Using A Dichotomous Keyby Cassie Zanca

The students will explore the benefits of creatingand using dichotomous keys as a means ofidentifying an organism or object.

GOAL

The student will use a dichotomous key toidentify seashells.

A dichotomous key is a tool that allows theuser to determine the identity of items and organismsin the natural world. It is the most widely used form ofclassification in the biological sciences because it offersthe user a quick and easy way of identifying unknownorganisms. Keys consist of a series of choices that leadthe user to the correct name of a given item.

“Dichotomous” means “divided into two parts.”That is why dichotomous keys always give two choicesin each step. In each step, the user is presented withtwo statements based on characteristics of theorganism. If the user makes the correct choice everytime, the name of the organism will be revealed at theend.

There are two kinds of descriptions that mightbe presented to the user of a dichotomous key:qualitative and quantitative descriptions. Qualitativedescriptions concern the physical attributes, orqualities, of the item being classified. Examples ofqualitative descriptions are such phrases as “contains

green striations on top surface” or “feels slick onbottom surface.” Quantitative descriptions concernvalues that correspond with the item being classified.Examples of quantitative descriptions are such phrasesas “has 10 striations on top surface,” “has 8 legs,” or“weighs 5 grams”. Knowing the difference betweenthese two types of descriptions can be immenselybeneficial for creators and users of dichotomous keys.

There are two ways to set up a dichotomouskey. One way is to present the two choices together,and the other way is to group by relationships. Whenthe dichotomous key is set up by presenting the twochoices together, it is easy to distinguish between them.However, relationships between various characteristicsare not emphasized. When the dichotomous key isgrouped by relationships, the choices are separated,yet it is easy to see the relationships between them.While this method may prove to be more difficult toconstruct, many users prefer it because it gives themmore information.

OBJECTIVE

BACKGROUND

BENCHMARKS (Louisiana) MIDDLE SCHOOL BENCHMARKS HIGH SCHOOL BENCHMARKS

SI-M-A1,A3,A4,A7 SI-H-A1,A3,A4,A5SI-M-B4,B6 SI-H-B2,B4PS-M-A1,A3 PS-H-A1LS-M-A2,A3 LS-H-C4,C5LS-M-C1

MIDDLE SCHOOL STANDARDSUC&P- Systems, order & organizationLS - Diversity & adaptations of organisms

NATIONAL SCIENCE STANDARDSHIGH SCHOOL STANDARDSUC&P - Systems, order, & organizationPS - Structure & properties of matter

1 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program

ANSWER KEYSee the answer key for the format used in class. Keys for both formats are provided.

• Create a concept map from the key.• Design a short dichotomous key using a particular item (examples: coke tabs, empty ketchup bottles,

paper clips, etc.) and have the students work through the key.• Have students work through an online dichotomous key.

EXTENSION STARTERS• Research ways dichotomous keys are used in other areas of school, society, culture, etc.• In geometry class, develop a dichotomous key of a formal proof.• Create a collage from the different levels of the key.

Download blackline masters 1,2,3, and 4 (pages 1-3) from the website. There you will find two styles ofdichotomous keys and pictures used in the keys. Pick one of the dichotomous key worksheets; both reflect thesame dichotomous process. If you are a verbal learner use the key on blackline master 2. If student is a visuallearner use blackline master 3. Duplicate as many keys as necessary for each student or student group. It issuggested that if you use this activity many times, you should laminate the pictures of the shells and then cutthem apart as sets for future use.Note to teacher: When making copies of the seashell pictures from your blackline master, lighten the density ofthe copies and if possible use the photo/text option. If you do not lighten the density, the images will be difficultto see.

ALTERNATIVE ASSESSMENT IDEAS

Timme, Stephen, 1991, Association for Biology Laboratory Education website, How to Construct and Use aDichotomous Key, accessed 02/16/01, http://www.zoo.utoronto.ca/able/volumes/vol-12/7-timme/7-timme.htmDescription: An excellent web-based activity on the construction and use of a dichotomous key that also describesthe use of a dichotomous key in the field and provides a key for prairie plants.

Frontier High School, Red Rock, OK, The Dichotomous Key, accessed 02/16/01, http://pc65.frontier.osrhe.edu/hs/science/hbotkey.htmDescription: Provides instructions on the two methods of constructing a dichotomous key as well as several onlinedichotomous keys. Grade level: High School.

Detka, Jon , California State University at Monteray Bay, Designing and Using a Dichotomous Key, accessed 02/16/01, http://www.monterey.edu/students/Students_D-H/detkajon/world/ron/dichotdesign.htmlDescription: Students first construct a simple dichotomous key and then use a basic key to identify some of thenative plants and the most unwanted invasive weeds of California. Grade level: 3-5.

