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Dear tardigrade enthusiasts, possibly you are on the verge of getting bored by tardigrade moulting which has been subject of our recent issues. But we have to come back to it once more in order to bridge over to a new and fascinating chapter of tardigrade anatomy.

We had found out that the empty cuticula of our Ramazzottius oberhaeuseri  water bear has a fine network structure. But we didn't go into a deeper discussion about the advantages and drawbacks of this kind of structure.
When studying the cuticula in an advanced Sherlock Holmes manner you will find out that it has in fact two 'ghost-eyes' which normally cannot be seen when looking at the complete, living animal.
Even though the photomicrograph shown below is not competitive in terms of aesthetic and technical quality - kind of cellar portrait - the two lens-like spots in the tardigrade head region become apparent. The cuticula is fully transparent in those areas and has no granulation.

[ Ramazzottius oberhaeuseri, cuticula, detail ]

Ramazzottius oberhaeuseri,
empty cuticula, detail of head with 'ghost eyes'.

The lens-like structures have a maximum diameter of about
8 to 10 µm.

As we are amateurs it should be allowed to use our phantasy in order to comment on our findings:

"We think that those areas are smooth and transparent as they are just above the eyespots and because a granulated area would spoil the optical properties of the eyes, similar to a frosted glass"

In order to maintain our hypothesis against potential protests we should play safe and better prove that the tardigrades have in fact usable eyes and that some kind of image viewing might be possible by means of those eyes. In particular a higly developed eye would definitely take profit from a transparent windowpane.

Therefore let's have a look at the scientific literature first in order to see what the experts are saying about the existence and properties of tardigrade eyes. Here is a choice of quotations picked out of some of the best tardigrade monographs:

"There are inverted pigment ocelles on the eye lobes of the upper parts of the brain which consist of a single eye cell each ... " (translated from: E. Marcus: Tardigrada. p. 7. Berlin 1936.)

"The cirri and the pigment beaker ocelles serve as sensory organs. The ocelles are placed on the upper part of the ganglion and consist of single eye cells" (translated from: A. Kaestner: Lehrbuch der Speziellen Zoologie. Vol. I: Wirbellose, 1st part. p. 589. 3rd ed. Stuttgart 1969.)

"There is a distinct anterior region which may or may not bear   eyespots  and  cephalic appendages " (C.I. Morgan, P.E. King: British Tardigrades. p. 2. London 1976.)

"Eutardigrades and Echiniscoidea have eyes (ocelles) positioned more or less rostral on the outer lobes of the upper ganglion. The eyes are said to consist of a single sensory cell in a pigment beaker which is opened towards the lateral front-end of the tardigrade body. The eye pigment is normally black, Echiniscidae have red eyes, 'blind' tardigrades have no pigment at all.
...  At present there are no ultra-structural investigations about the anatomy of those organs available." (H. Greven: Die Bärtierchen. p. 25. Wittenberg Lutherstadt 1980.)

"In molte specie, sia di Etero- che die Eutardigradi, esistono occhi posti sui lobi esterni del cerebro, e costituiti da una sola cellula fotosensibile. Spesso (ma non sempre) la cellula fotosensibile è circondata da macchie pigmentate costituite da granuli più o meno sparsi, neri negli Eutardigradi, generalmente rossi, ma talora marrone o neri, negli Eterotardigradi Echiniscidi" (W. Maucci: Tardigrada. p. 14. Bologna 1986.)

"A pair of cup-shaped pigmented eyespots associated with the lateral lobes of the cerebral ganglion is also present in many species" (I.M. Kinchin: The Biology of Tardigrades. p. 52. London 1994.)

In short, the scientists agree that tardigrades have eyes but state at the same time that these eyes are in fact very primitive and are made up of a single cell only, sometimes in combination with a little bit of pigment. Hartmut Greven goes on one step further and mentions the pigment beaker opened in an angle to the outside world, a construction which would possibly allow some kind of directional eyesight. So he doesn't completely destroy our secret hope that the tardigrades might at least be able to note the direction from which an incident light ray is arriving. Furthermore Hartmut Greven mentions the obvious lack of scientific work on the properties of tardigrade eyes.

As we have seen the miraculous miniaturization e.g. of tardigrade claws and  muscles   we might become a little bit depressed now.

Our fine, highly developed tardigrades which fight their way though the moss jungles, which are able to combat against bacteria in the relative size of a sausage and which do survive on the bottom of ice glaciers - those tardigrades are assumed to have a primitive eyesight only, like a worm?

Depending on your personal temperament you might end up in crying now or hurry up to the next public library or have intense looks at some more tardigrades under your microscope. We have decided to adopt one of the less dramatic pathways (library).
Already 100 years ago the scientists have been thinking about the eyesight properties of the so-called lower creatures. In William Carpenter's book of 1891 we even found an image photographed through (!) an isolated fly eye, illustrating the visual impression which a fly might have when looking around in the world.
One further fine example is the monograph by Richard Hesse (see literature). Hesse explains the various degrees of sophistication of eye construction, starting with a simple light detector, which is just able to tell 'light' from 'no-light', going over various development stages until to a lens-eye allowing image views. Hesse's book also helps to come to mental piece with the given fact that some tardigrades have eye pigment whereas others, moving and behaving identically, have no eye pigment at all. He simply states that also human albinos with little or no eye pigment at all are able to see in an equivalent quality as the rest of us humans.

And, in the end, we were successful and came across a very dusty issue of an old zoological journal with a very impressive statement concerning the tardigrade eye (translated):

"I just ... would like to add, that the tardigrade eye ... in contrary to previous statements by xxx ... is by no means a primitive pigment spot. The eye is rather complicated and has a homogeneously curved, very conspicuous lens."

Are we allowed to believe this author who might have been the father of our grandfather? We will continue exactly at this point in our next issue. See you.


Walter Maucci: Tardigrada. p. 256. Bologna 1986 (two figures represent the ghost eyes of Ramazzottius oberhaeuseri).

Richard Hesse: Das Sehen der niederen Tiere. Jena 1908 (very good, but only in German language).

William Carpenter: The microscope and its revelations. p. 908. 7th ed, London 1891.

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