Tardigrada

Echiniscus granulatus, from Kinchin (2008).

Belongs within: Ecdysozoa.
Contains: Parachela, Arthrotardigrada.

My first tardigrades
Published 16 April 2008
Tardigrade, from here.

Well, the lab course I’ve been tutoring reached the point yesterday that I’d been waiting for ever since I found out I’d be tutoring it—I’ve now seen my first tardigrades! I’ve covered the generalities of tardigrades elsewhere, but as I’ve noted before, there is nothing to compare with seeing an organism that you’ve previously only known from the literature in real life. And I have to say—tardigrades are just as adorable as I’d always imagined them to be. If not more so.

Many tardigrades store their eggs underneath the cuticle, then shed them along with the cuticle when moulting. This photo by William West, from here, shows a specimen of Milnesium tardigradum in the act of doing so.

While I couldn’t tell you exactly what species they were, but they looked much like the photo at the top of this section. Little sausage-shaped animals with four pairs of little stumpy legs, on which they bumbled about in a decidedly endearing manner. I wasn’t the only person impressed, either—I heard more than one student exclaiming aloud how cute they were when they looked down the microscope. When you’ve been trying a week previously to encourage enthusiasm in students about nematodes, it’s certainly nice to have an organism that pretty much sells itself.

Lecture on tardigrades, from tardigrades.com.

If you’d like to see your own tardigrades, collect some moss or lichen and sit it in a petri dish with some water (use distilled water or rainwater, not tap water). Leave it there for enough time for the tardigrades to become active and crawl out of the moss. (While they may be found in terrestrial habitats, tardigrades require at least a film of water to move in. If they dry out, they either die or enter a dormant resistant stage known as a tun.) After 3-24 hours, pipette some water out of the base of the dish (tardigrades don’t swim, so they settle to the bottom) into another dish or watchglass. Place your pipetted sample under a stereo microscope, and you should be able to see the tardigrades crawling around on the bottom! (As well as other small organisms such as rotifers, nematodes and possibly protozoa.) If you want a closer look, you can transfer a tardigrade onto a slide using a micropipette. A drop of alcohol added to the slide will knock the tardigrade out. Take a look at the Microbial Life page for more details.

However, in the interests of safety, I feel I must draw your attention to the warning given by www.tardigrades.com:

Please note the following mental and health risks: in some case addictive behaviour towards tardigrades has been noted. And, even worse, young people showed an increased interest in non-commercial, zoological and even philosophical topics. As a rule excited readers can be successfully calmed down by means of scholarly biology lectures, e.g. featuring the properties of allium cepa or the difference between mitosis and meiosis. Please note that it might be unwise to mention tardigrades in presence of those biology teachers who have never heard of them. We do not want to be held responsible for nervous breakdowns or any other possible consequences that might be caused by tardigrade abuse.

Return of the water bears
Published 30 November 2009
False colour SEM image of two tardigrades, from here.

Having told you how to find your own tardigrades, I supposed the next logical step would be to say a few things about tardigrade ecology. For that, I shall draw heavily from the excellent reviews of Nelson & Marley (2000) and Nelson (2002).

Tardigrades may live in salt water, fresh water or terrestrially among mosses and leaf litter. However, because all tardigrades require at least a film of water to live in, the boundary between freshwater and terrestrial species is a trifle blurry and many species can be found in both. Tardigrades feed on plants and algae; their mouthparts have a piercing stylus through which they suck the cytoplasm out of cells. Different techniques are used for collecting marine and limno-terrestrial species, and I mention that solely because it gives me an opportunity to note that one of the methods for collecting marine tardigrades (and other sand-dwelling meiofauna) involves sieving material through a fine mesh net referred to as “Higgins’ mermaid bra” (or, depending on author, “Gwen’s mermaid bra”, as it was Mrs Higgins who invented the tool used by her husband).

Close-up of the head of the tardigrade Macrobiotus. The stylet apparatus is visible inside the head; the stylets are everted when the animal is feeding. Photograph by Martin Mach.

Tardigrades are well known for their ability to form resistant tuns when exposed to unfavorable conditions, a process called cryptobiosis. Five different types of cryptobiosis have been identified in tardigrades: encystment (production of a dormant phase without significant water loss), anoxybiosis (resistance to low oxygen levels), cryobiosis (resistance to freezing temperatures), osmobiosis (resistance to elevated salinity) and anhydrobiosis (resistance to desiccation). Not all tardigrades share all five resistances—for instance, anhydrobiosis (the best-known form) is only found among terrestrial tardigrades—and different species will have different degrees of resistance. Much has been made of the resilience of at least some tardigrade tuns, such as their ability to survive immersion for up to eight hours in liquid helium at -272°C (Rebecchi et al. 2007; for comparison, absolute zero is calculated to be -273.15°C) and even to survive exposure to the vacuum of space (Jönsson et al. 2008). However, the often-repeated claim that tardigrade tuns can survive for more than one hundred years seems to be unsupported (Jönsson & Bertolani, 2001, reviewed the 1948 report generally cited in support of this claim and found that the tuns tested in that report in fact failed to revive); tuns have not yet been definitely shown to survive for more than ten years.

