daphnia

• IV


  Caffeine concentration
• DV


  Heart rate of Daphnia (beats per minute)
• control variables

  • Temperature – measure temperature with thermometer. Carry out in same place
  • Volume of solutions – use same number of drops on Daphnia each time
  • Stress of Daphnia – try to minimise stress of Daphnia (explained in Ethical Considerations section)
  • Size of Daphnia – try to pick Daphnia of around the same size for repeats
  • Time to acclimatise – leave Daphnia in caffeine solution for same amount of time to acclimatise (e.g. 5 minutes)
• ethical considerations

  • not complex enough to suffer physical and mental stress
  • however still cannot give consent
• minimise suffering by:

  • Returning the Daphnia to their natural habitat after use
  • Storing the Daphnia in conditions that replicate their natural environment
  • Working at a good pace to minimise time Daphnia are under any possible stress
  • Turning off the microscope lamp when not in use, as Daphnia are poikilotherms (cold-blooded)
  • Not using an excessive amount/concentration of caffeine
• why use daphnia?

  • Daphnia (otherwise known as water fleas) are very common and so there is no real threat to the species’ existence or its dependent species 
  • daphnia possess a less developed nervous system compared to humans, so they have a reduced awareness of pain.
  • Daphnia are transparent and so the heart is visible, which avoids the need for dissection.
• equipment

  • Microscope
  • Counter
  • Cavity Slide
  • Dropping pipettes
  • Stopwatch
  • 5 different caffeine concentration solutions
  • Distilled water
  • Beaker containing Daphnia in pond water
  • Tissue
• control


  Count heart rate of the Daphnia when caffeine concentration is 0M (in distilled water).
• method

  1. Remove 1 Daphnia with a pipette and place it in a cavity slide under a microscope.
  2. Dab around the Daphnia with a tissue to remove the pond water and replace with drops of caffeine solution
  3. Leave the Daphnia for 5 minutes to acclimatise and then observe & count the heart rate under the microscope for 30 seconds (multiply number by 2 to calculate beats per minute).
  4. Repeat this for measurements across 5 different caffeine concentrations (e.g. 0.2M, 0.3M, 0.4M and 0.5M). Repeats can then be carried out with two other Daphnia.
• RESULTS & CALCULATIONS

  • Make sure to present your data in an appropriate table and graph.
  • Work out any mean values from your repeats.
  • The standard deviation can be displayed on the graph using error bars.
• conclusion


  An increase in caffeine concentration results in a faster heart rate (more beats per minute, bpm)
• evaluation points

  • Left for too long under microscope, temp increased (due to lamp) = increased heart rate (random error)
  • Different Daphnia showed different results due to genetic variation (random error)
  • Too high concentration of caffeine kills Daphnia (systematic error) – use lower caffeine concentrations