OBJECTIVE
This activity page will offer an experience in:

• Growing and culturing a film of algae
• Observing the grazing behavior of pond snails
• Evaluating the effectiveness of snails as a biological control

MATERIALS (Part 1)

• Microscope slides
• Wax pencil
• Petri dish
• Dissecting microscope
• Four 250mL beakers
• Spring water
• Pond snails (removed from classroom or pet store aquarium tanks)
  
  TEACHER'S NOTE:
  Students should have access to a classroom aquarium that maintains a rich community of snails and algal species.

PROCEDURE
Part 1 - Grazing snails

1. Use a wax pencil to label the edge of two microscope slides with your initials.
2. Place these slides in an aquarium that supports a rich community of algal species. Make sure that the slides are positioned so that you can observe their surface without removing the slides from the tank.
3. Examine the slides daily and record your observations on the growth of algae.
4. At the end of one week, remove the slides from the tank. Place them in a petri dish filled with spring water. Position the dish on the stage of a dissecting microscope and examine the film of algae. Record your observations on the appearance of this algal mat.
5. Fill two 250mL beakers with spring water and add one slide to each beaker.
6. Add six aquarium snails to one beaker.
7. Place both beakers in direct sunlight and in a place where they will remain undisturbed.
8. Each day, observe the algal film and snail population. Record what you see. Following each daily observation, use fresh spring water to replace any water that has been lost.
9. Maintain a log of your observations over a one-week period. Decide what features you think should be observed and measured (such as snail numbers, mat size, etc.) Make a prediction of how these values will change. Each day, record your observations. Were your predictions correct? Explain.

QUESTIONS

1. Where did the algal mat that grew over your microscope slides come from?
   
2. Was it necessary to expose the beakers and their contents to light? Explain.
   
3. Where did the oxygen required by the snails come from?
   
4. Suppose the snails were predators and not grazers. Would this affect the results of the experiment?
   
   

PART TWO

MATERIALS

• Sprigs of Elodea or other aquarium plant
• Pond snails (removed from classroom aquarium or pet store tanks)
• Forceps

PROCEDURE

1. Remove two sprigs of Elodea or other aquarium plant from a classroom aquarium.
2. Examine the sprigs beneath a dissecting microscope. Use forceps to remove any snails or other organisms attached to the plant. Be sure to remove clusters of snail eggs that may be attached to the plant.
3. Fill two clean 250mL beakers with spring water.
4. Place a sprig of Elodea in each beaker.
5. Add several pond snails to one beaker, but not the other.
6. Place one of the algae-covered slides in each of the two water-filled beakers.
7. Place both beakers in direct sunlight. Make sure that these beakers will remain undisturbed.
8. Each day, observe the plants, the algae-covered slide, and snail population. Record what you see. Following each daily observation, use fresh spring water to replace any water that has been lost.
9. Maintain a log of your observations over a one-week period. As in Part 1, decide what features you think should be observed and measured (such as snail numbers, mat size, Elodea size, etc.)
   Make a prediction of how these values will change.
   Each day, record your observations.
   Were your predictions correct?
   Explain.

QUESTIONS

1. Why was it necessary to remove all of the snails and other organisms from the transferred aquarium plants?
   
2. Was it necessary to expose the beakers and their contents to light? Explain.
   
3. Why were both aquarium plants and algae-covered slides used in this experiment? How is this representative of the concerns raised by the use of biological control?
   

EXTENSIONS

Controlling Snail Populations
To limit the damage caused by snails, aquarium keepers rely on several techniques to control their population. In one method, physical traps are used to catch the snails. Using lettuce as bait, the trap draws snails into a one-way chamber. Ongoing removal and emptying of the trap eliminates the snails from the tank environment.

Can you design a one-way trap for snails? Think about it. Create the blueprints for this baited trap. Share your design with classmates. With your instructor's approval, build and test the effectiveness of your snail trap. Was it successful? Why? How might you improve its design?

WEB CONNECTION

Aquatic Concepts
Pest algae and biological control for the fish tank.

Controlling Snail Populations in the Home Aquaria
An article with photos about snail control in the home aquarium.

Freshwater Aquariums @ About.com
How to maintain your aquarium.

Caulerpa
An extensive online essay about Caulerpa's impact in the Mediterranean.

For more Web links on this topic - visit our Resources Section.

The activities in this guide were contributed by Michael DiSpezio, a Massachusetts-based science writer and author of "Critical Thinking Puzzles" and "Awesome Experiments in Light & Sound" (Sterling Publishing Co., NY).

Academic Advisors for this Guide:
Corrine Lowen, Science Department, Wayland Public Schools, Wayland, MA
Suzanne Panico, Science Teacher Mentor, Cambridge Public Schools, Cambridge, MA
Anne E. Jones, Science Department, Wayland Middle School, Wayland, MA