Tardigrades and polarized light (IV)
Normally, when looking at tardigrades under polarized light for the first time, you might
be slightly disappointed. Sometimes you will notice a few shiny particles in
the stomach-intestine region - nothing else. Like this:
Tardigrade stomach-intestine region as seen under polarized light.
Image width ca. 0.2 mm.
Most of these are simply mineral particles digested during feeding.
What about the tremendous polarized light colours which are well known from the study of
organic chemical compounds, artifical resins etc.?
Colours of a small piece of plastic film as seen under polarized light.
Image width ca. 5 mm.
The answer comes from optical theory: the tardigrades are simply too small
for most interference phenomena. Typical mineralogists' cross-sections
are prepared with a thickness of ca. 30 microns in order to optimize the chances
to receive interference colours. And we know that the tardigrade stylets are much thinner,
about 2 or 3 microns. But let's give it a try nevertheless:
Tardigrade (eutardigrade) head with stylets,
as seen under polarized light. Image width ca. 100 µm.
Technical annotation: the photomicrograph image background was taken in
an intermediate polarization situation, not in full extinction - for this reason
the background isn't black.
So, why do we still get interference effects though the
stylet thickness is very, very thin? First of all the existence of an interference effect
indicates that the stylet material is double refractive. And, in order to show
an effect at all at this low thickness it must be strongly refractive.
When looking at the mineralogical tables we find that e.g. quartz is very weakly refractive
(0,009), apatite as the main constituent of human teeth as well (0,003) but calcite
- according to the literature: the stylet material! - is one of the few "kings" of
double refraction (0,172).
So, even at very low thickness calcite is able to show the first interference colour (white-grey),
and in thicker layers at the stylet root the next interference colour, which is yellow.
In a nutshell, this interference effect actually confirms that the stylet chemistry is characterized by its
And do not forget that those minute calcite tools with a thickness of a few microns
are actually working tools for everyday feeding which is a kind of miracle, of course.
Moreover due to the fact that, even in this tiny dimension they are said to be hollow
for an optimum weight/durability ratio!
© Text, images and video clips by
Martin Mach (firstname.lastname@example.org).
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