Natural selection promotes evolution of predator-detection mechanisms and escape behaviors in species lacking physical or chemical defenses. This 3-5 hour exercise uses aquatic snails to illustrate predator-avoidance strategies used by many species of small animals. A laboratory experiment is conducted to test student-generated hypotheses about how snails might detect predators through chemical cues, and how they may respond to those cues (e.g., by increasing refuge use through crawling into structurally-complex habitat or out of water) to avoid being eaten. The actual experimental design should be determined from discussions among students and the course instructor. However, one experimental plan that I have used successfully is described here. Students record the numbers of snails that are visible in underwater aquarium habitats, and therefore vulnerable to fish predation, before and after adding water with (predator-cue treatment) or without (predator-free treatment) chemical cues originating from crushed snails or fish. Statistical tests are then used to determine if cues induced increased refuge use that should enhance snail survivorship.
Timothy W. Stewart
Department of Natural Resource Ecology and Management, Iowa State University, Ames, Iowa 50011, firstname.lastname@example.org
The experiment described here can be introduced and completed in 1-3 hours. Approximately two additional hours are needed to complete statistical analysis and discuss results. Time required for the experiment is affected by chemical cue concentration in water added to aquaria. Expose snails to high chemical cue concentrations to elicit a rapid response and reduce time needed to complete the experiment. Mechanisms for accomplishing this are described in the Notes to Faculty (see Comments on Challenges to Anticipate and Solve: 3. Generating snail responses to predator cues).
OUTSIDE OF CLASS TIME
The instructor will need 1-2 hours to set up the experiment. Additional time is required to obtain and care for animals. Biology majors need 6-10 hours outside of class to read background literature and complete a paper based on experimental results. Non-science majors need 1-2 hours at the conclusion of the experiment to complete calculations and answer questions related to the experiment.
Biology majors write a short paper using the format of a peer-reviewed scientific journal article. Non-science majors are evaluated on the quality of answers to specific questions and the accuracy of calculations and interpretations of results from statistical analyses. Methodological details of assessing student performance in this activity are described in the Description and Notes to Faculty sections below.
The study is conducted in the classroom/laboratory setting. Snails needed for this experiment can usually be collected from a local pond or lake. Fish can also be obtained from ponds or lakes, or from a hatchery. Animals used in this experiment survive well in laboratory aquaria, and breeding snail colonies are easily established on a diet of goldfish flake food.
I use this experiment in a general ecology course for biology majors, and a general biology course for non-science majors. Class sizes range from 10-25 students.
I have used this exercise while teaching at a large university (Bowling Green State University) and a small four-year undergraduate institution (Longwood University).
This activity is very adaptable to a variety of educational scales. I have conducted the experiment in freshman-level non-science majors courses in biology and environmental science, and used the experiment to illustrate components of experimental design and statistical analysis in a graduate-level course in scientific methods. Because it relies on live animals and collection and analysis of real data, this experiment should also stimulate interest in ecology among junior high and high school students, as well as students at lower educational levels. Snails used in this experiment occur almost worldwide in a variety of freshwater ecosystems, so this experiment can be conducted in a variety of geographic regions at any time of year. It is ideal for students with physical disabilities.
Andy Turner's (Clarion University) work on predator-avoidance behavior in freshwater snails was a primary inspiration for scholarship that culminated in this experiment. I thank Jeff Miner, Rex Lowe (Bowling Green State University), James Haynes (SUNY Brockport), and Rob Dillon (College of Charleston) for supporting my interest in freshwater snail ecology. I also thank Charlene Waggoner (Bowling Green State University) who encouraged me to develop this teaching activity. Comments by Kathy Winnett-Murray and two anonymous reviewers greatly improved the presentation of this laboratory activity.
A modified version of this experiment was presented at the 21st workshop/conference of the Association for Biology Laboratory Education (ABLE) at the University of Nebraska-Lincoln, June 1-5, 1999 and published in ABLE's Tested Studies for Laboratory Teaching, Volume 21 (Stewart and Waggoner 2000, http://www.zoo.utoronto.ca/able/volumes/volume21.htm).
Timothy W. Stewart. April 2006, posting date. Observing and Quantifying Predator-avoidance Behavior: Habitat Shifts by Snails in Response to Predator Cues. Teaching Issues and Experiments in Ecology, Vol. 4: Experiment #2 [online]. http://tiee.ecoed.net/vol/v4/experiments/habitat_shifts/abstract.html
Students looking for snails at the end of the experiment.
Photograph by T.W. Stewart.
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