Dissect a Scientific Investigation
CLASS COPY
Background Information
Daphnia and algae are small freshwater organisms. They can be raised in the
classroom. They also play an important role in our ponds and streams.
Adult daphnia are about the size of sesame seeds. The circles on the daphnia's back
are baby daphnia. In the summer all of the daphnia are females. They reproduce
asexually by cloning themselves. In the fall when conditions aren't favorable some
males are born. These males and females produce eggs. The eggs hatch in the spring.
Daphnia are crustaceans like crabs and shrimp. Crustaceans don't have bones. They
have a hard exoskeleton like a shell. When daphnia grow they become too large for
their exoskeletons. Then they discard their old exoskeletons and grow a new one. This
process is called molting.
Every 2 days adult daphnia molt. When they molt the baby daphnia are born. When
daphnia are born they are very small. The juveniles grow for about 8 days.
After 8 days of rapid growth daphnia start to reproduce. When the daphnia starts to reproduce almost 70% of its energy
goes into making more daphnia. If all conditions are perfect, daphnia live approximately 50 days. If there isn't enough
food the daphnia don't live as long.
Daphnia habitat and predators
Daphnia can be found in a wide range of water habitats. They can be found from huge lakes to very small pools, such as
rock pools (see figure) and even pools that only have water in them during the rainy season.
Daphnia are considered zooplankton. Zooplankton are very small animals that live in water and are an important part of
food webs.
Daphnia are very important because they take nutrients from algae into their bodies and pass those nutrients on to their
predators. As other predators eat the fish or other animals which ate the Daphnia, the nutrients get passed on again. This
means that Daphnia are an important part of the food chain, even to people, who don't eat Daphnia directly.
Daphnia has a hard outer shell which protects it from some predators. However, even with its tough covering, Daphnia are
food for many small fish, tadpoles, salamanders, newts, and aquatic insects.
When daphnia live in lakes with fish that eat zooplankton (such as
guppies, shown in the picture), the daphnia tend to be smaller and more
transparent, to make it harder
for the fish to find them.
When daphnia live in water
with no fish, they tend to be
larger and darker. The larger
daphnia are harder for aquatic
insects (such as the phantom midge, shown in the picture) to eat.
Food for Daphnia
Daphnia hunt for and eat algae. Daphnia are considered predators and algae are their prey. Algae and daphnia live in
ponds and puddles. Algae have chlorophyll. That helps them make sugar. It also makes the water green. Algae are small.
Like plants, algae need light and nutrients to grow. The nutrients are dissolved nitrogen in the form of nitrates, other
minerals and CO2. The nutrients come from bacteria that break down plant and animal wastes. If the water has a lot of
nutrients the algae can grow quickly, doubling in number every day. As they consume the nutrients their growth slows.
The algae get energy from sunlight. If the pond isn't under trees it gets plenty of sun in the summer. In the winter there is
less sun so the algae grow more slowly.
Temperature also affects algae growth. As the water temperature falls below 70 degrees algae grow slower. As the
temperature approaches freezing, algae stops growing.
The algae will start to die if the water is too dark or they run out of nutrients. Then the bacteria will break down the dead
algae producing CO2 and nutrients. If there are enough bacteria they can use up most of the oxygen in the water. Fish also
need oxygen. Some times this lack of oxygen kills large numbers of fish.
Daphnia eat algae and bacteria. Daphnia are small but you can see them with your unaided eye. While you can see
colonies or clusters of algae and bacteria, you cannot see individual algae or bacteria with your unaided eye.
Algae and daphnia live in ponds and puddles. Algae have chlorophyll. That helps them make sugar. It also makes the
water green. Algae are small. You need a microscope to see them.
The algae get energy from sunlight. If the pond isn't under trees it gets plenty of sun in the summer. In the winter there is
less sun so the algae grow slower.
Temperature also affects algae growth. As the water temperature falls below 70 degrees algae grow slower. As the
temperature approaches freezing algae stops growing.
Daphnia reproduction, growth, and development
In a good environment, most Daphnia are female and will reproduce without breeding, through a natural form of cloning.
