Fish Dichotomous Key

Reviewed by: BD Editors

A dichotomous key helps identify plants, animals, bacteria, rocks, insects and many other things. It consists of a sequence of identification steps or questions with two choices (called a couplet) at each branching point. Some keys have more than two choices depending on which feature is being examined. Dichotomous keys are a type of single-access key (each couplet pertains to one characteristic) which dictates the attribute for each couplet. Multi-access keys let the user choose which attribute(s) to examine.

Dichotomous Keys for Fish

Dichotomous keys can be created for saltwater or freshwater fish or more specifically for a single type of fish such as sharks or tuna. For example, there are about 28 families of fish in the Great Lakes that number about 160 species. Minnows alone have some 62 species. Dichotomous keys come in handy to differentiate a lake sturgeon from a longnose gar. These keys are also essential for identifying and classifying new species of fish.

Fish have several common characteristics that are used to create dichotomous keys. Some primitive fish species have a few unique characteristics that have allowed them to survive for millions of years. But overall, common features of fish include scales, fins, gills and bony skeletons.

Beyond these basic characteristics, couplets will address more specific characteristics that differentiate the fish even more. These include the shape of the head, where their mouths are located, the average adult size, color markings, vertical stripes, fin spots, fin type and fin location.

How to Use a Dichotomous Key

As an example, the Florida Museum of Natural History has an online dichotomous key for identifying US Atlantic shark species. Below are the first three couplets in the key. Use the key to identify the two sharks shown in the images below the chart.

1a) Body flattened dorso-ventrally, skate-like in appearance. Squatina dumeril – Atlantic angel shark
1b) Body round in cross section. Go to question 2
2a) Seven gill slits, single dorsal fin. Heptranchias perlo – sharpnose sevengill shark
2b) Six gill openings, single dorsal fin. Go to question 3
2c) Five gill openings, two dorsal fins. Go to question 4
3a) Snout short, blunt and broad; eye small; distance between rear base of dorsal fin and origin of caudal fun about 1.5 to 2.0 times length of dorsal fin base; lower jaw with six rows of teeth. Hexanchus griseus – bluntnose sixgill shark
3b) Snout more pointed and narrow; eye large; distance between rear base of dorsal fin and origin of caudal fin about 2.5 to 3 times length of dorsal fin base; lower jaw with five rows of teeth. Hexanchus nakamurai – bigeye sixgill shark

Cow Shark Hexanchus griseus
The image above shows the bluntnose sixgill shark, Hexanchus griseus. Using the dichotomous chart above, compare it to the bigeye sixgill shark shown in the image below. Note, to answer the first couplet, a cross-section of the body of both fish is round.

Hexanchus nakamurai
The image above shows the bigeye sixgill shark, Hexanchus nakamurai.


  • Burgess, G. (n.d.). ID Key to US Atlantic Shark Species. Retrieved October 10, 2017, from
  • Stewart, S. (November 11, 2014). What’s that fish? The (dichotomous) key to fish identification. Retrieved from

Cite This Article

MLAAPAChicago Editors. "Fish Dichotomous Key." Biology Dictionary,, 05 Nov. 2017, Editors. (2017, November 05). Fish Dichotomous Key. Retrieved from Editors. "Fish Dichotomous Key." Biology Dictionary., November 05, 2017.

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