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While reading this article about tardigrades that had been previously frozen, I came across some verbiage I wasn't sure of and couldn't find any real explanation given the context. The article kept making mention of the tardigrades "in tun form", yet the only references I could find to the meaning of tun dealt with weight or casks (like for holding liquid).

So does tun mean cask (like a casing) in this context?

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  • $\begingroup$ I was wondering the same thing for a good long while, but I have come to the same conclusion that tuns are a folded-up version of the tardigrade that slows down the metabolism of the creature. >>>Tardigrades are microscopic, yet almost invincible one-celled organisms.-Anonymous $\endgroup$ – Tardigrade01 Feb 16 '17 at 23:12
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From Illinois Wesleyan University:

One way in which tardigrades have adapted to various types of environments, has been to reversibly suspend their metabolism. This state is known as cryptobiosis [ 'hidden life' (Clegg, 2001) ] and is a truly deathlike state. Metabolism lowers to 0.01% of normal or is entirely undetectable and the water content of the body decreases to less than 1%. [...]

The most intensely studied type of cryptobiosis is anhydrobiosis, a form of cryptobiosis initiated by desiccation. [...] Anhydrobiosis is an almost complete loss of body water and the animal can stay in this state for an extended period of time. Tun formation, a vital part of the process [Fig. 1.], results in a body that is constricted and folded. The first step is invagination of the limbs, longitudinal contraction of the body, and infolding of the intersegmental cuticle. Wax extrusion covers the surface and may help to reduce transpiration (water loss by evaporation). The tun formation process requires active metabolism. [...]

Tardigrade tun state
Fig. 1. Tardigrade before and after entering the tun state. Note the decreased body length and infoldings. Source: Media & Society Journal.

Reference
- Clegg, Comp Biochem Physiol B Biochem Mol Biol (2001); 128(4): 613-24

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