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Activity: Identifying Butterflyfish Using Dichotomous Keys

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:


<p><strong>Fig. 1.14.</strong> Generalized fish anatomy</p>

  • Fig. 1.15
  • Table 1.15
  • Scissors
  • Construction paper (optional)
  • Glue (optional)


  1. Cut the butterflyfish in Fig. 1.15 into individual cards.
  2. Select the card for butterflyfish “M” from Fig. 1.15 to identify.
  3. Using the dichotomous key in Table 1.15, identify butterflyfish “M” by following these example steps:
    1. Starting at decision point 1, read the two statements describing a certain feature of the butterflyfish.
    2. Decide which statement fits the picture of your fish. For example, the description, “Pelvic fin dark,” is the correct description for butterflyfish “M.” Refer to Fig. 1.14 for fish anatomy terminology.
    3. Note the number to the right side of this statement. It is #2.
    4. Go to the #2 decision point as indicated by your choice in the first decision point, and again decide which statement better describes the fish. Here, the second statement, “Lacks two large white spots below dorsal fin,” is the correct choice.
    5. Note the number on the right side of this statement. It is #3.
    6. Proceed to the #3 decision point as indicated by the previous decision point.
    7. Again, decide which statement better describes the fish on your card. Between the two choices at the #3 decision point, “Tail with one dark bar at tip,” is the better choice. This decision identifies butterflyfish “M” with its correct scientific name: Chaetodon kleini.
    8. Record the name of your fish below its picture in Fig. 1.15. (Because all fish in Fig. 1.12 are in the genus Chaetodon, the name of the genus may be abbreviated as “C.” For example, Chaetodon kleini may be identified as C. kleini).
  4. Identify the butterflyfish assigned by your teacher in Fig. 1.15.
    1. Read the two descriptions at each decision point, then select the description that better matches the fish.
    2. Repeat the process, selecting the proper description and following the decision points indicated until you reach a choice that provides the identification of the fish.
    3. Record all key steps (decision point numbers) on the reverse side of each card. For example, for butterflyfish “M” the key steps followed were 1→2→3→C. kleini
    4. When you identify a fish, write its scientific name under its image on your card.
    5. Continue until you identify all of the fish on your cards.


Activity Questions: 
  1. Compare your butterflyfish identifications with those of your classmates.
    1. Were there any disagreements over species identifications? Which cards were disputed?
    2. If so, review the key steps written on the reverse side of each card. Which steps (decision points) were disputed between classmates? How did you resolve the discrepancies?
    3. Can you suggest any revisions to the dichotomous key that might improve future identifications?
  2. What kind of features should be used for constructing a dichotomous key to identify species within a group of organisms?
  3. How might dichotomous keys handle the problem of identifying fish species that have different color patterns in juvenile and adult stages? Or male and female fish?
  4. What are the advantages and disadvantages of using color patterns and markings for dichotomous keys rather than physical body shapes and forms?
  5. What is the purpose of using dichotomous keys in identifying organisms?
  6. Imagine that you have been hired to develop a dichotomous key to identify butterflyfish. How might your dichotomous key be different from Table 1.15 if written for the following three audiences?
    1. Tourists snorkeling at a resort on Maui, Hawai‘i
    2. A museum ichthyologist identifying Chaetodon spp. butterflyfish specimens collected from around the world
    3. Children visiting a public aquarium
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.