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Life in Extreme Environments
Who is on Top in the Food Chain
How much do they EAT?

Students will design experiments to gain some understanding of the feeding rates of Rotifers and/or Tardigrades (charismatic microfauna of .6-8.mmin length). These organisms are the largest found in the Lake Hoare in the Taylor Valley making them the top predator in the food chain of this lake ecosystem. The rate of feeding and the amount of food available may determine population size. Students will use organisms collected locally or provided by the teacher. They will explore differences between their own collected data on feeding rates with that available on the Antarctic organisms that live in Lake Hoare. Can feeding rates of rotifers serve as an indicator of energy availability and carbon balance in Lake Hoare?

As consumers, rotifers and tardigrades get their energy by eating other organisms. As the largest organisms in this simple aquatic ecosystem, rotifers and tardigrades are in the top trophic level. Their populations are directly related to how much food is available to provide energy and nutrients. Therefore, if one can determine the average feeding rates of these organisms, we will learn more about how much of the carbon captured during photosynthesis by phytoplankton is transfered to higher organisms like rotifers and tardigrades. Researchers are interested in learning more about the carbon balance and solar energy input into this simple ecosystem living under extreme environmental conditions. No good way of measuring feeding rates of rotifers has been found. Students can help with creative designs and ideas.

Grade Level/Discipline

  • Grade 6 - 12
  • Biology, Ecology, Environmental Science, Chemistry


  • discuss the design of experiments to determine how much and how fast rotifers feed
  • design experiments with a lab partner and in small collaborative groups
  • collect data from experiments
  • present results of experiments to the class and discuss reliability of data
  • share things that did not work as planned
  • collaborate about problems encountered and solutions
  • propose ideas for further experimentation
  • identify new questions that were identified as a result of the deliberations

National Standards and Frameworks

  • Content standard C grades 5-8 and 9-12; Life Science, Interdependence of Organisms, Matter, Energy and Organization in living systems
  • Content Standard D grades 5-8 and 9-12; Energy in the Earth Systems

Teacher Preparation Activity
For the class:

    microscopes (1 per student is ideal)
    medicine droppers
    a large supply of rotifers*
    rotifer food -stained yeast cells
    rotifer food (see rotifer food preparation )
    distilled water (get from grocery store)
    For each lab group:
      poster paper or transparency and vis-a-vis pens
      Pre - activity set up
      Order rotifers from a biological supply such as Carolina Biological Supply or Wards. Ask for Bdeltoid rotifers. Make sure to order lots of rotifers. Students will use rotifers, one at a time, however, they may unintentionally kill some rotifers and need to get more. Start with ~100 rotifers.Your students may want to repeat their experiments several times. If you worked with the anhydrobiosis experiments, this one follows nicely and can use the same supply of rotifers. Order them once and get lots! If you find a ready local supply, have students collect and bring them in. Expect variable results when rotifers come from a wide variety of locations.

      *Ordered cultures can be separated into petri dishes for each class doing the experiments.

      Look at the rotifers and become familiar with the anatomy. The movement of the teeth found within the visibly movable part called the mastex is an indication of feeding.

      Tools and techniques (from the web site)
      For serious work a stereo microscope and a good compound microscope are necessary. Besides the usual slides, cover glasses, petri dishes, etc., one also needs at least a couple of very fine wires or very fine insect pins attached to handles (I tape mine to the ends of pencils or old pens). To form a scoop-like structure, flatten the free tip of the wire or the pin by pounding on it with a hammer. This tool is used under a stereo microscope to pick up and transfer a bdelloid rotifer from one culture to another or to a drop of water on a slide. The use of a pipette to transfer an active bdelloid rotifer should be avoided, because, usually the rotifer remains attached inside the pipette regardless of how many times and how forcefully the pipette is filled and emptied. On the other hand, with some practice, one can learn how to scoop up and transfer a bdelloid rotifer with a fine wire. Keep another wire ready to dislodge a bdelloid rotifer should it attach itself to the first wire.

      I have also had success using thin plastic tubing with a capillary tube or small pipette inserted into one end. The capillary tubes or small pipette is heated until the glass is soft followed by a qquick pull to form a tiny opening. It is then separated at the thinest point and briefly fire polished to form a fine tiny opening. I then put one end of the fine plastic tube in my mouth and ³hunt² the single rotifers under the stereo scope, suck them into the pipette and blow them gently out onto a slide. this is not for students to do, but works well if you are picking up lots of simgle rotifers during many weeks of research. The tiny pipettes work fine if you are quick. :)

      Prepare the rotifer food.

