How does one measure the success of a given type of animal or a given type of evolutionary design? Longevity? The extent of the habitats it thrives in? How successfully it can survive to pass on its genes? By these measures and more, the Tartigrade is practically the definition of success, and is damn near invincible in the process.

Today's animal is brought to you by scalius, who requested that I write about it. It is a worthy and awesome subject.

There are more than a thousand species of Tartigrades, but they're all relatively similar as far as we are concerned. They are some of the smallest animals on Earth, ranging from 0.1 to 1.2 millimetres long, and baby Tartigrades are only 0.05 millimetres. They were discovered in the 1770s, and if you're wondering why something so small was found so early, it's because they're pretty much everywhere.

The Tartigrade can be found in essentially every habitat imaginable. They are found from the high Himilayas (20,000 feet above sea level) to the depths of the deepest oceans (more than 13,000 feet below it), from the equator to both poles. They are most common on mosses and lichens, in sand, in soil, and in both marine and freshwater sediments, where you can find up to 25,000 of them per litre. If you've ever sipped from an outdoor water source, chances are good you swallowed some Tartigrades.

Also known as water bears (because they move sort of like a bear) and moss piglets (because they love moss), the Tartigrade is tiny and chubby, with eight little legs, like the world's most infinitesimal caterpillar. And remember, these aren't bacteria or anything, they're animals. They have brains, digestive tracts, skin, sucking mouths, and some even have tiny segmented eyes. They are related to roundworms.

They have been around nearly forever. Though Tartigrades don't fossilze easily (there's not much there to fossilize), the oldest discovered fossil imprint of one stems from 500 million years ago. That's not only before the dinosaurs, that's before life even began to permanently colonize the land. And since that 500 million year old Tartigrade is actually one of the only fossils found of them, it's entirely possible that they were around for hundreds of millions of years before that.

But the most amazing and startling thing about the Tartigrade is how goddamn impossible it can be to kill one. They can survive conditions that would destroy most animals. The Tartigrade thinks that Die Hard was an amusing comedy about a wuss doing wussy things such as "bleeding," "being cold," and "giving a fuck."

What can the Tartigrade survive?

Temperatures: The Tartigrade can survive temperatures up to 151 degrees Celsius, and can live for days at -200 degrees Celsius. It can even survive for a little bit at -272 degrees Celsius, which if you're keeping track is one degree above absolute fucking zero, the point at which all matter stops moving. The Tartigrade just mans up, puts on a sweater, and tells the dead, frigid universe to stop being such a baby.

They survive by reducing the water content of their bodies to nearly zero. Tissue without water cannot freeze.

Pressure: The Tartigrade can easily withstand 1200 atmospheres of pressure, which is what you'll find near the bottom of the oceans. Some species have even been shown capable of surviving 6000 atmospheres, nearly six times the pressure of water at the bottom of the ocean, the sort of pressure that would crush almost any other animal to a little screaming ball of matter. The Tartigrade shrugs and has a nap.

If that wasn't enough, they can also survive in the vacuum of goddamn space. In 2007, just for kicks, scientists sent Tartigrades into a low Earth orbit and exposed to space for ten straight days. When brought back inside and returned to Earth, not only did they survive, they were perfectly healthy and laid healthy eggs that hatched normally. One of them had to pee a little.

Radiation: Some of those Tartigrades that were sent into space were also fully exposed to solar radiation. Some of them survived it, which no one really expected. Whereas humans are killed by five to ten grays of ionizing radiation, the Tartigrade can take between 5000 and 6200 grays and laugh about it. It is theorized that they think it tickles.

Dehydration: As noted before, the Tartigrade can achieve an extremely reduced water state, wherein its entire body dries up. You'd think this would kill it after a while, since cells need water to function, but it can actually survive ten years -- an entire frigging decade -- in such a state. Their water usage drops drastically, and their metabolism is reduced to a fraction of a percentage point of its usual amount. Hydrate them back up after a decade and they just yawn, stretch, and get back to that book they were reading before they dried out.

The Tartigrade can also enter this extreme low-metabolic state in response to environmental toxins which, you guessed it, consequently can't kill it.

Fun fact: a Tartigrade in this state, known as a cryptobiotic state, is called a Tun. I don't know why.

Bonus fun fact: I've decided I want to be a paleocryptozoologist when I grow up. I'll search the world for fossils of Bigfoot, the Loch Ness Monster, and mermaids. I'll teach a university class on it.