The Tardigrada comprise a phylum of over 1070 described species of minute aquatic animals. The metamerically segmented trunk possesses four pairs of squat, lobopodial limbs with terminating claws; the claw morphology is taxonomically important. Tardigrades are fluid feeders, piercing animal or plant cells with eversible stylets and ingesting fluid with a muscular, pumping pharynx. Two classes are recognised: the Heterotardigrada and Eutardigrada and heterotardigrades include the majority of marine genera. Most tardigrades, however, inhabit interstitial or temporary water bodies in soil, mosses and lichens, surviving periods of drying by cryptobiosis. During this process, animals may sustain a complete loss of ‘free’ liquid water and accompanying cessation of metabolism. In this dehydrated state, tardigrades display suspended senescence and remarkable resistance to environmental extremes. The phylogenetic status of tardigrades has long been contentious, but recent molecular analyses show that tardigrades are basal arthropods and should be reclassified accordingly.

Key Concepts:

  • Tardigrades, also known as water bears, are minute lobopodial arthropods occurring worldwide from Arctic to tropical latitudes.

  • All tardigrades are aquatic; they are particularly associated with interstitial water of mosses, lichens and soils.

  • Two classes are recognised: the Eutardigrada and the Heterotardigrada; the latter includes some marine species.

  • All tardigrades possess piercing stylets that are used to puncture the cells of plants or, in a few species, animal prey.

  • In many species of tardigrades, the eggs and adults are capable of surviving extreme dehydration and sub‐freezing temperatures by means of cryptobiosis.

  • Cryptobiotic organisms frequently accumulate disaccharides or polyols that may stabilise membranes and proteins and/or promote vitrification.

  • Cryptobiotic tardigrades may be ametabolic and show extreme longevity and high resistance to temperature, radiation and microgravity.

  • Recent molecular phylogenetic studies unite the Tardigrada and Eutardigrada as a natural group within the collective arthropod taxa (Panarthropoda).

Keywords: arthropoda; meiofauna; cryptobiosis; tun; Heterotardigrada; Eutardigrada; interstitial; lobopodial

Figure 1.

Representative Tardigrada. (a) Echiniscus testudo, a common and cosmopolitan heterotardigrade, showing the buccal sensilla and thickened cuticular plates with projecting filaments (Length 250–400 μm). (b) Macrobiotus richtersi, a large eutardigrade (600–1000 μm in length). Note the curved stylets, wide buccal tube and bulbous pharynx with masticatory placoids.

Figure 2.

Claw types of tardigrade genera.

Figure 3.

Morphological variation in the buccopharyngeal complex of tardigrade genera.

Figure 4.

Scanning electron micrographs of Hypsibius (Rammazottius) oberhaeuseri during desiccation and entry into cryptobiosis: (a) partially contracted, (b) tun (dorsal) and (c) tun (ventral). Length of tun approximately 120 m.



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Further Reading

Bertolani R (1987) Biology of Tardigrades, Selected Symposia and Monographs I. Proceedings of the 4th International Symposium on Tardigrades. Modena: Mucchi Editore, Collana U.Z.I.

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Nelson DR (1982) Proceedings of the 3rd international symposium on Tardigrades, 3–6 August 1980. Johnson City, TN: East Tennessee State University Press.

Ramazzotti G and Maucci W (1983) II Phylum Tardigrada (III edizione riveduta e aggiornata). Memorie dell'Istituto Italiano di Idrobiologia 41: 1–1012.

Weglarska B (1979) Proceedings of the 2nd international symposium on Tardigrades. Zeszyty naukowe Uniwersytetu Jagiellonskiego 25: 1–197.

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Wright, Jonathan C(Nov 2012) Tardigrada. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001611.pub3]