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DAPHNIA
Daphnia are tiny crustaceans, never growing larger than
a couple of millimeters no matter how well they eat. They are
sometimes called water fleas because they are about that size
and superficially look like fleas. But unlike fleas they are nearly
transparent, live in water, have a shell, and are graced with
an interesting array of modified appendages for sensing, feeding,
and swimming. Daphnia are free swimmers, propelling themselves
with surprising speed, considering they use a pair of modified
antennae to swim. As they travel they filter even tinier organisms
from the water. They feed on single-celled algae, yeast, and bacteria.
Daphnia in turn are eaten by fish and aquatic insects.
It seems that their role in life is to provide a snack for a larger
organism.
Reproduction. Reproduction is an interesting
business with Daphnia. Under optimum conditions of ample
food and moderate temperatures, most of the Daphnia in
a colony are female. They produce eggs without mating by a process
called parthenogenesis. The 50 or so eggs are held in the brood
chamber for a few days until hatching, and then out comes the
new generation of Daphnia. This will continue indefinitely
as long as conditions are favorable. When Daphnia are stressed,
however, they produce about equal numbers of males and females,
and the offspring mate. After mating, the female Daphnia
produce hard-shelled eggs (cysts) that can withstand harsh conditions
for a long time, even complete drying out. When conditions are
once again favorable, the cysts hatch and the process repeats.
Habitat. An aquarium full of green water spells
good conditions for Daphnia. In the absence of predators,
a modest number of Daphnia can explode into a colony of
thousands in a few weeks. When this happens, it is possible to
use them for food-chain investigations. A hand lens will clearly
reveal the green color (algae) in the gut of the transparent Daphnia,
and fish will demonstrate the next part of the food chain in which
Daphnia play a role.
Daphnia can be kept in green water (water with a high
concentration of single-celled algae) at room temperature or a
little cooler. Don’t place them in direct sunlight. If no
green water is available, they can be fed bacteria or yeast. To
prepare bacteria, mash half of a hard-boiled egg yolk in 1 liter
of water. Let it sit for a couple of days. The cloudy liquid will
be full of bacteria. A yeast suspension can be prepared by stirring
half a package of dry baker’s yeast in a liter of warm water.
To feed the Daphnia, simply remove 1/2 liter of water from
the Daphnia culture (pour it through a net to save the
Daphnia), and put 1/2 liter of the food suspension into
the Daphnia culture. When the water clears, feed them again.
What to do when they arrive. Upon arrival,
transfer into a larger container of dechlorinated or spring water,
using a large baster or by pouring contents directly from the
shipping jar. Keep container at room temperature out of direct
sunlight. Daphnia are scavengers and feed on microscopic algae
and protozoans normally found in pond water. If keeping for longer
periods of time, introduce aquatic plants into the aquarium which,
as they break down, will provide food.
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