My students always enjoy this lab!
This week our biology 1 classes are measuring the heart rate in an organism called Daphnia. Daphnia is a crustacean in the phylum Arthropoda. Daphnia is visible with the naked eye, but a microscope must be used to see the heart and to count the number of heart beats per minute.
The variable that we tested was temperature. How does an increase in temperature affect the heart rate of Daphnia?
If this sounds like an experiment you would like to try with your students, here are the steps to the procedure as well as the equipment needed:
- You must have some sort of container or mechanism in which to place the Daphnia so that it can be viewed under the microscope. We purchased little plastic containers (see photos) that have a removable "lid". Using a large bore dropping pipet, the Daphnia is transferred to the container.
- We had our students place a few threads from a cotton ball on top of the Daphnia. This helps to hold the Daphnia in one place while the heart rate is being measured.
- To make this procedure easier for our classes, we measured three different temperatures: a cold temp, room temp, and a warm temp.
- Fill a small plastic Petri dish with ice. Place the Daphnia/container on top of the ice. (See photos). After a few minutes, locate the heart, and count the number of beats per minute. We have our students count heart beats for 15 seconds and multiply by 4.
- Next students remove the ice from the Petri dish and fill the dish with water that is at room temperature. The heart rate is again counted at this new temperature.
- Finally, water is heated to a temp of 30 to 35 degrees Celsius and placed in the Petri dish.
- Students graph the relationship between temperature and heart rate.
The biggest problem with this lab is helping the student to 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.
The results are predictable: As the temperature increases, the heart rate increases. Even though the results are very predictable, I continue to do this lab for many reasons.
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 tolerate and 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!