Santa Cruz Productions, Wastewater Filamentous Bacteria Dichotomous Key, accessed 02/16/01, http://home1.gte.net/vsjslsk1/gramstainflowchart.htmDescription: A completely web-based dichotomous key designed to assist students in identifying wastewaterbacteria.

RESOURCES & WEBLINKS

TEACHER PREPARATION

2 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program

You will be using a dichotomous key to identify a group of shells or pictures of shells. At the endof the activity, every shell or picture should be accurately identified.

STUDENT INSTRUCTIONS

Some things to keep in mind when using a dichotomous key:• Always read and consider both choices, even if the first one seems to be appropriate.

Jumping to conclusions may lead to the wrong classification of the item.• Always understand the meaning of the words used in each choice. Define the term. If you

are not sure of the meaning, look it up in a dictionary. Never guess, as this could also leadto the wrong classification of the item.

• When there are measurements given in the choices, use the appropriate measuring tools oradjust them to match your own set of tools. For example, if a key measurement is given incentimeters but your ruler is divided into inches, convert the centimeter measurement intoinches. Do not approximate and do not guess. Measure.

• If you are classifying a living or once-living thing, do not base your conclusion on a singleobservation. Living things almost always exhibit variability, so it is better to study manyspecimens in order to be sure that your results are representative of the majority.

• If you are left with two possible answers, read the description of both and decide which oneseems to fit your specimen more precisely.

• Once you have identified all of your items, do not assume that it is correct. If there is anydoubt, recheck the description of the organism to see that it appropriately matches. If itdoes not, then an error was made somewhere in key development.

3 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College ProgramBlackline Master 1

1. Sort shells by one characteristic; for example, whether the shell is over 3 inches inlength or under 3 inches in length.

2. Next, those shells that are over 3 inches will be sorted by a differentcharacteristic; for example, has spiny protrusions or no spiny protrusions.

3. Continue to sort shells by one characteristic until each shell has been identified.

4. With the shells under 3 inches, repeat the same process until all of those shells areidentified.

How to use a dichotomous key:

“Dichotomous” means “divided into two parts.” That is why dichotomous keys give two choices ineach step. In each step, you are presented with two statements based on characteristics of theorganism. If you make the correct choice every time, the name of the organism will be revealedat the end.

Example:

Name _____________________________ Period ____________ Date ______________

A Dichotomous Key Activity for Seashells

Instructions: Using the images on the card set that your teacher gives you, you should beable to correctly identify where a shell goes in the identification key. Write the number ofeach shell (noted on the card) on the line provided.

I. Univalve ________________A. Outer projections (spikes) _____

1. Spikes at two ends _____a. Shell solid white in color _____b. Shell not solid white _____

2. Spikes at one end only _____a. Shell is longer than it is wide _____

(1) Shell has vertical stripes _____(2) Shell has no vertical stripes _____

b. Shell is wider than it is long _____B. Spikes not present _____

1. Shell is wider than longer _____a. Shell has speckles _____b. Shell has no speckles _____

2. Shell is longer than wider _____a. Shell is speckled _____

(1) Shell is approximately 7 cm from umbo to base _____(2) Shell is approximately 5 cm from umbo to base _____

b. Shell is not speckled _____ (1) Shell is striped _____

(2) Shell has no stripes _____ (a) Curved at pointed end _____ (b) No curve at pointed end _____

II. Bivalve ______________________A. Two shells present (articulated) _____

1. Umbo axial length is approximately 5.0 cm long _____2. Umbo axial length is less than 5.0 cm long _____

B. One shell present _____1. Shell has pointed umbo area _____

a. Shell is smooth _____b. Shell is rough _____

2. Shell has straight umbo area _____

4 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College ProgramBlackline Master 2

Name ___________________________________ Period ____________ Date _______________

A Dichotomous Key Activity for Seashells

Instructions: Using the images on the card set that your teacher gives you, you should beable to correctly place a shell in its correct position on the dichotomous key. Once youhave correctly identified where the shell goes in the key, write the number of each shell(noted on the card) on the line provided.

5 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program

Spikes at one end only ___

Outer projections (spikes) ______

Spike at two ends _____

Shell solid white in color ______

Shell not solid white ______

Shell is longer than it is wide _____

Shell has vertical stripes _____

Shell has no vertical stripes_____

Shell is wider than it is long ______

Shell is wider than it is long _____

Shell is speckles ____

Shell is not speckles ____

Sea Shells

Univalve ___

Spikes not Present ____

Shell is speckles ____

Shell is approximately 7cm from umbo to base ____

Shell is approximately 5cm from umbo to base ____

Shell is longer than it is wide ____

Shell is not speckles ____

Shell is striped ____

Shell is not striped ____

Curved at pointed end ___

No curve at pointed end ___

Bivalve ____

Two shells present (articulated) __

Umbo axial length is approx. 5cm long ____

Umbo axial length is less than 5cm long ___

Shell is not striped ____

One shell present ____

Shell has pointed umbo area ____

Shell has straight umbo area ____

Shell is striped ____

Blackline Master 3

Name ___________________________________ Period ____________ Date _______________

A Dichotomous Key Activity for Seashells

Instructions: Using the images on the card set that your teacher gives you, you should beable to correctly place a shell in its correct position on the dichotomous key. Once youhave correctly identified where the shell goes in the key, write the number of each shell(noted on the card) on the line provided.

