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Daphnia, Daphnia, How do I Love Thee?

Let Me Count The Ways (of Measuring Your Heart Rate!)

Valentine's Day does not have to go unnoticed in our high school biology classes!  We might not discuss the emotional affairs of the heart, but we can definitely discuss the physiology of the heart!

Each year, during the week of Valentine's Day, I take my biology students to the lab to measure the heart rate in the crustacean, Daphnia.  Since Daphnia is an ectotherm, its heart rate will vary with changes in its body temperature.  (I also pass out a few Dove chocolate hearts along with the lab supplies!)

First we place the Daphnia into a small, water-filled chamber like the one seen in this photo.  A few strands of cotton fibers placed in the chamber helps to restrain the Daphnia while viewing!

A Petri dish is filled with ice water, and the Daphnia viewing chamber is placed on top of this ice water to cool the internal body temperature of the Daphnia.  After a minute or two, the Daphnia is placed under a dissecting microscope, and the fun begins!

Some students have difficulty at first, finding the heart.  Since the exoskeleton of Daphnia is clear, the heart is easily seen.  Once the students find the heart, they are ready to start counting the number of heart beats per minute.

After repeating three trials at the ice water temperature, students fill the Petri dish with water that is at room temperature and repeat.  Finally the students fill the Petri dish with warm water to finish the experiment.  The data from the experiment is fairly consistent from year to year, and students can quickly conclude that the heart rate of Daphnia speeds up as the temperature increases.

My favorite part of the experiment comes on the next day!  On day 2 of this lab, I have my students design their own experiment.   They are asked to design an experiment to test the effect of caffeine and alcohol on the heart rate of Daphnia.  The students must state a hypothesis, describe their experimental and control groups, carry out the experiment, collect their data, graph their data, and come to a conclusion based on their data.

For my honors level and AP Biology students, I have them complete this worksheet to determine the Q10 Temperature Coefficient.

This lab, along with all of the worksheets and a teacher guide, has recently been added to my TpT store.  You can view it here:

Measuring the Heart Rate in Daphnia


  1. I think science is a really good subject for school, I love science.
    Do you like science at all?-Nate

  2. This blog is really interesting I would really like to do some of this science experiments.