Pinhead bugs are nature's hard cases

A TINY animal no bigger than a pinhead that lives in roof gutters and between the cracks of paving stones has been rated by scientists as among the toughest life forms on Earth.

Tardigrades, which are more commonly known as "water bears" because of their chubby appearance under a microscope, can survive pressures, temperatures and radiation exposure that quickly kill all other animals.

In their active state, tardigrades walk around on eight stumpy legs looking for food and are covered in what appears to be armoured plates.

Japanese scientists report in the journal Nature that tardigrades can survive pressures that are 6,000 times greater than sea-level barometer readings - more than twice the pressure that destroys the vast majority of the most rugged bacteria.

The capacity of tardigrades to survive extreme conditions is renowned. Some have been revived after lying dormant for more than 100 years in the dried moss collections of museums. Their extreme hardiness is due to their ability to undergo complete dehydration, where they entirely lose their body fluids and survive in low-oxygen conditions without ill-effects.

"Terrestrial tardigrades become immobile and shrink into a form known as the 'tun' state when the humidity decreases," said Kunihiro Seki and Masato Toyoshima, from Kanagawa University.

"In this state they can survive extreme temperatures, as low as minus 253C or as high as 151C, as well as exposure to a vacuum or to X-rays."

The scientists subjected the tardigrades in their active and dehydrated states to a range of very high pressures for 20 minutes at a time, using a special fluid called perfluorocarbon to prevent the tun tardigrades from rehydrating.

They found the dehydrated tardigrades could survive pressures three times higher than the pressure which killed the active forms of the animal.

Understanding how the tardigrade is able to survive extremes of humidity, pressure and temperature may help to develop new preservation methods, such as long-term storage of human organs for transplant operations, the Japanese scientists said.