Tardigrade

Presentation

Tardigrades form a phylum of microscopic aquatic invertebrates that typically measure less than a millimeter in length. Their segmented bodies bear four pairs of short legs tipped with claws, earning them the nickname 'water bear'. They live in the thin film of water surrounding mosses, lichens, and sediments across all continents, including polar regions. They are renowned for their exceptional ability to withstand desiccation and extreme conditions through a state of suspended animation called cryptobiosis. First discovered in the eighteenth century, more than a thousand species have now been described. They are inconspicuous animals, invisible to the naked eye, but studied intensively in extreme biology and astrobiology.

The tardigrade body is short, cylindrical, and segmented into four sections, each bearing a pair of clawed legs. They possess a flexible cuticle that they shed periodically as they grow. Their digestive system includes a piercing stylet used to suck out the contents of cells they feed on. They lack a differentiated respiratory or circulatory system; gas exchange occurs by diffusion across the cuticle.

Characteristics

Indicative values: they vary by subspecies, sex and region.

Habitat and distribution

Tardigrades live in the film of water surrounding mosses, lichens, damp soil, and sediments. They are found in both freshwater and coastal marine environments, from mountain peaks to the ocean floor. Their main requirement is the presence—even if intermittent—of moisture around their body.

Behaviour

Tardigrades move slowly across damp substrates using their clawed legs, which is the origin of their name meaning 'slow walker'. When faced with desiccation, extreme cold, or oxygen deprivation, they enter cryptobiosis: they expel nearly all water from their body and contract into a form called a 'tun', suspending their metabolism. This state can persist for years or even decades before the animal reactivates when moisture returns. Outside these periods, their activity is limited to feeding and reproduction.

Reproduction

Depending on the species, reproduction is either sexual or parthenogenetic, with females capable of reproducing without fertilization. Eggs are laid in the old cuticle during molting or directly into the environment. Development is direct, with no larval stage; juveniles resemble miniature adults. Growth continues through successive molts until adult size is reached.

Peculiarities

Tardigrades tolerate temperatures ranging from near absolute zero to over 150°C, as well as extreme pressures and radiation doses far exceeding the human lethal dose. Individual specimens have survived direct exposure to the vacuum of space during experiments in orbit. This resistance makes them a model organism for studying survival biology and astrobiology.

Threats and conservation

As a very widely distributed and resistant group, tardigrades are not considered threatened. Localized degradation of damp micro-habitats, such as desiccation of mosses in urban settings, can affect local populations without threatening the phylum as a whole. Their global conservation status has not been evaluated due to their abundance and inconspicuous nature.

References

Profile updated 31 August 2026 · Reviewed by the TunisieVet editorial team.

CONSERVATION STATUS

Data deficient (DD)

IUCN source, latest published assessment.

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