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July 28

Scanning electron micrograph of an adult water bear (tardigrade), Hypsibius dujardini. Scanning electron micrograph of an adult water bear (tardigrade), Hypsibius dujardini.
Photo Credit: Bob Goldstein & Vicky Madden, UNC Chapel Hill tardigrades.bio.unc.edu via Flickr

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Water Bears

The tardigrade, also known as the water bear or moss piglet, is one of the world's hardiest animals. It is an aquatic creature, but is best described as limnoterrestrial, living in the thin water film on moss and lichen.

Found in diverse ecosystems spread across all seven continents, this microscopic animal is only a quarter to a half a millimeter long. It possesses a segmented body, four pairs of legs (which give it its wobbly gait) that each end in spiky claws, and piercing mouthparts.

The water bears' most fascinating feature, however, is that a significant fraction of the population is able to withstand extreme conditions, including a lack of oxygen and water, exposure to both space vacuum and solar radiation, high and low pressure and temperature, and exposure to X-ray doses lethal to humans. They do this through a process called cryptobiosis, a form of death-like hibernation that has been known to last up to 100 years. In this temporary transformation, tardigrades severely lower their metabolism rate, replace the water in their cells with a protective trehalose sugar, curl their bodies into cylinders, and allow themselves to dry out into what's called tuns. When the environment becomes more hospitable and the tun is soaked in water, most water bears will return to their normal state after a few hours.

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