This will depend on the number of microscopes you have and access to a healthy culture of Daphnia. Students can readily follow this procedure working in pairs. Because of the variability of results between individual Daphnia it is not appropriate to make conclusions from one set of results, so each pair (or group) of students should carry out more than one investigation to contribute to the class set.
One option is to record a live video of a sample Daphnia, during a time period in which students count the heart beats. Then, you can replay the video in slow motion and count the heart beats again. This allows students to consider the accuracy of their counting.
If your time or access to chemicals is limited, you could allow the students to work through the procedure in order to evaluate it and then use the example results provided for analysis.
Apparatus And Chemicals
For the class – set up by technician/ teacher:
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 (in water baths) – see note 2 Ethanol, 1% and 10%, 10 cm3 of each (note 3)
For each group of students:
Microscope – low power, transmission
Small piece of cotton wool
Pasteur pipette (for water from the Daphnia culture tank)
Chemicals that may affect the heart rate – at low concentrations (note 4)
Health & Safety
Read our health and safety guidelines
With Daphnia cultured in the laboratory, fed on yeast, Liquifry No.1, Spirulina or egg-yolk medium, there are no significant hazards associated with this procedure. With pond water culture, or other sources of food, more careful hygiene precautions are necessary.
1 Keeping live cultures of Daphnia: These notes are based on information in the CLEAPSS Laboratory Handbook. You will find more details in section L56. Daphnia are crustaceans, commonly found in ponds and lakes and widely sold as live fish food, these animals are fascinating objects for observation and study in their own right. They feed by filtering minute particles, such as bacteria and algae, from the fresh water in which they live.
They can be kept in any watertight container containing tap water that has been allowed to stand for a few days. Keeping a few Daphnia is not difficult, but cultivating a vigorous, dense colony requires some care. A good supply of oxygen is necessary, either by aeration or by using a large shallow tank to ensure that a large surface area of water is exposed to the air. Warming the water to about 15 °C also ensures rapid growth of the colony.
You can purchase live cultures from suppliers, including pet shops and local aquarists. Some scientific suppliers sell viable dried Daphnia eggs and culture kits. Alternatively, you can collect adult Daphnia by pond dipping; in which case you must observe strict hygiene procedures, since pollutants and the bacteria causing Weil’s disease may contaminate pond water. Stock purchased from aquarists is usually free from this hazard.
The safest, most hygienic 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). Alternatively, you can buy food such as Liquifry No 1 or Spirulina powder from aquarists or scientific suppliers.
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.
2 Instead of heating water in a water bath, you could surround the Daphnia in the Petri dish with a...