Tardigrade was an informal term applied to an unknown alien first encountered through the so-called "Ripper". The species was so named as it shared some natural traits with with the Earth species of the same name, which was described as its cousin. It was highly evolved with a highly functioning nervous system, and neurological tests suggested it might be sentient. (DIS: "Choose Your Pain")
It was a multi-dimensional creature whose unique genetic makeup allowed it to navigate the mycelial network by incorporating Prototaxites stellaviatori DNA in its genome through horizontal gene transfer. Its relation with P. stellaviatori was described as symbiotic. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry", "Choose Your Pain", "Magic to Make the Sanest Man Go Mad")
Although nothing in its biology suggested that it would attack except in self defense, when it did defend itself it was formidable. One member of the species killed a dozen Klingons and was found without a single bat'leth scratch afterwards. Its claws could shred the hull of a Starfleet ship, and its hide could withstand the firepower of a phaser set to kill. One weakness, however, was that the creature had a natural aversion to light. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry")
The creature was noted to be incredibly regenerative, and assessed as being virtually indestructible. When faced with adverse conditions, the creature could go into a state of extreme cryptobiosis as a survival mode. This reduced the water content of its body to less then one percent and slowed its vital signs to the point that they could barely be detected.
The only known tardigrade was "Ripper", first properly documented by the crew of the USS Discovery in 2256. It was discovered that through the tardigrade's connection to the mycelial network, it could be used as an organic computer to navigate the network and dramatically improve the performance of the experimental spore drive. After the Discovery made the spore drive viable by utilizing the tardigrade, Starfleet directed every ship, colony, and starbase to search for more tardigrades. However, forcing the creature to navigate the mycelial network would cause it great pain and progressively damage its frontal lobe. "Ripper" was subsequently released and the role of the organic "computer" was taken by the Discovery's chief engineer, Lieutenant Paul Stamets. (DIS: "Choose Your Pain")