Our Taxonomy series - in cooperation with Rolf Schuster*
Episode #1: Adropion scoticum [Murray 1905]

The first description of Adropion scoticum dates back to 1905 and the British naturalist James Murray (1865-1914). He discovered the species in Fourth Valley, a region of Scotland and gave it the epithet "scoticum". The original name, Diphascon scoticum, is still frequently found in scientific literature and online. In 2014, the species was removed from the genus Diphascon and placed in a new group called Adropion.
Sceptics might remark that this is akin to suddenly renaming all Hubers or Millers to Schmidts and Mayers. However, such changes in taxonomic names are at least partially, or possibly entirely justified, simply because they reflect advances in our understanding of anatomy in better-founded species names.

The following description is based primarily on the monograph by Hieronim Dastych [Dastych 1988].

The body of Adropion scoticum appears elongated in microscopic top view (fig. 1). It typically measures between 180 and 370 µm. Eyes, or more precisely, ocular pigment spots are absent. The skin is smooth, without the pustules, pores, armor, spines etc., found in many other species.

Fig. 1: Full view of a medium-sized Adropion scoticum. This portrait condenses the most important species characteristics into the small, postage-stamp-sized format of 590 pixels, typical for our journal. It is therefore worthwhile, exceptionally, to get very close to the screen: In the anterior part of the body is an oval pharyngeal bulb with the species-specific, elongated macroplacoids and also dark-appearing, punctate microplacoids. From the anterior part of the pharyngeal bulb branches a flexible esophagus with spiral reinforcement, which transitions at the mouth end into a straight, rigid tube, which in turn bears the stiletto-like feathers of a crossbow.

If we examine the image more closely, we see that the mouth tube is quite long. In its middle and posterior parts, it exhibits a spiral reinforcement, a characteristic of the Adropion group (fig. 2).

Fig. 2: Virtually the trademark of the Adropion group (and the closely related groups Diphascon and Pilatobius) - a flexible mouth tube with a spiral-like sheath. Note that the spiral structure ends at the transition to the rigid mouth tube part located further forward. The transition between the spiral structure and the smooth structure, marked here with a red arrow, is characterized in some other species by a kind of knob (a "drop"). This is absent in Adropion. "Adropion" thus simply means "without a drop". To use a human analogy, we can imagine the esophageal system as a kind of flexible vacuum cleaner hose, conveniently equipped with a rigid crevice tool at the end. In this way, the tardigrade can suck up its food just as precisely as a diligent housekeeper vacuums up dust!

Taxonomists enjoy counting and measuring. To bring particular joy to this strict group of people, whose thinking is somewhat austere, the flexible part of the mouth tube is shown here at an even higher magnification (fig. 3).

Fig. 3: Close-up of the spiral part of the mouth tube. We counted 92 spiral ducts, which are quite close together, with only about 400 nm between each duct. This very narrow mouth tube has an extremely small outer diameter of about 2.6 µm (!). The spiral structure and the small diameter are, incidentally, clear indications of a vegetarian diet - with a pronounced preference for ultra-fine plant-juice muesli. After all, who with this anatomy would want to risk choking on a transversely positioned micro-ossicle?
Technical note: For this level of detail, we had to push the conventional light microscope to its performance limits. The image shown here was taken using an apochromatic oil immersion objective, to which oblique light, precisely tailored to the spiral structure, was directed via a decenterable condenser.

The oval pharyngeal bulb exhibits a sequence of three elongated macroplacoids followed by a posteriorly connected microplacoid. In Adropion scoticum, the second macroplacoid is usually the shortest, and the third the longest (fig. 4).

Fig. 4: Close-up of the pharyngeal bulb. The particularly elongated, rod-shaped reinforcements are especially prominent and therefore easily recognizable under a microscope.

The outer and inner claws of Adropions's eight legs are of different sizes. On the first three pairs of legs, cuticular ridges (thickenings, see figures 5 and 6) are present both at the base between the outer and inner claws and at the base of the inner claw.

Fig. 5: Close-up of the claw pair on a foreleg. The outer and inner claws are of different sizes on each of the eight legs of Adropion scoticum. Red arrows mark the aforementioned cuticular ridges between the claws and at the base of the inner claw. Phase contrast image.

Fig. 6: And, because it's so beautiful, not only for taxonomists, here's a second image, this time without didactic arrows, with less drastically enhanced contrast, but in color ...
Note well: Once you know what the cuticular ridges look like, you can recognize them much more easily even with natural image contrast.

Cuticular ridges can also be observed at the bases of the outer claws of the 4th pair of legs (fig. 7).

Fig. 7: The fourth pair of legs, again with asymmetrical claw pairs and clearly pronounced cuticular ridges (see red arrows) at the bases of the outer claws.

The eggs are smooth and are laid in the skin (exuvia) of the animal during molting.

Adropion scoticum is frequently found in the soil mosses of forests, especially coniferous forests, predominantly in rather damp locations.

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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