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Many plant and animal species settle as so-called aufwuchs (from the German term
meaning "upgrowth") on rocks or underwater plant parts. Many algae and even
some animals prefer a permanent, fixed home. The advantage is obvious: The
danger of simply being washed away by currents is lessened. But what does an
animal do that has grown in place but feeds on other animals? Precisely — it grabs
hold of whatever happens to swim by. One example of a feared predator from the
pond is the hydra, a freshwater polyp. Using its tentacles, it fishes for its food. But
that's not all! To make matters worse, this animal also has small harpoons loaded
with toxin with which it spears its prey in order to paralyze and ultimately kill it.
While these monsters of the pond can be found with the naked eye — some ani-
mals get up to 2.5 centimeters in length — their small prey can only be seen under
the microscope.
If you scrape the film off of rocks in order to collect pond specimens, you will
probably unwittingly destroy many of these organisms. So follow this tip: Simply
let algae and animals grow directly on the slide.
Fishing without a Net
You can fish for microscopic algae and animals in your rain barrel or, even bet-
ter, in a forest pond or your garden pond.
You will need:
• 4 slides
• 3 bottle corks
• a knife
• a string (the length depends on the depth of the location, see below)
• a plastic bag filled with rocks or sand
• kitchen towels (or other paper towels) and cover slips later for the micro-
scope
Assemble a "fishing" apparatus as shown in the figure. To do this, fill the
plastic bag (a zip-lock bag, for example) with rocks or sand and tie it closed at
the top using the string. The string needs to be long enough so that it reaches
from the bottom of the pond or water barrel (wherever your "plastic bag
anchor" will be lying) up to the surface. So, simply measure the depth using
a stick, add another 30 centimeters to be on the safe side and cut the string
accordingly. Now the glass slides are stuck into the corks. To do this, make
approximately 1.5 centimeter cuts into the corks from both sides with a knife,
then stick the slides into them. Have an adult lend you a hand with this tricky
and delicate task.
Prepare a second cork with slides and tie both corks to the string with spacing
of about 10 centimeter. Now lower the whole thing slowly into the experimen-
tal water until the plastic bag is sitting on the ground. Now you just need to
attach a cork or piece of styrofoam to the upper end of the string as a bobber
— and wait for a few days.
As a rule, in the summer, enough organisms will have settled on the slides af-
ter one to two weeks. You can then transport them in a bucket of pond water
and observe them at home under the microscope. To do this, pull a slide out of
one of the corks and clean off the bottom side with a paper towel. Place the
slide onto another piece of paper towel with the clean side down and let it dry
a little before you put the cover slip in place (or several cover slips next to each
other) and put it under the microscope.
At different magnifications, observe the colorful microcosmos that has settled
on your slide.
Later, you can put the apparatus in the pond again, collect more specimens,
and then observe how the composition of the aufwuchs has changed over
time. You could also study different areas of the pond. For example, you could
compare one location in the pond that is in full sun with one that is in the
shade of overhanging branches.
Diatoms
Water flea
A green hydra that
houses single-cell
algae in its body. In
addition to its prey, it
also gets nourishment
from its cohabitants.
"Fishing" apparatus
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