7724

• UCREFRP Catalog Number: 7724
• Author: Snyder, D. E.
• Title: Contributions to a Guide to the Cypriniform Fish Larvae of the Upper Colorado River System in Colorado.
• USFW Year: 1981.
• USFW - Doc Type: \
• Copyright Material: NO

The caudal fin is the first fin toaifferentiate
from the median finfold in many fishes. Such is
always the case in cypriniforms. The portion
involved first thickens along the ventral side of
the posterior end of the notochord, then begins to
differentiate into the hypural elements of the
caudal skeleton. Immediately thereafter, the first
caudal rays become apparent, marking the beginning
of the mesolarval phase, and the posterior portion
of the notochord begins to bend or flex upward.
Care must be taken not to confuse striations or
folds in the finfold with developing rays. As the
fin develops and the notochord continues to flex
upward, the hypurals and developing caudal rays,
all of which are ventral to the notochord, are
moved to a posterior or terminal position. The
first principal rays are medial; subsequent princi-
pal rays are progressively added posteriorly above
and anteriorly below. The principal caudal rays,
which are the first to attain their full adult
complement, articulate with the hypural bones of
the caudal structure and include all branched rays
plus one unbranched rayon each side. BranChing and
segmentation can be observed as or shortly after
the full complement of rays becomes evident.

The number of principal caudal rays is typically
very consistent within major groupings of fish.
Cyprinids, for example, generally have 19 principal
rays (ten based on the superior hypurals) while
catostomids usually have 18 principal rays.

Secondary or procurrent caudal rays, which are
added in an anterior direction, begin forming
immediately after the principal rays are formed or
nearly formed. They are often the last group of
rays to attain the full adult complement. Accord-
ingly, they are often ignored in larval work though
they may be of taxonomic value in juveniles and
adults.

The dorsal and anal fins, which typically form
either simultaneously (many cyprinids) or dorsal
first (most catostomids), usually begin development
prior to the attainment of the full complement of
principal caudal rays. Tissue first aggregates
in the vicinity of the future fin, and the basal
structures or pterygiophores soon become evident.
The latter structures permit limited use of dorsal
and anal fin position and meristics about midway
through the mesolarval phase. The anterior princi-
pal rays develop first with subsequent rays added
in a posterior direction; the first of the
secondary rays (anteri or to the pri nci pa 1 rays) are
frequently evident before all the principal rays
are formed; secondary rays are added in an anterior
direction.

The first or most anterior principal ray in
both dorsal and anal fins remains unbranched, while
all others branch shortly after or as segmentation
becomes evident. The last or most posterior princi-
pal 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
be mistaken for separate rays.

Principal dorsal and anal ray counts between
and within certain genera vary sufficiently to
often be of use in identification at the specific
level, especially the anal rays of cyprinids and
the dorsal rays of catostomids. The position of
the dorsal fin origin (anterior insertion) and

LARVAL CHARACTERISTICS 11

insertion (posterior insertion) relative to the
origin of the pelvic fins or fin buds and the vent
varies considerably among the cyprinids and is
useful in identification at the genus or species
levels. These position characters are relatively
more constant among the catostomids (e.g., dorsal
fin origin is always well in advance of the pelvic
fins), especially at the subfamily level, and
therefore of less value in larval identification.

The pelvic fins begin as buds at some stage
prior to or at the very beginning of the metalarval
phase. In cypriniform fishes, they originate in
an abdominal position 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 make their appearance;
the adult complement of segmented rays quickly
ensues. Within the cypriniform fishes, pelvic ray
counts are seldom used diagnostically. However,
both the position of the pelvic fin or fin bud
relative to other structures and its position in
the sequence of developmental events can be useful
in identification, especially in the family
Cypri ni dae.

The pecto1'al fins typically begin as buds
immediately behind the head during the late embryonic
phase. However, pectoral buds are not evident on
some species (including some cypriniform fishes)
until shortly after hatching. Though strongly
striated and occasionally with membraneous folds
and breaks, they typically remain rayless in cyprin-
iforms until late in the mesolarval phase when
most of the principal median fin rays are present.
With the exception of secondary caudal rays, the
rays of the pectoral 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 ray counts are generally of little value
in larval identification.

Other Countable Structures. Other characters
that may be treated meristically (and in some cases
morphologically) include branchiostegal rays, gill
rakers, pharyngeal teeth and scales. Branchiostegal
rays form early in larval development but counts
are usually constant within major taxon groups.
Within the order cypriniformes, all members of the
superfamily Cyprinoidea, which includes the
Cyprinidae and Catostomidae, have three branchio-
stegals (McAllister 1968). Due to later develop-
ment, small size and/or internal location, the other
characters are seldom used, and then usually only
on later metalarvae and juveniles. Gill rakers form
gradually with numbers increasing throughout much
of the larval period and the early portion of the
juvenile period. Pharyngeal teeth form relatively
early but may not be sufficiently 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, in most
cases, species can be more easily distinguished by
use of external characteristics. Scales typically
become apparent late in the larval period or early
in the juvenile period, but all are not typically
present until a short time later. First scales on