      • Use regular yeast that is available in any grocery store. (I have used both cake yeast and powder yeast.)
      • Follow the directions on the package to get a good culture of yeast cells growing. I usually start with 150 ml of water.
      • Take a small amount of the yeast culture and heat to boiling in the microwave or on a hot plate. The heating will break down the glyco-protein covering on the cells and allow the dye to penetrate the cells.
      • The add aceto-oricen stain. The yeast will become bright red and are easy to see.
      • Make a slide and check for concentration of cells.
      • You will probable have to dilute the cells for student use.
      • Use care so as to not have so many cells that students can not possibly keep track of feeding rates. Check the dillution before setting it out for your students.

      When students actually begin their experimenting, encourage them to observe and count feeding rates of only one organism at a time. Each student should have their own microscope, however, they can work in pairs having one student observe and call out information while the partner records the data. Encourage students to document everything that happens and stress that PATIENCE is ESSENTIAL!

      Log on to the web and read through the web site for Bdeltoid rotifers. The site has good information and pictures of rotifer anatomy. If facilities permit, you may want students to look at the site as well. The Bdeltoid rotifers are the ones with which I worked in Lake Hoare. Also check out my journal pages for January 20 - 24, 1997 to see the kinds of experiments that I did while in the dry valleys.

      Time Frame

      • Engagement, Exploration, and Explanation; 2 class periods
      • Exploration 2 class periods
      • Sharing results 1-2 class periods
      • Extensions can be at the discretion of the teacher. Some students may want more individual time to explore other ideas.

      Teaching Sequence

      Engagement and Exploration ( teaching sequence)
      Have several poster-sized pieces of paper ready to record results of the experimentation. Perhaps one or two students can assume the role of class recorders. Students can also document the discussion in their lab notebooks.

      Ask students to share their ideas about food webs and food chains in their local area. Then ask students to discuss in small groups what they think might be part of the food web in the perennially ice covered Lake Hoare in the Taylor Valley in the trans-antarctic mountains.
      Ask students to consider the kinds and numbers of things living in a local lake. Then compare the list to what they think might be in the Antarctic lake. Why might the populations and bio-diversity be different? What are some possible causes for the differences/ How big to you think the top organisms in the for chain are? How numerous relative to the organisms supporting them?

      Ask students whether eating rates make a difference? If so, how? Why? How could you measure feeding rates?
      Have students think about food availability. Does food availability influence population sizes? Where does food come from in Lake Hoare? How could we find some answers to these kinds of questions?

      Have students read the Journal entries of Barb Schulz on January 20, 21, 22, 24, 1997.

      These pages describe experiments done on feeding rates of Bdeltoid rotifers found along the wet shores of lake Hoare where a small moat of melt water exists for a short time during the austral (southern hemisphere) summer.. You might see some of the experiments and experiences as well as some of the problems encountered by this researcher!

      Let students see rotifers.
      Students should make slides and observe rotifers for both anatomy, movement patterns and possible feeding. Students should make accurate drawings of their rotifers. This frequently takes an entire class period depending on students skill level with microscopes. many students really like the rotifers and some even want to give their rotifers pet names!

      Explanation (discussing)
      Ask students to share observations of the rotifers.
      Make a class list of what students have seen and what they know from observations. Ask if anyone saw the rotifers eating? How can you tell? Tell students that they will be supplyibg the rotifers with a supply of stained yeast cells as a food source. Students should make a slide with a few yeast cells and be able to identify the yeast. The cells will be tiny and bright red under 40x lens. As they become digested, a change in pH levels may cause the yeast cells to become blue. See if students can see this change. Patience is required. Steady observation is a must.

      Ask students to design an experiment that would help scientists measure the feeding rates of rotifers.
      Work in small groups. Record your ideas both in your science notebook and have a group reporter record on a transparency to use with the overhead projector as you share ideas with the class. How can you tell if the rotifer eats ?

      Ask students to list possible ways to conduct feeding experiments.
      Identify variables that will need to be controlled. Students should agree on what makes a good experiment. Each lab group could work on a different variable if you wish. Discuss how many trials are necessary before you have enough evidence to make a statement about rotifer feeding rates.

      Ask students to come to agreement on how the class will conduct their experiments before you begin.
      Students can all do the same thing or each lab group can vary the procedure they develop. Be aware that the microscope light will heat up the animals and may influence the feeding. Consider the amount of daylight exposure as well.

