In the natural world, there lives a battle-hardened soldier against which all dangers have fought in vain. They laugh at fire and ice alike. They can live without oxygen and withstand pressures that would easily crush the strongest submarine hulls. They can even survive extreme radiation and the near vacuum of outer space. While hardy, this animal is extremely difficult to see. Which is surprising, since they can be found almost everywhere – despite being fond of the moss and lichen that abound in freshwater, these animals happily live in extreme climates ranging from scorching desert dunes to freezing Antarctic ice.
These animals are no other than tardigrades, perhaps better known by their more affectionate nicknames of “water bears” or “moss piglets”. A tardigrade resembles something of a large, lumbering bear – it is short and plump, and its 8-legged gait resembles that of bears. The tardigrade’s earliest names resulted from these distinctive characteristics – it was first christened Kleiner Wasserbär (“little water-bear”) in 1773 by German zoologist Johann August Ephraim Goeze, and then Tardigradum (“slow walker”) by Lazzaro Spallanzani in 1777. Tardigrades normally grow to about 0.5mm (0.02 in), making it possible to view them under a decent microscope. As mentioned earlier, tardigrades are small and chubby, and they also have eight appendages with 4-6 claws, or suction cups, attached to the end. Tardigrades even have specialized mouthparts, called bucco pharyngeal apparatuses, that allow them to suck up nutrients from plants or even other microorganisms.
All this information about tardigrades is well and fine, but how do they manage to ace the game of survival? The secret to their successful resilience lies in two mechanisms: one is a damage suppressor (Dsup for short) protein that protects the tardigrade’s DNA; the other is the ability to use cryptobiosis, during which metabolic functions stop. The Dsup protein, which functions by wrapping itself like a protective cloud around DNA-covered histones, is especially useful because it protects the tardigrade’s DNA from radiation exposure, which causes mutations in DNA (not a good thing). It is also able to preserve the tardigrade’s DNA during dehydration. Cryptobiosis is also an impressive technique – during cryptobiosis, tardigrades in effect shut down their body. They dry themselves out and curl into a ball, called a tun, to await better conditions while metabolic rates drop to 0.01% of normal. Instead of water, a protective sugar called trehalose enters the tardigrade’s cells. Revival from this state typically takes a few hours and depends on how long the said tardigrade was dormant. Tardigrades have even been revived from hundred-year moss, making them true survivors.
Another interesting thing to note is that there may be extraterrestrial tardigrades. During the numerous asteroid strikes in Earth’s history, debris ejected into space could have contained tardigrades. After traveling through space, some of this debris managed to land on Mars (with the tardigrades). More recently, an Israeli Moon launch contained some contraband – scientists had smuggled some tardigrades onto the vessel before it launched. While it was a serious breach of unofficial intergalactic travel laws (terrestrial organisms would disturb any possibly pre-existing organisms), the experiment still raised interesting questions. Unfortunately, recent lab experiments showed that tardigrades would be unlikely to survive high-speed impacts, meaning that Earth does not yet have much to fear from a growing army of aliens.