Natural Selection in Brine Shrimp

 

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AUSTRALIAN CURRICULUM ALIGNMENT: 

  • Natural selection occurs when selection pressures in the environment confer a selective advantage on a specific phenotype to enhance its survival and reproduction; this results in changes in allele frequency in the gene pool of a species (ACSBL090)

BACKGROUND: 

Brine shrimp (Artemia Salina) are a species of small crustaceans that are found in saline environments, specifically salt lakes, worldwide. Brine shrimp do not inhabit oceans due to the high presence of predators. Brine Shrimp are able to avoid predators by living in very high saline environments that other aquatic life are not able to. This makes them an excellent model for the study of natural selection and adaptations. When environmental conditions are optimal, female Brine Shrimp produce thin shelled eggs that develop steadily and hatch quickly into live young. Less ideal environmental conditions, such as low oxygen levels or extremely high salinity will trigger females to produce thicker shelled cysts that are covered in a hardened, brown outer layer called a chorion. The chorion maintains the embryos in a dry, oxygen-free environment. These encased embryos can survive for months or even years in this dormant state called diapause. The development of the cysts is suspended until changes in the environmental conditions cause the embryos to develop further, and eventually hatch. 

When environmental conditions are ideal once again, the cysts break open and the Brine Shrimp remains attached to the shell, surrounded by a hatching membrane. During the umbrella stage, the nauplius (larval brine shrimp) remains attached to the cyst for a few hours. In the first larval stage, the nauplius feeds on the yolk reserves until it sheds its exoskeleton for the first time. The nauplius begins to feed on algae during the second larval stage. In the 2-3 weeks it takes the Brine Shrimp to reach adulthood it moults approximately 15 times. 

Salinity levels within the environment greatly impact population growth of Brine shrimp. In this investigation, you explore how hatching viability is impacted by different saline level environments. To do this, you will attempt to hatch cysts in four different salt concentrations and measure the hatching viability by counting how many nauplius that emerge from the cysts. Brine Shrimp will hatch within 24 - 48 hours making this a great way to quickly observe how Brine Shrimp have adapted to survive in different environmental conditions.

PREPARATION - BY LAB TECHNICIAN

  1. First make a 4% salt solution, by adding 4g of salt to 100mL of distilled water. 
  2. To make the 2% salt solution, add 100mL distilled water to 100mL of 4% salt solution.
  3. To make the 1% salt solution, add 100mL distilled water to 100mL of 2% salt solution.
  4. To make the 0.5% salt solution add 100mL distilled water to 100mL of 1% salt solution.
  5. The 0% salt solution is simply distilled water.

METHOD - STUDENT ACTIVITY

Preparing Cysts for Hatching

  1. Using a permanent marker, label 4 petri dishes: 0%, 0.5%, 1.0%, 2.0%.
  2. Form your hypothesis. For example, “the cysts will have the greatest hatching viability in the solution with the highest salt concentration, because Brine Shrimp thrive in high saline environments”. 
  3. Cut approximately 6 cm of sticky tape. Adhere half of the strip to the inside bottom of a petri dish and fold the rest over to form a loop. Press down to secure it. Alternatively, cut and use approximately 3 cm of double sided tape and carefully adhere it to the inside bottom of a petri dish. 
  4. Lightly touch a fine brush to the side of the vial containing the brine shrimp eggs. Collect 25 eggs on the brush. Do not collect too many eggs as you will be required to count each one.
  5. To adhere the eggs to the double sided tape, lightly press the brush onto the tape.
  6. Repeat steps 3-5 for the remaining petri dishes.
  7. Using a magnifying glass, count the number of eggs on each strip of tape. Record this information in Table 1. 
  8. Measure 30 mL of each solution using a graduated cylinder and pour it into the appropriately labelled petri dish. Thoroughly clean graduated cylinder between each solution to avoid cross contamination.  
  9. Using a microscope, count the number of eggs on each strip of tape. Record this information in Table 1.
  10. Place the petri dishes under a light bank and allow to rest at room temperature for 24 hours.

Data Collection (Day 3) 

  1. Using a microscope, examine the contents of each petri dish. 
  2. By this stage, you should see some Brine Shrimp have hatched and are swimming in the salt solution. Count the number of hatched and unhatched brine shrimp eggs and record this information in Table 1.  
  3. Calculate the hatching viability of each dish at 48 hours by dividing the number of shrimp swimming by the initial number of eggs in the petri dish. Repeat this process for all 4 petri dishes and record the results in Table 1.  
  4. Round up your calculations to the nearest hundredth and contribute your results to the class data in Table 2.  
  5. Calculate the mean, standard deviation, and standard error of the mean for each saline concentration solution in the class data. Round your answers to the nearest hundredth and add them to Table 2. 
  6. Draw a bar graph that illustrates the sample means within 95% confidence (+ 2 SEM). 

OBSERVATION AND RESULTS

Below is an example of expected results for your experiment. This is an example only as individual results will vary. 

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Mean, standard deviation, and standard error of the mean will vary depending on class data results.

INVESTIGATIONS

  • Challenge students to describe 2 conditions that were controlled in this experiment.

  • Ask students whether their hypothesis was proven correct? Why/ why not?
  • Based on their individual and class data, ask students whether is there enough evidence to conclude environments of different salinities affect the hatching viability of brine shrimp.

EXTENSION EXERCISE

To further investigate the relationship between environmental conditions and hatching viability, challenge students to design an experiment that explores other conditions that may impact the hatching viability of Brine Shrimp. This may involve varying the temperature and access to light


 

 

icn-timer.pngTime Requirements
  1. 45 mins  

 

icn-material.pngMaterial List
  1. Brine Shrimp Eggs (Cysts) 

  2. 4 Petri Dishes 
  3. Salt Water Solutions (0.5%, 1.0 % and 2.0%) 
  4. 1 Fine Brush 
  5. 4 Microscope Slides 
  6. 4 Strips of Double-Sided Tape 
  7. 1 Stereo Microscope (or magnifying glass)
  8. 1 Permanent Marker 
  9. Distilled Water 
  10. 1 Graduated Cylinder 
icn-security.png Safety Requirements
  1. Wear appropriate personal protective equipment (PPE). 

  2. Know and follow all regulatory guidelines for the disposal of laboratory wastes. 
  3. Wash hands thoroughly before and after working with any organic materials.
  4. To dispose of brine shrimp, pour them down the sink. Alternatively, you may like to keep them for further observation.

icn-link.pngHelpful Links
 

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