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

There is often a noticeable amount of intra-
as well as interregional variability in many of
the characters to be discussed. This variability
necessitates confirmation of identity based on as
many diagnostic characters as possible.

Myomeres

Myomeres, because they are obvious mor-
phological features and relatively consistent in
number and position, are one ofthe most useful
characters available for identification of larvae
above (and sometimes at) the species level,
especially for proto larvae and meso larvae. They
begin as part of the embryonic somites and are
usually formed in their full complement prior to
hatching. Throughout the proto larval and much
of the meso larval phase, myomeres are
chevron-shaped, but by the metalarval phase
they evolve to their typical three-angled adult
form. Fish (1932) and many subsequent authors
observed that there is a nearly direct, one-to-one
correlation between total myomeres and total
vertebrae (including Weberian ossicles in
cypriniforrns). Snyder (1979) and Conner et al.
(1980) summarized myomere and vertebral
counts for many cypriniform fishes.
The most anterior and most posterior myo-
meres are frequently difficult to distinguish.
The most anterior myomeres are apparent only
in the epaxial or dorsal half of the body; the first
is often deltoid in shape and is located immed-
iately behind the occiput. The most posterior
myomere is defined as lying anterior to the most
posterior complete myoseptum. Siefert (1969)
describes a "false (partial) myoseptum" posterior
to the last complete myoseptum which adds to
the difficulty of discerning the last myomere.
Early in the larval period, myomeres are most
readily observed using transmitted light. Polar-
izing filters, depending on thickness and certain
other qualities of the preserved tissues, can dra-
matically increase contrast between the muscle
tissue of myomeres and the myosepta that separ-
ate them. Myomeres of some metalarvae and
most juveniles are difficult to observe even with
polarizing filters; reflected light at a low angle
from one side and higher magnification some-
times facilitates observation.
Typical counts used in taxonomic work
include total, preanal, and postanal myomeres.
Partial counts are frequently used to also refer-
ence the location of structures other than the

vent or anus. The most generally accepted
method of making partial counts was described
by Siefert (1969) for distinguishing preanal and
postanal myomeres: "postanal myomeres in-
clude all [entire] myomeres posterior to an
imaginary vertical line drawn through the body
at the posterior end of the anus . . . Remaining
myomeres, including those bisected by the line,
are considered preanal." The technique is
equally applicable with other structures or points
of reference such as origins of fins or finfolds.
The opposite approach was used by Snyder et al.
(1977), Snyder and Douglas (1978), Loos and
Fuiman (1977) and, according to the latter
authors, Fish (1932 )-only entire myomeres were
included in counts anterior to points of
reference. Siefert's method is recommended as
standard procedure because resulting counts
more nearly approximate the number of vertebrae
to the referenced structures.
In the United States and Canada, the range
of total myomere (and vertebral) counts for
cyprinids, 28 to 52, is slightly larger and nearly
includes that for catostomids, 32 to 53. Ranges
for preanal and postanal myomere counts also
overlap with 19 to 35 and 9 to 22, respectively,
for cyprinids and 25 to 42 and 5 (possibly 3) to
14, respectively, for catostomids. Despite the
magnitude of overlap in these ranges, propor-
tions of postanal to preanal and preanal to total
myomeres will distinguish most cyprinids from
catostomids (Snyder 1979). The postanal to
preanal myomere proportion is at least 2/5 (often
greater than 1/2) for cyprinids (exclusive of
subfamily Cyprininae, the carps) and less (often
less than 1/3) for catostomids. Also, the pro-
portion of preanal to total myomeres is 5/7 or
less (often less than 2/3) for cyprinids and
greater (often greater than 3/4) for catostomids.
For cypriniform fishes in the Upper Colorado
River System the degree of overlap in total and
preanal myomere counts is less and larvae with
fewer than 42 total or 32 preanal myomeres can
be cyprinids only.

Fins and finfolds

Fin-ray meristics and fin pOSItIons are
among the most useful characters for later
meso larvae and metalarvae, especially among
the cyprinids. These data can be detennined
from older juveniles and adults or gleaned from
published descriptions of adults. The sequence

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