Johann Goeze's 'kleiner Wasserbär' — the little water bear of 1773 (1773)
The German pastor and naturalist gave the group its enduring nickname
Science · Biology
Tardigrade
First described 1773; phylum named 1776
Biology — micro-animals, phylum Tardigrada
Tardigrades are microscopic eight-legged invertebrates, rarely more than a millimetre long, that make up a phylum of their own alongside the arthropods and velvet worms. They live in moss, leaf litter, soil, freshwater and the deep sea, and are famous for cryptobiosis: when their surroundings dry out they retract their legs, expel almost all their water and shrivel into a barrel-shaped "tun" in which metabolism falls to undetectable levels. In that state they have survived boiling, freezing to near absolute zero, the vacuum and radiation of open space, and radiation doses hundreds of times higher than would kill a human. Active, hydrated tardigrades are far more fragile, which is why the popular label "indestructible" is misleading.
How to spot them
- ▸Eight legs, a barrel-shaped body under a millimetre long, and a lumbering gait that earned it the nickname 'water bear'
- ▸Dries into a 'tun' and can be revived by a drop of water years later
- ▸Survived ten days of open space — vacuum and unfiltered solar ultraviolet — on a 2007 orbital mission
- ▸Tolerates radiation doses around a thousand times the human lethal dose
- ▸A phylum of its own, sister to the velvet worms and arthropods
Key facts
Lazzaro Spallanzani's 1776 name for them meaning 'slow steppers' (1776)
The Italian priest-scientist also showed they could be dried out and revived
Dries into a barrel-shaped 'tun', shedding almost all its body water, and revives when wetted
Cryptobiosis — metabolism drops to an undetectable fraction of normal
Sent into orbit on the FOTON-M3 mission and survived ten days of open space (2007)
The TARDIS experiment; some survived the vacuum plus full solar ultraviolet and went on to reproduce
Thousands of them were aboard the Beresheet lander that crashed on the Moon in 2019 (2019)
Carried, dried, in an Arch Mission 'lunar library'; whether any survived is unknown
A DNA-shielding protein nicknamed 'damage suppressor' was found in Ramazzottius varieornatus (2016)
Dsup binds chromatin and reduced X-ray damage when transferred into human cultured cells
'Moss piglet' is the other common nickname for this eight-legged micro-animal
From where they are most easily collected — a squeezed clump of damp moss
Key ideas
- Cryptobiosis: by replacing lost water with protective molecules — trehalose in some species, intrinsically disordered 'CAHS' proteins in others — the cell's contents vitrify into a glass-like solid, so there is nothing left to freeze, boil or chemically react.
- Extreme tolerance is a by-product of drying-out, not of space: the same machinery that protects a dehydrated cell in moss happens to protect it against vacuum, cold and radiation.
Quick-fire facts
- •Phylum Tardigrada; around 1,300–1,500 described species, in habitats from Himalayan scree to deep-sea trenches
- •Most are 0.3–0.5 mm long; the largest reach about 1.2 mm, and the smallest are under 0.1 mm
- •Milnesium tardigradum (Doyère, 1840) is the classic laboratory species; the radiation-tolerant Ramazzottius varieornatus was the one sequenced for the Dsup protein
- •Described in 1773 by Johann August Ephraim Goeze as the 'little water bear'; Lazzaro Spallanzani coined Tardigrada, 'slow steppers', in 1776
- •In the tun state they have survived temperatures from close to absolute zero (−272 °C) to over 150 °C, and pressures far above those of the deepest ocean
- •They withstand ionising radiation of the order of thousands of grays; 5–10 Gy is lethal to a human
- •They feed by piercing plant, algal or animal cells with a pair of needle-like stylets and sucking out the contents; some species eat other tardigrades
- •Closest relatives are the velvet worms (Onychophora) and the arthropods; together these form the Panarthropoda
- •Common misconception: they are not indestructible. A hydrated, active tardigrade is easily killed by heat, crushing or drying too fast — the toughness belongs to the tun
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Sources
- Takuma Hashimoto et al., Nature Communications — Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein
- K. Ingemar Jönsson, Elke Rabbow, Ralph O. Schill, Mats Harms-Ringdahl and Petra Rettberg, Current Biology 18 (2008) — Tardigrades survive exposure to space in low Earth orbit The TARDIS experiment aboard FOTON-M3