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<br />position of the cleithrum (part of its supporting
<br />structure), and it more nearly approximates the
<br />posterior margin ofthe operculum than does the
<br />posterior margin of the auditory vesicle.
<br />Accordingly, we recommend this definition of
<br />head length (Snyder 1983b) and have used it in
<br />all our descriptive work. For purposes of con-
<br />sistency, we apply it to juveniles as well as
<br />larvae. The measure is most precisely deter-
<br />mined while examining the specimen from
<br />above or below and, if necessary, holding the fin
<br />away from the body.
<br />Body depths and widths are measured in
<br />planes perpendicular to the horizontal axis of the
<br />fish. Many biologists report these as maximum
<br />or minimum measures (e. g., greatest-head depth,
<br />greatest-body depth, and least-caudal-peduncle
<br />depth). However, for comparative purposes, it
<br />seems more logical to specify standard reference
<br />points for such measures as was done by Moser
<br />and Ahlstrom (1970), Fuiman (1979), and
<br />Snyder and Douglas (1978). Five specific loca-
<br />tions, four corresponding to specific length
<br />measurements, are used herein: (1) immediately
<br />posterior to eyes, (2) origin of pectoral fin, (3)
<br />origin of dorsal fin, (4) immediately posterior to
<br />vent, and (5) at anterior margin of most posterior
<br />myomere (along the horizontal myosepta). It is
<br />often desirable to approximate position of
<br />reference points in larvae prior to formation of
<br />the referenced structure (e.g., origin of dorsal fin
<br />in protolarvae and flexion meso larvae based on
<br />position in later stages). Neither fins nor fin-
<br />folds are included in depth measurements
<br />herein. As mentioned earlier, care must be used
<br />in evaluation of depth and width measures
<br />affected by body condition and gut contents
<br />(e.g., measures at the origin of the dorsal fin).
<br />
<br />Other morphological characters such as
<br />position, size, and form of the mouth and gut,
<br />and related changes, can be among the more
<br />useful characters for identification to the species
<br />level. Size of the mouth, as well as its position,
<br />its angle of inclination, and the form of specific
<br />mouth structures are diagnostic for some cyprin-
<br />iforms, especially in metalarvae. Timing of
<br />mouth migration from terminal to inferior posi-
<br />tion can be especially useful for catostomid
<br />metalarvae. Gut-loop length, timing ofloop for-
<br />mation, and eventual degree and form of gut
<br />loops, folds, or coils can be diagnostic for the
<br />
<br />larvae of many fishes. Such characters are
<br />especially useful in distinguishing postflexion
<br />meso larvae, metalarvae, and early juveniles of
<br />certain catostomids.
<br />
<br />Pigmentation
<br />
<br />Basic patterns of chromatophore distribu-
<br />tion, and changes in these patterns as fish grow
<br />are often characteristic at the species level.
<br />Used with caution, preferably in combination
<br />with other characters, and with an awareness of
<br />both intra- and interregional variation, chro-
<br />matophore distribution and patterns for many
<br />fishes are among the most useful characters
<br />available for identification. However, in some
<br />instances, differences are so subtle or variation
<br />so great that use of pigmentation is impractical
<br />and may be misleading.
<br />In cypriniform and most other fishes,
<br />chromatophores other than melanophores have
<br />not been sufficiently studied for identification
<br />purposes. Such chromatophores are typically
<br />neither as numerous nor as obvious as melano-
<br />phares and their pigments are difficult to pre-
<br />serve. In contrast, melanin, the amino acid
<br />breakdown product responsible for the dark,
<br />typically black, appearance of melanophores
<br />(Lagler et al. 1977), remains relatively stable in
<br />preserved specimens. However, melanin is
<br />subject to fading and bleaching if specimens
<br />are stored or studied extensively in bright light
<br />for long periods of time, stored in highly
<br />alkaline preservatives, or subjected to changing
<br />concentrations of preservative fluids. To mini-
<br />mize the latter effects, as well as shrinkage and
<br />deformation, dilute formalin solutions (3-5%,
<br />unbuffered or buffered to near neutral) are
<br />strongly recommended over alcohol solutions as
<br />storage media. Most of the following discussion
<br />refers to chromatophores in general, but in this
<br />manual and others for freshwater species in
<br />North America, pigmentation typically refers to
<br />that of melanophores.
<br />According to Orton (1953), pigment cells
<br />originate in the neural crest region (dorsal por-
<br />tion of body and tail) and migrate in amoeboid
<br />fashion in waves to their eventual position. The
<br />first wave of chromatophores occurs late in the
<br />embryonic period or early in the larval period
<br />and establishes a relatively fixed basic or
<br />primary pattern of chromatophore distribution.
<br />
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