It's February ... And That Means Daphnia Love is in the Air!

This is the PERFECT lab for a high school biology class on Valentine's Day!

I soooo look forward to this lab each and every year.  Measuring the heart rate in Daphnia is FUN, is fairly easy to do, allows our biology students to work with living organisms in the lab, and perhaps most importantly, it provides an outstanding situation for having our students design and implement their own experiment.

Are you nervous about "student-designed experiments"?  Don't be!  If properly trained and instructed on the front end, the students will surprise you with their creativity and passion for designing an experiment.  But I am getting ahead of myself.  Let me go back to the beginning....

Daphnia are tiny crustaceans, closely related to a shrimp. They are often called "water fleas." They are small, but not microscopic.  They can be seen with the naked eye, but you will need a dissecting microscope to view the beating heart. Daphnia can be purchased from any of the companies that sell lab supplies and equipment. Since Daphnia are arthropods, they demonstrate the three major arthropod characteristics: exoskeleton, jointed appendages, and segmented body. The exoskeleton is clear, allowing the student to easily view the heart. Daphnia are ectotherms and their body temperature changes with the surrounding environment.  This further means that there is a direct relationship between the internal body activities and the external temperature of the water in which it lives.

This sets the stage for the first part of our experiment.  I have my students carry out an experiment where they measure the change in heart rate when Daphnia are exposed to different temperatures.  I provide the procedure, and the student simply follows the instructions and carries out the lab.  The Daphnia are exposed to three different temperatures, and the student counts the number of heart beat per minute in each environmental condition.

Students carry out my procedure, graph their results, and answer my questions.  On Day 2, the fun begins!  It is time for the students to design their own experiment!  They are asked to design an experiment to test the effect of caffeine 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. 

The student-designed experiment will take some time.  I require my students to write their experiment and submit it to me for approval before they begin. Once approved, they carry out their experiment and write a formal lab report. The best learning occurs as the student is carrying out their experiment.  They invariably realize that their procedure is flawed, and they must revise, revise, and revise until they get it right.

For my honors and AP students I require that they also complete a worksheet on how to calculate the Q10 temperature coefficient.

And this is what we do to celebrate Valentine's Day in my biology classroom.  Happy Valentine's Day everyone!

In my TpT Store: "Measuring the Heart Rate of Daphnia" contains handouts for all parts of this experiment, including the student designed experiment and the Q10 worksheet.  Teacher Guide included. 

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

Measuring Heart Rate in Daphnia

Watch Their Hearts Beat: A Daphnia Lab That Brings Biology to Life

An Unexpected Star in the Biology Lab:  Need a classroom lab that’s easy to set up, exciting to run, and packed with teachable moments? Meet Daphnia—a tiny aquatic crustacean with a see-through body and a heart you can actually see beating under a microscope. It’s weird. It’s wonderful. And it’s the perfect hook for teaching circulatory systems, environmental effects on physiology, and graphing real data.

This Daphnia heart rate lab checks all the boxes for a super-engaging, low-prep biology investigation. Students get hands-on experience with live organisms, observe real-time physiological changes, and collect data that sparks discussion and curiosity. This lab encourages deeper thinking, reinforces graphing skills, teaches the scientific method and experimental design, and turns abstract textbook concepts into visual, measurable experiences.

What Students Discover: When Daphnia are placed in environments of varying temperatures, their heart rates respond—dramatically. Students watch as these tiny creatures go from chilled-out to heart-racing, helping them understand the link between metabolism and temperature in ectothermic animals. 

Even better? Students can extend the lab activity by designing their own experiment to test a different variable —testing how caffeine, pollutants, or pH levels influence heart rate. Honors and AP students can take it further by calculating Q₁₀ temperature coefficients, analyzing their results like real researchers.

Please note: Teach students to respect all living organisms. No harm should come to the Daphnia in this experiment.

This isn’t just a fluff activity. The Daphnia heart rate lab builds core science skills:

• Graphing and analyzing data
• Comparing variables and controls
• Designing and running investigations
• Problem solving and critical thinking

It also supports NGSS and AP Biology objectives, from cellular processes to environmental impacts on organisms.

The biggest problem with this lab is helping students find the heart. The digestive system of Daphnia is quite active, and the students often think that the moving digestive system is the heart. The heart is located just behind the head on the dorsal surface of the body.  It will appear as a very small, clear and transparent, beating sack.

Classroom-Ready, Teacher-Approved: Looking for something that won’t eat up your prep time? This Daphnia heart rate resource comes with the following.

• Clear student worksheets
• Ready-to-go graphing templates
• Background reading for context
• Optional extensions for advanced learners
• A teacher guide to make implementation a breeze

Ready to Try It?

This lab isn't just about watching hearts beat—it’s about getting your students excited to learn. Whether you're teaching standard biology, honors, or AP, this investigation delivers science that sticks.

👉  Click here: Measuring the Heart Rate in Daphnia

The benefits of this lab? 

• Labs keep the student excited and interested in science.  
• If the student views your class as "fun", they are more likely to perform well in the parts of the class that are less fun.....think lecture days.
• Working with living organisms is a fundamental part of a biology class and should be included at every opportunity.
• Students gain an appreciation for the living world when they get to view organisms that they are unlikely to see or notice in nature.

What do you do with the Daphnia when the lab is over?  Well, our Daphnia will spend the remainder of their days in my Elodea tank!