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Tardigrades ( ), also known as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals.

They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär 'little water bear'. In 1776, the Italian biologist Lazzaro Spallanzani named them Tardigrada, which means "slow walkers". Tardigrades live in diverse regions of Earth's biosphere: mountaintops, the deep sea, tropical rainforests, and the Antarctic. They are among the most resilient animals known, with individual species able to survive severe conditions — such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvation — that would quickly kill most other forms of life. There are about 1,500 known species in the phylum Tardigrada, a part of the superphylum Ecdysozoa. The earliest known fossil is from the Cambrian, some 500 million years ago. They lack several of the Hox genes found in arthropods, and the middle region of the body corresponding to an arthropod's thorax and abdomen. Instead, most of their body is homologous to an arthropod's head. Tardigrades are usually about 0.5 mm (0.02 in) long when fully grown. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or sticky pads. 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 extreme conditions have brought them into science fiction and popular culture including items of clothing, statues, soft toys and crochet patterns.

Tardigrades as a group are cosmopolitan, living in many environments on land, in freshwater, and in the sea. Many tardigrades are predatory; Milnesium lagniappe includes other tardigrades such as Macrobiotus acadianus among its prey. Except for 62 exclusively freshwater species, all non-marine tardigrades are found in terrestrial environments.

Three of these are specific to tardigrades and have been called tardigrade specific proteins.

In 1773, Johann August Ephraim Goeze named the tardigrade Kleiner Wasserbär, meaning 'little water-bear' in German (today, Germans often call them Bärtierchen 'little bear-animal'). 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". All four of these are now the nominal species for higher tardigrade taxa. Ferdinand Richters worked on the taxonomy of tardigrades from 1900 to 1913, with studies of Nordic, Arctic, marine, and South American species; he described many species at this time, and in 1926 proposed the class Eutardigrada.

They have three pairs of legs rather than four, they have a simplified head morphology, and they have no posterior head appendages, but they share with modern tardigrades their columnar cuticle construction. Multiple lines of evidence show that tardigrades are secondarily miniaturised from a larger ancestor, probably a lobopodian, perhaps resembling the mid-Cambrian Aysheaia, which many analyses place close to the divergence of the tardigrade lineage. The oldest remains of modern tardigrades are those of Milnesium swolenskyi, belonging to the living genus Milnesium known from a Late Cretaceous (Turonian) aged specimen of New Jersey amber, around 90 mya. Morphological and molecular phylogenetics studies have attempted to define how tardigrades relate to other ecdysozoan groups; alternative placements have been proposed within the Panarthropoda. The Tactopoda hypothesis holds that Tardigrada are sister to Arthropoda; the Antennopoda hypothesis is that Tardigrada are sister to (Onychophora + Arthropoda); and the Lobopodia (sensu Smith & Goldstein 2017) hypothesis is that Tardigrada are sister to Onychophora.

Genomic studies across different tardigrade groups help reconstruct the evolution of their genome, such as the relationship of tardigrade body segments to those of other Panarthropoda.

The musician Cosmo Sheldrake imagines himself as a robust tardigrade in his 2015 "Tardigrade Song". He sings "If I were a tardigrade ...

Quick Facts

• They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär 'little water bear'.
• 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 extreme conditions have brought them into science fiction and popular culture including items of clothing, statues, soft toys and crochet patterns.
• They lack several of the Hox genes found in arthropods, and the middle region of the body corresponding to an arthropod's thorax and abdomen.
• They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or sticky pads.

Source material: Wikipedia - "Tardigrade". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.