Fear—Reactions of Daphnia
TITLE:
Fear —Reactions of Daphnia
SUBJECT/TOPIC:
Physiology
GRADES:
9–12
STANDARDS
ALIGNMENT:
Science as Inquiry
Life Science: Behavior of Organisms
TIME ALLOWANCE:
Two 50-minute periods
OVERVIEW AND PURPOSE / OBJECTIVE(S)
(information, concepts to be learned):
In this lesson students will:
Understand how an organism responds to a stimulus
Design an investigation of the response of an organism to a stimulus
VOCABULARY:
Adrenaline (or epinephrine)—A molecule that acts as both a hormone and a
neurotransmitter. Adrenaline is synthesized during times of stress and produces various
effects that include increased heart rate, sweating, and increased metabolism.
Fear—A distressing negative sensation induced by a perceived threat. It is a basic
survival mechanism occurring in response to a specific stimulus, such as pain or the
threat of danger.
MATERIALS:
-culture of water flea (Daphnia; note 1)
-water from Daphnia culture tank at different temperatures: 0 °C (in an ice bath), 10 °C
(by adding ice to a water bath), 20 °C, 30 °C, and 40 °C.
-ethanol, 1% and 10%, 10 ml of each (note 2)
-aspirin
For each group of students:
-microscope—low power, transmission
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-petri dish
-small piece of cotton wool
-Pasteur pipette (for water from the Daphnia culture tank)
ACTIVITY / ENGAGEMENT
(reinforcing lesson, making real-world connection)
Introduction
Fear is a chain reaction in the brain that starts with a stressful stimulus and ends with the
release of chemicals that cause a racing heart, fast breathing, and energized muscles,
among other things.
Besides the involvement of the brain in responding to fear, body hormones such as
adrenaline help the entire body respond.
For this investigation, students will be using the water flea as a way to explore how the
body responds to fear. Before starting the lab, ask your students the following:



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In a human, what can cause fear?
How does the body show fear—what happens?
In a simple organism such as a water flea, what might create fear or a response
similar to fear? (Answer: change in conditions)
Why must an organism have a mechanism to respond to fear?
Student Investigation
Ask students, working in groups of 2 or 3, to design an experiment that will show an
organism’s response to fear by exploring the heart rate changes in Daphnia under various
conditions.
Teacher’s note—students can try shining a light; introducing chemicals such as aspirin;
changing water temperature, etc (see note 4).
Be sure to ask students to create a hypothesis and then make additions to the included
worksheet.
Background information:
In the water flea Daphnia, the single, small heart is easily visible when viewed under
transmitted light under a low power microscope. The heart rate (which can be up to 300
beats per minute) can be monitored and counted in different conditions—for example,
changing water temperature or changing the type and concentration of chemicals added to
the water. A change in Daphnia heart rate may not be a predictor of a similar change in
human (or vertebrate) heart rate under the same conditions, but the procedure provides an
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interesting technique for investigating the effects of different chemicals on a metabolic
process.
Procedure:
Part 1
Observing the heart rate of Daphnia.
1. Take a small piece of cotton wool, tease it out and place it in the middle of a small
Petri dish.
2. Select a large Daphnia and use a pipette to transfer it onto the cotton wool fibers.
3. Immediately add pond water to the Petri dish until the animal is just covered by the
water.
4. Place the Petri dish on the stage of a microscope and observe the animal under low
power (see note 3). The beating heart is located on the dorsal side just above the gut and
in front of the brood pouch (see diagram). Make sure that you are counting the heart beats
and not the flapping of the gills or movements of the gut. The heart must be observed
with transmitted light if it is to be properly visible.
5. Use a stopwatch to time 20 seconds and count the number of heart beats in several
periods of 20 seconds. The heartbeat of Daphnia is very rapid, so count the beats by
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making dots on a piece of paper. Count the dots and express heart rate as the number of
beats per minute. At the end of the investigation, return the Daphnia to the stock culture.
Part 2
Investigating the effect of temperature.
1. Record the temperature of the water in the Petri dish.
2. Add pond water at a different temperature to the Petri dish.
3. Allow the Daphnia some time to acclimate (but monitor the temperature of the water in
the dish and add more hot or cool water if necessary to adjust the temperature).
4. Record the heart rate again as in Step 5 of Part 1.
5. Plot a graph of mean frequency of heart beats per minute against temperature.
Investigating the effect of chemicals.
1. Take a large Daphnia from the stock culture and record its heart beat at room
temperature in pond water (as in Part 1).
2. Add one drop of 1% ethanol to 5 ml of pond water in a beaker. Mix well. Draw the
pond water off the Daphnia with a pipette and replace it with 2 or 3 ml of the water
containing ethanol (note 2). Record the rate of heart beat again.
3. Repeat step l using 10% ethanol in place of 1%.
4. Repeat with other chemicals, such as aspirin (note 4).
Notes
1. You can purchase live Daphnia cultures from suppliers, including pet shops and local
aquarists. Some scientific suppliers sell viable dried Daphnia eggs and culture kits.
Small, regular supplies of food are required. Provide only sufficient to cause the water to
turn just faintly cloudy. After a few days the Daphnia will have filtered out the suspended
particles of food, making the water clear once more, which is your cue to add more food.
Clear scum from the surface of the water; but leave debris that sinks to the bottom – it
may contain Daphnia eggs.
The safest and most convenient ways to provide the necessary food for a colony of
Daphnia is to feed them on a few drops of a suspension of fresh yeast or of egg-yolk
medium (made by blending a hard-boiled egg in 500 cm3 of water).
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Alternatively, you can buy food such as Liquifry No 1 or Spirulina powder from aquarists
or scientific suppliers.
2. Ethanol diluted to 10% and 1% is low hazard for the students using the liquid.
3. Monitor heating due to the microscope lamp: When working with organisms under a
microscope, the effects of heating due to the microscope lamp itself can be significant.
Turning the lamp on only when observing the Daphnia will help, and LED microscopes
produce less heat than those with incandescent lamps.
4. Aspirin (o-acetylsalicylic acid) is harmful if swallowed, but a soluble tablet dissolved
according to the manufacturers’ instructions would give a suitable concentration to use in
the investigation at low hazard to the students. In each case, add one drop to 5 ml of
water before applying to the Daphnia.
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Student Worksheet for Reactions of Daphnia
Experiment Title: _________________Date: _____Name: ______________________
Student Hypothesis: ______________________________________________________
Materials:
Safety Precautions:
________________________________________________________________
________________________________________________________________
Procedure:
Lab safety equipment should be used, and safety protocols followed.
___________________________________________________________
___________________________________________________________
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Data and Observations:
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