10 Astounding Facts About Tardigrades

Tardigrades may be the toughest animals on Earth. They have evolved to live almost anywhere and survive almost anything. Some tardigrades can shrug off conditions that would obliterate most living beings, including extremes far beyond anything found on Earth.

They are also tiny, rotund, and strangely endearing, with nicknames like "water bear" and "moss piglet."

Since we're surrounded by these little juggernauts, and they seem unlikely to go anywhere anytime soon, we might as well get to know them a little better.

1. Tardigrades Are Microscopic, But Just Barely

Tardigrades are near the edge of visibility for most human eyes. A typical tardigrade is about 0.5 mm (0.02 inch) long, and even the largest ones are less than 2 mm (0.07 inch) in length. Some larger tardigrades can be visible to the naked eye, but since they're also see-through, we're unlikely to get a good view without at least a low-power microscope.

2. Tardigrades Are Their Own Phylum

Tardigrades comprise an entire phylum of life, which is one taxonomic rank below kingdom. Other phyla in the animal kingdom include groupings as broad as the arthropods (which includes all insects, arachnids, and crustaceans) and the vertebrates (all animals with backbones).

Tardigrades have been around for at least 500 million years or so, possibly sharing a common ancestor with arthropods. Over 1,000 species are known today, including marine, freshwater, and terrestrial tardigrades.

3. Tardigrade Bodies Are Like Walking Heads

At some point early in their lineage, tardigrades lost several genes involved with producing the head-to-tail body form of animals during development. They have lost a large intermediate region of the body axis, too, lacking segments that, in insects, correspond to the entire thorax and abdomen. According to a 2016 study published in Cell Biology, the tardigrade's body now seems to be made mainly from head segments, making its entire body "homologous to just the head region of arthropods."

4. Tardigrades Can Go Decades Without Food or Water

Perhaps the most famous thing about tardigrades is their uncanny durability. Tardigrades are not immortal, but they have a powerful adaptation that allows them to survive for decades in extreme conditions: cryptobiosis.

To endure environmental stress, tardigrades suspend their metabolism through a process called cryptobiosis. They curl up and enter a death-like state known as a tun. Their metabolism slows to 0.01% of normal, and their water content drops to less than 1%. They survive in this state by replacing the water in their cells with a protective sugar called trehalose, which preserves all the cellular machinery until water is available again.

Tardigrades have different kinds of tun states for different hardships. Anhydrobiosis helps them survive desiccation, for example, while cryobiosis protects against deep freezes. Tardigrades can survive long periods without food or water in a tun, then return to normal once they're rehydrated. Some have been reanimated from a tun after lying dormant for 30 years.

Outside of their tun state, tardigrades have a lifespan of up to two and a half years.

5. Tardigrades Perform Well Under Pressure

Some tardigrades in a tun can handle pressure as high as 600 megapascals (MPa). That's nearly 6,000 atmospheres, or 6,000 times the pressure of Earth's atmosphere at sea level, and it's about six times higher than the pressure found in the planet's deepest ocean trenches. Even half as much pressure, 300 MPa, would kill most multicellular life and bacteria.

6. Tardigrades Are the First Animal Known to Survive in Space

Two tardigrade species flew into low-Earth orbit on the FOTON-M3 mission in 2007, becoming the first animals known to survive direct exposure to space. The 12-day mission included active and desiccated tardigrades, exposing some of each group to either the vacuum of space, the radiation, or both. Exposure to the vacuum was no problem for either species, and the lack of gravity had little effect, either. Some tardigrades even laid eggs during the mission. They were not impervious, though, and the combined effects of the vacuum and UV radiation did take a toll.

Tardigrades also visited the International Space Station in 2011, with similar results pointing to an incredible tolerance of the space environment. In 2019, when the Beresheet probe crashed on the moon, a capsule containing tardigrades in a tun state may have survived the impact, scientists announced. The fate of the tardigrades remains unclear, but even if they are still up there, they can't reanimate without liquid water.

7. Tardigrades Are Resistant to Radiation

Research has shown tardigrades can survive roughly 1,000 times more radiation than a human. They often resist the damage of radiation exposure in both active (hydrated) and tun (desiccated) states, which researchers have noted is a little surprising since the indirect effects of ionizing radiation are expected to be much higher in the presence of water. Being in a tun does seem to confer more protection, though.

Tardigrades have not only survived massive irradiation; they've also gone on to produce healthy offspring following radiation exposure. Researchers believe this is due to tardigrades' abilities to both avoid the accumulation of DNA damage and to efficiently repair the damage that has been done. Still, as some space experiments have shown, even tardigrades have a limit for how much radiation they can take.

8. Tardigrades Aren't Picky About Temperature

Polar tardigrades have survived cooling down to minus 196 degrees Celsius (minus 320 Fahrenheit), and research suggests some might be able to withstand temperatures down to minus 272 C (minus 458 F), or just one degree above absolute zero. More heat-tolerant species, on the other hand, can survive temperatures as high as 151 C (300 F).

9. You Can Find Tardigrades Yourself

Tardigrades can live in almost any kind of environment on Earth. They've been found in hot springs, on top of Himalayan peaks, under layers of solid ice, in tropical rainforests, in mud volcanoes, and at the bottom of lakes and oceans. They're also abundant in many less exotic places, however, such as creeks, meadows, moss patches, leaf litter, stone walls, roof tiles, and even parking lots.

If you have access to a microscope, you could try to find tardigrades near you. The general advice for amateur tardigrade hunters is to collect a small clump of moss or lichen, then place it in a shallow dish to soak in water overnight. Remove the excess water, then lightly shake or squeeze water from the soaked clump into a Petri dish or a similar transparent receptacle. You can then study the water with a stereo microscope at low magnification — 15x to 30x should be enough to see tardigrades.

10. Tardigrades Will Probably Outlive Us

Tardigrades date back at least half a billion years, and they have already survived at least five mass extinctions. Combined with what we know about their tolerance of extreme temperatures, pressure, radiation, dehydration, and starvation, they seem better-equipped to survive any upcoming global disasters than we are.

Scientists have come to that conclusion, too. In a study published in Scientific Reports in 2017, researchers examined the risk that various cataclysmic events might wipe out all life on Earth, focusing on things that may have triggered past mass extinctions: asteroid impacts, supernovae, and gamma-ray bursts. "Surprisingly, we find that although human life is somewhat fragile to nearby events, the resilience of Ecdysozoa such as [tardigrades] renders global sterilization an unlikely event," the researchers wrote.