"Tell me, water bear: are you a predator or a vegetarian?"

When looking at the literature you will find out that some tardigrade species, in particular those with rather big individuals are considered to be carnivorous.

Already in the beginning of the 20th century the German water bear specialist Prof. Ferdinand Richters complained that it was difficult to study the feeding behaviour of those tardigrades: "How seldom do we encounter an eating tardigrade !"
What should then be done in order to find out more about a potentially carnivorous behaviour? There is a very simple answer: as most tardigrades are transparent we just have to look at the contents of their stomachs. When you are careful you will be able to study the stomach content without doing any harm to the tardigrade.
Milnesium tardigradum  is known to be a predator, so for the beginning we should watch out for big Milnesium water bears, or, not quite as good, big   Macrobiotus  water bears.
Here we will show as an example what we have found out when studying a big Milnesium tardigrade:

Big Milnesium tardigrade from a city bridge (Munich, Germany). Body length ca. 700 µm.

The nutrition pathway through the body is clearly visible: from the mouth (lower left) through the short and wide mouth tube, followed by the pear-shaped pharynx and the short oesophagus tube leading into the big stomach-intestine area.

As already announced we will look for traces, what kind of food our splendid and apparently well-fed tardigrade has been eating recently:

Milnesium tardigrade as above, image showing the stomach-intestine region of the tardigrade in detail. On the left side of the image the oesophagus (tube link to the pharynx) is visible. The areas marked in red will be discussed below. Image width ca. 400 µm.

The two red arrows pointing to spheroid objects indicate the remains of two rotifers - the apparently less digestible pharynx organs of the rotifers (see also detail image, left image in the row just below).
Furthermore our tardigrade has devoured a colleague from its own tribe, though not its own species! The oval contour line above and two images below (middle and right detail photographs) show what is left from a tiny tardigrade: claws, buccal tube and pharynx, possibly of a young Hypsibius oberhaeuseri tardigrade which apparently has been devoured as a whole (!)

Detail of the stomach-intestine region: pharynx of a rotifer

Detail of the stomach-intestine region: claws of a tiny tardigrade

Detail of the stomach-intestine region: pharynx and buccal tube of the tiny tardigrade

But most of the other tardigrades are vegetarians feeding on vegetary material only. E.g. the Echiniscus tardigrades dispose of their stomach-intestine content when moulting. It looks like a garbage bag, in this case filled with green plant juice only, nothing else:

"Garbage bag" of an Echiniscus tardigrade.
In order to keep the deposited eggs in the old cuticula clean the Echiniscus tardigrade disposes of its stomach content during   moulting .
Length of the object shown ca. 100 bis 150 µm.

Literature:
Ernst Marcus: Zur Ökologie und Physiologie der Tardigraden.
Zoologische Jahrbücher / Abteilung für allgemeine Zoologie. Vol. 44 (1928). Phys., p. 329-330.

© 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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