When reproducing this way, the eggs do not get fertilized, so the young are exact copies of their mothers. The unfertilized
eggs develop into live embryos inside the female's body, and the young are released into the environment within two to
three days.
When the environment becomes stressful, Daphnia adapt by producing male as well as female embryos. Once they
become mature, these individuals breed and produce fertilized eggs that are encased in tough protective shells. These are
released from the female's body and will have to go through several cycles of freezing and thawing before they hatch.
It is difficult to distinguish male and female Daphnia. They are basically identical except that males are generally smaller
in size, have larger antenules, and the first legs have a stout hook used in clasping the female during mating.
Daphnia and environmental pollution
The effects of pollution on daphnia
Environmental scientists often check a pond's health by looking for daphnia. If they
find daphnia the pond is healthy. If there are no daphnia the pond may be polluted. The
scientists take a sample of pond water back to the lab for analysis. They can then figure
out what in the water may have killed the daphnia by doing more experiments.
For example, scientist Gören Dave from the University of Gothenburg in Sweden put
daphnia in water with different amounts of copper. He found that more than 50
micrograms of copper killed daphnia. 25-50 micrograms of copper resulted in daphnia
that were shorter than normal, and the babies did not grow up and survive as well. 525 micrograms of copper had no effect on daphnia. Less than 5 micrograms caused
daphnia to grown and survive better than no daphnia. Similar experiments have been
repeated with lots of chemicals and have demonstrated that many heavy metals and
pesticides, amongst other chemicals, can kill daphnia.
The effects of medicine on daphnia
Much of the medicine humans take end up in our rivers and lakes. Sometimes people flush medicine they don’t need
down the toilets. Most of the time it is medicine we have taken is not entirely used up by our bodies and comes out in our
bodily waste. These medicines are not filtered out by sewage treatment plants, so end up in our rivers and lakes, where
daphnia live.
Colleen Flaherty is a Zoology student at the University of Wisconsin-Madison.
Colleen has studied the effects of different medicines on daphnia. She raised
daphnia in water with the same concentrations of medicines that are found in
lakes and rivers. She found that many different medicines, including Prozac
(used to treat depression) and clofibric acid, a drug used to lower cholesterol,
affected daphnia negatively. Many drugs, such as aspirin and nicotine, affect
daphnia immediately by causing changes in heart rate. Colleen put daphnia in
water with no nicotine, 10 micrograms of nicotine, and 100 micrograms of
nicotine. She found that 10 milligrams of nicotine increased daphnia’s heart
rate by 10%, and 100 micrograms increased it by 20%. Similar studies with
other drugs have found that many drugs affect daphnia and their heart rate.
Dissect a Scientific Investigation
Name
Now that you have some background information on Daphnia, see if you and your group can fill in the blanks for a
scientific investigation that was conducted on them. The only information you have is a data table and materials list!
Your challenge is to determine what the rest of the investigation looked like and to form a conclusion from the data.
Question:
Hypothesis:
Steps
1.
Materials
Live Daphnia culture (in fresh water)
Compound Microscope
Slides and Coverslips
Pipets to transfer Daphnia
Variables:
Heat Source (hot plate, set on 2)
Thermometer
Graphing capability (notebook or Microsoft Excel
software)
Manipulated
Responding
Controlled (3)
Data
Daphnia
(Individuals)
A
B
C
D
E
F
G
H
I
0
75
71
65
60
56
54
47
48
39
Daphnia Heart Rate in Different Temperature
Temperature (oC)
10
20
82
92
85
96
93
93
128
155
151
150
142
168
150
140
153
234
30
178
180
190
260
278
272
328
308
318
Data Analysis (Graph or other type of chart).
Conclusion
Write a conclusion for the Daphnia experiment performed.
In your conclusion, be sure to:
 Answer the experimental question.
 Include supporting data from the Time of Amylase Contact vs. Amount of Simple Sugar table.
 Explain how these data support your conclusion.
 Provide a scientific explanation for the trend in the data.
Question: (copy investigative question here)
Conclusion: