Tardigrades (also known as water bears or moss piglets) belong to a phylum of water dwelling 8-legged segmented micro-animals which can grow up to 1.5 mm. They were first discovered in 1773 by a German zoologist and have been found in the fossil record as long ago as the middle of the Cambrian Period, over 500 million years ago. Tardigrades are ubiquitous organisms. They live on every continent (yes, including antarctica) and are found in diverse biomes, from freshwater, oceans, deserts, rainforests to the highest mountain peaks.
Tardigrades are known to be one of the most resilient organisms which can survive exposure to extreme temperatures (between -458°F to a scorching 303°F), high levels of radiation, extreme pressures (both high and low), dehydration, starvation, air deprivation etc. Tardigrades have been boiled and frozen and come back to life. Some species of tardigrades are known to survive in about 6 times the pressure at the bottom of Marianas trench (about 1000 atmospheres).
Tardigrades can survive without water due to a process called Anhydrobiosis (Greek for way of life without water). When faced with an adverse environment, like extreme heat or cold, they expel almost all of their water (more than 99%) and become a dried barrel called a "tun". They lower their metabolism via cryptobiosis. During this, the cells produce a special matrix which locks in position the water sensitive cell components, such as proteins and DNA, to stop them from unfolding, breaking apart or fusing. When favourable conditions return, this matrix dissolves and leaves behind the unharmed functional cells.
In 2007, tardigrades hitched a ride on the FOTON-M3 spacecraft as it was launched into a low-Earth orbit for ten days. When tardigrades exposed to space vacuum came back to life on Earth, the scientists discovered that 68 percent lived through the ordeal and reproduced even after combined exposure to space vacuum and radiation from the Sun. They are the first known animal to survive after exposure to outer space.
Scientists have also discovered a protein called “Dsup” (damage suppressor) in tardigrades that prevents the destruction of its DNA by radiation.
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