, with the first work at the atomic level investigating the way the animal survives extreme stress.

In most other organisms, these sorts of stresses destroy the DNA in cells, but tardigrades have a damage-suppressor protein (Dsup) that somehow shields the DNA…

The researchers’ modelling of all the atoms in the protein and all their electrostatic interactions shows that the protein is “intrinsically disordered” and highly flexible, and seems to be able to adjust its structure to precisely fit DNA’s shape.

“Our study reveals that the electrical effects underlying the positive-negative charge attractions determine the dynamics of the structural changes of Dsup in its interaction with DNA,” says Mínguez-Toral. “We believe this electric shielding is paramount in protecting DNA from radiation.”

Figuring out precisely how tardigrades tolerate such extremes could be useful in several ways. “Right now the main applications we are actively working on are the stabilisation of pharmaceuticals and the engineering of stress tolerant crop plants,” says Thomas Boothby, who works with tardigrades at the University of Wyoming.

So, who knows? Maybe tardigrades will turn out to be good for something after all. Take that, c. elegans!

* * *

I cannot but think that in a time of economic precarity and ecological stress, even before the pandemic, the “I will survive!” nature of the tardigrade appealed to something in the zeitgeist beyond cuteness. National Geographic:

Tardigrades are microscopic eight-legged animals that have been to outer space and would likely survive the apocalypse. Bonus: They look like adorable miniature bears.

From Live Science:

A team of scientists considered a series of doomsday scenarios that would be catastrophic for humanity, including nearby supernovas, the expansion of our own sun to a red giant star, and a massive asteroid colliding with Earth. In every scenario, tardigrades were just fine, confirming that when it comes to life on Earth, they are as close to indestructible as it gets, the researchers said in a statement.

Examples could be multiplied. But not so fast. This is a disquieting, and recent, factoid. From Nature, “Thermotolerance experiments on active and desiccated states of Ramazzottius varieornatus emphasize that tardigrades are sensitive to high temperatures“:

Here, we investigate the tolerance to high temperatures of Ramazzottius varieornatus, a tardigrade frequently found in transient freshwater habitats. Using logistic modelling on activity we evaluate the effect of 24 hour temperature exposures on active tardigrades, with or without a short acclimation period, compared to exposures of desiccated tardigrades. We estimate that the 50% mortality temperature for non-acclimated active tardigrades is 37.1 °C, with a small but significant increase to 37.6 °C following acclimation. Desiccated specimens tolerate much higher temperatures, with an estimated 50% mortality temperature of 82.7 °C following 1 hour exposures, but with a significant decrease to 63.1 °C following 24 hour exposures. Our results show that metabolically active tardigrades are vulnerable to high temperatures, yet acclimatization could provide a tolerance increase. Desiccated specimens show a much higher resilience—exposure-time is, however, a limiting factor giving tardigrades a restricted window of high temperature tolerance. Tardigrades are renowned for their ability to tolerate extreme conditions, but their endurance towards high temperatures clearly has an upper limit—high temperatures thus seem to be their Achilles heel.

Well, if the earth’s surface temperature is 82.7°C (180.86°F) none of us are going to be worrying very much about the tardigrades. As for 37.6°C (99.68°F), that’s awfully close to equatorial temperatures now. Perhaps there are studies on tardigrades in tropical soils, though a cursory Google search doesn’t turn up any. Nevetheless, from Live Science:

“Tardigrades are definitely not the almost-indestructible organism as advertised in so many popular science websites,” said Ricardo C. Neves, a postdoctoral scientist in biology at the University of Copenhagen, who co-authored the new paper on tardigrade toughness [quoted above]

So, perhaps another use for tardigrades is as a sentinel species, since conditions that would kill a tardigrade would kill anything else. Personally, though, I think the tardigrades will make it through this era, the Cenozoic, into the next, whatever that may be. Tardigrades have form. And what a wonderful world, that can evolve such a creature!

NOTES

[1] The tardigrade’s original name, “little water bear,” from its lumbering gait, was coined by German pastor J.A.E. Goeze in 1773. Italian naturalist Lazzaro Spallanzani coined il Tardigrado, meaning “slow-stepper”, because they moved so slowly (or, as Bob and Ray would have it, “Slow. Stepper”). I cannot find who coined of “moss piglet.” The phrase does not occur in my Shorter Oxford English Dictionary.

[2] The tun is just one of the amazingly prolific tardigrade’s survival strategies. From Comparative Biochemistry and Physiology, “New Insights into Survival Strategies of Tardigrades,” a passage so dense I’m just going to quote it without comment. It’s the caption to Figure 2.

Fig. 2.Schematic illustration of survival strategies in tardigrades, including cryptobiosis, diapause and regulation in the active state. Cryptobiosis can be induced by a range of extreme environmental conditions. Anhydrobiosis, osmobioisis and possibly chemobiosis are cryptobiotic sub-states characterized by formation of a tun (compare to Fig. 1). In striking contrast, anoxybiosis (prompted by oxygen depletion) and cryobiosis (induced by extremely low temperatures) do not lead to tun formation. A turgid and elongated body characterizes oxygen depleted tardigrades, while tardigrades in cryobiosis may have a partly contracted body. Importantly, true cryobionts seem to lack a lower lethal temperature. Diapause in the form of encystment and cyclomorphosis implicate cyclic changes in the morphology and physiology of the tardigrades and, noteworthy, are directly related to the molting process. The hatching of resting eggs requires an environmental cue such as rehydration after a period of desiccation. In addition, tardigrade eggs may also enter any of the cryptobiosis forms endured by adults. Active state tardigrades can be extremely tolerant to environmental stress, osmoregulating when facing large fluctuation in external salinity, avoiding freezing by supercooling and handling extreme levels of ionizing radiation. Interestingly, however, tardigrades are sensitive to high temperatures and even the otherwise highly resistant tun, seems to have a restricted timeframe for high temperature tolerance.

I count five, in addition to the tun. No wonder the tardigrave has survived our entire eon!

[3] From the Online Etymology Dictionary, tun (n.) “Large cask,” especially one for wine, ale, or beer, Old English tunne “tun, cask, barrel,” a general North Sea Germanic word (compare Old Frisian tunne, Middle Dutch tonne, Old High German tunna, German tonne), also found in Medieval Latin tunna (9c.) and Old French tonne (diminutive tonneau); perhaps from a Celtic source (compare Middle Irish, Gaelic tunna, Old Irish toun “hide, skin”)

[4] Unsettlingly, because of what we hear about billionaires’ thirst for blood, tardigrades are being investigated to see if one of the sugars they manufacture for their “glass armor” can be used to help store dehydrated blood for long periods.

[5] I read, but could not find again, a professor’s story of how they gave a student the project of finding a new species of tardigrade, which they did, then gave it a scientific name, and then published a paper on it. Learning, as opposed to credentialing.