We have finished the rather controversial  alien story   in the last issue and will turn to a much more neutral topic: the respiratory and circulation system of the water-bears.
First of all, there is a simple statement in the chapter "tardigrades" within Kaestner's comprehensive biology textbook: "respiratory system and circulation system are missing".

So what? Why should we discuss systems here that do not exist at all?

But the situation is not as simple as that.

It will take a few sentences to explain. As we have mentioned already previously biologists encountered problems when trying to place the tardigrades in an existing category within the kingdom of animals. So they decided to contribute a tribe to them, which is quite an honour when keeping in mind that e.g. all the vertebrates are packed in a single tribe as well. The terrestrian tardigrades can be divided into two distinct groups: The Heterotardigrades (armoured, hairy) and the Eutardigrades (unarmoured, without hairs). When we focus our microscope on a Heterotardigrade back, we get an image like the one shown below::

Living Heterotardigade (Echiniscus sp.) with grainy armour plates and hairs. Image width ca. 250 µm.

Obviously, the organism within those armour plates will need some oxygen to breathe. As there there are no lungs, no heart and no circulating blood, it must be assumed that the oxygen enters the tardigrade just by means of diffusion, i.e. that the tardigrade body is small enough that the oxygen can pass through it without problems. So, once again miniaturization turns out as an advantage: tardigrades are not only so tiny that many potential enemies just overlook them but moreover they will never suffer from heart diseases and lung cancer as they can live without heart and lungs (and BTW: smoking is not possible under water ;-).

But some of the bigger Eutardigrade water-bears would encounter problems when just relying on oxygen diffusion only. E.g.  Milnesium tardigradum  is rather big and moving quickly, so it will need more oxygen than those smaller and slower armoured Heterotardigrades. As always, there is a way out of it. Remember what we are doing when we try to enhance diffusion, e.g. of milk in coffee. Right, we are stirring the system a little bit. The waterbears do just the same thing: they are stirring themselves! You will notice in the film below how Milnesium is moving and by this movement is pumping its body fluid through the body cavity.

Eutardigrade water-bear Milnesium tardigradum.
Length ca. 500 µm. Dark field illumination.
The body fluid is pumped through the body cavity when the animal is moving. So the higher oxygen amount needed for the movement is provided by the movement itself - which is a fine example of an ingenious self-regulating system.

This kind of "circulation" has the advantage of being very simple and effective at the same time.

Literature

Hartmut Greven: Die Bärtierchen. P. 21. Wittenberg 1980.

© 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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