What are tardigrades and why are they nearly indestructible?

Tardigrades, often called water bears or moss piglets, are near-microscopic aquatic animals with plump, segmented bodies and flattened heads. They have eight legs, each tipped with four to eight claws or digits, and somewhat resemble the hookah-smoking caterpillar from "Alice in Wonderland." Though tardigrades are disarmingly cute, they are also nearly indestructible and can even survive in outer space.

Tardigrades were discovered in 1773 by the German zoologist Johann August Ephraim Goeze, who dubbed them "little water bear." Three years later, Italian biologist Lazzaro Spallanzani named the group "Tardigrada," or "slow stepper," for their toddling gait, according to the Science Education Resource Center at Carleton College (SERC). There are currently about 1,300 known tardigrade species within the Tardigrada phylum (a classification category) according to the Integrated Taxonomic Information System (ITIS), a resource for species names and classifications created by a partnership of U.S. federal agencies.

What makes tardigrades so indestructible?

Tardiges have an unusual strategy for surviving harsh conditions: They enter an almost death-like state called cryptobiosis, expelling more than 95% of the water from their bodies, retracting their heads and legs and curling into a dehydrated tun. 

By the 1970s, scientists determined that different forms of cryptobiosis in tardigrades could be caused by four environmental triggers: desiccation, freezing, lack of oxygen and excess salt, reported a 2020 study published in the journal Scientific Reports.

During cryptobiosis, a tardigrade's metabolic activity drops to as little as 0.01% of normal levels. Its cells are protected from damage by water-soluble proteins that are unique to tardigrades, known as tardigrade disordered proteins, or TDPs. When tardigrades expel their body's water, TDP molecules form a tough, glasslike cocoon around cells. This keeps cellular material safe while the tardigrade is a tun and enables it to reanimate in water when conditions are more hospitable, according to a 2017 study published in the journal Molecular Cell. 

"Tardigrades are fascinating little beasties," Sandra McInnes, a tardigrade researcher with the British Antarctic Survey, who has been studying species that occur in the frozen snowscapes of Antarctica since 1980, previously told Live Science. "Tardigrades have this ability to cope with extreme environments by shutting down their metabolism. This ability to cope with drying out or freezing is what gives them their durability in the Antarctic."

However, tardigrades do have a fatal weakness: They wilt under heat, which could be a problem as climate change increases temperatures. A 2020 study published in the journal Scientific Reports found that tardigrades in water temperatures of about 100 degrees Fahrenheit (37.8 degrees Celsius) can die in just one day.

"Tardigrades are definitely not the almost-indestructible organism as advertised in so many popular science websites," study co-author Ricardo C. Neves, a postdoctoral scientist in biology at the University of Copenhagen, previously told Live Science.

How big are tardigrades?

Water bears can range from 0.002 to 0.05 inches (0.05 to 1.2 millimeters) long, but they usually don't get any bigger than 0.04 inches (1 mm) long, according to the World Tardigrada Database.

A tardigrade's body typically consists of only 1,000 cells, according to an article published in the journal Arthropod Structure and Development in 2019. In comparison, the human body is made up of many trillions of cells.

Where do tardigrades live?

As their name implies, water bears live just about anywhere there's liquid water, inhabiting the ocean, freshwater lakes and rivers, and the water film that coats terrestrial mosses and lichens. 

They can survive a wide range of environments: from altitudes of over 19,600 feet (6,000 meters) in the Himalayan mountain range to ocean depths more than 15,000 feet (4,700 m) below the surface, according to the University of Michigan's Animal Diversity Web (ADW). 

Related: Tardigrades probably see in black and white 

Not all tardigrades live in extreme environments, but water bears are known for surviving extreme conditions that would kill most other forms of life, by transforming into a dehydrated ball known as a tun. 

What extremes can tardigrades survive?

Researchers have found that tardigrades in a tun state can withstand temperatures as low as minus 328 degrees Fahrenheit (minus 200 degrees Celsius) and hotter than 300 degrees F (148.9 C), Smithsonian magazine reported. They can also survive exposure to radiation, boiling liquids, and up to six times the pressure of the deepest part of the ocean, according to the Science Education Resource Center at Carleton College in Minnesota. 

A 2008 study published in the journal Current Biology revealed that some species of tardigrades — when dehydrated — could weather a 10-day trip into low-Earth orbit, and return to Earth unharmed by solar ultraviolet radiation and the vacuum of space. 

More recently, desiccated tardigrades have been shot from a high-speed gun, traveling nearly 3,000 feet per second (900 meters per second) and surviving a crushing impact of about 1.14 gigapascals of pressure. 

