Summary: Working with a set of Great Lakes fish cards, students identify distinguishing characteristics of fish and use a dichotomous key to identify 10 common fish families.
Time: 30-60 minutes
- 30-60 minutes
Discuss the importance of observing distinguishing characteristics of living organisms in order to classify and identify them.
- Hold up the generic fish illustration.
- Explain that this is not a real fish, but rather a composite showing a variety of physical traits.
- Point out the location and names of the various fins and other special features such as barbels (a scientific term for the “whiskers” used by bottom-feeding fish to sense food), and adipose fin (a fleshy fin located behind the dorsal fin).
Arrange the students in small working groups.
- Provide each group with the generic fish illustration, and a set of fish family cards.
- Explain that the species on the cards belong to 10 Great Lakes fish families.
- Ask the groups to sort the fish cards based on any set of physical characteristics they choose or for which they have some prior knowledge. (This is an important step in drawing out students’ previous knowledge and creates motivation for students to learn more.)
Discuss the results.
- Ask students: How did you sort the fish? What features did you look at? Was it easy or hard?
- As the discussion progresses, ask students: Did anyone sort by tail shape, by presence of a barbel, by mouth shape, or fin rays? This way, the discussion becomes informative to students about what features scientists consider important, and informative to the teacher with regard to gauging students’ current knowledge.
Explain that in this next part of the activity, students will use a dichotomous key to sort and identify fish based on the characteristics that ichthyologists — fish scientists — view as important.
- Divide the class into small groups. Explain the need to use classification systems to organize living organisms.
- Introduce the concept of a dichotomous key. Emphasize that this system uses a set of logical steps based on distinguishing characteristics and results in the correct classification or identification of an organism.
- Pass out a set of fish family cards and a dichotomous key to each group. (The Fish characteristics fact sheet may be helpful for reference.)
- Remind students that the species on the 12 cards belong to 10 Great Lakes fish families.
- Explain that each group will use the dichotomous key to identify the correct families. (Families include Trout and Salmon, Pike, Sturgeon, Lamprey, Sunfish and Bass, Perch, Sucker, Goby (invasive), Catfish and Freshwater Cod.)
- Using the set of fish family cards, have students begin by selecting one fish and “keying it out” by answering the questions and following the arrows as indicated on the key.
- For each fish, they should identify the correct fish family. As they identify each illustration, have them write the name of the family on a post-it note and label each card.
- Review the results. Hold up the enlarged fish illustrations (labeled Teacher Master) and tell about each species, pointing out distinguishing characteristics and family.
- Did everyone correctly identify all fish families?
- Was it difficult to distinguish some of the characteristics?
- How else might some of the characteristics be described?
- Remind students that these variations, or adaptations, help fish survive in their environment. Explain that one limitation of a dichotomous key is that all fish of a given family or species do not look exactly alike (as with humans). There will always be individual differences. Fisheries scientists often use many additional physical characteristics, such as scale counts, fin location, and body depth, in combination with factors such as geographic distribution to correctly identify fish.
The process of using a dichotomous key is valuable in many disciplines. Have students create their own dichotomous keys using a group of commonly found objects, such as pasta, buttons, dried beans or fruit.
- Explain that the goal is to sort a large set of items into smaller sets based on distinguishing characteristics, until the items can no longer be sorted.
- Divide the participants into smaller groups of four to five people, and provide each group with a collection of items to sort. Have them develop a key by forming questions about the objects. An example might be: Is the object textured or smooth? Is the shape curved or straight? Let students work for a few minutes and then stop them to make sure they are using terms everyone will understand.
- Once the groups have finished sorting the items, ask them to switch their items and keys with another group. Other groups should be able to use the key to sort the items in the same way. Once all the groups have completed this, ask one group to share and demonstrate their key.
Anna Switzer, Ph.D. candidate in science education, University of Michigan School of Education; and Gerald Smith, Professor Emeritus of Geological Sciences and Professor Emeritus of Ecology and Evolutionary Biology, University of Michigan College of Literature Science and Arts.