"What is the life expectancy of a typical water bear?"

At first sight this seems to be a rather simple question. Nevertheless there is no clear answer. In fact it turns out to be rather tricky because of the water bear desiccation periods. Those dry periods might last up to at least 7 years (each one!). It is quite obvious that in theory 10 of those long desiccation periods would sum up to 70 years and we would arrive at the life expectancy of man, elephant or even tortoise.

Water bears individuals have been kept alive in culture dishes for time periods of up to one year, so it is quite clear that their minimum life expectancy is one year at least. But nobody knows whether those water bears kept in artifical environment encountered some kind of natural death or possibly died too early because of the restrictions within the culture environment.

It is impossible to study water bear individuals in their natural environment. Terrestrial tardigrades can feed on the moss plants they live in. And, as a rule, they will be able to fill up their stomach within five minutes or less. They are completely surrounded by food in the moss cushion - land of milk and honey. Furthermore the moss tends to protect them against sudden humidity changes, it provides optimum oxygen supply and serves as a light barrier. Possibly the water bears simply feel protected and at home there, too, and therefore might live longer.

Because of all those complications we will not come to a conclusion here as far as life expectancy is concerned. But it is a tempting task to study water bear aging in general and to study the varying properties of the ageing water bears in particular.

Human experience has taught us to interpret signs of age: age spots, "misplaced" thick hairs emerging out of nose and ears, even forming tiny eye-brow jungles. Furthermore old humans tend to move in a strictly controlled and economic manner whereas youngster simply jump around. And, last but not least, each year of human life has a tendency to deposit additional fat on the bone structure. And, with increasing age there is a continously increasing need for a nap from time to time.
Funny enough, we do find many of those signs of age with water bears, too. Old individuals appear to be more hairy, their body is more massive and forceful. And the old ones tend to have pigment spots. In contrary the young ones have a slim body and a broad head and some of them move like elegant dancers. Let's look a the massive backside of a senior water bear:

Back and hind legs of a senoir Echiniscus water bear (Echiniscus mediantus).
Image width ca. 150 µm.

The general visual impression is similar to that of a rhinoceros, a symbol of body mass and considerable age:

Rhinoceros image on one of the famous bronze doors of the dome of Pisa, Italy.

Tiny bronze sculpture "Rhinoceros"

I analogy to human age spots we notice an increasing number of discolorations and vacuolae when looking at senile water bears. Furthermore the structure of the armour plates reminds of an old leather with lots of scratches, scars and imperfections but lots of character as well.

Armour plates on the back of a senile Echiniscus water bear.
Many intensively red vacuolae indicate advanced age.
High magnification.
Image width ca. 50 µm.

A further detail, shown below, depicts a tardigrade hind leg with red vacuolae and pigment spots.

Detail of a hind leg of a senior water bear. Many vacuoles and pigment spots are typical for old individuals.
Note the dentate collar which can be found only on the last pair of legs.

When becoming more familiar with water bears you will notice that senile animals sometimes need more time in order to return from the dry state and that some of them will not survive the rehydration process - hit by a tragical and sudden death during the fight for revival.

The next issue of the  Water Bear web base  will discuss the typical signs of (happy) youth. We will show touching portraits of water bear babies.

© 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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