Passing from anhydrobiosis via coma (asphyxis) to active life

A typical tardigrade life story seems to be characterised by many risks and a continuous lack of shelter. Furthermore the tardigrade universe can be extremely small, just a tiny droplet of water. When this minute droplet of water is going to evaporate the tardigrade has no choice and has to resign to its fate - just letting things happen, folding its body to a dry 'tun', having no guarantee that a new droplet of water will actually be there somewhere in the future.
Just imagine if you would have to spend the night in a gutter of a big city being aware of the fact that you will be asleep but many other activities around you will actually continue and that you will not be able to control anything anymore.

Apart from the dry state (anhydrobiosis) of the tardigrades there is a further extreme situation called asphyxis. Asphyxis is caused by a temporary lack of oxygen which occurs under natural conditions from time to time. The famous tardiologist Ernst Marcus describes the phenomenon of tardigrade asphyxis as follows (translated from German, see literature)

"Tardigrades that were dry and that are revived by moisture either return to active life immediately  or   become stretched in an immobile passive state, the so-called   a s p h y c t i c     s t a t e    of a quite variable duration ..."

An explanation is given by Hartmut Greven in his nice German language tardigrade monograph (translated from German, see literature):

"Anoxybiosis happens when the oxygen content of the water sinks below a critical level. After an initial body contraction the animals stretch out (relax) to maximum length and are no more able to remove water that is penetrating through the cuticula. It is said that some species are able to survive up to five days in the asphyctic state."

The asphyctic state can be easily recognized due to the perfectly symmetrically stretched and completely immobile tardigrade bodies - in particular when most or all animals of a sample exhibit a similar, like-frozen behaviour.
As we have already seen in a previous issue, the tardigrade muscles work against the pressure ot the body fluid. When all muscles are relaxed the body therefore looks like a fully inflated swimming-pool crocodile. The following images are intended to illustrate the asphyctic state for typical eutardigrade and heterotardigrade water bears:

Water bear (eutardigrade), in the asphyctic state. Ventral view.
Raking light. Body length ca. 400 µm.

A further water bear (eutardigrade), in the asphyctic state. Dorsal view.
Body length ca. 500 µm.

Water bear (heterotardigrade),
in the asphyctic state.

Body length ca. 250 µm.

Tardigrades can undergo an asphyctic state at any time within their life history, e.g. also during a moulting process. In this case they will be found lying still within their cuticula:

Tardigrade (eutardigrade), in asphyctic state,
during a moulting process.
Body length ca. 350 µm.

At the moment when fresh water is added or when the oxygen content of the water rises the tardigrade comes back to active life. It doesn't lose time and goes on with the moulting process by means of rapid movements within the old cuticula. Pauses have to be avoided as the tardigrade can easily be attacked during moulting and as there is no chance to get some nutrition within the closed cuticula.

Revival from asphyxis (I).

Muscle activity control is signalized by some kind of trembling and an increasing body curvature.

Image width ca. 150 µm.

Revival from asphyxis (II).

The tardigrade continues its moulting procedure. Note the round eutardigrade "kiss" mouth and an eye-spot.

Image width ca. 150 µm.

Revival from asphyxis (III).

Clearly perceivable: mouth tube, strong curved stylets und delicate stylet supports.

Image width ca. 150 µm.

During asphyxis the muscles and the nerves of the targrades show a better contrast than usual and therefore can be studied more easily in detail. Just keep in mind that some basic insight into the nerv-muscle joint came from tardigrade studies!

We will have a closer look at those muscles and nerves in the next issue. Keep cool until then and avoid to become asphyctic or to fall into the dry state!

Literature (about asphyxis):

Hartmut Greven: Die Bärtierchen. p. 61. Wittenberg Lutherstadt 1980.
Ian M. Kinchin: the biology of tardigrades. p. 83-84. London 1994.
Ernst Marcus: Bärtierchen (Tardigrada). p. 28-29. Jena 1928.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

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