Materials Included In Kit

Additional Materials Required

Safety Precautions

Caution must be practiced when cutting with single-edge razor blades or other sharp, cutting devices. See Teaching Tips for an alternative method to razor blades. Students should thoroughly wash their hands upon completion of this laboratory.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Predators may be used again for feeding once digestion is complete. Return living organisms to their culture containers for a rest period in between feeding trials. 

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs, or for 15 groups of students. The activity requires patience and careful observation on the part of students. Allow plenty of observation time. The activity can be completed in several class periods, but students enjoy working with these live organisms and may request more time.
• The organisms used in this activity are as follows:

Planaria—predator (Phylum Platyhelminthes, Class Turbellaria)
Hydra—predator (Phylum Cnidaria, Class Hydrozoa) Use tan, brown or white species of hydra, not green.
Lumbriculus variegatus—prey (Phylum Annelida, Class Oligochaeta)

• Establishing permanent cultures of all three organisms in advance of their use in the laboratory is recommended. Directions are provided with the organisms for subculturing. An aeration system is key for the long-term maintenance of these aquatic experimental organisms.
• McWorm Quiz/Discussion Photos: Included in the kit are 15 colored photo sheets with photos labeled A–N. Also, there is a student quiz with behavior descriptions that can be matched with the colored photos. Use these materials as a discussion of predator–prey behaviors after completing the lab work or use the materials as a post-lab quiz. With some reasoning, students should have success in matching the pictures with the behavioral descriptions.
• White paper should be available to place beneath containers to enhance viewing contrast.
• Prior to class, it is strongly suggested to have small predator species (such as hydra and planaria) sorted individually and acclimated to small containers, such as small Petri dishes. For transfer, it is often necessary to carefully dislodge these organisms from the underwater surfaces to which they are stuck. Sometimes this can be done by gently squirting them from the side with a jet of spring water from a pipet. Once dislodged, use a large-tipped pipet to quickly transfer them to separate containers of spring water; then let them acclimate for at least 15 minutes or more.
• All activities described here may be done without the aid of magnification or microscopy. However, microscopic viewing of feeding may be especially fascinating to students, especially for planaria and hydra. If feeding is studied under magnification, use a stero dissecting microscope (or hand-lens) with weak illumination. Avoid situations that may generate too much heat for the organisms. Before trying to actually feed worms or worm fragments to any predators, let the predators adjust for a few minutes to these “dimmed” conditions; do not poke or disturb them during this adjustment period. This will maximize the chances that the predator’s feeding behavior will be stimulated when it is presented with living food. In general, it is always good to minimize the disturbance to predators before studying their feeding behavior because almost any predator’s “appetite” may suddenly disappear if it is sufficiently disturbed or stressed.
• Many other organisms can be used as predators for Lumbriculus including: crayfish (any size); predatory leeches (use genera Nephalopsis obscura and Haemopis marmorata. (Note: Parasitic leech species will not attach to or feed on Lumbriculus); small tropical fish (e.g., guppies, zebrafish, goldfish); Triops (tadpole shrimp); many aquatic insect larvae (dragonfly, damselfly, midge, caddisfly), aquatic amphibia or reptiles (small specimens); freshwater fiddler crabs; and freshwater glass shrimp. The leech species are commonly sold at bait shops in the northern United States.
• Feeding whole Lumbriculus to other predators (e.g., crayfish, leeches, fish, Triops, aquatic insect larvae, aquatic amphibia).

1. 1. Using a plastic pipet, very slowly and gently transfer one or several whole Lumbriculus worms into a container in which one of the optional predators has been previously kept for at least 24 hours. Try to deposit the worm near the predator.
   2. What possible cues might the predator use to detect a worm?
   3. Describe the predator’s attack and feeding behavior. How does the predator attack and hold onto the worm? (Consult a biology text or other reference materials for details about the predator’s feeding structures, feeding behavior, and digestive tract). Make labeled sketches of what you observe.
   4. Does the worm attempt to escape from the predator? If so, describe its escape movements.
   5. If the predator is repeatedly offered and eats a worm for several consecutive days, does the predator attack more quickly with each feeding? That is, does it learn? If so, what is the predator learning?

• In general, try to use predators that have not been recently fed. If possible, place large predator specimens (such as leeches and crayfish) into individual, medium-size containers of clean spring water (0.5–1.0 liters) overnight with no food. In the case of leeches, make sure a tight-fitting lid is placed on each container to prevent escape.

