Tardigrada (Water Bears or Moss Piglets)

"Tardigrades, or water bears, are aquatic animals, being active only when surrounded by water. These microscopic metazoans are found in a diversity of extreme habitats, ranging from the deep sea, at depths to 6000 m, to the highest elevations in the Himalayas, and from hot springs to the ice cathedrals inside the ice cap of Greenland. Tardigrades are amongst the smallest Metazoa with most of the 718 described species ranging in length from 0.1 to 0.5 mm. The adults of the smallest marine species are only 0.85 mm long but some terrestrial forms may reach 1.2 mm.

The body is bilaterally symmetrical and the generally cylindrical shape may be extremely dorsolaterally flattened in some species. The majority of tardigrade species are white to translucent but some terrestrial species are strongly coloured — yellow, orange, green, red to olivine-black. There are five distinct body segments including a cephalic segment and four trunk segments each bearing a pair of segmented legs. Marine forms may have telescopic legs with up to 13 claws or four to six toes with complex claws or distal adhesive discs. In terrestrial and limnic forms, the segmentation of the stumpy legs is reduced and they bear two or four claws which may be missing in some soil inhabiting forms. Although tardigrades have no circulatory or respiratory structures, other systems, such as the digestive system, are quite complex.

The phylum comprises three classes: Heterotardigrada, Eutardigrada and Mesotardigrada. Heterotardigrada, consisting of two orders, Arthrotardigrada and Echiniscoidea, have cephalic appendages. The arthrotardigrades are marine forms which usually have a median cirrus and telescopic legs with or without digits, while the echiniscoids are terrestrial armoured forms and marine unarmoured forms. The echiniscoids have no median cirrus and the legs are non-digitate. In Eutardigrada, which are primarily terrestrial and comprise the two orders Parachela and Apochela, cephalic appendages (which are not homologous to those of the heterotardigrades) are absent in the former but present in the latter. The cuticle of eutardigrades is not armoured and the legs are non-digitate. The class Mesotardigrada was established on the basis of the description of a single species found in a hot spring in Japan (Rahm 1937). The type material does not exist and the species description has been criticised.

...The Tardigrada have been known and studied since 1773 when Goeze first described ‘kleinen Wasserbaren’ (Goeze 1773, cited in Ramazzotti & Maucci 1983). In 1776, Spallanzani named the group ‘Il Tardigrado’, which means ‘the slow walkers’ after the way most terrestrial species moved. Spallanzani was also the first to discover that terrestrial tardigrades have cryptobiotic stages. Cryptobiosis (latent life) is a stage in which all metabolic activity is suspended for a period of time (Wright, Westh & Ramløv 1992)."

Source: https://www.biodiversity.org.au/afd/taxa/TARDIGRADA

Class: Eutardigrada

Order: Parachela

Family: Macrobiotiidae

Class: Heterotardigrada

Order: Echiniscoidea

Family: Echiniscidae

Class: Mesotardigrada (taxon inquirendum or nomen dubium)

Order: Thermozodia

Family: Thermozodiidae

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