9550

• UCREFRP Catalog Number: 9550
• Author: Snyder, D. E. and R. T. Muth.
• Title: Catostomid Fish Larvae and Early Juveniles of the Upper Colorado River Basin - Morphological Descriptions, Comparisons, and Computer-interactive Key.
• USFW Year: 2004.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

The first or most anterior principal ray in
both dorsal and anal fins remains unbranched
while all other principal fin rays branch distally
as or after ray segmentation becomes evident.
The last or most posterior principal ray in each
fin is considered to be divided at the base and
therefore usually consists of two elements that,
except for their close proximity and association
with the same pterygiophore, might otherwise be
considered as separate fin rays.
Principal dorsal- and anal-fin-ray counts
between and within certain genera often vary
sufficiently to be of use in identification at the
species level, especially anal-fin rays of cyprin-
ids and dorsal-fin rays of catostomids. Positions
of dorsal-fin origin (anterior attachment) and
insertion (posterior attachment) relative to origin
of pelvic fins or fin buds and the vent vary
considerably among cyprinids and are useful in
identification of genera or species. These posi-
tion characters are more consistent among catos-
tomids (e.g., dorsal-fin origin is always well in
advance of the pelvic fins), especially at sub-
family level, and therefore, are of less value in
identification.

The pelvic fins begin as buds before or upon
transition to the metalarval phase. In cyprini-
form fishes, they originate in an abdominal posi-
tion along each side of the preanal finfold. They
may erupt shortly after dorsal-and anal-fin
development begins or be delayed until just
before or shortly after all principal rays are
present in the median fins. Pelvic rays begin to
form shortly after the buds appear and the adult
complement of rays quickly ensues. Among
cypriniform fishes, pelvic-ray counts are seldom
used diagnostically. However, position of the
pelvic fins or fin buds, relative to other struc-
tures, and their formation in the sequence of
developmental events can be useful in identifi-
cation, especially among cyprinids.

The pectoral fins typically begin as buds
immediately behind the head in the late embryo.
However, pectoral buds are not evident in some
cypriniform fishes until shortly after hatching.
Though strongly striated and occasionally with
membranous folds and breaks, they typically
remain rayless in cypriniforms until late in the
meso larval phase when most of the principal
median-fin rays are present. With the exception
of rudimentary caudal-fin rays, the rays ofpec-

toral fins are often the last to establish their full
complement. For this reason and because the
number of pectoral rays is usually relatively
large and difficult to count without excision
(especially the smaller ventral rays), pectoral-
fin-ray counts are generally of little value in
larval fish identification.

Other countable structures

Other structures that may be treated meris-
tically (and in some cases morphologically)
include branchiostegals, gill rakers, pharyngeal
teeth, and scales. Branchiostegals form early in
larval development, but counts are usually
constant within major taxon groups. Within the
order Cypriniformes, all members of super-
family Cyprinoidea, which includes Cyprinidae
and Catostomidae, have three branchiostegals
(McAllister 1968). Due to later development,
small size or internal location, the other charac-
ters are seldom used to diagnose fish larvae.
Gill rakers form gradually in postflexion meso-
larvae or metalarvae with numbers increasing
throughout much of the early portion of the
juvenile period. The adult complement of gill
rakers on the first gill arch is not achieved in
many Catostominae until they reach about 70
mm standard length (Smith 1966). Pharyngeal
teeth form relatively early but may not be suffi-
ciently well developed to be readily removed
and observed until late in the larval period or
early in the juvenile period. Detailed study of
gill rakers and pharyngeal teeth might reveal
some useful diagnostic qualities, including size,
shape, and number. However, most specimens
are more easily identified using external char-
acters. Scales typically become apparent late in
the larval period or early in the juvenile period.
First scales on cypriniforms typically appear
midlaterally on the posterior half of the body
and from there spread anteriorly, dorsally, and
ventrally toward adult coverage. Scales of
large-scaled species are sometimes sufficiently
obvious by late in the metalarval phase to distin-
guish certain species or genera.

Morphology

The shape or form of larvae and specific
anatomical structures (e.g., gut, air bladder, yolk
sac, and mouth) changes as fish grow and pro-
vides some of the most obvious characters for

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