The Remarkable Resilience of Tardigrades
A tardigrade dropped into liquid helium at -272°C, boiled in a beaker, irradiated with a dose that would kill a human a thousand times over, or exposed to the raw vacuum of low Earth orbit will, in most cases, do the same thing: pull its eight legs inward, fold its cuticle into a wrinkled barrel about a quarter of a millimetre long, and wait. Drop water on it hours or decades later and the legs unfurl. The animal walks away.
The barrel is called a tun. The waiting is called anhydrobiosis, a reversible metabolic shutdown that researchers describe as a near-complete pause on being alive. And the substance doing most of the structural work inside the tun appears to be a glassy matrix of disordered proteins and sugars that takes over the jobs water used to perform. It holds membranes apart, keeps proteins from collapsing into useless tangles, and shields DNA from the chemical havoc of dehydration.
The Animal That Should Not Exist
Tardigrades, often affectionately called water bears, are about half a millimetre long, eight-legged, and related to arthropods. Under the microscope, they appear almost comically chubby. Their ubiquity is astonishing—they’ve been documented in soil, moss and rain gutters across Denmark, where environmental DNA surveys consistently reveal many tardigrade sequences that don’t match any known species.
These resilient creatures inhabit some of the most extreme environments on Earth, from the icy peaks of Mount Everest to the crushing depths of deep-sea trenches. Thousands have even been aboard the International Space Station, enduring the rigours of space travel. In 2019, a payload of tardigrades was inadvertently spilled across the lunar surface when the Israeli Beresheet lander crashed.
Despite this reputation for extreme survival, none of these hostile environments are where tardigrades thrive. Given the choice, they prefer a damp film of water on a piece of lichen. Their extraordinary resilience shines through only when water disappears.
Inside the Tun: A Metabolic Pause
When drying begins, a tardigrade contracts, actively folding its cuticle inward through muscular action. This reduces its volume and surface area, drastically slowing water loss. Inside the cell, a remarkable transformation unfolds.
In hydrated tardigrades, two families of proteins—CAHS (Cytoplasmic Abundant Heat Soluble) and SAHS (Secretory Abundant Heat Soluble)—float freely without fixed structure. These intrinsically disordered proteins are floppy and shapeless, doing little under normal conditions. As drying progresses, they snap into amphiphilic α-helices and assemble into a robust internal scaffolding. Genomic studies on Ramazzottius varieornatus have identified six families of desiccation-related proteins, with CAHS being the largest. Notably, this species keeps its protective machinery on standby, rather than activating genes only upon stress.
This scaffolding, along with sugars such as trehalose found in many tardigrade species, replaces the hydrogen bonds normally maintained by water. This prevents membranes from fusing, keeps proteins properly folded, and vitrifies the cell interior into a rigid, transparent, and chemically inert glass-like state—a process called vitrification. In this state, all measurable metabolic activity halts.
Two Species, Two Survival Strategies
Not all tardigrades employ identical tactics. Comparative genomics of Hypsibius dujardini and Ramazzottius varieornatus reveals two distinct approaches. H. dujardini requires a prolonged preconditioning period, ramping up expression of hundreds of genes before it can survive drying. In contrast, R. varieornatus simply dries, as its protective proteins are already primed.
This latter strategy, enabling survival of sudden desiccation without warning, garners the most attention. It is also why R. varieornatus is frequently used in radiation experiments, vacuum exposure trials, and genetic studies isolating the remarkable Dsup protein.
The Protein That Hugs Your Chromosomes
Dsup, short for damage suppressor, is a tardigrade-specific protein that binds directly to nucleosomes—the DNA spools comprising chromatin. When a cell undergoes dehydration or irradiation, damaging hydroxyl radicals form and can cleave DNA strands. Dsup acts as a physical shield, latching onto chromatin and absorbing these radicals before they damage the genetic material.
Interestingly, the conserved domain allowing Dsup to bind nucleosomes is structurally similar to vertebrate nucleosome-binding motifs. This similarity has led laboratories focused on radiation therapy and long-duration spaceflight to attempt splicing Dsup into human cells. While some studies suggest Dsup-expressing human cells exhibit reduced DNA damage under X-rays, they do not gain the near-indestructibility of tardigrades.
