Plankton Rainbow
Brine shrimp swim in different directions in response to light. But do they respond equally to all colors of light?
- Clear container or aquarium (flat tissue-culture flasks work well)
- Clean salt water (1 tablespoon salt per pint of water) to fill container
- Newly-hatched or adult brine shrimp (the quantity you need will depend on the size of your container)
- Flashlight
- Several glow sticks in multiple colors (blue, green, red) and/or colored gel filters large enough to cover the lens of your flashlight
- Black paper or cloth
- Optional: cardboard box (big enough for your container to fit in); a variety of data-collection materials (rulers, cameras, timers, graph paper, etc.)
- Fill the clear container with salt water and add the brine shrimp.
- Place black paper or cloth under or behind your container. Arrange it so that the brine shrimp are easily visible to the naked eye. Depending on your container, it may be easier to see the brine shrimp from the top rather than from the side.
Observe the behavior of the brine shrimp under the ambient light in the room. What do you notice?
Turn off the lights in the room. To maximize darkness, you can also put the container in a cardboard box that’s open on one end.
Shine a flashlight into the brine-shrimp container. What do you notice about their response the white light? What happens when you move the flashlight to a new position?
Snap the glow sticks to turn them on, or cover the end of the flashlight with a colored gel filter. Using one color at a time, hold the glow stick or gel-covered flashlight near the container. How do the brine shrimp respond? Do they respond differently to different colors? Try putting different colors on different sides of the container. How do the brine shrimp respond?
You probably noticed that your brine shrimp were attracted to the white light from the flashlight. In the wild, brine shrimp feed largely on phytoplankton, which tend to live near the water’s surface, where sunlight is available for photosynthesis. It’s likely that a natural attraction to the white light from sunlight helps guide brine shrimp to their next meal.
Newly-hatched brine shrimp, or nauplii, are strongly attracted to white light or sunlight. Adult brine shrimp may show different behaviors than the nauplii, and may be either attracted to white light or sunlight or repelled by it (a behavior called phototaxis). The reason for this variable response is a matter of some debate. It may depend on environmental conditions (such as the brightness of the light or the water’s temperature or salinity), the presence of predators, whether the brine shrimp are hungry, or some combination of circumstances. Can you design experiments to test any of these ideas?
You may also have noticed that your brine shrimp were most strongly attracted to blue light, and less so to other colors, especially red. Sunlight is a mixture of all different colors of light. But as sunlight travels through water, red and yellow light are absorbed, leaving only blue and green to be transmitted. Like many water creatures, brine shrimp have evolved to be most sensitive to blue light, the color of light that’s best transmitted in water.
Our understanding of the phenomenon explored in this Science Snack is built on the work of many scientists.
Dr. Aditi Pant was one of the first Indian women to participant in the Indian Antarctic Exhibitions, and was honored with the Antarctica Award for her contributions to the Indian Antarctic Program. Dr. Pant was inspired to pursue oceanography after reading about the life of plankton in the book The Open Sea. She attended the University of Hawaii, where she conducted research on photosynthesis in plankton communities, and completed her PhD at Westfield College London University, where she focused on the physiology of marine algae. Following her studies, she returned to India to join the National Institute of Oceanography in Goa. With the Plankton Rainbow snack, you can also take a closer look at light and its effect on organisms in the water.
Your students might generate many hypotheses about how brine shrimp respond to their environment. They may consider whether the brine shrimp respond to different wavelengths of light, whether they prefer particular temperatures of water, or other questions, all of which can be investigated.
In the classroom, this Snack can also provide an opportunity for students to test, evaluate, and argue for different methods of collecting data. In an investigation to determine which colors of light brine shrimp prefer, ask students to consider how they will gather evidence: Can they quantify the behavioral response, or is there another way to describe the data? What will they use as control condition(s) in their experiments?
Provide a variety of materials to encourage different approaches to data collection, such as different types of lights, graph paper, black and white paper, tape, stopwatches or timers, rulers, transparencies and transparency markers, cameras, and so on. Given these supplies, some students may choose to count the number of brine shrimp in a given area after a given amount of time exposed to light, while others might follow the paths of individual shrimp. Some students might examine how the brine shrimp respond to a single light source, while others set up competition trials using multiple colors shining at once.
Ask students to share and discuss the evidence they’ve gathered, focusing on the methods they’ve used. What were the pros and cons of each experimental protocol? Did each group gather enough data to make good conclusions? How accurate did they think each method was? Did everyone’s evidence agree? This discussion can be a good model for the behavior of a community of researchers, in which different people carry out different investigations to approach similar problems, and then share, compare, and debate the resulting data.
