Tardigrades, also known as water bears, are microscopic animals famous for surviving conditions that kill nearly all other life forms. They are fundamentally aquatic creatures, requiring water for all biological functions in their active state. However, when their environment dries up, they suspend life through a unique mechanism. This allows them to endure prolonged desiccation, emerging unharmed when moisture returns.
Life’s Dependence on Water
Tardigrades are classified as limnoterrestrial, meaning that even species found on land require a thin film of water surrounding their bodies to function normally. This water layer is necessary for essential biological processes like gas exchange, allowing them to take in oxygen and expel carbon dioxide across their cuticle.
Water acts as the universal solvent for metabolic chemical reactions, providing the medium for all cellular activity. It also helps maintain cell structure through turgor pressure, keeping the organism’s tissues functional. Without sufficient water, an active tardigrade cannot feed, move, or reproduce.
Entering the Tun State
When the water film around a tardigrade begins to evaporate, the animal initiates anhydrobiosis, a form of cryptobiosis or suspended metabolism. This survival strategy is triggered by slow desiccation, giving the tardigrade time to prepare its body. The first physical sign is the formation of the “tun” state.
The tardigrade retracts its head and eight legs, rolling its body into a compact, barrel-shaped structure. This shape minimizes the surface area exposed to the air, slowing water loss. During this process, the animal can lose up to 97% of its body water, and its metabolic activity drops to a level less than 0.01% of its normal rate.
The Molecular Survival Strategy
Tardigrades survive extreme dehydration using a sophisticated molecular defense system that replaces the lost water. Unlike many other desiccation-tolerant organisms, which rely on the sugar trehalose, tardigrades primarily use a unique set of proteins. These are called tardigrade-specific intrinsically disordered proteins (TDPs), which include Cytoplasmic-Abundant Heat-Soluble (CAHS) proteins.
TDPs are unusual because they lack a fixed three-dimensional structure in their hydrated state, remaining flexible. As the last remaining water molecules leave the cell, these TDPs condense to form a non-crystalline, amorphous solid known as a “bioglass” or vitreous state. This process, called vitrification, effectively turns the cell’s cytoplasm into a protective, glass-like substance.
The glassy matrix stabilizes and immobilizes all the desiccation-sensitive cellular components, including membranes, proteins, and DNA. By trapping these molecules in a rigid structure, the bioglass prevents them from unfolding, aggregating, or breaking apart, which are the typical causes of death during drying. When rehydrated, water re-enters the body, the bioglass “melts,” and the TDPs revert to their flexible state, allowing the tardigrade to resume active life within minutes to hours.
Beyond Desiccation
The protective tun state and its molecular mechanisms grant tardigrades resistance to a host of other environmental extremes. Anhydrobiosis is one form of cryptobiosis, a broader category of survival states that includes resistance to freezing temperatures. While in the tun, tardigrades can survive being heated to over 149°C (300°F) and frozen to temperatures near absolute zero, or around -272°C.
In this suspended state, they can withstand pressures six times greater than those found in the deepest ocean trenches and survive the vacuum of outer space. The cellular stabilization that prevents damage from dehydration also protects the organism against the physical stress of extreme cold, pressure, and high levels of radiation. This hardiness solidifies the tardigrade’s reputation as one of the most resilient animals on Earth.