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.”
1. They’re 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. They Are Their Own Phylum
Tardigrades comprise an entire phylum of life, which is one taxonomic rank below kingdom.
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. Their 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. They 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. They 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. They’re the First Animal Known to Survive in Outer 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.5 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. They’re 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.7 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. They 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).