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In this issue we will have a look at the fascinating ability of the tardigrades to survive in a completely dry state (so-called anhydrobiosis). We will above all concentrate on the visual side of this process, whereas the next issue will discuss the chemistry behind.
The dry state tardigrades were called "little tuns" (in German: "Tönnchen") by the early scientists due to their compact tun shape. It is not easy to find them under the microscope, in particular when searching among dry moss particles without addition of water. The tuns appear roundish, wrinkled, more or less transparent, sometimes with very little symmetry. But you might be lucky and come across one of them on a protruding "glass hair" of a moss plant:

[Echiniscus tardigrades on moss glass hair]

Echiniscus tardigrades in dry state on a moss glass hair.
Length ca. 0.15 mm.

When the process of desiccation begins tardigrades tend to cling to protecting plant shelters. This has several advantages, e.g. it reduces the danger of being flooded away by water but above all the process of desiccation has enough time to proceed in an optimum manner. Furthermore the less protected ventral side of the tardigrade body is covered by the plant.
In contrast to the dry forms of rotifers (with vertical streaks) tardigrade dry forms have horizontal streaks. In dry state it is often not possible to see the highly characteristic buccal tube, but when flooded with water those structures become clearly visible (see image series below).
In case of slow desiccation more compact and regular tuns tend to be formed whereas quick desiccation causes "accident" tuns with deformations and unsymmetric shape (all this being less favourable, as the surface area vs. the outer world and, as a consequence, the vulnerability increase).

[ tardigrades ; dry form of an eutardigrade water bear]

Very regular, symmetric tun of an eutardigrade water bear formed after slow desiccation. Length ca. 0.2 mm.

Although the visual appeal during dry state appears to be harmonic and peaceful it should be kept in mind that the animals are in fact close to death and have to trust blindly on the development of a better future. It happens from time to time that in particular senile water bears do not find back to active live after rehydration.

[dry state short after addition of water droplet]

Eutardigrade in the phase of stretching short after the beginning of rehydration.

It is possible to trace different phases of rehydration, all of which are finished after about 10 to 15 minutes. The series of fotos below shows the revival of a tardigrade of the Macrobiotus hufelandi group. The initial size of tun is 0.3 mm.

Stage 1 Stage 2 Stage 3

 Ca. 90 seconds after soaking

 Swelling ...

 Continued swelling ...

Stage 4 Stage 5 Stage 6

 Extremities coming out

 First active movements

 Ready for real live in a
 water droplet universe!

A critical observer might have noticed a logical gap between photomicrograph 4 and photomicrograph 5. The reason for this is that at this moment the cover glass had to be lifted carefully and removed at this moment in order to provide the tardigrade with sufficient space and oxygen.

In the next issue we will discuss the physiological and chemical background of desiccation. In the long range we will also consider species determination and anatomy here, but it should be kept in mind that above all reliable species determination is clearly beyond the scope af the microscope amateur. There are only a few professional specialists for this task worldwide. So it might be more comfortable to behave as humble amateurs just enjoying the miracle of water bear life.

With best wishes for the year 2003 and with the hope that there will be no reason for you to wish that you would prefer to hide in a dry state like the water bears!


Hartmut Greven: Die Kryptobiose der Bärtierchen. Mikrokosmos 62 (1973) S. 65-69.

© 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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