Morphometric & Meristics laboratory
Taxonomic Characters
The first step in successfully working with fishes is correct identification. Similar species
(i.e. Great Barracuda and Guaguanche) require in depth examination to discern the few
differentiating characteristics. Many times these examinations require accurate
measurements and counts of fin ray elements. Remember, fin ray elements are the
supports found in the paired and median fins of actinopterygans. The purpose of this lab
is to introduce you to these characters and instill the necessary precision and accuracy
needed when working with fishes.
The first characteristics we will discuss are those, which are measurable. These
characters are usually measured in the millimeter scale and are referred to as
morphometric characters. The most common of these measurements is standard length
(SL), which is typically measured from the tip of the snout to the base of the caudal fin
(Figure 1). Other measurements used in fisheries applications (regulatory size limits)
include total length (TL) and forked length (FL); these will be covered later in this
exercise. Statistically speaking, morphometric measurements are continuous variables.
This means that theoretically there are an infinite number of possible decimal places,
which can be placed on a measurement (i.e. 2.5, or 2.53, or 2.536…). Be sure to use
figure 1 for this exercise and in the future when making various morphometric
measurements.
Another useful character when examining fishes is meristics. These are counts of fin
elements, i.e.: the number of dorsal fin spines and rays. Statistically speaking meristics
are discrete variables. This means that there are either 5 or 6 dorsal fin spines not 5.5.
So, how do you tell the difference between a spine and a ray? The table below should
help.
Spines
Rays
Hard and pointed
Unsegmented
Unbranched
Solid
Segmented
Sometimes branched
Bilateral with left and right halves
When making counts it is important to examine the base of each fin where each element
inserts into the body. Since most fin elements are difficult to discern without
magnification, for this exercise you will use a dissection microscope. Transmitted light
usually works best. Since rays are often branched, examining fins at the element tips
would result in 2 or more counts for what is only a single element. As with most things
in ichthyology meristic counts are written in a strict format. Below is a guide you should
use you for this lab when expressing meristic counts. This same technique is also
employed in many field guides, so learn it now!
Dorsal - D
Pectoral – P1
Anal - A
Pelvic – P2
Caudal – C
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Spines are written using roman numerals and soft-rays are written using Arabic numbers,
so:
D: VII, 10 - is a fish that has seven dorsal spines followed by 10 soft rays
P2: 8 - is a fish which has 8 pelvic rays and no spines
It is not uncommon for field guides to list a range on meristic counts (i.e.: Inshore
lizardfish (Synodus foetens) D: 10-13, A: 10-14, P: 12-15). This illustrates the fin
element variability inherent in a single species of fish.
Qualitative Characters
Qualitative characters are not easily expressed as numbers. These traits are usually body
shape, fish color and are expressed using descriptive words. Some researchers use predesigned data sheets that assign numbers to qualitative characters. For example body
background color might be expressed on a scale from 1-5 with (1) designating light color
and (5) dark color.
Precision and accuracy
Throughout this class we will frequently use the terms of accuracy and precision. Many
students think these terms can be used interchangeably; this is not the case! Accuracy
refers to the ability to acquire a true measurement or count. For instance, if a fish's true
length is 11.0mm. and you measure the fish to be 11.0mm. that is considered accurate.
Additionally, if you were to measure that same fish 3 further times and acquire
measurements of 10.9, 11.0 and 10.9 this repeatability is referred to as precision. It is
important to note that you can be precise without being accurate!
Laboratory exercise
In this exercise we will be making various measurements and counts on several species of
fish. Record the appropriate information in the attached table.
Some important considerations:
 Take care to not let the specimens dry out. Use the provided trays with a thin layer of
water.
 Dividers should be used on most measurements and then placed on a ruler
 All measurements and counts on paired fins and other structures should be made on
the left side of the fish
 All measurement should be made to the nearest mm. (Except when using the vernier
calipers.)
 Familiarize yourself with the use of the vernier caliper (these are used for juvenile,
larval fish and when measuring small structures)
 Check and recheck your counts and measurements, we will be using this information
in a future lab when we discuss morphological variation and character trends
 When counting fin elements using a dissecting scope try both transmitted light and
direct light to see which is better for viewing
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Figure 1. Cailliet et al. (1986) Fishes: A field and laboratory manual on their structure,
identification, and natural history. Waveland Press, Inc.
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Schreck, and Moyle (1990) Methods for fish biology American Fisheries Society
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Name__________________
Section____________
Complete the table below for two species of fish.
Counts
Common Name:
Common Name:
Species:
Species
Common Name:
Common Name:
Species:
Species
Dorsal fin elements
Anal fin elements
Pectoral fin elements
Scales along the lateral
line
Branchiostegal rays
Total gill rakers on first
arch
Measurements
Standard Length
Body depth
Caudal peduncle depth
Predorsal length
Length of dorsal base
Length of anal base
Height of dorsal fin
Height of anal fin
Length of pectoral fin
Length of pelvic fin
Length of longest dorsal
spine
Head length
Head width
Snout length
Suborbital width
Eye diameter
Upper-jaw length
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Qualitative
Common Name:
Common Name:
Species:
Species:
Position of mouth:
Inferior, terminal, superior
Snout Profile:
Convex, concave, straight
Upper-jaw teeth shape:
Simple pointed, simple
blunt, multicuspid
Shade of body background
color:
Light, dark
Pattern of body color:
Plain, complex
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