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Mediterranean On the Rocks:
The Green Invader

An alien algae is choking the waters of the Mediterranean. With no natural predators here, the algae Caulerpa taxifola grows an inch per day and threatens the region's fishing and tourism. Marine biologists are puzzled at the algae's ability to survive and thrive in these waters. A tiny sea slug could provide a biological solution, but its potential release raises other questions.

Curriculum Links
National Science Education Standards
Activity: Alien Algae



adaptation, algae, taxonomy






(Please visit the Subject-Area Search feature on this website for related Frontiers shows and activities!)


5-8: Structure & Function in Living Systems; Reproduction & Heredity; Populations & Ecosystems; Diversity & Adaptations of Organisms
9-12: The Cell; Molecular Basis of Heredity; Biological Evolution; Interdependence of Organisms; Behavior of Organisms
5-8, 9-12: Understandings About Science & Technology
5-8: Populations; Resources & Environments; Natural Hazards; Risks & Benefits
9-12: Population Growth; Natural Resources; Environmental Quality; Natural & Human-induced Hazards
5-8: Science as a Human Endeavor; Nature of Science
9-12: Science as a Human Endeavor


Tracing the origins of the quick-growing Caulerpa taxifola alga that is threatening the Mediterranean required some scientific sleuthing. Biologists first had to identify the alien species and its origin, then explore the adaptations that enabled it to survive in the Mediterranean ecosystem. Then they hypothesized and tried ways to control the algae. A unique control has been suggested in the form of a sea slug, but that solution raises other questions.

In this activity, you will determine how temperature can affect the growth of algae and perhaps discover a way to control it.


  • 3 medium-sized jars or beakers with covers
  • culture of freshwater algae added to tap water that has been allowed to stand for a few days (If that is not available, pond water may be used, but be aware that microscopic organisms other than algae will be present. Cultured algae may be purchased through scientific supply companies or aquarium supply stores.)
  • journal for recording purposes
  • microscope
  • slides
  • slide covers
  • droppers

  1. Observe the water containing the algae culture. What does it look like? Can you see anything unusual? What color is the water?

  2. Prepare a slide with a drop of the water containing algae. Observe it under a microscope. Conduct a population count by counting the number of cells visible in the field of view. Record this number.

  3. Divide the algae solution into three separate sterilized jars. Place one solution in a warm area like the top of a refrigerator or radiator, one in a cool area (inside the refrigerator) and the third at room temperature. Make certain each jar has an available light source. (Algae are photosynthetic and cannot survive without light.) Jars should be loosely covered to permit a gas exchange but prevent evaporation.

  4. Create a hypothesis about what you think will happen to the algae culture in each of the jars over a three-week period.

  5. Observe the jars of algae for a period of three weeks. Conduct a population count twice each week following the exact procedure you used for your first count. Record this information and your observations in the journal.


  1. Which jar grew the most algae? Do you think temperature has an effect on growth of algae? What effect did you observe? What effects could global warming have on the presence of algae throughout Earth's waters?

  2. The algae you grew in this activity are different from C. taxifola. List the differences and similarities between the types of algae. Unique characteristics of the algae species in the Mediterranean include its enormous size, its ability to adapt to the Mediterranean's climate and its ability to reproduce asexually. (The algae seen in this story actually form one huge cell.)


  1. Who owns the Mediterranean Sea? Identify the countries bordering the sea and brainstorm or role-play scenarios as suggested by Alan Alda's questions about releasing the slugs into the Mediterranean to address the problem of the Caulerpa's proliferation. What if one country doesn't want to release the slugs? Is releasing slugs a good idea? What might be some consequences of a deliberate release?

  2. Research how algae are controlled in aquariums. Using your jars of algae, try to discover a non-toxic way to get rid of the algae.

  3. Research other invasive algae and how they are being controlled. Red tide has invaded estuaries along coastal areas of the U.S. Its toxic blooms can kill many dolphins and other marine mammals. In California, biologists at the Monterey Bay Aquarium have reported an algal bloom with a fatal neurotoxin that has killed hundreds of sea lions.

  4. Simulation models of algae may be seen online at

  5. Research the biology and strategies for controlling the algae in the Mediterranean, then prepare your recommendations.

  6. Compare the biology of green algae and plants. How are they different? Similar? See


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