Illinois Natural History Survey - University of Illinois

The Naturalist's Apprentice: So Many Fishes! How can You Tell Them Apart?

Objective: to make students aware that there are many species of fish in Illinois, and to introduce students to the dichotomous key.


Materials needed: a copy of "So Many Fishes! How Can You Tell Them Apart?" and a copy of "Fish Anatomy" for each student.


Other ideas: Before handing out the key to students, have them distinguish several classroom objects by use of a simple key. Example: book, pencil, chalkboard eraser, chalk, box of pencils.


1. a) object has square or rectangular shape with corners-- Go to 2 
b) object not as above (cylindrical or round)--Go to 3


2. a) object cannot be opened and has soft, porous surface--eraser
b) object can be opened--Go to 4


3. a) object flat on both ends, has no core--chalk
b) object comes to point on end, has a central core of dark material--pencil


4. a) object has several paper sheets between hard covers--book
b) object not as above--box of pencils


Have the students make a key of their own to distinguish five or six other objects in the classroom. Remind them that the statements in the key only need to distinguish the object or objects from the other objects, and only those that have not yet been eliminated. A key must be followed from the first couplet for each object. Notice that the couplets 2 and 9 in the fish key are identical. If couplet 1 had been skipped, then couplet 9 would incorrectly identify the channel catfish and tadpole madtom.



a) blackspotted topminnow
b) brook stickelback
c) quillback
d) spotfin shiner
e) tadpole madtom
f) blue gill
g) channel catfish
h) orangethroat darter
i) black redhorse
j) blackside darter


There are more species of fish in Illinois than any other group of vertebrates (animals with backbones) except birds. Currently, there are 194 (179 native and 15 introduced) species of fish living and reproducing in the lakes, ponds, streams, rivers, and backwaters around the state. Have you ever wondered how ichthyologists (scientists who study fish) are able to tell one species from another? While some are distinct, many more are difficult to tell apart. Scientists use a tool known as a dichotomous key to identify different unfamiliar plants or animals. These keys are written by other scientists (experts on the particular group of organisms) as an aide to others who have a need to identify plants and animals.


A dichotomous key is a series of paired opposite statements (couplets). The scientist will read the first statement in the couplet that describes some characteristic of that organism. If the statement is true, then the scientist will follow the directions given after that statement. If the statement does not fit the organism, the scientist will read the second statement in the couplet. If the first statement did not match the organism, the second statement should. As the scientist works through the key, he or she will either identify the organism or continue to another couplet. By following the directions given at the end of each statement that most closely matches the description of the unknown organism, the plant or animal can ultimately be identified.


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