Zoological systematics

- How to fit water bear individuals into the kingdom of animals -

WB web base #18

On the cover of one of the tardigrade monographs by Ernst Marcus we find the Latin inscription: Sine systemate chaos . The user of the book by this is asked to accept that any study of zoology doesn't make much sense without an inherent intellectual framework. As the Water Bear web base is aimed towards microscope amateurs we might of course simply point out our amateur status and pass on all the difficult problems of taxonomy to the professional scientists.

It should be a warning for us that the key for determining water bear species in Hieronim Dastych's book about the Tardigrada of Poland comprises ten full text pages.
The water bear species in particular have the reputation of being difficult to discern. Heinz Streble und Dieter Krauter, famous authors of the most popular textbook on pond-life in Germany (see literature) discuss this problem - as everything else in their book - in a straightforward, short and honest manner:

The exact species determination on the basis of claw type, egg form and sclerified inclusions (macroplacoids) within the buccal aparatus is difficult.

On the other hand zoological systematics provide us with a unique tool for precise communication with other people.
Each animal within the Kingdom of Animals has its own unique name as defined by the rules of Linné.

Keeping in mind the complications of tardigrade species determination we will have a very simplified look at species definition here.

First of all it is important to understand the basic nomenclature of species. Let's look at an example:
The water bear species name   Macrobiotus areolatus MURRAY, 1907  bears systematic information as follows. The first term "Macrobiotus" indicates the order (genus) name which is the same for a group of similar (related) species. The genus name is unique for all animals and therefore is a very sharp criterion. The second term "areolatus" indicates the exact species within the genus group, i.e. it is lower in systematic hierarchy and at the same time even more precise. As a rule the species category is defined by the ability to produce children wheras different species will not be able to mate successfully.
"MURRAY" is the name of the person who discovered (described) the animal for the first time with sufficient professional quality. "1907" is the year of the publication. It is not absolutely necessary to add the name of the discoverer and the year of the publication, as a rule these data are added just for additional clarity.
When we look up the genus name "Macrobiotus" within the reference literature, we will also find the higher levels of the zoological hierarchy. In our example the genus Macrobiotus is a part of the higher grouping  Eutardigrada. The Eutardigrada again are a part of the water bear  phylum  .

The red cells within the table below show the line of determination beginning with the phylum, passing on to the order Eutardigrada, the genus Macrobiotus and, as an example, the exact species Macrobiotus areolatus.


Animals (zoology)


Tardigrada (water bears)

overall 30 phyla with an estimated total of ca. 1 mio. species






and others

Hypsibius, Macrobiotus,
Milnesium ...



several hundred species

Macrobiotus areolatus and about hundred further species

only a single species: Thermozodium esakii RAHM

Some species, e.g.  Milnesium tardigradum  can be recognized easily. Nevertheless the genus  Milnesium  has developped into a more complex system with several newly discovered species recently, so even this island of simplicity has become deceptive.
After some basic study the reference literature will allow a rough determination e.g. down to the genus level. On the other hand it should be kept in mind that most species determinations methods can be applied only to killed and chemically preserved animals. As amateurs we should prefer to enjoy and respect the living animals though their exact species names might remain nebulous sometimes.


Ernst Marcus: Tardigrada. Berlin 1936.
Some definitions by Marcus meanwhile are considered as outdated by more recent work. Nevertheless, still today the book is very interesting to read as it provides numerous examples how to discuss and how to bridge controversial taxonomic points of view.

Heinz Streble, Dieter Krauter: Das Leben im Wassertropfen (i.e. "Life in the water droplet"). Stuttgart 1973, S. 99. In Germany the "Streble-Krauter" is the bible of the microscope amateur for water animalcules. It contains many hundreds of line drawings for species determination of the more abundant microscopic species. The very short chapter about water bears is worth reading as well.

The probably best and most modern taxonomic book is:
G. Ramazzotti und W. Maucci: Il Phylum Tardigrada. Memorie dell'Istituto Italiano di Idrobiologia 41 (1983) S. 1-1012 (in Italian language only). But there seems to be an English translation by professor Beasley.

Hartmut Greven's book "Die Bärtierchen" (1980) is the only modern German language book. Many people are trying to get one at antiquarian book stores. A second edition is planned for end of 2004 or 2005. The first edition has lots of interesting information but contains no table for taxonomy down to the species level.

Ian M. Kinchin, The Biology of Tardigrades, London 1994, is very expensive, in particular when counting the pages and moreover the white pages, and it offers no species determination as well. But it contains fine illustrated descriptions of the most abundant water bear species like  Milnesium tardigradum  and  Macrobiotus hufelandi  .

Hieronim Dastych: The Tardigrada of Poland. Warszawa 1988. Here we have a species determination table. If you should stumble upon a copy of this book I would recommend to buy it.

Besides the classical literature focusing on species determination by means of morphological characteristics there is an increasing number of publications featuring the genetic relation as a measure and additional assistance to classical taxonomy. See e.g.
James R. Garey, Diane R. Nelson, Laura Y. Mackey, Jia Li: Tardigrade Phylogeny: Congruency of Morphological and Molecular Evidence. no date (probably about 1995), but it should (hopefully) be quite close here !

WB web base #18

©Text and images by  Martin Mach