Tardigrades: color and transparency

When running a web image search on tardigrades you will find out that most of the images shown are in fact Scanning Electron Microscope images (SEM images). Those SEM images will provide excellent three-dimensionality and detail.

But at the same time they are also somewhat misleading, giving a wrong impression of the actual tardigrade visual appeal: a scanning electron microscope is not able to gain color information from the objects under investigation (as electrons do not bear any color information). As a consequence all those SEM images are gray-scale images only. Film producers and print media managers are afraid that those primarily black-and-white photomicrographs might be considered as dull and old-fashioned by the broader audience. We are living in a world boasting of highly color saturated image media. One way out of this situation is to paint the SEM images with artificial color (so-called pseudo-color) in order to impress the multimedia consumer. But when looking closer and when comparing with the real thing you will notice that the artificial colors are by far too homogeneous and too saturated: the moss leaves are usually painted with a highly saturated green, with the visual appeal of an artifical pizzeria lawn. And the tardigrades are lacking the fines shades of color caused by their inner anatomy, their stomach content, their eyes and pigment spots.

But, as an estimated 99.9% of the spectators will never have seen a real tardigrade in its real environment the well-meant paint fraud will usually remain unnoticed. And yes, we might actually accept it as some kind of didactic help. But of course we should be aware that all this is far from reality. It goes without saying that no tv news spectator would accept a painting book image of Donald Trump or Beyoncé instead of the much more realistic color photographs.

In case of the tardigrades the problem lies in the fact that the artificial mix cannot be recognized as such by an average image consumer with a limited knowledge of microscopy and science in general. The following image might help to understand why a monochrome tardigrade body surface paint is far from reality:

Light microscope photograph, with partial incident light: Echiniscus sp. tardigrade, crawling around the tip of a moss leaflet. A complex color and structure mix is arising from the partial transparency (note the stomach-intestine line within the inner volume) in combination with the reddish inner body fluid and the light refraction play due to the physical characteristics of the thin cuticula (a color of thin plates effect, some of you will possibly remember from their physics lessons at school).

As a consequence the above picture is portraying the tardigrade in a manner which is close to our human visual perception (in particular incident microscope illumination is very similar to what we are considering as normal everyday light). But please keep in mind that we are not talking about morals: the body painting of the SEM images can be accepted - but for the sake of seriosity one should hope to find a clarification that the respective images are artificially colored.

And we should point out here that the ordinary "transmitted" light of the bright field microscope might as well be critizised as being somewhat artificial, though not as artificial as the SEM color constructs. Transmitteld light has a tendency to reveal inner anatomy, similar to an X-ray image. When looking at our intro video you will probably understand that the inner structures (stylets, stomach) are enhanced by means of the transmitted light, whereas the "skin" of the tardigrade and its outer appearance might be better accentuated by purely incident light.

The SEM is marking an utmost extreme in concentration on surface detail, completely neglecting all inner detail. As a consequence a SEM image cannot depict the tardigrade eyes: the tardigrade eyes become invisible. A common walk-around is the "use" of tardigrade skin wrinkles. Those wrinkles will perform the job in so far that human perception can accept them as some kind of eyes and sum up everything to a "face". Overall the SEM image is based on artefacts but it might work as well - as long as you have never seen a true tardigrade under a microscope.

The tardigrade scientist Ferdinand Richters characterized the tardigrade Milnesium tardigradum as a "glass house", thus pointing out the immense importance of tardigrade transparency. The video below will help to understand that the classical light microscope is far superior in its ability to penetrate the tardigrade inner anatomy:

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Light microscope video, mixture of incident and transmitted light: head region of Milnesium tardigradum in natural movement. Only the light microscope is able to reveal the inner bucchal anatomy and the eyes with their transparent lenses (!) above the eye pigment.
The SEM electrons would be stopped from penetrating in this scenario immediately after having passed the first few microns of the cuticula. As a consequence they cannot depict the eye pigment situated below this cuticula.

Last but not least you will not come across SEM live videos in the internet, as the tardigrades would have to be filmed in water - and water cannot be penetrated by electrons either. In contrast an ordinary light microscope can provide live videos of tardigrade birth, of the cell divisions in the egg, of nerves and muscles, of the moulting process, the deposition of the eggs, the desiccation and awakening of the tardigrades and much more.

To sum up we are far from blaming the SEM for its defects but we would like to point out that only the light microscope will be able to supply live biology in the true sense of the word. So we would like to end with the following encouragement:

"Simply enjoy the false-color images - but don't forget about reality!"

© 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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