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

Tardigrades have relatively few anatomical features suitable for classification by family, genus and species. In this Part I of the taxonomy introduction, we first examine the particularly chracteristic pre-digestion region and the shape of the claws.

Part II of the taxonomy introduction, with other features that are also important for identification, will follow in August. From September 2026 onwards, exemplary illustrations of individual species are planned - primarily species that are easily even for amateur microscopists and that are relatively common in Central Europe. And, of course, with photos!

1) Taxonomically important features in the anterior digestive region
The stylets of tardigrades (fig. 1) consist of particularly resistant, mineral-reinforced composite material. In their function, they are therefore quite comparable to sharp daggers. The stylets can be thrust outwards through the mouth opening and are typically used to pierce plant cells. The predominantly liquid contents of these plant cells are sucked in by the tardigrade, passing through the buccal tube into the rounded pharyngeal bulb. From there, the food travels towards the stomach, is chemically digested, and finally excreted ventrally in front of the hind legs.
The pharyngeal bulb is contractile (see fig. 2). It can contain so-called macro- and microplacoids. These consist of hard material and, as can be seen in fig. 2, might press concentrically against each other (though this function is subject to contradictory discussion among scientists). So, they might crumble solid food particles like a nutcracker - or possibly be unable to do so ;-).

Fig. 1: Feeding and predigestion area of tardigrades (as seen in top view). One the left, a representative of the genus Macrobiotus; on the right, a representative of the genus Diphascon, each equipped with a mouth opening (at the very top), buccal tube and the so-called pharyngeal bulb. The pharyngeal bulb is sometimes - and perhaps even more vividly - referred to as a "chewing stomach" in reference to its (possibly wrongly assumed) "chewing" function. Drawing after [Kinchin 1994].

List of distinguishing features in the anterior digestive tract:

• Shape and number of macroplacoids
The number of macroplacoids is counted from front to back on one side.
In fig. 1, we would thus have two rounded, bean-shaped macroplacoids on the left and three, more elongated macroplacoids on the right.

• Presence or absence of the microplacoid?
Only the right pharyngeal bulb in fig. 1 has a microplacoid.

• Round or elongated pharyngeal bulb shape?
Fig. 1 shows a particularly common, rounded to slightly oval shape of the pharyngeal bulb on the left, and a rarer, elongated pharyngeal bulb variant on the right.

• Shape of the pharyngeal tube and "drop"
The pharyngeal tube can be rigid and straight, or partially coiled and partly flexible. For species with an elongated, flexible pharyngeal tube, there is a special feature that is important for identification: at the transition from the rigid to the flexible part, there may or may not be a bulge ("drop" formation, see fig. 1, right).

A cross-section through the pharyngeal bulb (fig. 2) illustrates the three-rayed arrangement and the assumed function of the macroplacoids in the (potential) grinding of food:

Fig. 2: Tardigrade pharyngeal bulb, in cross-sectional view, at the level of the macroplacoids. Drawing after [Kinchin 1994].

If the tardigrades are compressed too tightly under a coverslip, it can happen that not just two, but three rows of macroplacoids are seen simultaneously (because all three are then on approximately the same focal plane).

2) Taxonomically important claw characteristics
Within the phylum Tardigrada, there are three classes. Heterotardigrada, Apotardigrada and Eutardigrada. These three classes can be distinguished by claw shape:

a) Heterotardigrada - claw characteristics
This class includes, among other groups, the Echiniscus species, which are easily recognizable by their mostly reddish bodies. They have four approximately equal claws on each leg (see fig. 3), which may have spurs near the base.

Fig. 3: Leg of a heterotardigrade with four claws.
Drawing after [Kinchin 1994].

Heterotardigrada - list of claw characteristics:
• Presence/absence of the papilla
• Number and shape of the denticles on the dentate collar
   (found only on the 4th pair of legs)
• Length of the claws
• Presence/absence of the spurs at the claw bases
• Shape and orientation of the spurs: curved downwards or upwards

b) Apotardigrada - claw characteristics
The Apotardigrada are represented by only one group (one genus) called Milnesium. The claws of this group will be pesented in a later issue of the journal, under the species Milnesium tardigradum.

Fig. 4: Typical pair of claws on one leg of a eutardigrade.
Drawing after [Kinchin 1994].

c) Eutardigrada - claw features (see fig. 4)
The majority of tardigrades found in our region belong to the Eutardigrada. These animals have two double claws on each of their eight legs: an inner and outer double claw. Each claw can be interpreted as a combination of a large main branch and a smaller secondary branch. Additional accessory points (tips) are often found at the ends of the main branch. A cuticular stiffening, the so-called lunula, may be present at the base. Furthermore, cuticular bars may be found both between the outer and the inner claws and next to the inner claw (only one cuticular bar is visible in fig. 4, to the left of the inner claw).

Eutardigrada - list of claw charcteristics:
• Shape and size of claws
• Presence/absence of accessory points
• Presence/absence and shape of the lunula
• Presence/absence and shape of cuticular bars

Further identifying characteristics based on other anatomical regions will follow in the second part of this taxonomy introduction (August issue). See you soon!

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.

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.

Main Page