Mosses tend to terrify some of those among us who own
small gardens. On the other hand the mosses are a paradise for the water bears.
In the literature we find quotes of population densities up to 22,000 individuals
per gram of dried moss (see e.g. in Ernst Marcus: Bärtierchen. Jena 1928, p. 18).
Very old publications indicate that the water bears are found most typically in roof
gutters. In his first description of
Macrobiotus hufelandi Oken mentions the
existence of water bears in roof gutters but doesn't forget the roofs themselves.
Later authors simplified this information in so far as they skipped the roofs
and mentioned the roof gutters only.
E.g. Ludwig Schmarda writes in his biology textbook "Zoologie"
(2nd ed., Leipzig 1878) about the tardigrades:
which can be found in moist mosses, in ditches
und in roof gutters.
We can easily understand why the water bears can be found in roof gutters.
It is a kind of accident. Strong rainfall tends to carry away
the water bears from their favourite places, the moss cushions on the roofs.
The water bears then end up in the gutters as homeless animals:
Meanwhile many people on earth live in houses covered
by new, sometimes glazed tiles where the tardigrades cannot live anymore.
What are the advantages of those mosses on old roofs?
Let's have a glance at the properties of the mosses first. We can perform
a simple experiment in order to understand.
Just take one of those dark, dry moss cushions, e.g. of Grimmia
pulvinata , from a wall exposed to full sunlight and weigh it.
By means of a letter balance we can tell that the weight of the moss cushion
shown below in the figure is a little bit less than 5 g.
Now we flood the moss with deionized water or tap water
until it is saturated. In the experiment shown here it took about seven minutes
until the moss was fully soaked by the water. We let drop down the remaining
unbound water and weigh again. Now the balance reading is 24 g. We conclude
from this experiment that the moss is able to take up its fourfold mass in water.
And, possibly even more strange, we notice
that it has not only grown in size but that its colour has turned
to an intense green again. Before it had an ashgrey colour, like dry
brushwood. You need lots of light in order to present this very dark green
colour on the crt:
Furthermore we note that some part of the moss
seems to behave differently. Apparently this part is no more alive and therefore
cannot return to the green state.
Now we have lots of water caged within the moss. Due to the many leafs the outer
surface area is great and therefore we will always have enough oxygen in the water.
In the last issue of the Water Bear web base we have learnt that the water bears
cannot breathe actively and that the oxygen
therefore has to penetrate into their tiny bodies just by means of diffusion.
So a well-ventilated system will provide an optimal environment continuously
saturated with oxygen.
When looking at moss cushions on earth overloaded by water for several days
we will find out that the number of bacteria and funghi tends to increase rapidly
and that the moss will become smelly after some time. The tardigrades within
those too wet moss cushions are threatened by the bacteria:
Though many of us are afraid of bacteria as well we will have to
admit that the scale relation is much more favourite for us humans:
From the perspective of a tardigrade a single bacterium has the size of
a hot dog sausage - which is really a terrible situation!
Due to their water storage capacity the mosses offer rather long periods
of constant humidity to the water bears. In contrast to the mosses on earth
the mosses on roofs will receive lots of water during a rainfall but will
never bind more water than what can actually be retained by the moss plants.
For this reason there is no dammed up water within the mosses and no stagnant
humidity. From time to time the moss becomes even completely dry and the number
of bacteria will become smaller. So after the next rainfall the tardigrades
will have a fair chance to revive in a clean environment without any 'hot dog'
It is really a pity that the number of old roofs tends to diminish worldwide ...
© Text, images, and video clips by
Martin Mach (firstname.lastname@example.org).
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.