So, yes, tardigrades have a phylum to themselves, backward-facing rear legs and are cute in their weird, boneless way, but as we indicated earlier, that's not why they're famous. What makes water bears really special is that they're extremophiles. This means they can survive just about anything we throw at them (or throw them in). They even put cockroaches to shame.
Scientists have frozen tardigrades down to "functional" absolute zero (0.05 kelvins, -272.95 degrees Celsius, -459.31 Fahrenheit) for 20 hours. When thawed out, the little water bears went right back to business as usual. They've also been put on ice for nearly two years at -200 Celsius (-328 Fahrenheit) and brought back to life. Researchers have boiled them alive, tried to crush them with 40,000 kilopascals (5,801 psi) of pressure, attempted to suffocate them with (variously) carbon monoxide, carbon dioxide, nitrogen and sulfur dioxide, and even shot them into space to see if they could endure the intense ultraviolet radiation from the sun (they could and did).
All this has won them the status of absolutely, hands down, the toughest creatures in existence. But how the #&¡?!! do they do it? In two non-simple words: anoxybiosis and cryptobiosis. These are two of the three states in which tardigrades can exist. The other one is "active," or what we commonly call "life." That's the state in which they crawl around adorably and eat, sleep, dream, wake, have sex, get in fights, etc. But if, for some reason, their oxygen supply is suddenly lowered, they can blow themselves up like the Stay Puft Marshmallow Man and just hang out in this anoxybiosis state until they can breathe again.
Then there's cryptobiosis. Tardigrades like or, to be more precise, need water. But, incredibly, if the water dries up, they can shed 97 percent of their own moisture, wither to one-third their normal size and stop metabolizing. It's basically a form of suspended animation. This dehydrated, non-metabolizing version of a tardigrade is called a "tun," and a tun is, for all intents and purposes, indestructible. Add a bit of a water, and a tun quickly transforms back into a cuddly water bear and trundles off as though nothing happened.
And just to add yet another layer of ridiculous invincibility, in their tun state, water bears produce huge quantities of antioxidants that essentially neutralize the ill effects of intense radiation. That's why they were able to survive outer space [source: Herkewitz].
These amazing strategies are not shared by all tardigrades, but only by the terrestrial species who live in small films of water on leaves, moss and lichen. The ones who live in ponds or oceans have fairly stable environments, so they haven't needed to be equipped with quite as many superpowers. But if you're a water bear who likes to hang out in a little micro-puddle on a maple leaf, you've got to be ready for anything. When the sun comes out and dries up that water, you have to follow suit [source: Miller]. And at that point, you should probably be prepared for quite a journey if a gust of wind hits; stiff breezes are the tardigrade's main mode of long-distance travel.
So now we know what tardigrades can withstand and why they developed such extreme adaptive strategies. The next question is — how? How did they manage to evolve such unique characteristics?