Their findings, however, suggest that several thousands of tun-state tardigrades that were carried on the Israeli lunar mission Beresheet would not have survived after the lander crashed on the moon, on April 11, 2019. The shock pressure of the metal lander hitting the moon would've been "well above" the limits tardigrades could survive. "We can confirm they didn't survive," Alejandra Traspas-Muina, who led the research as a PhD student at the Queen Mary University of London, told Science magazine.

How do tardigrades enter a 'tun' state?

While scientists have long known that to survive extremes, tardigrades enter their tun state through a metamorphosis known as anhydrobiosis, exactly how they do this has been a longstanding mystery. In a study published in the journal PLOS One in 2024, researchers discovered the molecular underpinnings that allow them to enter their near-invincible state. 

The team exposed the tardigrade species Hypsibius exemplaris to a series of life-threatening conditions, such as dangerous levels of hydrogen peroxide and temperatures of minus 112 degrees Fahrenheit (minus 80 degrees C) and measured the chemical environment inside the tardigrades' cells. 

They found tardigrades produce free radicals — oxygen atoms with an additional unpaired electron that emerge in animal cells during a phase known as oxidative stress. In most animlas, this is harmful, but in tardigrades, free radicals react with the amino acid cysteine to transform them into their tun state. When they inhibited the cysteine oxidation process, the tardigrades were incapable of entering the tun state. 

What do tardigrades eat?

Most tardigrades suck fluids from cells in plants, algae and fungus, puncturing cell walls with needlelike stylets in their mouths and hoovering up the liquid inside. 

However, some species can consume entire living organisms, such as rotifers, nematodes and even other tardigrades, according to Illinois Wesleyan University's Species Distribution Project (SDP).

How do tardigrades reproduce?

Reproduction in tardigrades may be sexual or asexual, depending on the species. For egg-layers, females produce up to 30 eggs at a time, and eggs may be fertilized either inside the female's body; in her shed cuticle after the male ejaculates his sperm there; or while attached to sand or substrate, according to ADW. Other tardigrade species are self-fertilizing hermaphrodites that reproduce through parthenogenesis — a process in which an embryo develops without external fertilization. 

Embryos typically are fully developed within 14 days of fertilization, though their development can last up to 90 days depending on environmental conditions such as dryness and temperature, according to ADW. Young tardigrades do not have a larval stage and resemble miniature adults upon hatching, though they usually have fewer claws and spines than fully-grown water bears do. The youngsters grow in several stages by molting their external cuticle "skin," and each molt can take five to ten days to complete.

Tardigrade anatomy

Inside the tardigrade's tiny body, you won't find any bones, according to research published in the Proceedings of the National Academy of Sciences (PNAS) in August 2021. Instead, they are supported by a hydrostatic skeleton. This is a fluid-filled compartment called a hemolymph. Similarly to human blood, the hemolymph is filled with nutrients. 

Although they lack a spinal cord, tardigrades have a ventral nervous system, according to the book Forest Canopies, published in 2004. This sends signals between the tardigrade's brain and body and is the functional equivalent of a vertebrate's spinal cord. 

Water bears have a complete digestive system, but no circulatory or respiratory system. Instead, oxygen from the water enters their bodies through their cuticle walls. To aid circulation, they have muscles which contract to transport the nutrients in their hydrostatic skeleton.

Tardigrade tuns can be revived even after decades have passed. In 2016, scientists revived two tuns and a tardigrade egg that had been in cryptobiosis for more than 30 years, Live Science previously reported. Reanimation from even longer tun states might be possible. In 1948, a researcher in Italy purportedly revived a tun from a dried-out piece of moss that was over 120 years old, the BBC reported in 2015. However, no other researcher has since reanimated a tardigrade from a tun that old, according to the BBC.

And in some tardigrades, fluorescence could lend them protection against radiation by transforming UV rays into harmless blue light, Live Science previously reported. 

Are tardigrades endangered?

Tardigrades have not been evaluated by the International Union for Conservation of Nature, the global organization that monitors conservation status for animals and natural habitats; and they aren't on any other endangered list. In fact, tardigrades have survived all five mass extinctions on Earth since the group evolved about half a billion years ago, according to the University of Wisconsin Madison, and water bears could survive after humanity is long gone, researchers found.

Related: The 5 mass extinction events that shaped the history of Earth 

In 2017, scientists from Harvard and Oxford universities looked at the probabilities of certain astronomical events — Earth-pummeling asteroids, neighboring supernova blasts and gamma ray bursts, to name a few — that could take place over the next several billion years. Then, they evaluated the likelihood of those events wiping out Earth's hardiest species. While such disasters would likely eradicate humans, the researchers found that little tardigrades would survive most cosmic cataclysms, they reported in their study published in the journal Scientific Reports.

"To our surprise, we found that although nearby supernovas or large asteroid impacts would be catastrophic for people, tardigrades could be unaffected," David Sloan, a co-author of the study and a researcher at Oxford, said in a statement.