I. Univalve 2,3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 16A. Outer Projections (spikes) 3, 4, 5, 7, 8, 10

1. Spikes at two ends 8, 10a) Shell solid white in color 8b) Shell not solid white 10

2. Spikes at one end only 3, 4, 7a) Shell is longer than it is wide 3, 4

(1) Shell has vertical stripes 4(2) Shell has no vertical stripes 3

b) Shell is wider than it is long 7B. Spikes not present 2, 5, 6, 9, 11, 14, 16

1. Shell is wider than longer 2, 16a) Shell has speckles 16b) Shell has no speckles 2

2. Shell is longer than wider 5, 6, 9, 11, 14a) Shell is speckled 6, 11

(1) Shell is approximately 7 cm from umbo to base 6(2) Shell is approximately 5 cm from umbo to base 11

b) Shell is not speckled 5, 9, 14(1) Shell is striped 5(2) Shell has no stripes 9, 14

(a) Curved at pointed end 14 (b) No curve at pointed end 9

II. Bivalve 1, 12, 13, 15, 17A. Two shells present (articulated) 12, 15

1. Umbo axial length is approximately 5.0 cm long 122. Umbo axial length is less than 5.0 cm long 15

B. One shell present 1, 17, 131. Shell has pointed umbo area 1, 13

a) Shell is smooth 13b) Shell is rough 1

2. Shell has straight umbo area 17

6 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program

AnswerKey

Blackline Master 4

Name ___________________________________ Period ____________ Date _______________

A Dichotomous Key Activity for Seashells

Instructions: Using the images on the card set that your teacher gives you, you should beable to correctly place a shell in its correct position on the dichotomous key. Once youhave correctly identified where the shell goes in the key, write the number of each shell(noted on the card) on the line provided.

AnswerKey

7 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program

Curved at pointed end 14

No curve at pointed end 9

Shell is longer than it is wide 5, 6, 9, 11, 14

Shell is speckles 6,11

Shell is not speckles 5, 9, 14

Shell is approximately 7cm from umbo to base 6

Shell is approximately 5cm from umbo to base 11

Shell is striped 5

Shell is not striped 9,14

Spike at two ends 8, 10

Shell solid white in color 8

Shell not solid white 10

Outer projections (spikes) 3-5, 7, 8, 10

Spikes at one end only 3, 4, 7

Shell is longer than it is wide 3,4

Shell has vertical stripes 4

Shell has no vertical stripes 3

Shell is wider than it is long 7

Shell is wider than it is long 2,16

Shell is speckles 16

Shell is not speckles 2

Spikes not Present 2, 5, 6, 9, 11, 14 16

Sea Shells

Univalve 2-11, 14, 16

Two shells present (articulated) 14,15,17

Umbo axial length is approx. 5cm long 14

Umbo axial length is less than 5cm long 15,17

Shell is rough 15

Shell is smooth 17

Bivalve 1,14,15,17,20

One shell present 1,20

Shell has pointed umbo area 20

Shell has straight umbo area 1

Blackline Master 5

Seashell Pictures for Dichotomous Key Activity

Shell 1 Shell 1 Shell 1 Shell 1

Shell 2 Shell 2

Shell 3 Shell 3 Shell 3

Shell 4 Shell 4

Shell 5 Shell 5 Shell 5

Shell 6 Shell 6 Shell 6

8 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College ProgramBlackline Master 6 - page 1

Shell 7Shell 7

Shell 8 Shell 8

Shell 9 Shell 9

Shell 10Shell 10

Shell 11 Shell 11

Shell 12 Shell 12 Shell 12 Shell 12

Shell 13 Shell 13

Shell 10

9 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College ProgramBlackline Master 6 - page 2

Seashell Pictures for Dichotomous Key Activity

Shell 14 Shell 14

Shell 15 Shell 15 Shell 15

Shell 16 Shell 16

Shell 17 Shell 17 Shell 17 Shell 17

Shell 18 Shell 18 Shell 18 Shell 18

Shell 15

10 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College ProgramBlackline Master 6 - page 3

Seashell Pictures for Dichotomous Key Activity