This phylum of animal that I happen to love, despite being relatively unknown, is more commonly refereed to as Water Bears or Moss Piglets. In their own way, they are just as adorable as their namesake.
It just looks so hugable.
All Tardigrades have a few key features in common throughout the entire phylum. They all have eight stubby little legs, each having four to eight tiny claws. They consist of four segments and a head. Their bodies are covered by a cuticle of chitin which it molts occasionally. To feed, they project a stylet into the cell of the organism it intends to feed off of and simply slurps up the contents. They primarily feed on moss and algae but a few species have been known to prey on other invertebrates. The adults are eutelic, meaning they have a fixed number of cells that they can posses. The largest having around 40,000 cells. Due to their small size, they do not require a respiratory system and instead rely on cellular diffusion for their needed gas exchange.
Despite having a name that means 'slow walker', these tiny creatures have found there way to nearly every environment we can think to look for them. They have been found in the Himalayas at 6,000 meters (20,000 ft) and in the deep oceans at 4,000 meters (13,000 ft). What is also astonishing is their numbers. In a single liter of sediment water, around 25,000 Tardigrades may be found.
They also posses a level of survivability that is beyond absurd. They are so hardy that researchers sometimes have difficulties finding environments that will harm them. When an environment starts to become too hostile for them, they enter a state known as cryptobiosis. This is essentially a freeze-dried state that allows them to endure just about anything. The do this by producing trechalose, a non-reducing sugar that replaces water and protects its tissue. This substance, after being noted in Tardigrades, has been used in various dry vaccines.
As I mentioned, while Tardigrades are in their cryptobiotic state, they are rather impervious. In this state they have survived exposure to near absolute zero temperatures. They were exposed to -273 C (459 F) and were still able to be revived (absolute zero is -273.15 C). Next came extreme heat with survivability up to 151 C (304 F). They are even resilient to pressure, surviving up to 6,000 atm, which is six times the pressure found at the bottom of the Mariana Trench. Wondering what to try next, researchers tried radiation. The median survivability limit was 5,000 Gy for gamma-rays and 6,200 Gy for heavy ions. To put this in perspective, 5-10 Gy is enough to kill your average healthy adult human. Going for what can only be considered the "Why the hell not" award, Tardigrades were exposed to the vacuum of space on the FOTON-M3 mission for ten days. Upon retrieving the experiment, they found the little critters hadn't really noticed as they had laid and hatched eggs during that time as if life were simply normal.
Thankfully, of the 1,000 known species, there has yet to be a case of a parasitic or otherwise harmful Tardigrade. After learning that they are a step away from invincibility, this is quite a good thing.