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

monly used as a synonym for period or phase
(e.g., Kendall et al. 1984), should be reserved
for instantaneous states of development.
The larval phase terminologies most com-
monly used in recent years, particularly for
descriptive purposes, are those defined by Hardy
et al. (1978-yolk-sac larva, larva, prejuvenile;
modified from Mansueti and Hardy 1967),
Ahlstrom et al. (1976-preflexion, flexion, post-
flexion; expanded upon by Kendall et al. 1984),
and Snyder (1976b and 1981-protolarva,
mesolarva, metalarva). Definitions for all three
terminologies were presented by Snyder (1983b)
and Kelso and Rutherford (1996). During a
workshop on standardization of such terminol-
ogies, held as part of the Seventh Annual Larval
Fish Conference (Colorado State University,
January 16, 1983), it became obvious that these
are not competing terminologies, as they often
are treated, but rather complementary options
with subdivisions or phases defined for different
purposes. As such, it is possible to utilize all
three terminologies simultaneously to: (1) facili-
tate comparative descriptions and preparation of
keys based on fish in similar states of develop-
ment with respect to morphogenesis of finfold
and fins; (2) segregate, for fishes with homo-
cercal tails, morphometric data based on stan-
dard length measured to the end of the noto-
chord prior to and during notochord flexion
from those measured to the posterior margin of
the hypural plates following notochord flexion;
and (3) approximate transition from at least
partially endogenous nutrition (utilization of
yolk material) to fully exogenous nutrition
(dependence on ingested food) based on
presence or absence of yolk material.
The combined terminology presented below
and utilized herein effectively integrates princi-
pal subdivisions and functions ofthe three com-
ponent terminologies. In doing so, Ahlstrom's
"preflexion- flexion-postflexion" terminology is
treated, for fishes with homocercal tails, as a
subset of Snyder's mesolarva phase. Since noto-
chord flexion in the caudal region usually begins
when the first caudal-fin rays appear and is
essentially complete when all principal caudal-
fin rays are well defined, and since presence of
fin rays can be more precisely observed than the
beginning or end of actual notochord flexion, fin
rays are used as transition criteria. As a result,
all proto larvae are preflexion larvae, and all

metalarvae are postflexion larvae. Although
most fish pass sequentially through all phase
subdivisions designated, some pass pertinent
points of transition prior to hatching or birth and
begin the larval period in a later phase or
possibly skip the period entirely.
The definition for the end of the larval
period is necessarily a compromise deleting all
requirements (some taxon-specific, others diffi-
cult to determine precisely) except acquisition of
the full complement of fin spines and rays in all
fins and loss of all fin fold (last remnants are
usually part of the preanal finfold). Provision
for taxon-specific prejuvenile (or transitional)
phases are also deleted. In some cases, finfold
persists through the endpoint for such special
intervals, which are then effectively included in
the larval period.
Timing of complete yolk absorption varies
from well before notochord flexion and initial
fin ray formation, as in most fishes with pelagic
larvae, to postflexion stages after all or most of
the fin rays are formed, as in many salmonids.
Accordingly, the interval during which fish
larvae bear yolk should not be represented
generally as a separate phase preceding phases
based on fin formation as it has been treated by
Kendall et al. (1984). The Hardy et al. termin-
ology effectively distinguishes between larvae
with and without yolk by modifying the period
name with the adjective "yolk-sac" when yolk
material is present. Any period or phase name
of the combined terminology can be similarly
modified to indicate presence or absence of yolk
material (e.g., yolk-bearing larva, yolk-sac meta-
larva, postflexion meso larva with yolk, proto-
larva without yolk).
The combined terminology is designed to be
relatively simple but comprehensive, precise in
its transition criteria, applicable to nearly all
teleost fishes, and flexible. It can be utilized in
part (essentially as one of its component termin-
ologies) or its entirety depending on purposes of
the user. For example, if it is necessary to
acknowledge only that the fish is a larva and
whether it bears yolk, the terms "yolk-sac larva"
and "larva without yolk" are all that is needed.
Biologists who formerly utilized one of its com-
ponent terminologies should have no difficulty
in adapting to the combined terminology-essential
features and terms of the original terminologies
have been retained.

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