One fascinating facette of the tardigrade world are tardigrade eggs. There are hundreds of different shape varieties some of which exhibit an extraordinary symmetry and a tremendous geometric complexity. In particular in the Italian and early German publication reviews we find an enormous wealth of different egg designs some of which might be misunderstood as a funny joke. E.g. Marcus presents an egg of  Macrobiotus polyopus   the surface of which consists of a complete network of regular hexagons.
But first of all we should have a short look at the the framework conditions which rule tardigrade egg development.
The number of eggs depends on the tardigrade species und the nutrition state of the water bears. Some authors think that tardigrades are even able to redigest their own eggs in order to survive poor nutrition conditions. The highest numbers of eggs reported in literature range between 30 and 40. The overall maturation time until to the moment of hatching is in the range of several weeks. In a similar way as with the grown-up water bears the eggs are also able to survive complete desiccation. In the dry state the process of maturation comes to a halt.
The eggs are deposited in two different manners: some tardigrade species deposit their eggs freely, whereas others like Milnesium tardigradum  leave the eggs in the old empty cuticula after the sloughing process.

Under the microscope you will come across situations like the ones depicted below:

Empty skin of Milnesium tardigradum with two eggs (diameter ca. 100 µm)

Four eggs of another tardigrade species (Echiniscus group) in cuticula. The structure of the skin of the mother is still visible as well as the red colour of the youngsters. This is by the way the species represented on the header of this magazine.

In contrary to those eggs deposited in the cuticula as shown above, freely deposited eggs have so-called egg processes which are characteristic for the different species. As an example you will see one egg of the cosmopolitan water species  Macrobiotus hufelandi  below.

Single egg of Macrobiotus hufelandi deposited freely. Diameter ca. 100 µm

Unfortunately, the eggs are problematic objects for a light microscope. First of all they are difficult to find, mostly of transparent or greyish appeal and moreover often glued to non-transparent material like stones and plant fragments.
In an early stage of development the eggs are not transparent at all and as with any spherical object under the light microscope there is an inherent problem, the fight against insufficient focus depth.

The b/w images shown below are computer generated images which have been calculated by means of a raytracer software. It is not so easy and an interesting mathematical problem to have those egg processes distributed all over the surface rather evenly but not in a strict manner. Ian M. Kinchin (see literature) explains the function of the egg processes in his water bear monograph as follows:

- Anchor to fix the egg to a substrate or a transporting medium
- Defensive against being eaten by other animals
- Water reservoir which slows down the desiccation process
- Regulation of gas exchange between egg and environment

And furthermore, they are nice to look at:

Computer models of tardigrade eggs. Right: Slightly oval egg by Macrobiotus hufelandi

Very nice drawings of tardigrade eggs can be found in the monograph by Ernst Marcus quoted below. There are egg processes resembling flowers or even Easter bunnies - really unbelievable.
Modern scanning electron microscope (SEM) images can be found in the more recent international literature in particular those publications by anglo-saxon and italian authors.

In the next issue we will continue to discuss the properties of the tardigrade eggs.

Links und literature

Ernst Marcus: Bärtierchen (Tardigrada). Gustav Fischer Verlag, Jena 1928.
- with many drawings of eggs. Even today among the very few publications which discuss egg development in detail. -

Ian M. Kinchin: The Biology of Tardigrades. S. 62-67. Portland Press, London 1994.
- good overview and SEM images -

© 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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