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.
Hartmut Greven: Die Bärtierchen. P. 21. Wittenberg 1980.
© Text, images and video clips by
Martin Mach (firstname.lastname@example.org).
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.