      Allow students to work on their experiments. Offer suggestions and ask them questions. Provide the stained yeast as a food source to start with.
      Have students:

      • think about temperature
      • think about edible things that might already be in the water that that rotifers might eat
      • think about when rotifers might get hungry
      • think about the chlorine in tap water (make distilled water available)
      • possibly provide lake water and sterilize in a pressure cooker or autoclave
      • think about when the rotifers reproduce and how that might influence the feeding rates
      • think about light and how that might influence feeding rates, etc

      Try not to tell them how they should do the experiment. Perhaps a student lab group will creatively design a useful technique to measure feeding rates and thereby make a contribution to the ongoing studies of ecosystems. See student data sheets and procedure sheets at the end of this document.

      Students should collect and analyze their data. What to they know for sure? Finding what does not work is as good as finding what does work. It is important to stress this aspect of experimentation. This is not an exercise in rediscovering what we already know. This is an activity that can help the scientific community discover a way to measure and collect data that is significant in the energy relationships of the ecosystem being studied. Scientists must figure out ways to solve problems. Here is the chance to do real science. Students should present their methods and data to the class. Collectively the class should decide what they have discovered.

      Barbara Schulz
      Biology Teacher, Lakeside School
      Seattle, WA
      Among those who have studied bdelloid rotifers, the Scottish naturalist James Murray (1865-1914) deserves special mention. He started his professional carrier as a sculptor, but moved to natural history in his early 30s (Greaves, 1996). Although Murray had not had any formal training as a biologist, he quickly became an expert in the study of microscopic organisms. In a relatively short span of 11 years, Murray described a large number of new species of bdelloid rotifers from locations around the world he visited during various expeditions, including Great Britain, Antarctica, New Zealand, Australia, Hawaii, South America, Canada and Africa. He spent more than a year in Antarctica with Ernest Shackleton's expedition. Murray apparently continued working even under the most adverse physical conditions. For example, in his report of the rotifers of Canada (Murray, 1911), which he crossed by train on the way back from Antarctica, Murray complained that "the train did not offer great facilities for microscoping, the Microscope having to stand on a yielding cushioned seat." Nevertheless, he recorded 42 species of bdelloid rotifers, including 5 new ones, from Canada. Murray also studied monogonont rotifers and tardigrades. He knew David Bryce and the two often consulted each other on rotifer matters. Murray and several others were tragically killed while attempting to reach the land from their icebound ship during Vilhjalmur Stefansson's ill-fated expedition to the Arctic.


      "If we use excessively elaborate apparatus to examine simple natural phenomena Nature herself may escape us."

      Karl von Frisch, preface to The Dancing Bees, 1953

      "People who have never tried to fathom the mysteries of the bottom of brook or pond are to be pitied."

      Anna Botsford Comstock, Handbook of Nature Study, 1939

      Student handout pages on following pages.

      Bdeltoid Rotifer Observation
      • microscope
      • slides
      • coverslips
      • drawing paper and pencils
      • medicine dropper


      • Get materials
      • Make a slide without a coverslip first.
      • Look at the slide only under 10x if there is no coverslip.
      • Watch and follow the rotifer by moving the slide around
      • Draw a rotifer with as much detail as you can.
      • Write a brief description of the behavior you see
      • Share your results with your lab group

      Experimental Design
      Rotifer feeding rates

      Methods: record how you plan to make the slides and collect data.

      List variables that you will control such as

      • the kind of water used
      • how many drops of water you use
      • how much food you use in drops on the slide
      • approximently how many yeast cells per drop
      • how long you leave the slide under the microscope exposed to the light
      • temperature
      • time of day data is collected


      • Write what you see and what happened with each slide that you make!
      • Documentation of what you do is very important!!
      • Give numbers when ever possible
      • Document any change in conditions from one slide to another
      • write down any questions that come to mind while you do your experiments
      • share ideas with your lab partners
      • draw findings if you think it will help clarify what you see


      • Organize your information with your lab partners
      • create a data table that is useful for the whole lab group
      • collaborate and discuss what each of you found


      • Decide what you know with supporting evidence in the form of numbers (example 2 rotifers ate 6 yeast cells that I saw inside their bodies)
      • Select a reporter to share your findings with the class
      • Write a lab report. Your teacher will tell you if it can be a group report or if each of you needs to submit an individual one with shared data presented.

        Please review this activity.

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