Photo Credit: Hans Hillewaert / CC-BY-SA-3.0 , via Wikimedia Commons
To develop an understanding of how the growth and survival of an organism (brine shrimp) depends on physical conditions. This is accomplished by designing an experiment to determine the optimum salinity of water needed to hatch brine shrimp.
This lesson is the first of a two-part series on brine shrimp. These lessons relate to the idea in the central benchmark that in any particular environment, the growth and survival of organisms depend on the physical conditions.
Animals that live in salt marshes, mangrove swamps, and tidal flats all have built-in adaptations to deal with saltwater and changing levels of salinity. For example, brine shrimp can survive in seawater and brackish water and can be found in salt lakes and brine ponds all over the world. However, brine shrimp have no defense against predation, so can best thrive in conditions in which their predators cannot, such as high salinity and temperature.
In Brine Shrimp 1: Hatching Brine Shrimp, students will design a test to determine the optimum salinity for hatching brine shrimp.
In Brine Shrimp 2: Brine Shrimp Survival, students will raise these brine shrimp, designing an artificial environment in which they can survive.
You should read Brine Shrimp and Ecology of Great Salt Lake, from the Water Resources of Utah website, for background information on brine shrimp before you begin this lesson.
Begin the lesson by discussing the characteristics of saltwater wetlands. A nature video or picture book that describes such an environment can be used to give students a more vivid impression. Then briefly explain to students how some animals and plants in a salt marsh deal with the changing conditions.
Next, sprinkle some brine shrimp eggs on a large piece of paper and invite students to view the eggs through a magnifying glass and describe what they see. What do they think they are seeing?
Tell students that they are looking at the eggs of an organism that lives in saltwater lakes and, like the animals that live in saltwater wetlands, is adapted to salty conditions. Explain that the eggs they are looking at can be hatched in saltwater, and that they will design an experiment to figure out just how salty the water should be in order to hatch the most eggs.
Ask students to brainstorm about ways in which they can determine the optimum level of salt in the water for hatching (i.e., the level of salt that allows the most eggs to hatch).
Next, have students work in groups to design an experiment to determine this optimum level of salinity for hatching. For example, students could set up several containers of water, putting freshwater in one and water that's increasingly salty in the others.
After the groups have devised preliminary plans for their investigations, discuss some of the following questions. These questions are meant to help students refine their investigations so that the only variable is salinity.
- In this experiment, what is the dependent variable? (It is the number of eggs to hatch.)
- What is the independent variable? (Salinity.)
- How many different salinities will you test?
- What other variables will you keep constant? (Temperature, the number of eggs, etc.)
- How will you keep those variables constant?
Now have students carry out their experiments. Students should use only spring water or aged tap water in the mixtures, as brine shrimp are sensitive to poor water quality. In addition, instruct students to label all containers with the ratio of salt (rock salt) to water used. After the containers have been filled with salty water and labeled, have students add some eggs to each container and observe what happens (it will take a day or two for the eggs to hatch). Remember that strong illumination is necessary for hatching.
FYI: One tablespoon [15 ml] of salt mixed with one cup [240 ml] of water is usually a good mixture for hatching the brine shrimp eggs. However, do not tell this to students; let them experiment with their own ratios.
Once students have completed their experiments, have them present their results to the class, highlighting what they found to be the optimum salinity for hatching (i.e., the salinity that allowed the most eggs to hatch).
Students should display summaries of their experiments and conclusions on poster boards and accompany these with written reports. They should include tables or graphs to show the hatching rate for the various ratios of salt to water tested. Power Point presentations, Excel graphs, etc. can be used to enhance these reports.
In addition, have students provide written responses to the question: "How could this have been a better experiment?" You should expect students to note things such as better control of variables and more precise equipment, for example.
Use the class discussions and student presentations to assess student understanding. To further assess student understanding, ask these questions:
- What would happen to the population of brine shrimp in the Great Salt Lake if the salinity increased? If it decreased? Use examples from your hatching experiment to support your answer.
- Under what conditions would it be least likely for the brine shrimp to survive? Support your conclusion with results from your investigation.
- Why does salinity fluctuate in the Great Salt Lake? Why are brine shrimp suited to this habitat?
Finally, ask students:
- In the salinity experiment, you tried to hold temperature constant. What if you wanted to see if temperature affects the hatching of brine shrimp eggs? Briefly outline an experiment you could conduct to determine this, indicating the independent and dependent variables, as well as the variables to be kept constant. Try to design a tight experiment, keeping in mind how you felt the salinity experiment could have been done better.
It is recommended that you follow this lesson with the second one in the Science NetLinks series on brine shrimp: Brine Shrimp 2: Brine Shrimp Survival.
Students may want to design other brine shrimp experiments. Brine shrimp, like many creatures that live in saltwater, can tolerate very different salinities. These animals can remain relatively active even if the salinity isn't at an "optimum." Questions to explore could include the following:
- What happens if you transfer the hatched shrimp to water of different salinities? Do the shrimp survive?
- How big a salinity change can they stand?
- Does temperature affect the hatching of brine shrimp eggs?
- Does acid rain affect brine shrimp?
Brine Shrimp and Ecology of Great Salt Lake has information on how brine shrimp live in the Great Salt Lake and how they are harvested there.