7723

• UCREFRP Catalog Number: 7723
• Author: Snyder, D. E. and R. T. Muth
• Title: Editor
• USFW Year: Series
• USFW - Doc Type: 1990
• Copyright Material: NO

head, snout, eye, and fm lengths, as well as
snout-to-vent and snout-to-origin-of-fm (dor-
sal, anal and pelvic) lengths.

Snout-to-vent length is measured to the
posterior margin of the vent or anus. It is a
primary diagnostic character for many spe-
cies, especially at the family and sometimes
subfamily level. In the Upper Colorado
River System, most cyprinid larvae are readily
differentiated from catostomid larvae by
snout-to-vent lengths less than 72% SL.
Exceptions are most larvae of common carp
( Cyprinus carpio) and occasionally mesolarvae
of Colorado squawfISh. The term "preanal
length" is often applied to this measure but
might be misinterpreted as length to origin of
the anal fm. For many fishes, including
cypriniforms, the latter measure is approx-
imately the same as snout-to-vent length since
the anal fm begins at or near the posterior
margin of the vent.

Head length is typically measured to the
posterior margin of the operculum in juven-
iles and adults, but the operculum may be
absent or incomplete throughout much of the
larval period. Accordingly, many biologists
have redefmed head length for larvae to be
measured to the posterior end of the auditory
vesicle or the anterior or posterior margin of
the cleithrum, one of the first bones to ossify
in fish larvae (Berry and Richards 1973).
Unfortunately, the auditory vesicle and
cleithrum are not always easy to observe,
especially in postflexion mesolarvae and
metalarvae. Also, resultant measures to the
auditory vesicle are considerably anterior to
the eventual posterior margin of the oper-
culum. Snyder et al. (1977) and Snyder and
Douglas (1978) measured larval head length
to origin (anterior insertion) of the pectoral
fin. This measure has distinct advantages
over the alternatives -- the base of the
pectoral fm is readily observed throughout
the larval period (except in the few species
that hatch prior to pectoral bud formation),
it somewhat approximates the position of the
cleithrum (part of its supporting structure),
and it more nearly approximates the posterior
margin of the operculum than does the pos-
terior margin of the auditory vesicle. Accord-
ingly, we recommend this definition of head
length (Snyder 1983b) and have used it in all
our descriptive work. For purposes of consis-
tency, we apply it to juveniles as well as
larvae. The measure is most precisely deter-

mined while examining the specimen from
above or below and, if necessary, holding the
fm away from the body.

Body depths and widths are measured in
planes perpendicular to the horizontal axis of
the fISh. Many biologists report these as
maximum or minimum measures (e.g., great-
est head depth, greatest body depth, and least
caudal peduncle depth). However, for com-
parative purposes, it seems more logical to
specify standard reference points for such
measures as was done by Moser and Ahl-
strom (1970), Fuiman (1979), and Snyder and
Douglas (1978). Five specific locations, four
corresponding to specific length measure-
ments, are used herein: (1) immediately
posterior to eyes, (2) origin of pectoral fm,
(3) origin of dorsal fin, (4) immediately
posterior to vent, and (5) at anterior margin
of most posterior myomere (along the hori-
zontal myosepta). It is often desirable to
approximate position of reference points in
larvae prior to formation of the referenced
structure ( e.g., origin of dorsal fm in
protolarvae and flexion mesolarvae based on
position in later stages). Neither fms nor
fInfolds are included in depth measurements
herein. As mentioned earlier, care must be
used in evaluation of depth and width mea-
sures affected by body condition and gut
contents (e.g., measures at the origin of the
dorsal fm).

Other morphological characters such as
position, size, and form of the mouth and gut,
and related changes, can be among the more
useful characters for identification to the
species level. Size of the mouth, as well as its
position, its angle of inclination, and the form
of specific mouth structures are diagnostic for
some cypriniforms, especially in metalarvae.
Timing of mouth migration from terminal to
inferior position can be especially useful for
catostomid metalarvae. Gut loop length, tim-
ing of loop formation, and eventual degree
and form of the gut loops, folds, or coils can
be diagnostic for the larvae many fishes.
Such characters are especially useful in
distinguishing postflexion mesolarvae, meta-
larvae and early juveniles of certain catos-
tomids.

Pigmentation

Basic patterns of chromatophore distribu-
tion, and changes in these patterns as fish

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