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 - I use 20g in a litre of aged tap water), 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 or two, and can be left another day or maybe two before using for the activity (I wait 2.5 days after adding the eggs to the salt water, before harvesting) After too many days without food or water replacement, that remove ammonia and other toxins, the young shrimp (called nauplii, singular: nauplius) 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. ("Phototaxis" is the term for movement in response to light, and attraction to light is called positive phototaxis.) This observation introduces the activity.
Ask all students to observe the attraction of shrimp to (white) light using these steps:
1. Gently swirl the dish to distribute the shrimp evenly, then carefully place the lid on.
2. Hold or rest the flashlight over the open corner of the lid, and wait one minute, without disturbing the dish.
3. 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 shrimp are more attracted to.
Hand out the filters.
Ask students to repeat the steps as for the white light (1. swirl dish, 2. light over dish, 3. look at distribution), but with filters between the dish and the light, so that the shrimp are exposed to different colours.
(We used two of each filter colour, which I had previously determined to give best results i.e. for blue light, use two blue filters, for red light use two red filters, for green light use one blue and one yellow filter etc.)
Make sure that all students try blue, red and green light, then they can mix pairs of filters to try any other light combinations they wish. 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.
Allow students to speculate as to why brine shrimp might be more attracted to certain colours. Encourage thought around what might help these animals survive (e.g. food, habitat etc). Encourage them to come up with other experiments to try (and a hypothesis to test) to find out.
During discussion, inject some ideas:
1. The food source of brine shrimp is blue-green algae - it might be advantageous to be sensitive to the colour of a food source, and to swim towards it.
2. The attraction to certain light colours correlates with how deep different colours penetrate water
Our shrimp were also strongly attracted to green and purple light, which penetrate fairly deeply into water, though not as deep as blue light.
Could shrimp have an adaptation that allows them to follow blue light to the surface of the water (where their food lives?)
(Note: 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.)
Super interesting aside: as red light does not penetrate deeply into the ocean, deep ocean animals are red-coloured, so that they are camouflaged in the blue light of the deep ocean.
Look at this image through a blue filter, and see the red animals of the deep ocean disappear!
Light attraction of other pond organisms
Other pond organisms show light attraction to white light (I have not tried other colours). Collect a dense population with a fine net to test.
Use the same set up indoors as for shrimp. Outdoors, the light of the sun can be used: collect pond life in a blacked-out dish, and place it on the ground in bright light. Leave the lid on for a few minutes, then remove the lid and immediately look for the distribution of the animals - they will be under the portion of the lid that was open to the light (see last photo). Just like the brine shrimp, they are attracted to light ("positive phototaxis").