The transformation into a tun
In the mosses of Victoria Land, Antarctica, microscopic animals called tardigrades endure some of the most extreme conditions on Earth. Species like Acutuncus antarcticus and the more recently discovered Mopsechiniscus franciscae inhabit these frozen landscapes. To survive, they employ a remarkable process called anhydrobiosis, or "life without water".
When the thin film of water they live in evaporates, the tardigrade must desiccate slowly along with its environment. This triggers a transformation into a desiccated, ametabolic state known as a "tun". The animal retracts its eight legs and head, contracting into a shriveled ball that reduces its surface area and slows further water loss. During this process, it can lose up to 97% of its bodily water.
Once in the tun state, the tardigrade's metabolism slows to almost undetectable levels, a form of suspended animation. This state allows it to withstand environmental stresses that would kill nearly any other animal, including extreme temperatures, radiation, and the vacuum of space.
The physics of biological glass
The secret to the tun's resilience lies in how it protects its cellular machinery from the damage of dehydration. While it was once thought the sugar trehalose was the primary protectant, studies show that tardigrades produce it at low levels. Instead, they rely on a suite of unique, tardigrade-specific intrinsically disordered proteins (TDPs).
As water leaves the cells, these proteins—particularly Cytoplasmic-Abundant Heat Soluble (CAHS) proteins—begin to form a gel-like network. This network eventually solidifies into a non-crystalline, amorphous solid; a biological glass. This process, called vitrification, physically locks cellular components in place.. By turning their insides to glass, tardigrades prevent their proteins and membranes from denaturing, breaking, or fusing, which would otherwise be catastrophic upon rehydration.
This glassy state is what allows the tun to survive temperatures from just above absolute zero (−272°C) to over 150°C. It also provides protection against immense pressure and radiation doses thousands of times higher than what a human can withstand. Other specialized proteins, like the Damage suppressor protein (Dsup), specifically bind to the tardigrade’s DNA, shielding it from radiation and hydroxyl radicals. When water returns, the glass dissolves, and the tardigrade can resume its active life, sometimes within minutes.