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Activity

Brine shrimp light attraction

Summary: 
Expose brine shrimp to light and notice how they are attracted to different colours. Discuss as a possible adaptation.
Science content (2016 curriculum): 
Biology: Features, Adaptations of Living Things (K, 1, 3, 7)
Biology: Sensing, Organ Systems (4, 5, 6)
Biology: Evolution, Natural Selection (7)
Lessons activity is in: 
Materials: 
    To grow the brine shrimp:
  • brine shrimp eggs (aquarium stores carry them)
  • clear plastic box
  • sea salt
  • tap water, left to stand overnight to release the chlorine
  • pipette to transfer brine shrimp
  • small glass jar or other transparent container, for transporting shrimp to the lesson
    For shrimp observation and light attraction activity:
  • brine shrimp, about 100 per dish. Juveniles worked well for me, 1 or 2 days after hatching.
  • optional: 20X - 40X microscope(s), ideally with an adapter to view image on a classroom screen. I use a microscope attachment for my smartphone (Carson HookUpZ) then view the camera image on a screen via Apple Airplay
  • petri dishes (or other shallow dish with lid), completely covered in black paper and black tape, except for one corner of the lid (see photo)
  • flashlight
  • coloured filters - red, yellow and blue ideal (I use scrap acetate sheets left over from industrial colour printing, from Urban Source in Vancouver)
Procedure: 

Grow the brine shrimp for the lesson:
Start growing the shrimp two or three days before the lesson. Prepare 2% sea salt in dechlorinated tap water (i.e. 2g salt in 100ml water or equivalent), enough to fill the plastic box to a depth of 3-5cm. Sprinkle brine shrimp eggs over the surface, to make a sparse coating. Leave in a space with natural light, undisturbed. (Some sources say to add a bubbler to the water, to provide more oxygen, but for my use I have found that this is not necessary, though my hatch rate is likely lower.)
The shrimp should hatch after a day, and can be left another day or maybe two before using for the activity. After this, without food or water replacement for removing ammonia and other toxins, they will start to die.
To harvest the shrimp for the lesson, place near a bright window, with one corner of the box facing the window. If a window is not bright enough, point a flashlight at the corner of the box. Raise up the end of the box facing the light with a book.
The shrimp will move towards the light, so wait a few minutes, then use the pipette to suck up a concentration of shrimp in the shallower water nearest to the window. Transfer to a small jar or other container to bring to the lesson. Avoid the empty egg cases floating on the surface of the water.

Observing brine shrimp closely:
At the start of the lesson transfer about 100 shrimp to each blacked-out petri dish. Ideally have one dish per pair of students.
Hand out one flashlight for each dish, and ask the students to keep the lid off, and use the flashlight to find the shrimp in their dish. Show them by scanning the flashlight over the dish at an angle, the shrimp can be found more easily. Ask students to watch the movement of the shrimp - they will move along in a jerky way, following a zig-zag path.
This is a good time to use a higher power microscope to look at a shrimp closely. Ideally project the image on a screen so that the parts of the shrimp can be discussed together more easily. The juvenile shrimp use their antennae as paddles to push (or "row") through the water, hence the jerky movement. As they get older they will grow legs and swim with a smoother motion. The juvenile has one eye at the front of the head. They will later grow a pair of eyes, located on the sides of the head.
The juvenile will initially eat the yolk remaining from the egg it was born from, then start to feed on tiny algae in the water.
Brine shrimp are found naturally in salt water, such as the Great Salt Lake in Utah, the Caspian Sea, inland salt swamps, as well as in coastal waters near San Fransisco. They need salt water, but can live in salt concentrations as low as sea water (about 3%) or as high as 50%!

Light attraction of brine shrimp:
While looking closely at the shrimp, some students may have noticed that they are attracted to the light. This observation introduces the activity.
Ask all students to observe the attraction of shrimp to (white) light using these steps:
Gently swirl the dish to distribute the shrimp evenly, then carefully place the lid on.
Hold or rest the flashlight over the open corner of the lid, and wait one minute, without disturbing the dish.
Carefully lift the lid, without jostling the dish, and immediately look to see where the shrimp are distributed. Quickly scan the flashlight over the dish to find the location of all the shrimp.
Most of the shrimp will be gathered under the location of the light. Not all of the shrimp will have moved there though. This distribution of shrimp is called a "strong attraction" to light. (See photo of instructions and shrimp distribution images.)
Discuss why the shrimp might be attracted to the light: they eat algae, which are near the surface of the water where there is more light. (The algae need light for photosynthesis so they collect where there is more light.)

Explain that different colours of light attract shrimp differently, and that the students will find out which colours they are more attracted to.
Hand out the filters (I use two of each filter colour, which I had previously determined to give best results with the filters I use.)
Ask students to repeat the procedure as above (see photo of instructions), adding two filters at a time to change the light colour. i.e. for blue light, they use two blue filters, for red light they use two red filters, for green light they use one blue and one yellow filter etc.
Students use a worksheet to record whether the shrimp are "strongly attracted", "weakly attracted" or "not attracted" to each light colour.

The students should find that the shrimp are strongly attracted to blue light, and not attracted (or weakly attracted) to red light. We found that shrimp are also strongly attracted to green and purple light. We did not have enough data points to graph yellow and orange light.
See the data photo for our results (S indicates strong attraction, W indicates weak attraction, - indicates no attraction, with the number of dishes reporting for each). See the graph for our results of those colours with 11 or 12 dishes reporting. With more time, more groups can try all the colours.

Discussion of possible reasons for attraction to certain light colours.
Show students an image of how far different wavelengths of light penetrate water e.g. https://disc.gsfc.nasa.gov/education-and-outreach/additional/science-foc... e.g. http://oceanexplorer.noaa.gov/explorations/04deepscope/background/deepli...
Ask why shrimp may be more attracted to light that reaches greater depths in lakes and oceans (blue light penetrates the furthest) and less attracted to light that is absorbed rapidly by the water (red light). Any answer should lead to discussion on animal behaviour, as this topic is far from understood by scientists. Include in the discussion, if students do not bring it up, the possibility that shrimp are most attracted to the light colours that penetrate to deeper water, so that even if they are in deep water, they can move towards the light source and find their way to the surface, where the algae (their food) gathers.
Our shrimp were also strongly attracted to green and purple light, which penetrate fairly deeply into water, though not as deep as blue light.
(I have not found much scientific research on light colour attraction specifically by brine shrimp, though there are many papers on light attraction by collections of small ocean animals, and how attraction varies with time of day, season, age of the animal, and wavelength of light - this is very current research that the students are doing their own experiment alongside.)

Other discussion points, if results are not aligned with the penetration depths of different light colours:
The filters being used are not passing a narrow band of light colours, but let other wavelengths through (I doubled up my filters to tighten up the data).
By adding filters, the intensity of the light is also reduced, which shrimp may also be sensitive to.
The wavelengths given off by the flashlight will also likely change the data. (I used a cool-white LED flashlight.)

Notes: 
Grades tested: 
Gr 4
Gr 5
Gr 6
Teacher: 
Kevin Dwyer
Pascal Spino
Teaching site: 
Britannia Elementary
Activity originally developed and delivered: 

With the Vancouver School Board's Scientist in Residence Program http://scientistinresidence.ca