Name: _______________________________________________ Period: _________________

Using and Making a Dichotomous Key

Background information: The dichotomous key is the most useful tool for species identification. It consists of series of paired statements describing the physical features of the organism in question. By following the key, the species can be identified by its scientific name.

Objectives: Students will 1) use a dichotomous key to identify fourteen shark species and 2) practice constructing their own dichotomous key.

Part I:

1.      Use figure 32-1 as a guide to the various shark features described in the dichotomous key.

2.      Read sentences 1A and 1B of the key. Then study shark 1 in figure 32-2 for the characteristics referred to in statements 1A and 1B. Follow the directions on the dichotomous key until a common name for shark 1 has been identified.

3.      Write the scommon name for the shark in the space provided below the shark.

4.      Repeat this process until all of the sharks have been identified. You will only use each name once.

 

 

Dichotomous Key for Sharks 

1.   a.   Body shape is like a kite when viewed from above….…………………Go to statement 12

     b.      Body shape is not like a kite when viewed from above……………...........Go to statement 2 

      2.   a.   Has no pelvic fin and nose is looks like a saw……...........………… (American Saw Shark)

      b.      Pelvic fin is present……………………………………………..……….Go to statement 3 

      3. a.  Has 6 gill slits………….……………………..…………………………………. (Cow Shark)

     b.      Has 5 gill slits……….…………………….…………………….…………Go to statement 4

       4. a.  Has only 1 dorsal fin……………………......…….… …………………………...(Cat Shark)

      b.  Has two dorsal fins……………………….………………………………Go to statement 5

       5.   a.  Mouth is at front of head with eyes behind mouth…………………………... (Whale Shark)

      b.      Mouth is back along underside of head……………………………….......Go to statement 6

       6.  a.  Head is elongated sideways with eyes at end of extensions…..……  (Hammerhead Shark)

        b . Head not elongated sideways……………………………….…………….Go to statement 7

   7.  a. Top half of caudal fin is exactly the same size and shape as bottom half.... (Mackerel Shark)

        b. Top half of caudal fin different in size and shape form bottom half……....Go to statement 8

        8.  a.  First dorsal fin is very long, almost half the total body length................  (False Cat Shark)

         b.      First dorsal fin length much less than half the body length…………...Go to statement 9

9.     a.   Caudal fin very long, almost as long as entire body……....................... (Thresher Shark)

          b.        Caudal fin length much less than length of entire body……………Go to statement 10 

        10.     a.  Nose with long needle-like point on end………………………………… (Goblin Shark)

       b.      Nose without needle-like point……………………………...………Go to statement 11

11.        a.   Anal fin absent………………………………………………………… (Dogfish Shark)

         b.      Anal fin present………………………………………………………. (Requiem Shark)

        12.        a.  Small dorsal fin present near tip of tail…………………….....………………….. (Skate)

        b.      Small dorsal fin absent near tip of tail………………………………Go to statement 13

        13.        a.  Horn-like appendages at front of body (head)………….......…………………. (Devilray)

        b.      No horn-like appendages at front of body (head)…………………………….. (Stingray)

 

Part II: Constructing a Dichotomous Key

Now it is your turn to make a dichotomous key. The organisms your dichotomous key will need to identify are the creatures on the following page. Keep in mind the following when constructing your key:

1.       Each number should consist of two statements, A and B. HINT: You cannot have three statements because the “di” in dichotomous means “two”.

2.       The paired statements should be inclusive. For example:

Correct

1A: Creature has wheels…….

1B: Creature does not have wheels…….

                                Incorrect

1A: Creature has wheels…….

1B: Creature has scales…….

This is incorrect because someone trying to identify Hairy birdius would have been excluded and the person would not be able to figure out the name of the creature.

3.       Each statement should either identify the creature by name or tell the person using the key to move to another statement.

4.       There should not be “dead ends” in the key. A person using the key should not come to the point where they have nowhere to go unless they are finished.

5.       Be very accurate with your descriptions. Descriptions should not be subjective. When in doubt, measure with a ruler to be accurate.

 

Dichotomous Key for Creatures