A Daphnia is a tiny crustacean (related to shrimp) that has a clear outside skeleton (carapace) and
jointed legs. Like other arthropods, its heart is on its back. The environment challenges each living thing to respond. It includes the air, the water, heat and light, and the chemicals which enter our bodies. Because Daphnia are ECTOTHERMS (cold-blooded), their body temperature changes with the surrounding environment. Since chemical reactions are speeded up in warmer temps, what would you predict the effect of temperature changes would be on their rate of metabolism (and heart rate)?
I PREDICT THAT: __________________________________
Chemicals which enter their bodies can also change their heart rate by interfering with the chemicals that nerves use to transmit signals. Chemicals that speed up heart rate are known as stimulants, whereas chemicals that slow down the heart rate are known as depressants.
4 Daphnia in culture liquid (per table)
A clean depression slide
A small container for “used” Daphnia
1% ethanol in a dropper bottle
** Keep the light for your microscope OFF as much as possible to avoid overheating your Daphnia!
1. Using a clean pipette, carefully transfer a Daphnia and ONE drop of liquid onto a slide. Keep the drop small so that the Daphnia can’t swim out of your field of view.
2. Place the slide under the microscope and focus on the Daphnia so that you can see the beating heart. REMEMBER: its heart is on its back!
3. Count the number of heart beats that occur in 10 seconds. Have your lab partner time 10 seconds for you as you count heartbeats. You want to make your measurements quickly, so that the Daphnia does not become stressed in the small volume of water.
4. Record the number of heart beats in the data table on the next page. Multiply the number by 6 to get the number of beats per minute.
5. Take at least three separate heart rate measurements for each individual Daphnia and calculate the average of the three measurements. REMEMBER: if the three numbers are NOT within 10% of each other, repeat your trials!
6. When you have finished recording the heart rate in water (the CONTROL solution), add ONE DROP of the 1% ethanol solution to the slide. Turn the light OFF and wait 30 seconds.
7. Turn the light back on and count the number of heart beats for 10 seconds again, repeating at least 3 times. Multiply each count by 6 to get the heart rate per minute. Record in data table.
8. Rinse the Daphnia into the “used” container, then repeat steps # 3-9 with a new Daphnia.
Independent Variable: What did you add or change?__________________________________________
Dependent or Responding Variable: What you measured: ______________________________________
Average BPM in Water: _________________ Average BPM in Ethanol: _______________________
Did the ethanol increase or decrease the heart rate? ____________________
Did the data support your hypothesis? ______________________________________
Would you classify the ethanol as a stimulant or a depressant? _____________________________
Compare your results with your lab partner. Did you get the same results? __________________
Was there a difference between your average BPMs and your partners? _____________________
If yes, why? _____________________________________________________________
EXTENSION: Propose an experiment where you will measure the change of the heart rate of a daphnia at different temperatures. DESIGN the experiment with a sketch to show how you will change the temperature of your daphnia.