Tardigrades: The Microscopic Animals That Can Survive Almost Anything

Eight Legs, Half a Millimeter, Nearly Indestructible

Tardigrades — informally called water bears or moss piglets — measure 0.1 to 1.5 millimeters as adults. They have eight stubby legs, a barrel-shaped body, and a toothed tube for a mouth. They were first described by German pastor Johann August Ephraim Goeze in 1773, named Tardigrada ("slow walkers") by Italian biologist Lazzaro Spallanzani in 1776, and have fascinated biologists ever since. More than 1,300 species have been described. They live on every continent, from Antarctic ice to Himalayan glaciers, from the deepest ocean trenches to the tops of mountains. None of this is why they are famous. They are famous because of what they can survive.

The Tun State: Life on Pause

When environmental conditions become lethal — extreme desiccation, temperature extremes, radiation, vacuum, or toxic chemicals — tardigrades can enter a cryptobiotic state called the tun. The process takes several hours. The animal retracts its legs, contracts into a barrel shape, loses up to 97% of its body water, replaces water molecules in its cells with a sugar called trehalose that forms a glass-like protective matrix, and reduces its metabolism to less than 0.01% of normal activity. The result is a desiccated, inert structure that is alive — but barely.

In the tun state, tardigrades produce large amounts of specific proteins called tardigrade-specific intrinsically disordered proteins (TDPs). These proteins — unique to tardigrades and found in no other animals — vitrify when dried, forming a glassy matrix that physically protects cellular structures from damage. Research published in 2017 in PLOS Biology demonstrated that introducing tardigrade TDP genes into yeast and human cell cultures made those cells more resistant to desiccation — suggesting the tardigrade survival mechanism could theoretically be transferred to other organisms.

Documented Survival Extremes

The Space Experiment

In 2007, ESA's FOTON-M3 mission carried tardigrades (Richtersia coronifer and Milnesium tardigradum) in external containers exposed to open space — vacuum, ultraviolet radiation, and cosmic rays — for 10 days. After return, rehydration revived the majority of animals from the M. tardigradum group. Many of those exposed to full solar UV — the most damaging component — died, but a significant proportion survived the vacuum and cosmic ray exposure alone. This remains the only confirmed case of animals surviving open-space exposure without any physical shielding.

In 2019, an Israeli lunar lander called Beresheet crashed into the Moon while carrying a payload that included thousands of desiccated tardigrades in tun state. The scientific consensus is that the tardigrades survived the crash intact — desiccated and inert on the lunar surface — and might remain viable for decades. Whether they could be revived would require retrieval and rehydration, currently impossible.

Radiation Resistance Mechanisms

Tardigrades survive radiation doses 1,000 times the human lethal dose through multiple mechanisms. Research published in Nature Communications in 2016 identified a tardigrade-specific protein called Dsup (Damage Suppressor) that physically shields DNA from radiation damage by binding directly to chromosomes. Human cells transfected with the Dsup gene showed approximately 40% less radiation-induced DNA damage. This finding triggered substantial interest in potential biomedical applications, including radiation protection for cancer patients undergoing radiotherapy and for astronauts on deep-space missions.

Evolution and Phylogenetic Position

Tardigrades are not insects, not worms, and not crustaceans — they occupy their own phylum, Tardigrada, within the superphylum Ecdysozoa (animals that molt). Their closest relatives are thought to be the velvet worms (Onychophora) and possibly arthropods. Tardigrade fossils are rare due to their soft body and small size, but amber-preserved specimens have been dated to the Cretaceous (approximately 90 million years ago). Molecular clock analyses suggest the phylum is far older — possibly 600 million years, predating the Cambrian explosion.

Distribution and Ecology

Tardigrades are cosmopolitan and abundant where their preferred microhabitats exist — thin films of water on mosses, lichens, soil particles, leaf litter, and marine sediments. A 2019 survey estimated that a square meter of temperate forest soil contains hundreds of thousands of tardigrades. They feed on plant cells, algae, and in some species on other tardigrades. Their ecological role in nutrient cycling and as prey items for soil nematodes and mites is modest but real.

The conditions required to revive a tardigrade from the tun state are simple: add water. Animals that have been desiccated for months or years can resume movement within minutes of rehydration. The record for revival after desiccation is disputed — widely cited figures of 30+ years come from historical museum specimens, with modern controlled studies confirming revival after at least 9 years. The molecular mechanisms underlying this capacity for indefinite suspended animation remain one of biology's most actively studied and genuinely remarkable phenomena.