The tardigrade is a microscopic invertebrate, widely recognized as one of the most resilient animals on Earth. Commonly known as water bears or moss piglets, these creatures typically grow to about half a millimeter, making them difficult to see without magnification. They inhabit a vast array of environments, from the highest mountain peaks and Antarctic ice to the deepest ocean trenches, but are most often found in the thin film of water on mosses and lichens. This ubiquitous presence is possible because their biological structure allows them to tolerate conditions that would instantly kill nearly all other forms of life. The tardigrade’s fame is due to its unparalleled capacity to endure these extremes by temporarily suspending its life processes.
The State of Cryptobiosis
The strategy a tardigrade employs to survive catastrophic environmental changes is entering a reversible, ametabolic state called cryptobiosis. This is not merely hibernation, but a temporary suspension of all measurable life processes. Cryptobiosis reduces the creature’s metabolism to less than 0.01% of its normal rate and is induced by various stresses, including extreme cold, lack of oxygen, and high salinity.
To enter this state, the tardigrade undergoes a significant morphological change. It contracts its eight legs and head while shriveling its body into a compact, barrel-shaped structure known as a “tun.” In this form, the creature minimizes its surface area and wraps itself in a protective, thickened cuticle.
The tun state grants the tardigrade its legendary durability against otherwise lethal physical forces. It can maintain this state of suspended animation for decades, only to rapidly revive—often within minutes—when conditions become favorable again. This ability to completely shut down and restart is the fundamental mechanism underlying its survival of all other extremes.
Surviving Desiccation
The most studied form of cryptobiosis is anhydrobiosis, the survival mechanism used when a tardigrade’s habitat dries out completely. This process allows the water bear to withstand the loss of up to 97% of its body’s water content. This level of dehydration causes irreversible damage to the cells of most organisms. The primary challenge of desiccation is preventing cellular structures, such as membranes and proteins, from collapsing and aggregating as the necessary surrounding water molecules disappear.
To counter this, some tardigrade species, such as Adorybiotus coronifer, synthesize and accumulate the non-reducing sugar Trehalose. This sugar is thought to replace the lost water molecules, forming a protective, glassy matrix that stabilizes cellular components and preserves their structure. However, many other tardigrade species produce low or undetectable levels of Trehalose, indicating a reliance on alternative protective agents.
A distinct line of defense comes from a family of proteins known as Tardigrade-specific Intrinsically Disordered Proteins (TDPs). These proteins are highly hydrophilic and lack a fixed three-dimensional structure when the tardigrade is active. Upon desiccation, TDPs vitrify, forming a non-crystalline, glass-like solid inside the cells that acts as a structural scaffold. This amorphous glass matrix physically prevents the denaturation and aggregation of other vital biomolecules, allowing the tardigrade to survive for periods that can extend to more than 30 years without water.
Resistance to Physical Extremes
While in the cryptobiotic tun state, the tardigrade can shrug off physical stressors that would be instantly lethal to almost any other animal. Regarding temperature, the creatures can survive brief exposures to both the super-heated and the near-absolute-zero ends of the scale. They have been shown to endure temperatures as high as 151 degrees Celsius (304 degrees Fahrenheit) for a few minutes.
At the opposite extreme, some species can tolerate temperatures as low as -272 degrees Celsius (-458 degrees Fahrenheit), which is only one degree above absolute zero. The structural integrity of the tun, stabilized by its internal protective molecules, is believed to prevent the formation of damaging ice crystals at these freezing temperatures.
Tardigrades also display remarkable resilience to pressure, enduring forces from both extremes. They can survive the crushing pressures found in the deepest parts of the ocean, with some species tolerating up to 6,000 atmospheres. Conversely, they can also survive the vacuum of outer space, as demonstrated by experiments on the FOTON-M3 mission. This survival of vacuum conditions is strongly linked to their capacity for anhydrobiosis, as the space environment rapidly desiccates the organism.
Resilience to Radiation
Tardigrades exhibit an extraordinary tolerance for ionizing radiation, being able to survive doses that are 1,000 to 3,000 times higher than the lethal dose for humans. Radiation primarily causes damage by breaking DNA strands and generating reactive oxygen species within cells. Most organisms rely on DNA repair mechanisms to fix this damage after it occurs.
The tardigrade, however, possesses a unique protective mechanism involving a specific protein called Damage Suppressor (Dsup). This protein is not involved in repairing DNA but rather in physically shielding it from damage in the first place. Dsup binds directly to the chromatin, the complex of DNA and proteins that forms chromosomes.
By binding, the Dsup protein forms a protective physical barrier around the DNA helix. This active shielding reduces the number of initial breaks and suppresses the harmful effects of radiation-induced free radicals. This mechanism is so effective that it provides a high degree of radiotolerance even when the tardigrade is in its active, non-tun state.