Investigation: What Factors Affect the Heart Rate of Daphnia


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)?

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.

**Develop a hypothesis that predicts how the heart rate of the daphnia will change when it is exposed to a depressant.



Daphnia in culture liquid (per table) | Transfer pipette / depression slide
Compound microscope or Stereoscope | A small container for “used” Daphnia
1% ethanol in a dropper bottle


Caution: 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.
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.

Analysis and Conclusion

Average BPM in Water: _________________    Average BPM in Ethanol: _______________________

1. Did the ethanol increase or decrease the heart rate?

2. Did the data support your hypothesis?

What was the independent (manipulated) variable in your experiment?

What was the dependent variable?

3. Would you classify the ethanol as a stimulant or a depressant? Why?

4. Compare your results with your lab partner. Did you get similar results? If no, propose an explanation for why your results might have been different.

Part 2: Design an Experiment

Now that you have an idea how to measure the heart rate of the daphnia, choose ONE new variable to test. Your instructor will provide you with additional materials. Variables: temperature, sugar, artificial sweetener, caffeine. You will turn in a lab report that includes: