Water Bear

Milnesium tardigradum

Water Bear (Milnesium tardigradum) is a species in the genus Milnesium, family Milnesiidae, order Apochela.

Scientific classification

Description

Milnesium tardigradum is a tardigrade — a micro-animal under a millimetre long with four pairs of stubby clawed legs and a lumbering gait that earned the group the name "water bear". Unlike most tardigrades, which graze on plant cells and algae, Milnesium is a predator, feeding on rotifers, nematodes and other tardigrades.

Cryptobiosis

Tardigrades survive conditions that kill essentially all other animals, by entering an ametabolic state called a tun: the body contracts, the legs withdraw, water content falls below one per cent of normal, and metabolism becomes undetectable. In this state Milnesium and its relatives have survived temperatures near absolute zero and above 150 °C, pressures many times those of the deepest ocean, ionising radiation at doses hundreds of times the human lethal level, and — in a 2007 orbital experiment — direct exposure to the vacuum and unfiltered solar ultraviolet of low Earth orbit.

The protective machinery involves trehalose in some species and, more generally, intrinsically disordered proteins that vitrify the cytoplasm into a glass, together with a damage-suppressor protein that shields DNA.

None of this makes tardigrades hard to kill in their active state. A hydrated tardigrade is a delicate animal; it is the tun that is nearly indestructible.

Distribution

Cosmopolitan, in the water films around mosses, lichens and leaf litter, where the ability to dry out and revive matches a habitat that does the same.

References

• Jönsson, K. I. et al. (2008). Tardigrades survive exposure to space in low Earth orbit. *Current

Biology* 18.

• Møbjerg, N. et al. (2011). Survival in extreme environments. Acta Physiologica 202.