9492

• UCREFRP Catalog Number: 9492
• Author: Snyder, D. E.
• Title: Computer-Interactive Key to Sucker Larvae and Early Juveniles of the Upper Colorado River Basin with Description of Longnose Sucker.
• USFW Year: 2003.
• USFW - Doc Type: Fort Collins.
• Copyright Material: NO

fin to its most distal margin) and length of the base of the fin correlate well with the number of
principal fm rays discussed above. As would be expected, these measures are greatest for
razorback sucker and least for mountain sucker, but not much less than for longnose, white, and
bluehead suckers. Length to the insertion of the dorsal fin is also greatest (farthest back) for
razorback and least for mountain sucker, whereas length to the origin of the fm is least (most
forward) for tlannelmouth and razorback suckers and greatest (farthest back) for white,
bluehead, and mountain suckers.
Among the remaining measures, only eye diameter is useful for all developmental
intervals. As protolarvae, mountain sucker generally have the greatest eye diameters and
mountain and longnose suckers the greatest head lengths (measured to the origin of the pectoral
fin bud) relative to standard (notochord) length. Bluehead and tlannelmouth protolarvae
typically have the smallest eyes and heads. For subsequent developmental intervals, differences
in eye diameter are best considered as a percentage of head length. For these later stages Utah
sucker usually have the largest eyes whereas tlannelmouth sucker continue to average the
smallest eyes, although not by much. Head length among juveniles is often greatest for
razorback and white suckers and least for bluehead, tlannelmouth, and mountain suckers.
In addition to dorsal fin lengths discussed above, pectoral and caudal fin lengths are also
useful for specific developmental intervals. Pectoral fin length is sufficiently diagnostic only for
proto larvae, and then only with respect to the maximum values which are greatest for white,
longnose, mountain, and razorback suckers and least for Utah and bluehead suckers. Caudal fin
length is sufficiently diagnostic only for metalarvae and juveniles. Among metalarvae, caudal
fin length is greatest for razorback sucker and least for mountain sucker. Among juveniles, it is
greatest for razorback and Utah suckers and least for mountain and longnose suckers.
Lengths from snout to pelvic fin origin and posterior margin of the vent are the only
remaining length characters considered sufficiently diagnostic to include in Table 3. Snout to
pelvic fin origin lengths, like lengths to the origin of the dorsal fm, are typically greatest (farthest
back) for bluehead sucker and least for tlannelmouth sucker metalarvae and juveniles, thereby
maintaining the pelvic fin origins at a more-or-less similar horizontal distance behind dorsal fin
origins. For posttlexion mesolarvae, length to origin of the pelvic fin is also greatest for
bluehead sucker but least for Utah, longnose and razorback suckers. Snout to vent lengths are
greatest for Utah and razorback sucker posttlexion mesolarvae and razorback sucker juveniles.
As noted above, body depth measured at the origin of the dorsal fm retlects the amount of
yolk remaining in proto larvae and meso larvae and the health or condition of the fish in later
stages, but especially for larger juveniles, it also represents differences in structural depth. The
upper end of the range for this measure is notably greater for razorback sucker juveniles than
other species and is probably due, at least in part, to enlarging interneural bones behind the head
which will eventually form the distinctive predorsal "razor" or keel of older juveniles and adults.

Pigmentation. - Capture of these suckers prior to initial eye and body pigmentation is
rare. If not pigmented at hatching, at least the eyes and some body pigmentation is usually
evident by emergence from the spawning substrate. Longnose, white, and mountain suckers are
usually well pigmented by 9 nun SL and Utah, bluehead, and tlannelmouth suckers by 11 nun
SL (Table 4). Pigmentation throughout early development is generally lightest for tlannelmouth
sucker and especially razorback sucker.
Of all pigment characters, the most diagnostic for later larvae and early juveniles of
bluehead and mountain suckers is the extent of peritoneal pigmentation (Table 4). In the ventro-

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