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The Big Feast  or  The Feast of the Big Ones (III)

About 15 years ago we had abandoned our periodical Classical citation feature, simply because it was hidden behind too many clicks (could be found by tardigrade fanatics only ;-). One of the citation victims was a remark by Prof. Ferdinand Richters:

"Wie selten sieht man ein Bärtierchen fressen!"
(in English: How rare is it to encounter a tardigrade in a feeding situation!)

Source: Ferdinand Richters, Arktische Tardigraden.
Fauna Arctica 3 (1904) p. 497.

Admittedly, this remark is referring to most big, well-fed terrestrial tardigrades. But we shouldn't forget that some marine tardigrades are actually grazing continuously on sand grain surface areas, thus collecting tiny algae within a time schedule coming close to a 24/7 shift working style.

On the other hand, a big terrestrial tardigrade is well able to shred one of those large, moon-shaped green Closterium algae within seconds. The target is ending up as a bunch of green debris and there is definitely no need for a second dish course.

When watching through the microscope on a regular basis we will also come across tardigrades which spend more time on devouring a single victim. This may take several minutes, in some cases even many minutes. And it is quite interesting to note that the feeding on larger prey can cause problem for the tardigrade. When trying to incorporate a long algal thread the tardigrade is encountering difficulties as it is not able to cut the thread. Instead it tries to suck it in like long spaghetti. This takes time and is posing a potential risk in case an other predator is attacking the tardigrade during the process.

Macrobiotus richtersi appears to have solved this kind of longitudinal problem. When attacking a lengthy nematode it is able to grasp it sideways, as shown in the image below:

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Fig.: Macrobiotus richtersi has attacked the nematode from a lateral angle and is sucking out its gut. We found out that this behaviour was the most typical feeding manner of the tree moss tardigrades under investigation. Though clearly visible under the dissecting microscope it was not easy to document this photographically - as the tardigrades were moving vigorously during the process. The V-shape geometry with the nematode in front of the tardigrade mouth does look like a kind of moustache. We didn't come across a single longitudinal, spaghetti type nematode feeding in this tree moss tardigrade population.
For comparison please have a look a the carnivore Milnesium tardigradum which has a much broader mouth tube, therefore being able to devour bigger victims as a whole. And, judging from the remains in the stomach, there can be no doubt that Milnesium tardigradum is cannibalistic as well (cf. the claws of a tiny tardigrade in its stomach).

So we can conclude that crosswise eating is a good strategy when the only available food has spaghetti geometry. And one should never forget the tremendous risks when feeding on a 100 m spaghetti thread for hours ;-)

© 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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