Return of the Water Bears (Taxon of the Week: Tardigrada)

False colour SEM image of two tardigrades, from here.

If you're a long-time reader of this site, you're probably already aware of the existence of tardigrades or water bears, microscopic stumpy-legged invertebrates. Previous posts on Tardigrada have given an overview of the main subgroups of tardigrades, and suggested how you might find your own specimens. The next logical step, I suppose, would be to say a few things about tardigrade ecology, and 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.

In one of my earlier posts, I referred to the well-known ability of tardigrades to form resistant tuns when exposed to unfavorable conditions, a process called cryptobiosis. What I did not explain at that time was that 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.

REFERENCES

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.

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

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

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