Tabula rasa II: Cellular boundaries and the limits of perception

Okay, we might have called this chapter "Bin shed roof population (part V)". And in fact it is actually based on our moss tardigrade findings on a bin shed roof - which had been thoroughly cleaned by somebody else afterwards (and, as some people might suspect with a typically German cleaning zeal). But our PR advisor suggested otherwise, arguing that the respective title might diminish our modest share of public attention.

Fig. 1: Bin shed roof, as in July 2019. In this place we had found a rich tardigrade population, as already reported previously (cf. this year's magazine issues from February to April). We had found - not surprisingly in an urban area - the carnivore tardigrade Milnesium tardigradum, but moreover rich quantities of charming red Echiniscus tardigrades.

In times of increasing travelling restriction it might be worth while to note that you needn't necessarily travel to one of Planet Earth's poles in order to find fascinating tardigrades. But please don't get us wrong: we like travelling to distant oceans and we hope that the globalization of a common human awareness might survive - somehow.

Already in our previous magazine issue we had shown the well-fed female Ramazzottius tardigrade from the bin shed roof:

Fig. 2: Ramazzottius oberhaeuseri tardigrade, as found in one of our moss samples before the radical cleaning. Body length ca. 0.3 mm and in perfect condition!

For a deeper understanding of the Ramazzottius anatomy we would like to resort to one of our older photomicrographs - taken in the stretched, so-called asphyctic state which is one (of several ingenious) tardigrade survival tricks:

Fig. 3: Ramazzottius oberhaeuseri in the so-called asphyctic state (here in an older photomicrograph, just for comparison). The asphyctic state is occurring in situations with lack of oxygen, e.g. when there is little water in combination with lots of soil. The tardigrades can temporarily cope with this situation by entering the asphyctic state which might be considered as a kind of unconsciousness. In the asphyctic state some anatomical details come out more clearly than in the normal, active state of the tardigade. E.g. the absence of eye pigment ('no black eye spots') is more apparent. Furthermore the pharyngeal bulb and its characteristics can be studied more easily.

In addition we have a further image - not shown previously - representing the head area of Ramazzottius oberhaeuseri and furthermore a photomicrograph of the characteristic egg, which resembles a youngster's hair fashion:

Fig. 4: Head area of Ramazzottius oberhaeuseri. Image width ca. 100 µm.

Fig 5.: Egg of Ramazzottius oberhaeuseri (older photomicrograph). In this later stage of egg development some details are becoming apparent: a fully developed mouth tube, the stylets and the stylet springs. Egg diameter ca. 70 µm.

With a little bit of microscopist's luck we can perceive single cells and cell nuclei in the tardigrade epidermis (~ the tardigrade skin).

Fig. 6: Detail of the back side of Ramazzottius oberhaeuseri. Active state. Image width 180 µm.

As explained by Hartmut Greven in his fine tardigrade monograph, the specialists tend to count the numbers of cells in rows and columns. The outcome of this work was that in the case of Ramazzottius oberhaeuseri there is a constant pattern, only changing with age. And there was a subsequent hyothesis that the number of tardigrade cells was a kind of a constant. Tragically an italian tardiologist found out a few years later that cell divisisons do occur in the adult tardigrade as well - thus destroying a beautiful hypothesis of simplicity!

The total number of tardigrade cells can be counted only approximately, because most of the cells cannot be visualized in a similar manner as in the epidermis. The total cell numbers as reported in the internet vary between 1,000 und 40,000 (!) cells. But even the lower number of assumed cells is quite impressive when taking into account the overall tinyness of a tardigrade. Just keep in mind that a similar-sized Paramecium caudatum or an amoeba are made up of a single cell!

In case you would like to direct the interest of your children away from biology, e.g. towards business is is advisible to let them study the cell structure not on the basis of tardigrades but on the basis of onions. You might as well use a bottle cork:

Fig. 7: The cell structure shown here, taken from a wine bottle cork, is marking a milestone in the history of science: Robert Hooke (1635-1702) used it as a demonstration object that most living organisms are actually made up of tiny compartments which he called cellulae.

In case you should prefer the tardigrade as an object for cell study you might have a look at the early egg devolopment of the tardigrade shown here:
Tardigrade evolution biology!

Please keep in mind that the tardigrade is considered as the smallest articulate animal on earth - as a consequence it is transparent and offering live insight into all its life affairs!

Literature

Hartmut Greven: Die Bärtierchen. p. 18/19. Lutherstadt Wittenberg 1980.

Robert Hooke: Micrographia. London 1665.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
The 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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