Teacher Tips

• For teachers who prefer not to use single-edge razor blades in their classrooms, there is an alternative tool (a “substitute razor blade” made from an acetate transparency sheet) with which worms can be cut into fragments. Use a pair of scissors to cut a 2 cm–wide strip from along the outside edge of a heavy acetate (transparency) sheet. Keep track of which is the original outside edge of the sheet because it should be extremely straight and will therefore work well as a cutting edge for subdividing worms. Cut this acetate strip into 3 cm lengths, keeping track of the original edge of the sheet. Wrap the opposing edge of the strip lengthwise with tape, thus making a flexible and safe tool that looks somewhat like a single-edge razor blade.

• Obtaining Worm Fragments—Transfer a Lumbriculus worm to a flat-bottomed dish (plastic Petri type) containing a small volume of spring water. Place this dish on a white background. It is important that the container is flat enough that a Lumbriculus worm can be readily cut into many small pieces using a new single-edge razor blade or a razor blade substitute. Use the blade to cut the worm into many pieces, each about 1 mm in length. Only light pressure is needed to cut the worms.
• Making Microtip Pipets—It is easy to modify standard disposable plastic pipets so that the tips are thin and long, making them more useful for transferring small Lumbriculus fragments to a predator, such as hydra. These modified pipets are also useful for the transfer of tiny aquatic organisms or small volumes of fluid from one container to another. To make these pipets a pair of pliers will be used. Place a wide stem plastic pipet with the tip extending over the edge of a desk or counter top. Grip the bulb-end of the pipet firmly with one hand that rests securely on the counter. Using the other hand, grip the tip of the pipet with the pliers. Pull very slowly, firmly, steadily and straight out until the tip is forcefully stretched out to a length of about 1 to 1.5 inches; then stop pulling. Stretching beyond this, or stretching too rapidly, may cause the tip to break away. When properly done, the stretched region will be transparent, very uniform in diameter (1–2 mm), and several centimeters long. Use sharp scissors or a new razor blade to cut off the flattened tip that was gripped by the pliers.
• When drawing up a very small fluid volume with the worm fragment, grip and squeeze the pipet with the thumb and forefinger along the tubular portion of the pipet rather than the bulb-end where it is much harder to get fine control of suction.

Further Extensions

Many optional experiments (perhaps student-designed) can easily be designed or be used as science projects and research projects. Two are briefly described as examples:

1. How many fragments may be eaten by a planarian in a 24-hour period? To study this, transfer numerous pieces of worm fragments into the planarian’s dish. Count the fragments, cover the dish, and place it in the dark. Re-count the fragments after 24 hours. Note: Worm fragments might conceivably die and disintegrate overnight, even without the presence of a predator. Therefore, it is advisable to compare the number of “consumed” fragments in the planarian’s dish to a separate “control” dish which contains only worm fragments and no predator.
2. Test whether substances in “worm water” (that is, water from a dish in which numerous worm fragments were cut up) might stimulate feeding behavior in a fresh, unfed hydra. To test this hypothesis, use a thin-tipped pipet to obtain a few drops of the worm water from the dish. Then, very, very slowly expel this “worm water” onto the tentacles of an unfed hydra whose tentacles are extended. Describe and try to compare this hydra’s reaction to a hydra that was fed a worm fragment or one that received a volume of pure spring water (experimental control). Note any changes in the shape of the hydra, its tentacles and its mouth on the hypostome.

Sample Data

Feeding Planaria

Observation of planaria and Lumbriculus fragments

Observations will vary.

Observations will vary.

Answers to Questions

Feeding Planaria

1. What sensory cues from the worm might the planaria be sensing?
   

   Perhaps movement or chemicals produced by the prey.
   

2. Did the worm fragment react to the planaria? If so, describe the response.
   

   Answers will vary. Usually the worm fragments wiggle vigorously in an attempt to escape.
   

3. How does the fragment reach the mouth of the planaria?
   

   Planaria uses its ciliated ventral surface to glide over the fragment, wrap around it, and move it posteriorly to the mouth.
   

4. Where does the food go after entering the mouth?
   

   The food goes into the pharynx and into the digestive system.

5. How long did it take to ingest the Lumbriculus fragment?

   Answers will vary. Usually the fragment is ingested in a few minutes.

1. What might cause the Lumbriculus fragment to stick to the hydra tentacle?
   

   Tentacles are naturally sticky due to secretions and discharge of nematocysts.
   

2. What sensory cues might the hydra be responding to?
   

   Perhaps movement or chemicals produced by prey.
   

3. Did the Lumbriculus fragment respond to the hydra?
   

   Answers will vary. Usually worm fragments respond to nematocyst stinging by localized twitching.
   

4. Describe the appearance of the hydra as it ingests the fragment.
   

   The hydra’s mouth opens widely to surround the fragment and the fragment slowly passes into the hydra’s digestive cavity.
   

5. Describe the appearance of the worm fragment within the hydra’s digestive cavity.
   

   The fragment is still in one piece and pulsations in the fragment’s dorsal blood vessel are still apparent.
   

