Tardigrade Species Identification (Part II)
- Guest Contribution by Dr. Rolf Schuster *

Tardigrades have relatively few anatomical features suitable for classification by family, genus and species.

Part I of the taxonomy introduction (previous journal, July 2026) covered
1) The pharyngeal bulb and
2) The claws.

Part II of the taxonomy introduction (here) describes the cuticula - essentially the skin - as well as the eggs of tardigrades.

From September 2026 onwards, exemplary illustrations of individual species are planned - primarily species that are easily identifiable even for amateur microscopists. And, of course, with photos!


3) Taxonomically important features of the cuticula ("skin")
Heterotardigrada: The most common representatives of this group in Central Europe are the Echiniscus species. Their dorsal cuticula consists of a highly segmented, usually reddish appearing armor (figs. 1 and 2). This armor is typically equipped with body appendages (threads, also called filaments, and spines). The nomenclature used in scientific literature for the armor plates and appendages follows the basic principle below:

Fig. 1: Echiniscus back

Fig. 2: Echiniscus filaments and spines

Explanations: The Roman numerals in fig. 2 refer to trunk plates I to IV. K denotes the head plate; trunk plate I is also called the shoulder plate in literature.
The red labels S1 to S3 indicate the position of the so-called intercalated plates, which are located closer to the body than the more massive cover plates.
The capital letters A to E describe the positions where the body appendages my be found. In the example shown, filaments are only found at positions A and C; they are absent at B, D, and E. The filaments at level C are referred to as lateral, dorsal, or dorsolateral due to their different positions. In the animal shown here, position D also has a dorsally positioned spine on each side.

Depending on the species, the armor plates exhibit different surface structures:

Fig. 3: Surface of Echiniscus armor plates, and spines - top left and right.
Close-up at high magnification

Long filaments or short spines may also be found at the edges of the plates (fig. 4). Cuticula identification characteristics are therefore:
- Type and ornamentation of the plates
- number and position of the filaments and spines

Fig. 4: Surface of an Echiniscus back with spines and filaments

Eutardigrada: In eutardigrades, the cuticula is often smooth. However, some species also exhibit additional features such as spines, thickenings, or other structures.

Fig. 5: Eutardigrade cuticula with 8 rows of dorsal spines.
Phase contrast image.

Fig. 6: Eutardigrade cuticula with a uniformly distributed pore structure.
Phase contrast image.

Fig. 7: Eutardigrade cuticula with unevenly distributed, scarred structure.
Brightfield image.

4) Taxonomically important characteristics of the eggs
Eggs can either be left behind in the cuticula during molting and develop there, or they are laid freely in the environment, for example on moss leaves. In many cases, the freely laid eggs have species-specific structures. therefore, the eggs are particularly important identification characteristics in eutardigrades.

Fig. 8: Echiniscus eggs with a (typical) smooth surface, deposited in the cuticula. The middle egg has already undergone its first cell division. Image width approx. 0.3 mm

Fig. 9: Freely laid Eutardigrade egg, example 1.
Image width approx. 0.1 mm. Image taken under brightfield illumination.

Fig. 10: Freely laid eutardigrade egg, image example 2.
Phase contrast image.

Notes and literature

(*) Dr. Rolf Schuster, tardigrade specialist, partner and co-author of this taxonomy series, will be happy to advise you on more in-depth taxonomic questions and on the identification of tardigrades you have found. Simply send an email to Rolf Schuster !

Hieronim Dastych, The Tardigrada of Poland. Monografie Fauny Polski 16, 1-255. 1988.
Hartmut Greven, Die Bärtierchen. Wittenberg Lutherstadt 1980.
Ian M. Kinchin, The Biology of Tardigrades. London 1994.

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