Open-Ended Inquiry Design Your Own Lab

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Open-Ended Inquiry • Design Your Own Lab

Chapter 18 Lab Dichotomous Keys

Problem Can you construct a dichotomous key that could be used to identify organisms?

Introduction In May 2007, scientists and other volunteers gathered in Rock Creek Park, Washington, D.C., to participate in a BioBlitz—a quick, 24-hour survey of species living in the park. Teams worked in 4-hour shifts throughout the park. By the time they were done, the teams had identified more than 650 species! Teams included experts on different types of organisms such as birds, beetles, fungi, and plants. The experts used identification guides, or keys, to help them identify the organisms they found. In this lab, you will first use a dichotomous key to identify sharks. A dichotomous key is built around pairs of statements that describe a visible trait. The reader must select the statement in each pair that best describes an organism. By following the steps in the key, the reader narrows down the list of choices and finally names the organism. After you have learned to use a dichotomous key, you will design your own key for a group of organisms.

Skills Focus Observe, Classify, Compare and Contrast, Sequence

Materials • reference materials

Build Vocabulary

Term Definition dorsal On, in, or near the back or upper surface of an animal projection Something that juts out, or projects, beyond the rest of a surface identify To recognize or show to be the person or thing described; to figure out the identity of

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Pre-Lab Questions 1. Observe Name three different physical traits that are used in the shark dichotomous key.

2. Apply Concepts After you make a list of physical traits that you can use in your dichotomous key, how will you decide which trait to pick for the first step?

Procedure Part A: Use a Dichotomous Key Before you try to identify sharks, you need to understand a bit about shark anatomy. Figure 1 is a general shark drawing with labels showing the possible locations of fins. Refer to Figure 1 as you use the dichotomous key to identify the sharks in Figure 2. 1. Tear out the sheet with the shark drawings (pages 115–116). Choose one shark from Figure 2. 2. Read statements 1a and 1b in the key. One of the statements describes the shark; the other does not. Choose the statement that describes the shark and follow the directions for that statement. 3. Continue following the steps in the key until you can identify the shark. Record the scientific and common name of the shark in the data table. 4. Repeat Steps 2 and 3 for the other sharks in Figure 2.

Figure 1 General external anatomy of shark

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Dichotomous Key for Sharks Step Characteristic Species

1a Anal fin present . . . Go to Step 2

1b No anal fin . . . Go to Step 6

2a One dorsal fin Notorynchus cepedianus, Sevengill shark 2b Two dorsal fins . . . Go to Step 3

3a Spines on dorsal fins Heterodontus francisci, Horn shark 3b No spines on dorsal fins . . . Go to Step 4 4a Mouth at front of head Rhincodon typus, Whale shark 4b Mouth at bottom of head . . . Go to Step 5 Sphyrna zygaena, Smooth 5a Eyes on ends of hammerlike hammerhead projection 5b Carcharodon carcharias, No hammerlike head Great white shark

6a Flattened body (like ray) Squatina squatina, Angel shark 6b Body not flattened . . . Go to Step 7 Pristiophorus schroederi, 7a Long sawlike projection from Bahamas sawshark snout Somniosus 7b No sawlike projection microcephalus, Greenland shark

Data Table Shark Scientific Name Common Name

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Part B: Construct a Dichotomous Key You will be working with a group of organisms of your choice, such as fish, amphibians, insects, birds, or mammals. Choose 5 organisms from one of these groups that you are familiar with. You will need to include an illustration of each organism with your dichotomous key. 5. Choose a group of 5 related organisms. Below, make a list of visible physical traits that vary among the species in the group.

6. On a separate sheet of paper make a simple drawing of each species. Use a letter to label each drawing. Record the scientific name and common name of each species next to the appropriate letter below – NOT next to the drawing. You will need to use resource books, guide books, etc. or the Internet to find the scientific names. A: B: C: D: E: 7. Use the space on page 113 to construct a dichotomous key for your group of organisms. Use the key for sharks as a model. Start by choosing a trait that divides your organisms into two smaller groups. Keep dividing each of the smaller groups with pairs of statements until you have a final identifying statement for each of your species. HINT: Start your key with a more generalized characteristic and work towards more specific traits. 8. Check the usefulness of your key by making a copy of your key and asking another student to use it to identify your drawings.

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Dichotomous Key

Analyze and Conclude 1. Predict How would the dichotomous key for sharks need to change if you wanted to use it to identify ten different sharks?

2. Evaluate What was the most challenging part of making your own dichotomous key?

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3. Infer Suppose you had actual samples instead of drawings. What other traits could you use to build a dichotomous key?

4. Compare and Contrast The shark dichotomous key groups three species that lack anal fins together. But a recent cladogram of sharks indicates that the Greenland shark is actually most closely related to the Sevengill shark, which has an anal fin. What does this tell you about the difference between a dichotomous key and a cladogram?

5. Draw Conclusions In what way are the characters used to design a dichotomous key more limited than the characters that are used to build a cladogram?

6. Infer The dichotomous keys in this lab are used to trace organisms to the species level. Could keys be designed which classify unknown organisms to higher levels of the Linnaean system—to a family or order, for example? Why or why not?

Build Science Skills Find a cladogram in your textbook or other reference that lists derived characters. Which of the derived characters could be used as traits in a dichotomous key? Which of the traits could not be used, and why?

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Figure 2 Shark species

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