Tardigrade

Tardigrades form the animal phylum Tardigrada. They have a compact body with four pairs of legs, each usually ending in claws or pads, and a feeding apparatus suited to piercing or taking up food. Depending on species, they occur in the sea, freshwater, and moist terrestrial microhabitats such as moss and lichen. Although popularly dubbed water bears, they are animals rather than microscopic bears or insects. Scientific reviews report body lengths from tens of micrometres to around two millimetres, with exact size depending on species and life stage.

Many terrestrial tardigrades need a thin film of water to remain active. When conditions become unfavorable, some can enter cryptobiosis: a state in which measurable activity is greatly reduced. Anhydrobiosis is the form associated with loss of water. In drying-tolerant species, the animal may contract into a compact form and undergo physiological changes that reduce damage while water is absent. Tardigrades are not uniformly resistant: ability to survive drying, and the conditions needed to do so, differ among species and populations. Drying rate matters; experimental work has found that some species require gradual dehydration to survive.

There is no single explanation for tardigrade resilience. Research has identified tardigrade-specific intrinsically disordered proteins (TDPs) that contribute to desiccation tolerance in studied species. Boothby and colleagues showed that disrupting particular TDPs reduced tolerance, while expressing them in other cell systems could improve protection from drying; the proteins formed glass-like solids in dry conditions. Other proteins and processes may also contribute, and their roles vary across species. The common claim that trehalose alone explains tardigrade desiccation tolerance is not supported: reported trehalose levels are low or absent in some species, and the wider molecular picture remains under investigation.

Some tardigrades withstand unusually high radiation doses, but results depend on species, physiological state and the measured outcome. A 2024 study across three species found that radiation induced genes involved in DNA repair and a tardigrade-specific protein called TDR1. The protein interacted with DNA, and expressing it in human cells improved resistance to the DNA-damaging drug bleomycin; this is evidence for a possible role, not proof that TDR1 alone accounts for whole-animal radiation tolerance. Earlier space experiments found that tardigrades could survive exposure to space vacuum under experimental conditions, while ultraviolet and other radiation reduced survival. These results do not mean tardigrades can live indefinitely in space: space exposures, species and survival endpoints differ, and active reproduction requires suitable conditions.

Tardigrades are useful subjects for studying how animals respond to dehydration, radiation and other stresses. Their survival strategies have inspired research into cell preservation and stress-resistant materials, but practical applications remain experimental. Comparing species and carefully defining the exposure and outcome are essential: spectacular survival records can otherwise obscure the limits of what a particular experiment demonstrated.