Image from Microbial Life Educational Resources.

Although a few species may be categorized as aquatic, most members of the Phylum Tardigrada are limno-terrestrial. That is, they are regular inhabitants of the ephemeral films of water associated with mosses, lichens, etc. Of more than 910 tardigrade species in the world, only 62 are strictly aquatic. Twenty of these in 5 genera (3 families) occur in North America. In contrast, 194 spp. in 34 genera (8 families) are known from Nearctic limno-terrestrial microhabitats (Garey et al., 2008). The phylum is divided between two classes, Eutardigrada and Heterotardigrada. A third class (Mesotardigrada) is almost surely spurious, based on a single species from a single collecting event from a locality that no longer exists.

Tardigrades are minute (μm-mm) animals with an arthropod-like body plan. They are commonly referred to as “water bears“ for their slow, lumbering ursid type locomotion. Tardigrades are basically cylindrical, with four segments bearing stumpy, clawed walking legs. Their epidermis secretes a cuticle that must be molted 6-12 times during their life cycle. Tardigrades lack jaws. Instead, they have a pair of piercing stylets that they use, in combination with their sucking pharynx, to relieve plant and algal cells of their fluids. Sexes are generally separate, although (as would be expected for tiny aquatic-terrestrial taxa, there is some evidence of both hermaphroditism and parthenogenesis.

Tardigrades are notable for their ability to withstand extreme conditions. During dry (or low-oxygen) conditions, adults withdraw their head and appendages into their body cavity and enter a stage of cryptobiosis known as a tun. In this way, tardigrades can take advantage of their briefly aquatic limno-terrestrial habitats and wait-out intermittent dry spells.

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Bibliography

• Zhang, Z.-Q. 2011. Animal biodiversity: An introduction to higher-level classification and taxonomic richness. Zootaxa 3148: 7-12. [Available online]
• Guidetti, R. & R. Bertolani. 2011. Phylum Tardigrada Doyère, 1840. In: Zhang, Z.-Q. (Ed.) Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness. Zootaxa 3148: 96-97. [Available online]
• Nelson, D.R., R. Guidetti & L. Rebecchi. 2010. Tardigrada. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition: 455-484.
• Garey, J.R., S.J. McInnes & P.B. Nichols. 2008. Global diversity of tardigrades (Tardigrada) in freshwater. Hydrobiologia 595: 101-106. [Available online]
• Guidetti R. & R. Bertolani. 2005. Tardigrade taxonomy: An updated check list of the taxa and a list of characters for their identification. Zootaxa 845: 1-46. [Available online]
• Brusca, R.C. & G.J. Brusca. 2003. Invertebrates, 2nd edition. Sinauer Associates, Inc., Sunderland, Massachusetts. 936 pp.
• Smith, D.G. 2001. Tardigrada (Water bears). Pennak's Freshwater Invertebrates of the United States, 4th edition: 227-243.
• Underhill, J.C., W.D. Schmid & D.E. Gilbertson. 1982. Taxonomic Keys to the Common Animals of the North Central States, exclusive of the parasitic worms, terrestrial insects, and birds, 4th edition. 205 pp.
• Marcus, E. 1959. Tardigrada. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 508-521.
• Pennak, R.W. 1940. Ecology of the microscopic Metazoa inhabiting the sandy beaches of some Wisconsin lakes. Ecological Monographs 10(4): 537-615. [Available online]

Student Essays

• "Tardigrada Explained" by Laura Van Remortel (BIOL/WATR 361, Fall 2013 )
• "Dormancy Strategies in Tardigrades" by Crystal Buchholtz (BIOL/WATR 361, Fall 2013 )
• "Suitability of Terrestrial Tardigrada for Bryophytic Habitats" by Taylor Lockwood (BIOL/WATR 361, Spring 2015 )

Web Pages

• Animal Diversity Web
• Discover Life
• Integrated Taxonomic Information System
• Microbial Life Educational Resources
• Synopsis of the Described Tardigrada of the World
• Tree of Life
• WikiSpecies
• Wikipedia
• World Register of Marine Species