Using Dichotomous Key to Identify Sharks

Classification is a way of separating a large group of closely related organisms into smaller subgroups. The scientific names of organisms are based on the classification systems of living organisms. The identification of an organism is easy with a classification system. To identify an organism, scientists often use a key. A key is a listing of characteristics, such as structure and behavior, organized in such a way that an organism can be identified.

 

Procedure

1. Use the image below as a guide to the parts of a shark's body that are used in the dichotomous key.

Image of shart with body parts identified

Image source

2. Read statements 1A and 1B of the key. They describe a shark characteristic that can be used to separate the sharks into two major groups. Then study organism #1 in the image below for the characteristic referred to in 1A and 1B. Follow the directions until the name for Shark 1 is determined.

For example, to key a shark that has a body that is NOT kite-shaped when viewed from above and has a pelvic fin and six gill slits, follow the directions of 1B and go directly to statement 2. Follow statement 2B to statements 3. At statement 3A, identify the shark as belonging to Family Hexanchidae.

3. Continue using the key, always starting back at Statement 1a and 1b for each shark until all have been identified. Indicate the family name for each shark in Figure 2 on your word processing document. Then complete the questions under 'Conclusion." Click here to download or print a copy of the image below.

Image depicting 14 shark or shark-like fish

Shark Dichotomous Key

1

A

Body kite like in shape (if viewed from above)

Go to statement 12

 

B

Body not kite like in shape (if viewed from above)

Go to statement 2

2

A

Pelvic fin absent and nose sawlike

Family Pristiophoridae

 

B

Pelvic fin present

Go to statement 3

3

A

Six gill slits present

Family Hexanchidae

 

B

Five gills present

Go to statement 4

4

A

Only one dorsal fin present

Family Scyliorhinidae

 

B

Two dorsal fins present

Go to statement 5

5

A

Mouth at the front of the head rather than back along underside of head

Family Rhinocodontidae

 

B

Mouth back along underside of head

Go to statement 6

6

A

Head expanded on the side with eyes at the end of expansion

Family Sphyrnidae

 

B

Head not expanded

Go to statement 7

7

A

Top half of caudal fin exactly same size and shape as bottom half

Family Isuridae

 

B

Top half of caudal different in size and shape from the bottom half

Go to statement 8

8

A

First dorsal fin very long, almost half the length of the total body

Family Pseudotriakidae

 

B

First dorsal fin length much less than  the half the total length of body

Go to statement 9

9

A

A. Caudal fin very long, almost as long as the entire body

Family Alopiidae

 

B

Caudal fin length much less than length of entire body

Go to statement 10

10

A

Nose with long needlelike point on end

Family Scapanorhynchidae

 

B

A light stripe extending the length of the body, a marked constriction at the base of the tail

Go to statement 11

11

A

Anal fin absent

Family Squalidae

 

B

Anal fin present

Family Carcharhinidae

12

A

Small dorsal fin present near tip of tail

Family Rajidae

 

B

Small dorsal fin absent near tip of tail

Go to statement 13

13

A

Hornlike appendages at front of shark

Family Mobulidae

 

B

Hornlike appendages not present at front of shark

Family Dasyatidae

 

Conclusion

1. What is a classification key and how is it used?

2. List four different characteristics that were used in the shark key.

3. Which main characteristic could be used to distinguish shark 4 from shark 8?

4. Which main characteristic could be used to distinguish shark 4 from shark 7?

 



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