---
type: Object
title: Tardigrade
description: A microscopic invertebrate ("water bear") that survives near-total desiccation
  through cryptobiosis and has withstood direct exposure to the vacuum and UV radiation
  of space.
aliases:
- water bear
- water bears
- Tardigrada
tags:
- biology
- zoology
- extremophile
- space
resource: raw/feature-article.pdf
timestamp: '2026-07-16T15:09:32Z'
citadel_version: 0.3.0
---

Tardigrades, commonly called water bears, are microscopic invertebrates that endure near-total desiccation by entering [cryptobiosis](../concepts/cryptobiosis.md), a dormant survival state.[^s1]

As a tardigrade dries, it loses roughly 97 percent of its body water and pulls its legs in to form a shrunken, barrel-shaped structure called a "tun".[^s2] To survive, it floods its cells with the sugar trehalose and with tardigrade-specific intrinsically disordered proteins (TDPs); as the animal dries, these vitrify — turn glassy — cradling its proteins and cell membranes so they do not shatter.[^s3]

In 2007, in an experiment called TARDIS aboard the European FOTON-M3 satellite, tardigrades were carried into low Earth orbit and exposed directly to the vacuum of space and the Sun's unfiltered ultraviolet radiation; some survived the vacuum exposure, and a number of those also survived the radiation and went on to reproduce after returning to Earth.[^s4]

A dried tun can remain dormant for years, and rehydration revives it within minutes to hours.[^s5]

## See also
- [Cryptobiosis](../concepts/cryptobiosis.md)

## Sources
[^s1]: [raw/feature-article.pdf](../../raw/feature-article.pdf), p. 1 — water bears defined; cryptobiosis as their dormant survival state (ingested 2026-07-16)
[^s2]: [raw/feature-article.pdf](../../raw/feature-article.pdf), p. 1 — 97 percent body-water loss and tun formation while drying (ingested 2026-07-16)
[^s3]: [raw/feature-article.pdf](../../raw/feature-article.pdf), p. 1 — trehalose and TDPs vitrify to protect proteins and membranes (ingested 2026-07-16)
[^s4]: [raw/feature-article.pdf](../../raw/feature-article.pdf), p. 2 — TARDIS experiment aboard FOTON-M3, 2007: survival of vacuum and UV exposure, reproduction after return (ingested 2026-07-16)
[^s5]: [raw/feature-article.pdf](../../raw/feature-article.pdf), p. 2 — dried tun dormancy duration and rehydration revival time (ingested 2026-07-16)
