Death of a waterbear - from myth to reality

Agreed, many people including ourselves, do have a noticeable tendency to glorify the survival characteristics of the tardigrades. There are notions of extraterrestrial, beyond Darwinism, hyper-adapted, extremophile etc. properties. We have learnt about the tremendous resistance of the dry "tuns" against dilute mineral acid and organic solvent attack, against zero Kelvin temperature, vacuum and extreme pressure. And, honestly, who of us humans by nature has the physical capability to survive in temporary glacier cavities and on top of the Himalayas? Has each tardigrade to be considered as a James Bond survivor among the rest of other, more modestly equipped creatures?

As we have plenty of byte space within our magazine there is no need to package all tardigrade content within one page and we can afford to differentiate and to be a little more realistic, in particular with a look to the fate of single tardigrade individuals (single individuals among billions!). Already in previous issues we have learnt that the older tardigrade reveal clearly perceivable senile behaviour  and, moreover, that  motion-restricted  tardigrades appear from time to time. In order to visually remember we once more have a look at the leather-type backside of a senior tardigrade with its typical red pigment spots, early signs of decay and inevitable death:

Senior tardigrade, detail of the backside with red pigment spots.
High magnification, image width ca. 40 µm.

When looking at large tardigrade populations under the microscope we will find a few dead animals as well. Their life apparently has come to a halt due to natural high age death. When studying the scientific literature one might come to the conclusion that most tardigrades will be devoured by carnivorous giant amoebae, nematodes or by the few tardigrade species that are known to be predators. But quite the contrary is the truth. A typical tardigrade dies due to age, due to changes in environment, bacterial or funghus infection, in a similar manner as man. In particular ageing and its consequences become apparent also during casual inspection: older tardigrades need a longer time to fill their stomachs with green moss juice, they move slower, react slower and need more time in order to revive at rehydration.
In the end the tardigrade simply ceases to move and stands still in the water. There are clear visual differences when compared with the  Dry state  and the  Asphyctic state . E.g. the red  Echiniscus  tardigrades lose their vivid carrot colour and become grey with shades of brown. Decay processes within the increasingly heterogeneous body fluid tend to form dark aggregations and spots:

Dead tardigrade. Body length ca. 250 µm.

At higher magnifications the decay of all vital processes becomes evident. Transparent tissue becomes opaque, looses its highly ordered, complex structure and turns into a mess of destroyed protein.

Right hind leg of the dead tardigrade with partially decayed,
already slightly hard-contoured claw gland

In the end the micro marvel tardigrade looses its fascinating properties. The contours of the inner organs become foggy and disappear.

Body of a dead tardigrade

A remaining pharynx and a red eye (see below) leave no doubt that this tardigrade has died and that we have not just come across an empty cuticula after moulting - otherwise the inner cavity would appear empty and crystal clear (or possibly filled with eggs).

Head region of a dead tardigrade

The outer skin (cuticula) and the sclerified parts of the bucchal system tend to keep their structures a little longer.

Back of a dead tardigrade

Man hopes to reach eternity after death. So it appears to be morally justified to expect a similar harmonifying fate for our peaceful and fully vegetarian tardigrade. Everything else has to be considered as indecent and as a an apparent lack of ecological justice - and this is by no means meant to be a funny joke.

Dead tardigrade, as seen in a mixture of incident and transmitted light

Literature

Walter Neubert: Geburt, Leben und Tod eines Rädertierchens. Mikrokosmos 83 (1994) p. 17 - 30. [comment: the author has followed the life of rotifer individuals from birth to death]

© 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.

Main Page