Published online 5 November 1998 | Nature | doi:10.1038/news981105-1

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Pressure brought to bear

Life is tough, but among the toughest of all life-forms are shy, deceptively charming little creatures called tardigrades. These small animals - never more than 1.2 millimetres long - live in water. About 400 species have been described, from habitats as varied as the deep sea to the thin films of water that coat the leaves of mosses. The endearingly chubby appearance of tardigrades, their slow, bumbling movements and eight stumpy legs have earned them the nickname ‘water bears’.

This soubriquet is deceptive - for beneath their cuddly exteriors lie unmatched reserves of endurance. In times of drought, tardigrades pull in their legs, contract and shrivel, and turn into ‘resting’ stages called ‘tuns’ in which metabolism all but stops. It is the tun, not the tardigrade, that is tough.

Tuns can withstand temperatures as low as -253 °C, just twenty degrees above absolute zero (some reports have them withstanding immersion in liquid helium, at -272 °C). They can stand being heated to 151 °C. Specimens have recovered after immersion in brine, ether and absolute alcohol. But give a tun a drop of water, and it will rehydrate to form a tardigrade, as if nothing had happened. It’s hard to top a tardigrade.

But perhaps most remarkable of all is the ability of tuns not to crumple under extreme pressure. Most living things, even bacteria, will be killed by pressures above 300 megapascals (MPa), or about 3,000 atmospheres. But tardigrade tuns can double that - in a report in the 29 October edition of Nature, Kunihiro Seki and Masato Toyoshima of Kanagawa University in Japan report that tuns can withstand pressures of 600 MPa, equivalent to six times the pressure of sea water at a depth of 10,000 metres.

The researchers forced live tardigrades to become tuns by dehydrating them on filter paper. They then immersed the tuns in an inert solvent, perfluorocarbon, so that the tuns wouldn’t rehydrate - which could have happened, had water been used. The tuns were then squeezed inside a pressure capsule for 20 minutes at a time, with the pressure increasing in 100-MPa steps, one step per minute, After a peak of 600 MPa, the pressure was removed at the same rate.

Given the pressure applied, and the speed of pressurization and decompression, it is amazing that the tuns weren’t pounded into paste. But after the squeeze, and some restorative water, 95% of the tuns of one species, Macrobiotus occidentalis, and 80% of the tuns of another species, Echiniscus japonicus, survived to live another day.