The (al­most) in­de­struc­tible wa­ter bear

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Tardi­grades, mi­cro­scopic wa­ter-dwelling an­i­mals also known as wa­ter bears, are among the most fa­mous sur­vival­ists in the world. Since they were first de­scribed by a Ger­man zo­ol­o­gist in 1773 they’ve been found in the depths of the ocean, at the top of Hi­malayan moun­tains, at the poles and in hot springs. De­spite this amaz­ing re­silience, they’re tech­ni­cally not ex­tremophiles. This clas­si­fi­ca­tion is re­served for those crea­tures that ex­ploit and thrive in ex­treme con­di­tions, while tardi­grades cope by sus­pend­ing their me­tab­o­lism and en­ter­ing dor­mancy. Dur­ing pe­ri­ods of drought, wa­ter con­tent in the body drops to one per cent of the nor­mal level. To pre­vent dam­age to the body, unique pro­teins take the place of lost wa­ter in the cells and be­come glass-like. In their de­hy­drated tun state some species of tardi­grade are ca­pa­ble of en­dur­ing tem­per­a­tures close to ab­so­lute zero and, though hot tem­per­a­tures will even­tu­ally kill them, they can last for sev­eral min­utes at 150 de­grees Cel­sius. With their me­tab­o­lism barely func­tion­ing, star­va­tion can be en­dured for years. They can also sur­vive ra­di­a­tion lev­els hun­dreds of times higher than the dose that would kill a hu­man and have even re­turned alive from a jour­ney to the vac­uum of space. Many tardi­grades can sur­vive in this des­ic­cated dor­mant state (known as cryp­to­bio­sis) for five years and will re­sume nor­mal ac­tiv­ity within hours of ex­po­sure to mois­ture. A few ex­cep­tional spec­i­mens have been ‘brought back to life’ af­ter al­most a decade, es­tab­lish­ing tardi­grades as some of the crea­tures most likely to sur­vive a nu­clear war or apoc­a­lyp­tic as­tro­phys­i­cal event.

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