In search for a way out - the Milnesium contest

For our new readers we will provide a short summary of the visually perceptible tardigrade development stages between moulting with egg deposition and hatching. A small series of photomicrographs featuring  Milnesium tardigradum   is well suited in order to demonstrate the various activities.

During the moulting phase the tardigrade mother spends some time within the old cuticula. By depositing its eggs within the shed skin it provides an effective shelter for the babies from the very beginning.

Zoological development is constant change and constant stress. After the exhausting (full diet) moulting phase and after the painful egg deposition the tardigrade mother has to work its way out of the cuticula through a small opening of the cuticula front end.

Tardigrade  Milnesium tardigradum  during moulting with egg deposition,
still in the old cuticula.

Tardigrade Milnesium tardigradum (other individual) during the moulting phase with parallel egg deposition, still in the old cuticula.

Same as above, detail view of the two eggs, cell division borderlines visible.

Finally the tardigrade is crawling a little bit on the outside of the cuticula.
The situation resembles a technical inspection - but the behaviour might as well be caused by the lack of natural moss substrate in the surroundings:

Tardigrade Milnesium tardigradum, crawling on cuticula with deposited eggs.

Depending on the nutrition state and on the kind of species we will come across tardigrade egg deposits with far more than just two or three eggs:

Tardigrade egg deposit with eleven eggs in the so-called morula stage (mulberry cell cluster, multi-cellular but still undifferentiated). Image with ca. 0.5 mm.

After about a week the eggs will have reached a stage of maturity which does already reveal many characteristics of the adult tardigrade: a wide bucchal tube (remember that Milnesium is a predator sometimes devouring its prey as a whole), small stylets, transparent tissue etc.

Detail view of a Milnesium tardigradum egg deposit with eggs in a more mature stage.

A little bit later, typically in the morning, we will note a kind of common tardigrade baby haste. Some have hatched already, some are still vigorously moving within their eggs. As the tardigrade body volume is drastically increasing after hatching and as the tardigrades start moving across the cuticula in search for a way out, we encounter a very lively, chaotic situation. The passive and active elements within the cuticula bounce against each other thus e.g. revealing the elastic properties of the empty egg shells:

Detail of a Milnesium tardigradum egg deposit. Empty eggshell, baby tardigrade and eggs in earlier state of development.

The baby tardigrades inspect all the cavities of the cuticula and constantly cross each others pathway. Some are simply searching at the wrong end:

Others look for an exit within the leg cavities of the cuticula:

We notice the heavy traffic within the center of the cuticula. From time to time we will be able to perceive anatomic details as well, like buccal tubes (first image below, two bucchal tubes in the dark area on the left) or a double claw (second image below, red arrow):

The correct direction to the actual way out on the front end is being inspected as well but still obstructed by a cuticula fold which tends to redirect the tardigrades in a bow back - misleading them:

... further, unsuccessful trials at the leg cavities follow:

But, finally one of our freedom fighters will succeed. We have devoted the following animated GIF to her and her success. Please have some patience and follow the development of the animated GIF:

Bravo! The other baby tardigrades will soon make use of the pathfinder escape and will follow within minutes and hours.
Please allow an emotional amateur microscopist's comment: have we become eyewitness of a sweet success now - a success arising from a permanent struggle for survival and from a permanent brother-and-sister-competition? Possibly, yes, probably, yes - we are sure that you will be able to note some parallels with regard to your own existence on earth as well.

© 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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