Ichthyology Lab
ESS 345: Ichthyology
23 Jan 2017
Dichotomous Keys
Readings: Chapter 6 and 7 in Stauffer et al. 2016
Due Date: Next Lab Period
Using the color fish images provided, construct your own dichotomous key for the fishes on the
following page.
The way to build this key is to carefully examine at all the images provided. Think of what
comparisons and what differences you see. This could be shape (morphology), color, fin
position, mouth position, tail shape, etc. Don’t worry if you don’t know what the correct
terminology is. Build your dichotomous key based on the characters you can easily describe.
Think external characters.
You should create a series of couplets that identify each of the pictures on the color sheet. It is
not important that you know the names of the fish, or their evolutionary relationships.
Things to remember when constructing a dichotomous key:
1. Dichotomous means “in 2 parts”. Therefore, we create a couplet: 2 statements where each
leads to a more precise description of an organism.
2. Always read both couplets, even if you think the first one is appropriate. Jumping to
conclusions may lead to incorrect identification.
3. Always understand the meaning of the words in each choice. Define the term. If you aren’t
sure what the meaning of a word is, use a dictionary. Never guess- this can also lead to incorrect
identification.
4. When there are measurements given in the key, always use the appropriate measuring device.
For example, if the key lists measurements in millimeters but your ruler is in inches, you need to
either find a metric ruler or be ready to make conversions. Do not approximate and do not guess.
Always measure or count when needed.
5. If you are classifying a living or once-living thing, do not base your conclusion on a single
specimen or observation. Living things always exhibit variability. It is best to study many
specimens so that your results are representative of the majority.
6. If you are left with 2 possible answers, read the description of both and decide which one most
closely matches your specimen.
7. Once you have identified your specimen, do not assume its correctly identified. If there is any
doubt, recheck the description of the organism to see that it appropriately matches. If it does not,
a mistake was made somewhere in key development.
Ichthyology Spring 2017
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Ichthyology Spring 2017
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