Using Planarian Reproduction as a Path to Hands-On Inquiry Learning
Elizabeth Paine, PhD
Planarians are fascinating animals best known for their ability to regenerate from even very small pieces of their bodies. A common classroom lab involves cutting planarians into an anterior and a posterior half and watching them regenerate. What some people don't realize is that splitting in half—voluntarily!—is also a planarian reproductive strategy. Devising experiments to learn more about this reproductive strategy is a great hands-on inquiry opportunity for students. Have students design and set up experiments to determine what conditions prompt planarians to reproduce in this manner. First have them do background research on planarian care and reproduction.
Reviewing background information
There is a large variety of freshwater planarian species. Different species and even different strains within a species reproduce in different ways. Some planarians reproduce sexually, others asexually, and some are capable of both.
During sexual reproduction, planarians mate with each other and then—since planarians are hermaphrodites—each planarian lays a cocoon, a small circular egg sac, tethered to a fixed surface in the environment.
To reproduce asexually a planarian fissions into 2 halves. During this process, the animal attaches its tail end to a solid surface, and the anterior end of the animal pulls away from the tail. Eventually, the animal splits in 2, and the 2 halves each regenerate to form 2 complete, smaller animals that will each grow to a larger size. Past studies on a variety of different species of planarians found that multiple variables can control how often planarians of a given population will fission in this way. These variables include an animal's size, the light-dark cycle under which the animals live, temperature, the composition of the water, and population density. There is also evidence that the cephalic ganglia, in the head of the animal, control the fissioning event. The thought is that the cephalic ganglia, in response to certain external stimuli or lack of stimuli, send a signal that inhibits fission. Currently, complete understanding of this process eludes scientists.
Studying reproduction in the classroom
Students may find both sexual and asexual forms of reproduction intriguing; however, studying the asexual form of reproduction is easier. The laying of egg sacs by planarians has been observed to typically, but not always, be a seasonal phenomenon that occurs during the spring. The relative unpredictability of the egg sac laying, and the fragility of the worms during this phase, make studies of planarian sexual reproduction difficult for students in a classroom setting.
Studying asexual reproduction is less challenging. However, in order to obtain meaningful data students need to design their experiments carefully. Have students perform their experiments as a whole class inquiry activity in which the class works together to test a few variables. Together the students should figure out what question they are interested in asking and how they should design the experiment to answer the question.
Students need to design their experiments to use animals in numbers sufficient to ensure valid data. Higher numbers of animals help ensure that any differences seen between 2 groups of worms—kept under different conditions—are real and not due to random variation. Large numbers of worms also make small differences between groups easier to discern. Students should use at least 25 to 40 worms for testing each variable. Have your students plan their experiments before they collect or purchase planarians.
Students need to carefully manage the variables in their experiment and establish standard procedures for collecting data. For example, if students choose to test the effect of population size on the rate of fission, they must keep the volume of water per worm constant even as the number of worms in the population is varied. They should introduce only 1 variable at a time.
Students must establish a consistent method for counting and reporting the number of planarians that have undergone fission. How often are they going to count the number of fission events? Will they remove the head and tail fragments of fissioned planarians from the culture once they have counted that fission event? Are they going to report their numbers as "cumulative number of fission events per group" or as "number of fission events counted per group each time they count"? What are the advantages of each type of data reporting? Students also need to decide how many days over which they are going to collect data. In some previous studies of fission rates, collection of data occurred over 5 to 10 days, while in other studies it occurred over a period of several months.
Establishing a habitat
The planarians must have careful and diligent care during the experiment. Use springwater to culture the animals, because even dechlorinated tap water may contain chemicals to which planarians are sensitive. You will need a culture dish or another shallow, soap-free container made of glass, plastic, enameled steel, or stainless steel. Petri dishes work well. Pour fresh springwater into the container to a depth of 0.5 to 5 cm. Use a small brush such as a clean art brush or a Drosophila sorting brush to move the planarians. Maintain the planarians at 21 to 23° C (70 to 73° F). If students choose to study the effect of different temperatures on the fission rate, they should not maintain the animals at temperatures lower than 10 to 12° C or higher than 29 to 32° C. Be aware that planarians do not tolerate high temperatures very well. Planarians prefer dark places, so it is best to maintain them in the dark unless students are studying the effect of the light-dark cycle on fission.
Transfer planarians to a fresh dish of springwater every day or every other day. However, planarians will tolerate the same dish with water changes every other day. If you maintain them in this way, transfer them to clean dishes every week. If they are not too crowded, planarians will tolerate being in the same water over the weekend, but do not make a practice of leaving them in old water this long. Thoroughly wipe and rinse old dishes, then allow them to air dry. Do not use soap. Always handle the animals gently.
Feed planarians once or twice a week; suitable foods include fresh beef liver, hard-boiled egg yolk, Lumbriculus, pieces of earthworm, and crushed aquarium snails. For up to 50 planarians, feed a pea-sized portion. Allow the planarians to feed for 1 to 2 hr, then transfer them to a fresh dish of springwater.
The materials needed for studying planarians are easy to obtain. You may be able to collect planarians by dangling a strip of liver in a stream. Be aware that there is a possibility of collecting a population that reproduces exclusively in a sexual manner. You can purchase planarians and springwater from Carolina. You can also purchase springwater from a grocery store.
Best, J. B., W. Howell, V. Riegel, and M. Abelein. 1974. Cephalic mechanism for social control of fissioning in planarians. I. Feedback cue and switching characteristics. Journal of Neurobiology 5: 421–42.
Best, J. B., A. B. Goodman, and A. Pigon. 1969. Fissioning in planarians: Control by the brain. Science 164: 565–66.
Jenkins, M. M., and H. P. Brown. 1963. Cocoon-production in Dugesia dorotocephala (Woodworth) 1897. Transactions of the American Microscopical Society 82: 167–77.
Kenk, R. 1937. Sexual and asexual reproduction in Euplanaria tigrina (Girard). The Biological Bulletin 73: 280–94.
Newmark, P. A., and A. Sànchez Alvarado. 2002. Not your father's planarian: A classic model enters the era of functional genomics. Nature Reviews 3: 210–19.