Using a Dichotomous Key
Classification is a way of separating a large group of closely related
organisms into smaller subgroups. Identification of an organism is easy
with a classification system. The scientific names of organisms are based
on this system. To identify organisms, 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 by a scientific name.
PROCEDURES:
1. Study the shark anatomy in the diagram.
2. When using the key, always start at #1 and select the correct response.
3. The response will tell you what number to go to next--continue until you
come to the name.
4. The name identifies the shark- label each unknown shark in the diagram
-------------------------------------------------------------------------------------ANALYSIS:
1. What is a dichotomous key and how is it used?
2. List 4 different characteristics that were use in the shark key.
a.
c.
b.
d.
3. Which main characteristic could be used to separate shark #4 from # 8?
4. Which main characteristic could be used to separate shark #4 from # 7?
5. Can two different sharks have the same name?
1.
A. Body kite like in shape (if viewed from above)
B. Body not kite like in shape
2.
A. Pelvic fin absent and nose saw like
B. Pelvic fin present
Family Pristophoridae
Go to statement 3
3.
A. Six gill slits present
B. Five gill slits present
Family Hexananchidae
Go to statement 4
4.
A. Only one dorsal fin present
B. Two dorsal fins present
Family Scyliorhinidae
Go to statement 5
5.
A. Mouth in front of head rather than back along underside of head
B. Mouth back along underside of head
6.
A. Head expanded with eyes at end of expansion
B. Head not expanded
Family Sphyrnidae
Go to statement 7
7.
A. Top half of caudal fin exactly same size and shape as the bottom half
B. Top half of caudal fin different in size and shape from the bottom half
Family Isuridae
Go to statement 8
8.
A. First dorsal fin very long, almost half total length of body
B. First dorsal fin length much less than half the total length of body
9.
A. Causal fin very long, almost as long as entire body
B. Caudal fin length much less than length of entire body
10.
A. Nose with long needlelike point on end
B. Nose without needlelike point
11.
A. Anal fin absent
B. Anal fin present
12.
A. Small dorsal fin present near tip of tail
B. Small dorsal fin absent near tip of dorsal fin
13.
A. Hornlike appendages as front of shark
B. Hornlike appendages not present at front of shark
Go to statement 12
Go to statement 2
Family Rhinocodontidae
Go to statement 6
Family Pseudotriakidae
Go to statement 9
Family Aloplidae
Go to statement 10
Family Scapanorhynchidae
Go to statement 11
Family Squalidae
Family Carcharhinidae
Family Rajidae
Go to statement 13
Family Mobulidae
Family Dasyatidae
Prepare your own key for the five fish pictured. Use the same format as
the dichotomous key you just practiced. To help you get started, the first
statements are supplied. Next, choose another characteristic which will
divide fish into smaller groups. Write your key in the space below.
Dichotomous Key
1
A. Fish with long tube like body --------------------------- FISH #1
B. Fish with body shape not like a tube------------------- GO TO ____
2.
B.
A.
3.
B.
A.
4.
B.
A.
**Make a scientific name for fish #1 based upon its looks?