A Trick at Least 250 Million Years Old
Tardigrade fossils are rare, with only four amber-trapped specimens ever discovered. In 2024, Marc Mapalo of Harvard’s Museum of Comparative Zoology and colleagues re-examined two tardigrades encased in a single Canadian amber pebble dated between 84 and 72 million years ago. High-contrast microscopy of their claws—a key taxonomic feature—revealed a new genus and species, Aerobius dactylus, and led to a reclassification of the previously described Beorn leggi.
By placing these species on an updated family tree, the team inferred that the two main tardigrade lineages capable of cryptobiosis diverged during the Carboniferous period, between 359 and 299 million years ago. This timeline situates the evolution of their suspended-animation ability before the Permian extinction—also known as the Great Dying—252 million years ago, which wiped out approximately 96% of marine species and 70% of terrestrial life.
Tardigrades survived this cataclysmic event by curling into tuns and waiting it out.
How Long Can the Wait Be?
The honest answer is: no one truly knows the upper limit. Tardigrades have been revived after decades of cryptobiosis, particularly Antarctic moss-dwelling species reanimated from frozen moss samples. While cryptobiosis is not unique to tardigrades—certain nematodes, rotifers, and brine shrimp also undergo similar states—recent research has pushed the boundaries of dormancy. In 2023, scientists radiocarbon-dated permafrost sediments containing nematodes revived after an astonishing 46,000 years of frozen dormancy.
Though mechanisms vary, the principle remains consistent: remove water, vitrify the interior, halt metabolic chemistry, and wait for favorable conditions to return.
The Limits of Indestructibility
Despite their legendary toughness, tardigrades are fragile in their active, hydrated state. A gentle squeeze with tweezers can kill them, changes in pH are lethal, and hungry rotifers prey on them. Their extreme survival abilities apply only when in tun form.
Even then, mortality exists. The 2007 FOTON-M3 experiment exposing tardigrades to the vacuum of low Earth orbit found that while most survived vacuum alone, the combination of vacuum and unfiltered solar UV radiation was fatal to the majority. Similarly, boiling water kills most tardigrades that have not been desiccated first. The secret to their survival lies in the glass-like matrix formed during a controlled descent into dormancy.
These challenges have driven researchers to develop novel methods to study tardigrades without destroying them. In April 2025, a technique was reported for tattooing tardigrades with micrometre-scale patterns of biocompatible ink while they are in their tun state. This innovative method may enable tracking individual specimens through multiple drying and rehydration cycles.
Why Anyone Other Than a Biologist Cares
The extraordinary survival mechanisms of tardigrades have clear applications in biomedicine and pharmaceuticals. If CAHS proteins can vitrify a tardigrade’s cellular interior, they might be used to stabilize vaccines without refrigeration—a critical advancement for global health. Several laboratories are trialing dry-stabilized blood products and biologics using tardigrade-derived proteins, with promising but preliminary results. Agricultural scientists are also exploring whether expressing these proteins in crop plants could help them survive droughts that currently devastate yields.
Space agencies have a particular interest as well. Biological cargo aboard spacecraft traveling years through interplanetary space must be protected from radiation and temperature extremes. A tardigrade in a tun represents an ideal model passenger. The Beresheet lunar payload was an early, albeit somewhat reckless, test of this concept.
The Thing on the Moss
None of these wonders require expeditions to the Himalayas or the International Space Station. Tardigrades are everywhere—in moss on roof tiles, lichen on park benches, and leaf litter behind garden sheds. Scrape a patch of dry moss into a dish of water and view it under a 40x microscope, and chances are high you’ll see one, walking with the rolling, deliberate gait that earned them their German name, Bärtierchen, meaning little bear.
The numbers behind their revival are staggering. Cryptobiosis-capable lineages date back 359 to 299 million years. Nematodes have emerged from 46,000-year-old permafrost. Tardigrades withstand ionizing radiation doses roughly a thousand times the human lethal limit, the vacuum of low Earth orbit as shown by FOTON-M3 in 2007, and temperatures from near absolute zero up to brief spells above 150°C.
Rehydration is swift. Add water to a dry tun, and within minutes to hours, the cuticle unfolds, CAHS scaffolds dissolve, trehalose redistributes, membranes reseat, and metabolism resumes exactly where it left off. Whether the wait was ten minutes or ten years leaves no measurable trace on the animal’s physiology.
For more detailed insights into the extraordinary capabilities of tardigrades, see the source article Here.