Cryobiosis, the ability to withstand freezing, allows tardigrades to inhabit cryoconite holes like the one shown above in a photo from here. Cryoconite holes develop when darkly-coloured dust accumulates in patches on a sheet of ice; the increased heat absorption by the dark dust melts the surrounding ice, forming a small patch of liquid water. This water may then become home to bacteria, algae and other microscopic organisms released by the melting ice—a self-contained microscopic ecosystem where a nematode may be the most fearsome predator in town. The cryoconite hole may freeze up again when the winter comes, of course, but its inhabitants can wait in the ice for the sun to come again.

Systematics of Tardigrada
<==Tardigrada
    |  i. s.: Renaudarctus Kristensen & Higgins 1984 [Renaudarctidae]R-M86
    |           `--*R. psammocryptus Kristensen & Higgins 1984R-M86
    |         Dactobiotus ambiguusKF00
    |         Murryon pullariKF00
    |         Paradoxipus orzeliscoides Kristensen & Higgins 1989B92
    |         Chrysoarctus flabellatusTK98
    |         Wingstrandarctus corallinus Kristensen 1984PN19
    |--Thermozodium [Mesotardigrada, Thermozodia, Thermozodiidae]N02
    |    `--T. esakiiN02
    |--EutardigradaR-M86
    |    |  i. s.: Beorn [Beornidae]GE05
    |    |           `--B. leggi Cooper 1964W93
    |    |--ParachelaRS04
    |    `--Milnesiidae [Apochela]N02
    |         |--LimmeniusN02
    |         |--MilnesioidesN02
    |         `--MilnesiumGE05
    |              |--M. swolenskyiGE05
    |              `--M. tardigradum Doyére 1840RS04
    `--HeterotardigradaR-M86
         |  i. s.: Bryodelphax parvulusNH90
         |--ArthrotardigradaR-M86
         `--EchiniscoideaR-M86
              |--Oreella [Oreellidae]N02
              |    `--O. mollisNH90
              |--Carphania Binda 1978R-M86 [CarphaniidaeN02]
              |    `--C. fluviatilis Binda 1978R-M86
              |--EchiniscoididaeN02
              |    |--Echiniscoides sigismundiMS98
              |    `--Anisonyches Pollock 1975N02, R-M86
              |         `--A. diakidius Pollock 1975R-M86
              `--EchiniscidaeR-M86
                   |--HypechiniscusN02
                   |--Pseudechiniscus suillusNH90
                   `--EchiniscusRS04
                        |--E. blumiMS98
                        |--E. granulatusNH90
                        |--E. spinigerNH90
                        |--E. trisetosusNH90
                        `--E. viridissimus Péterfi 1956RS04

*Type species of generic name indicated

References

[B92] Bussau, C. 1992. New deep-sea Tardigrada (Arthrotardigrada, Halechiniscidae) from a manganese nodule area of the eastern South Pacific. Zoologica Scripta 21 (1): 79–91.

[GE05] Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press: New York.

Jönsson, K. I., & R. Bertolani. 2001. Facts and fiction about long-term survival in tardigrades. Journal of Zoology 255 (1): 121–123.

Jönsson, K. I., E. Rabbow, R. O. Schill, M. Harms-Ringdahl & P. Rettberg. 2008. Tardigrades survive exposure to space in low Earth orbit. Current Biology 18 (17): R729–R731.

[KF00] Kristensen, R. M., & P. Funch. 2000. Micrognathozoa: a new class with complicated jaws like those of Rotifera and Gnathostomulida. Journal of Morphology 246: 1–49.

[MS98] Margulis, L., & K. V. Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth 3rd ed. W. H. Freeman and Company: New York.

[N02] Nelson, D. R. 2002. Current status of the Tardigrada: evolution and ecology. Integrative and Comparative Biology 42 (3): 652–659.

[NH90] Nelson, D. R., & R. P. Higgins. 1990. Tardigrada. In: Dindal, D. L. (ed.) Soil Biology Guide pp. 393–419. John Wiley & Sones: New York.

Nelson, D. R., & N. J. Marley. 2000. The biology and ecology of lotic Tardigrada. Freshwater Biology 44 (1): 93–108.

[PN19] Poinar, G., & D. R. Nelson. 2019. A new microinvertebrate with features of mites and tardigrades in Dominican amber. Invertebrate Biology 138 (4): e12265.

Rebecchi, L., T. Altiero & R. Guidetti. 2007. Anhydrobiosis: the extreme limit of desiccation tolerance. Invertebr. Survival J. 4: 65–81.

[RS04] Regier, J. C., J. W. Shultz, R. E. Kambic & D. R. Nelson. 2004. Robust support for tardigrade clades and their ages from three protein-coding nuclear genes. Invertebrate Biology 123 (2): 93–100.

[R-M86] Renaud-Mornant, J. 1986. Tardigrada. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 254–262. E. J. Brill/Dr W. Backhuys: Leiden.

[TK98] Todaro, M. A., & R. M. Kristensen. 1998. A new species and first report of the genus Nanaloricus (Loricifera, Nanaloricida, Nanaloricidae) from the Mediterranean Sea. Italian Journal of Zoology 65: 219–226.

[W93] Wills, M. A. 1993. Miscellania. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 555–560. Chapman & Hall: London.

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