1 Practical
Investigating the metabolic rate of Daphnia
Safety
Care should be taken in using mains-operated microscopes with water/solutions.
Students should wash their hands after handling biological material.
Apparatus and materials
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microscope
cavity slide
dropper pipette
dropper pipette with glass tube reversed
Petri dish base
cotton wool
modelling clay
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digital temperature probe or thermometer, to
measure temperatures in the range 5–30 °C
beaker of water containing Daphnia
beaker of ice–water mixture
beaker of water at room temperature
beaker of lukewarm water at about 40 °C
Introduction
Daphnia is a species of small freshwater crustacean, commonly known as a water flea. Unlike humans
and other mammals, it is unable to control its body temperature, which changes with the temperature
of the surrounding water. In turn, changes to its body temperature will affect the animal’s metabolic
rate. Under a microscope, the body wall of the organism is fairly transparent, and the beating heart can
usually be seen through the carapace.
In this practical, you will:
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measure the heart rate of a Daphnia at different temperatures in the range 5–30 °C.
Procedure
1 Place a very small piece of cotton wool onto the depression
in a cavity slide. Just a few strands are enough – if you use
too much it will make it difficult to see the Daphnia.
2 You are provided with a dropper pipette that has
had the glass part reversed so that it has an opening
wide enough to accommodate a Daphnia. Use the
pipette to transfer a drop of water containing a large
specimen onto the cavity slide. The Daphnia will get
caught on the strands of cotton wool, restricting its
movements. Do not use a coverslip.
3 Place the slide under the low-power objective lens
of the microscope. Locate the Daphnia and bring it
into focus. Identify the head of the animal, where
the eye is clearly visible. Also note the legs,
protruding from the underside (Figure 1.1).
COAS Biology 2 Teacher Resources
Figure 1.1
Simplified drawing of Daphnia.
Original material © Cambridge University Press 2009
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1 Practical
4 Now identify the intestine, a C-shaped tube running the length of the animal’s body. The heart
should be visible just above the gut, behind the head. Increase the contrast by reducing the aperture
of the microscope iris. This will make it easier for you to see the heart, which is an oval sac,
beating rapidly. If you have used a large specimen, the heart rate should be slow enough for you to
count.
5 Make sure that the water on the slide doesn’t dry up from the heat of the lamp. If necessary, add a
drop more water from the beaker containing the Daphnia, to prevent this happening.
6 You may need to look at several specimens. When you have found a good specimen, where you
can see the heart beating and the beats are not too fast to count, proceed to the next step.
7 Place two small pieces of modelling clay in the bottom of a Petri dish to support the slide about
5–7 mm above the base of the dish (Figure 1.2). Mount the slide on these supports and place it
back on the stage of the microscope. Focus the microscope on the Daphnia again. You will need to
raise the lens several millimetres to allow you to focus on the organism.
8 Turn off the microscope. Without moving the Petri dish from the stage of the microscope, use a
pipette to add water at room temperature to the dish. Stop when the water touches the underside of
the cavity slide, without allowing the water to run over the top of the slide (Figure 1.2).
Figure 1.2
Cavity slide supported in Petri dish with water added.
9 The glass of the slide, and the drop of water containing the Daphnia, will soon be at the
temperature of the water in the dish. Measure the temperature by placing the temperature probe or
thermometer in the water.
10 Turn on the microscope. Measure the heart rate of the animal in beats per minute. Make more than
one count to check the reliability of your measurement. Record the rate in a suitable table.
11 Turn off the microscope again. Without moving the Petri dish from the stage, remove most of the
water from the dish using a pipette, and replace it with water from a beaker of iced water, making
sure that the water touches the slide as before.
12 Leave the slide for a few minutes until both the slide and Daphnia have equilibrated to the
temperature of the water in the dish. Now record the temperature of the water.
13 Turn on the microscope again. Measure the heart rate of the Daphnia as before. Record your
results in the table. Don’t forget to take more than one measurement to check for reliability in your
results.
14 Now use the method described above to measure the heart rate at a number of temperatures
between about 5 °C and 30 °C. Do not go above 30 °C, or this will kill the Daphnia. You can use
mixtures of ice-cold water, water at room temperature and lukewarm water for this. Aim for at
least six different temperatures.
15 Calculate the mean heart rate of the Daphnia at each temperature. Plot a graph of mean heart rate
against temperature.
16 How does changing the temperature of the Daphnia affect its heart rate? Explain as fully as you
can why the metabolic rate of the animal is affected by temperature. Why does a change of
metabolic rate affect its heart rate?
COAS Biology 2 Teacher Resources
Original material © Cambridge University Press 2009
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