6. How long did it take for the ingestion to occur?
   

   Answers will vary. Usually ingestion occurs in 10–30 minutes.

Teacher Handouts

10324_Teacher1.pdf

References

Special thanks to the late Dr. Charlie Drewes and Kacia Cain, Chair of Science, East High School, Des Moines, IA, for providing this activity.

Introduction

Detect, attack, devour, escape—all of these behaviors occur with regularity in the living world. Most often, however, they go undetected by humans. Sharpen your senses and witness some predator–prey interactions.

Concepts

• Predator–prey interactions

• Feeding behavior and digestion
• Escape behavior
• Sensory cue triggers
• Invertebrate toxins

Background

In this activity predator–prey interaction and feeding behavior in several invertebrate species will be studied. Such interaction usually requires (1) production of sensory cues (e.g., movement, chemical “odor” by the potential prey), (2) detection of these cues by the predator, (3) predatory attack and feeding behavior by the predator and (4) escape and/or defense by the prey.

Invertebrates, such as soft-bodied oligochaete worms (Phylum Annelida, Class Oligochaeta), are frequent prey of vertebrate or invertebrate predators in freshwater wetlands. Predators that eat oligochaete worms use a variety of structures and behaviors to capture their living food. The worms, in turn, may use various means to avoid attack and capture. Blackworms or mudworms, Lumbriculus variegatus, are free-living oligochaete worms that inhabit freshwater wetlands throughout North America and Europe. In the laboratory, they may be used as a year-round food source for a wide variety of freshwater invertebrate and vertebrate predators. For example, living worm fragments are rapidly, reliably and aggressively eaten by hydra and planaria. Whole worms are readily eaten by crayfish, leeches, some aquatic insect larvae, aquatic amphibians and most tropical fish. An interesting feature of Lumbriculus worms is that body fragments actually are produced in nature by fragmentation (a form of asexual reproduction). Worm fragments that survive an attack are able to rapidly regenerate missing segments and develop into whole worms. This is a major advantage for survival in this species. In the laboratory, survival of these fragments is also an advantage because uneaten worm fragments do not decompose and foul the water.

Materials

Prelab Questions

Refer to your textbook or other reference materials to learn the anatomy and structure of a planaria and a hydra. Label the numbered drawings on the McWorm Worksheet.

Safety Precautions

Caution must be practiced when cutting with single-edge razor blades or other sharp cutting devices used during this laboratory. Be sure to wash hands thoroughly upon completion of this laboratory. Follow all other appropriate laboratory safety rules.

Procedure

Feeding Planaria

1. Obtain 1–2 small Lumbriculus fragments (the prey) in a small Petri dish of spring water, or make the fragments as directed by your teacher.
2. Obtain a planarian (predator) that has been previously isolated in a small Petri dish of spring water.
3. Using a microtip pipet, transfer a small Lumbriculus fragment to the dish with the planarian. Try to eject the worm fragment very slowly from the pipet so that the fragment is in the path of the moving planarian.
4. Now, carefully observe details of the planarian’s behavior as it approaches and makes contact with the fragment. Do not disturb the dish during your observations.
5. Carefully observe the planarian’s feeding behavior as it ingests the fragment. Write a detailed description of your observations on the McWorm Worksheet. Then answer the questions about the planaria on the McWorm Worksheet.

Feeding Hydra

1. Obtain 1–2 tiny Lumbriculus fragments (the prey) in a small Petri dish of spring water, or make the fragments as directed by your teacher.
2. Obtain a hydra (the predator) that has been previously isolated in a small Petri dish of spring water.
3. Using a microtip pipet, transfer a tiny Lumbriculus fragment to the dish with the hydra. Try to eject the worm fragment very, very slowly from the pipet tip in such a way that the fragment comes in direct contact with one of the hydra’s tentacles. The idea is to have a fragment stick to a tentacle. (Be careful, however, not to touch the pipet tip to the tentacle. This can cause the hydra to retract its tentacles, dislodge the hydra completely or even cause injury.)
4. Now carefully observe details of the hydra’s behavior as it comes in contact with the fragment.
5. Carefully observe the hydra’s feeding behavior as it ingests the fragment. Write a detailed description of your observations on the McWorm Worksheet. Then answer the question about the hydra on the McWorm Worksheet.
6. Return all living materials to their appropriate culture containers as directed by your instructor.

Post-Lab Activity

Have a class seminar to share observations and answers to the questions on the worksheet. Consider additional questions:

1. How are hydra and planaria alike in their feeding behavior? How are they different?
2. How are they structurally similar? different?
3. What relationship might exist between hydra and planaria if placed together?

Student Worksheet PDF

10324_Student1.pdf