Tardigrades are prevalent in mosses and lichens and can readily be collected and viewed under a low-power microscope, making them accessible to students and amateur scientists.

Their clumsy crawling and their well-known ability to survive life-stopping events have brought them into science fiction and popular culture including items of clothing, statues, soft toys and crochet patterns.

There are no lungs, gills, or blood vessels, so tardigrades rely on diffusion through the cuticle and body cavity for gas exchange.

These are connected by paired commissures (either side of the tube from the mouth to the pharynx) to the dorsally located cerebral ganglion or 'brain'.

[3] The tardigrade Dactylobiotus dispar can be trained by classical conditioning to curl up into the defensive 'tun' state in response to a blue light associated with a small electric shock, an aversive stimulus.

A pair of salivary glands secrete a digestive fluid into the mouth, and produce replacement stylets each time the animal moults.

[3] Courtship occurs in some aquatic tardigrades, with the male stroking his partner with his cirri to stimulate her to lay eggs; fertilisation is then external.

Their eggs and resistant life-cycle stages (cysts and tuns) are small and durable enough to enable long-distance transport, whether on the feet of other animals or by the wind.

[6] The majority of species live in damp habitats such as on lichens, liverworts, and mosses, and directly in soil and leaf litter.

[8] Tardigrades consume prey such as nematodes, and are themselves predated upon by soil arthropods including mites, spiders and cantharid beetle larvae.

[10] Tardigrades are not considered universally extremophilic because they are not adapted to exploit many of the extreme conditions that their environmental tolerance has been measured in, only to endure them.

[3] Terrestrial and freshwater tardigrades are able to tolerate long periods when water is not available, such as when the moss or pond they are living in dries out, by drawing their legs in and forming a desiccated cyst, the cryptobiotic 'tun' state, where no metabolic activity takes place.

[18] In 2011, tardigrades went on the International Space Station STS-134,[19] showing that they could survive microgravity and cosmic radiation,[20][21] and should be suitable model organisms.

[30] The Dsup proteins of Ramazzottius varieornatus and H. exemplaris promote survival by binding to nucleosomes and protecting chromosomal DNA from hydroxyl radicals.

Schulze gave the first formal description of a tardigrade, Macrobiotus hufelandi, in a work subtitled "a new animal from the crustacean class, capable of reviving after prolonged asphyxia and dryness".

[39] The zoologist Hartmut Greven wrote that "The unanimous opinion of all later researchers is that Doyère's 1842 dissertation Memoire sur les Tardigrades is an indisputable milestone in tardigradology".

[40] In 1937 Gilbert Rahm, studying the fauna of Japan's hot springs, distinguished the class Mesotardigrada, with a single species Thermozodium esakii;[48] its validity is now doubted.

Another fossil species, Beorn leggi, is known from a Late Campanian (~72 mya) specimen of Canadian amber, belonging to the family Hypsibiidae.

[53] Morphological and molecular phylogenetics studies have attempted to define how tardigrades relate to other ecdysozoan groups; alternative placements have been proposed within the Panarthropoda.

[70] "Arthrotardigrada" Echiniscoidea Milnesiidae Isohypsibiodea Macrobiotoidea Hypsibioidea Possibly the first time that tardigrades appear in non-scientific literature is in the short-story "Bathybia" by the geologist and explorer Douglas Mawson.

Published in the 1908 book Aurora Australis and printed in the Antarctic, it deals with an expedition to the South Pole where the team encounters giant mushrooms and arthropods.

If they are dry, they can be reanimated on a microscope slide by adding a little water, making them accessible to beginning students and amateur scientists.

[76] Live Science notes that they are popular enough to appear on merchandise like clothes, earrings, and keychains, with crochet patterns for people to make their own tardigrade.

[77] The Dutch artist Arno Coenen [nl] created statues for St Eusebius' Church, Arnhem of microscopic organisms including a tardigrade and a coronavirus.

[78] The tardigrades' traits, including their ability to survive extreme conditions,[80] have earned them a place in science fiction and other pop culture.

[84] The biologists Mark Blaxter and Arakawa Kazuharu describe tardigrades' transition to science fiction and fantasy as resulting in "rare but entertaining walk-on parts".

[85] They note that in the 2015 sci-fi horror film Harbinger Down, the protagonists have to deal with tardigrades that have mutated through Cold War experiments into intelligent and deadly shapeshifters.

The protagonist, the xeno-anthropologist Michael Burnham, explains that the Ripper can "incorporate foreign DNA into its own genome via horizontal gene transfer.

[79] The scholar of science in popular culture Lisa Meinecke, in Fighting for the Future: Essays on Star Trek: Discovery, writes that the animal shares some of the real tardigrade's characteristics, including "its physical resilience to extreme environmental" stresses.

[87] She adds that while taking on fungal DNA is "ostensibly grounded" in science, it equally carries a "mystical impetus of what [the French philosophers] Deleuze and Guattari call a becoming",[87] an entanglement of species that changes those involved "and ties together all life".