# Tardigrade — Facts, Threats & Conservation

*Phylum Tardigrada — water bears and moss piglets*

> A tardigrade is a microscopic invertebrate in the phylum Tardigrada, typically 0.3–0.5 mm long, that survives extreme drying, freezing and vacuum by entering cryptobiosis — a reversible shutdown of metabolism — and individuals exposed to open space on a 2007 orbital experiment were revived afterward.

**IUCN status:** Not Evaluated at phylum level — most of the roughly 1,300 known species lack individual Red List assessments  ·  **WARN range:** Worldwide, Moss and freshwater films, Marine sediments

## Quick facts
| Fact | Value |
| --- | --- |
| Body length | Typically 0.3–0.5 mm |
| Legs | Four pairs with claws — not six like insects |
| Habitat | Water films on moss, soil, lichen and sediments |
| Survival state | Cryptobiotic tun when desiccated |
| Space experiment | Revived after 2007 FOTON-M3 orbital exposure |
| IUCN status | Most species not individually assessed |

## Scientific classification
- **Kingdom:** Animalia
- **Phylum:** Tardigrada
- **Class:** Heterotardigrada and Eutardigrada (two main lineages)
- **Order:** Varies by lineage — multiple orders described
- **Family:** Varies — more than 140 genera described
- **Genus:** Examples include Hypsibius, Milnesium, Ramazzottius
- **Species:** More than 1,300 described; many await assessment

## Conservation status
- **Status:** Not Evaluated (phylum and most species)
- **Population:** No population estimate exists
- **Trend:** Unknown
- **Assessed:** Not assessed for most species
- **CITES:** Not listed
- Tardigrades are widespread and rarely targeted by conservation programmes individually, but they illustrate how little is known about meiofauna abundance and how microhabitat loss from drying climates or pollution might affect even tolerant species.

## Key facts: Tardigrade
- Tardigrades are microscopic invertebrates in their own phylum, Tardigrada — not insects, not crustaceans.
- They live in water films on moss, soil and lichen, and in marine and freshwater sediments on every continent including Antarctica.
- Cryptobiosis lets them survive almost complete desiccation by curling into a dry tun with metabolism near zero.
- In the tun state, tardigrades have survived vacuum, temperatures near absolute zero and radiation doses that would kill humans — though they are not immortal and die under sustained high heat.
- The 2007 ESA FOTON-M3 experiment exposed tardigrades to open space for ten days; some were later rehydrated and revived.
- Most tardigrade species have not been individually assessed by the IUCN; their conservation status as a group is effectively unknown.

## What cryptobiosis actually is
Cryptobiosis is not a vague superpower. It is a defined physiological state in which an organism's metabolic activity becomes undetectably low while structure is preserved. When a tardigrade's habitat dries, it loses water through its cuticle, retracts its legs, and forms a tun — a compact barrel shape protected by specialised proteins and sugars that replace water in cells and stabilise membranes.

Rehydration reverses the process if the tun has not been damaged. The animal uncurls, reabsorbs water and resumes feeding within hours. The same machinery helps some species survive freezing: ice crystals are excluded or tolerated in ways that rupture most animal cells. The limits are real. Tardigrades die at sustained high temperatures — roughly above 40 °C for many species — and a tun left dry for decades may fail to revive if storage conditions were poor.

## The 2007 space exposure experiment
In September 2007, the European Space Agency's FOTON-M3 mission carried tardigrades of the species Richtersius coronifer and Milnesium tardigradum on the outside of the spacecraft, exposed to the vacuum of space, solar ultraviolet radiation and the temperature swings of low Earth orbit. The exposure lasted ten days.

After return, some individuals from both species were rehydrated and survived. The experiment demonstrated that cryptobiotic tardigrades can endure combined vacuum and radiation in space — not that tardigrades live naturally in orbit, but that their tun state protects against environmental extremes far beyond those of their mossy habitats. Subsequent work has explored which proteins — notably cytosolic abundant soluble proteins in some lineages — confer desiccation tolerance.

## Where tardigrades live and what they eat
Most tardigrades are found in terrestrial microhabitats that hold water intermittently: moss cushions, lichen, rotting leaves and soil pores. Marine and freshwater species live in sediments. They are part of the meiofauna — animals too small to see clearly without magnification but abundant in almost every sampled environment.

Feeding is by piercing or sucking. Many species have a stylet in the mouth that punctures plant cells, algae or smaller invertebrates, and pharyngeal muscles pump in the contents. Life cycles include moulting — tardigrades shed their cuticle to grow — and eggs laid in the old cuticle or attached to substrate. Generation times range from days to months depending on species and temperature.

## What WARN does
WARN publishes free wildlife education that separates documented science from viral exaggeration. Tardigrade hardiness is real and mechanistically understood — accurate guides keep cryptobiosis from becoming folklore that crowds out species facing genuine extinction risk.

If this guide helps you understand wildlife and the pressures it faces, a gift to WARN supports habitat protection and free public education in our partner countries.

## Frequently asked questions: Tardigrade
### Are tardigrades dangerous to humans?
No. They are far too small to bite or sting people, and none are parasites of humans. The only contact most people have is through a microscope.

### How long do tardigrades live?
Active adult life spans weeks to a few months in many species, though timing varies with temperature and food. In the dry tun state, some have been revived after years of desiccation; claims of decades depend on storage conditions and species.

### How big is a tardigrade?
Most adults are about 0.3 to 0.5 millimetres long — visible as a speck to the naked eye but requiring magnification to see detail. Giant marine species can approach 1.2 mm.

### Did tardigrades really survive in space?
Yes, in cryptobiosis. On the 2007 FOTON-M3 mission, tardigrades on the spacecraft exterior endured ten days of vacuum and radiation; some were rehydrated and survived after return. They were not living actively in space.

## Sources
- [European Space Agency — tardigrades survive space exposure](https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Research/Tardigrades_survive_exposure_to_space)
- [National Museum of Natural History — tardigrade facts](https://naturalhistory.si.edu/education/invertebrate-zoology/tardigrades)
- [Current Biology — cryptobiosis and desiccation tolerance (review context)](https://www.cell.com/current-biology/fulltext/S0960-9822(17)30982-9)

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Full guide: https://worldanimalrescuenetwork.org/wildlife-guides/tardigrade