Tardigrades and polarized light (II)
Frankly speaking, there is a series of potential questions by our readers which we are afraid of.
So far we are feeling lucky that no one appears to ask them.
One of those modestly impertinent questions might be:
"Why on earth are you using this ugly Comic font?"
Well, normally this might end up as a tricky subject but, thanks to the internet,
we are able to refer to somebody else
who will assure you that Comic is exacty the right font for our tardigrade pages!
But now let's go back to our main topic, the polarized light.
Before actually making use of the respective switch we should pause for a moment and wonder
what we are expecting from this particular illumination.
Polarized light shows strong colour effects with some inorganic crystalline materials
and fiber-like bundled structures. And we know that e.g. the tardigrade stylets
are said to consist of a hard, possibly crystalline inorganic material, similar to human teeth.
So, this material which might reveal specific properties when investigated by means of polarized light.
For a start let's look at some of the tardigrade stylets in normal brightfield illumination.
Obviously they differ in size, strength and curvature, some of them in addition might
come up with tiny morphological specialities:
The eutardigrade Macrobiotus hufelandi has strong,
curved stylets and fine stylet springs.
The giant eutardigrade Adorybiotus coronifer
is equipped with similar strong stylets. In this case the curvature is less pronounced and the
stylet springs look quite different from those of Macrobiotus hufelandi shown above.
The carnivore eutardigrade Milnesium tardigradum
comes with comparatively weak stylets and extremely small stylet springs.
When looking closer at the photograph you will notice that the stylet springs
are resting in stylet sheaths. Possibly Milnesium tardigradum
doesn't need strong stylets because it has a tendency to devour its victims as a whole!
This heterotardigrade Echiniscus sp.
reveals fine linear stylets, typical for all members of the genus Echiniscus.
a marine heterotardigrade shows similar stylets as its terrestrial Echiniscus
Once more, similar Echiniscus-style stylets of a marine Florarctus sp. heterotardigrade,
but with stronger curvature
So, with some species the stylets and in particular the style springs
are not easily visible at low magnifications. The stylet diameters can be as low as a few micrometers (!)
which is quite astonishing for a tool which must serve reliably for everyday object piercing and feeding.
Hartmut Greven mentions in his tardigrade book that some of those stylets
- even though being that much tiny - can be hollow, making use of the physical fact that
a tube has similar mechanical properties as a rod of the same diameter.
The chemical composition of the stylets
Many microscopists are using chemicals extensively. For this (brute) reason
they happened to find out already long ago that the tardigrade stylets tend to disappear when
kept in slightly acidic media. The simple conclusion is that the stylets are
made up of some acid-soluble compound. We didn't find respective chemical analyses in
the literature yet. But e.g. Hartmut Greven states in his book that the tardigrade
stylets appear to consist at least partially of calcite. Possibly this statement goes back to
Ferdinand Richter who talked about "stylets consisting of calcium carbonate" (Mikrokosmos, 1908).
Now the question is: how could we possibly make use of polarized light to
check the chemical character? In order to do so whe will have to look closer at the properties of calcite
from other sources when studied under polarized light. And we will start with this task in the next issue.
Hartmut Greven: Die Bärtierchen. p. 31-33. Wittenberg Lutherstadt 1980.
Ferdinand Richters: Die Bärtierchen (Tardigraden). Mikrokosmos 1 (1908) p. 53-57.
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