Studying Living Organisms

Comparative Embryology Using Japanese Medaka Fish

Eugene Shultz
Woodrow Wilson Biology Institute

Target age or
ability group:
This activity on embryology and evolution is easily adapted for students in grades 9-12.
Class time
This activity will require 10 minutes per day for approximately 12 days. An additional 1-2 class periods is required for completion of the activity.
Materials and equipment: The materials and equipment for raising Japanese Medaka fish are available in complete kit form from any reputable biological supply company. These kits include live adult fish and comprehensive instruction manuals. If you choose to buy the equipment separately, you will need the following:
1 10-20 gallon aquarium with filter and light, set up for receipt of live fish
6 female and 4 male adult Japanese Medaka fish (other numbers will be satisfactory, but 6 and 4 will give plentiful embryo supplies)
1 electrical timer
1 aquarium dip net
1 pair of stainless steel forceps with fine tips
1 Petri dish per day of egg collection (dishes with four part partitions work the best, but are not necessary)
Flake fish food
Medaka embryo rearing solution (available from Carolina Biological Supply Co.) or mix the following solution:
NaCl, 10% 1 ml
KCl, 0.30% 1 ml
CaCl2 . 2H2O, 0.40%1 ml
MgSO4 . 7H2O, 1.63%1 ml
Methylene blue, 0.01%1 ml
distilled H2O 95 ml
Steam sterilize the stock solutions before mixing.

Dissecting microscope for each group of students or microscope video projector for entire class

Materials and equipment (continued):
1 5-10 gallon aquarium without filter or bubbler set up for receipt of newly hatched fry
1 protozoan culture for feeding newly hatched fry (a hay infusion works well)
1 brine shrimp culture for feeding newly hatched fry
You will need the following materials for the comparative embryology activity:

1 copy of the student activity worksheet per student

Slides of chicken embryological development. Many sources are available. I recommend Optical Data's The Living Textbook: Principles of Biology. Life Sciences Sides 1-4. Frames 1804 through 1828 show chicken embryological development. You may wish to develop chicken embryos in conjunction with the fish embryos if you have the ability to do so.

Slides of human embryological development. Many sources are available, but I recommend the book A Child Is Born by Lennart Nilsson(1977).

Other resources for embryology studies are listed in the Resources Available section of this activity.

Summary of activity: A common source of evidence used to support evolutionary theory is comparative embryology. In this activity, students control the breeding of Japanese Medaka fish, collect the fertilized eggs, and daily view their embryological development. These embryos are surprisingly easy to culture in any school laboratory. The students then compare and contrast the fish embryological development to pictures of embryological development in chickens and humans. The students are guided to make inferences about evolutionary relationships among these animals based on their observations.
Prior knowledge, concepts or vocabulary necessary to complete activity:
The students may need to know some very basic information about embryological development, but I find that most of the information is gathered by the students as the activity progresses.

Teacher Instructions

Caring for and breeding Medaka fish is a simple activity that can be accomplished by teachers and students by following the procedures outlined here. I prefer to have students care for the fish and collect the eggs.

1.Prepare the aquarium several days before receiving the live fish. A standard aquarium preparation will suffice. Gravel and plants will aid in maintaining the aquarium. A filter of some type is necessary to keep the water clean. Medaka survive best at water temperatures of 20° to 25° C. Temperatures above 30° C are often lethal. The aquarium must be equipped with a light and timer.

2.Introduce the fish to the aquarium and allow them to adjust to their new environment for several weeks. Feed the fish sparingly two or three times a day. Do not allow the floor of the tank to become littered with food. I find that adding a bottom feeding fish to the tank helps keep the tank clean.

3.To induce the reproductive cycle of the fish, set the timer so that a regular daily photoperiod of nine to fifteen hours of light is provided. Within several days, the female Medakas will release eggs one to two hours after the start of the light period. As the eggs are released, the males fertilize the eggs. The fertilized eggs hang as a cluster from the female's cloaca.

4.To remove fertilized eggs, use a soft net to lift a female from the tank. Gently remove the eggs from the female with fine-pointed stainless steel forceps. Place the eggs in a Petri dish containing Medaka embryo rearing solution. It is recommended that one egg per ml of solution be used. Label the Petri dish with the current date. The Medaka eggs will now begin their embryological development.

5.The developing embryos should be maintained at a temperature of 20° to 25° C. It is important to change the embryo solution daily by using a pipette to remove the old solution and replacing it with fresh solution.

6.Many female Medaka will ovulate each morning for several weeks. Be sure to feed the fish well during the induced breeding season. An occasional feeding of live brine shrimp will help keep the females well nourished.

7.The students can view development in the embryos daily by placing the Petri dishes under dissecting microscopes. Alternatively, embryos can be viewed individually by placing a single embryo in a well slide and viewing it through the microscope. I generally prefer placing the Petri dish of embryos under a video microscope projector so the entire class can see the embryos on the classroom television. I usually do this at the beginning or end of each class period for ten minutes daily until the embryos hatch into fry. This takes approximately eleven days after fertilization, depending on the temperature at which the embryos were maintained.

8.The newly hatched fry should be placed in a small aquarium without a filter or bubbler assembly. The fry should be fed protozoans for the first 2 weeks, after which they may be fed finely ground flake fish food. Be careful not to overfeed the small fish. You will have to change the water occasionally. Keep the fry in this small aquarium until they are large enough to be transferred to a large aquarium with a filter assembly.

To complete the comparative embryology activity, have the students follow the instructions and answer the questions on the student activity sheet. The teacher will have to provide the appropriate slides and informational materials at the properly scheduled times as determined by the rate of development of the Medaka fish embryos.

Noticeable processes and developments that you may wish to compare and contrast among the three embryos are listed below. This is not an exclusive list. You and your students may notice other items of interest.

  • blastulation
  • gastrulation
  • yolk formation
  • neutral tube formation
  • somite development
  • optic disk formation
  • heart and circulatory system development
  • organogenesis

Resources Available

Campbell, Neil A. Biology. The Benjamin/Cummings Publishing Company, Inc. 1987.

Curtis, Helena, and Barnes, N. Sue. Biology, 5th Ed. Worth Publishers, Inc. 1989.

Fish Embryology, Videotape. Carolina Biological Supply Company. Catalog Number F6-49-2255-V.

Kirchen, Robert V., and West, William R. The Japanese Medaka: Its Care and Development, Carolina Biological Supply Company, 1976.

Nilsson, Lennart. A Child is Born. Dell Publishing Company, Inc. 1977.

The Living Textbook: Principles of Biology, Laserdisk. Optical Data Corp. 1994

Comparative Embryology

Student Activity Sheet

1.Create a data table in your biology notebook for making daily sketches of the developing embryos. The boxes below can serve as a pattern. Simply duplicate the row of boxes twelve times in your notebook. Record in each box the day of development. Be sure to label the appropriate outstanding features of each embryo as you make the sketch. Your teacher will have resources available to assist you in sketching and labeling each embryo.

Medaka Fish



Day ____________ Day ____________ Day ____________

2.As you make the sketches and do the labeling, notice the similarities and differences in each embryo at each stage of development. In your notebook, complete the following activities as you observe the embryological development:

A.Make a detailed outline of similarities among the three embryos at various stages of development.

B.Make a detailed outline of differences among the three embryos at various stages of development.

C.Based on your observations of similarities and differences in the developing embryos, what inferences can you make about the evolutionary relationships among the three organisms studied? Be sure to justify your inferences with concrete observations.

On to Using Magnetotactic Bacteria
to Study Natural Selection

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