9550 - Laserfiche WebLink

Home Browse Search

identification, particularly at family and sub- family levels. Within genera, morphological differences among species are usually much more subtle, but may still be of diagnostic value. Much shape or form-related information can be quantified via proportional measurements or morphometrics. Morphometric data emphasize the relative position and relative size of various body com- ponents and dimensions and may be critical to species identification. Such measurements may be allometric, changing in proportion as the fish grow; thus morphometric data should be related to size, at least for proto larvae and meso larvae. Some morphometric data, particularly body depths and widths, may be directly affected by the condition of individual specimens and volume and form of food items in their digestive tracts. The source of specimens and the preser- vative in which they are stored also may affect morphometric data. Some measures in wild fish may differ from those oflaboratory-reared spec- imens (e.g., fin lengths). Shrinkage and defor- mation are notably greater in alcohol than in formalin preservatives. Morphometric data in this guide are reported as percentages of standard length (% SL). Use of standard length (SL) avoids the allometric influence of caudal fin growth included in percentages based on total length (TL). As explained later (Methods), data can be easily converted to percent TL (% TL) for com- parison with other works. Prior to hypural plate formation and completion of notochord flexion (proto larvae and flexion meso larvae ), SL is the length from snout to posterior end of the notochord (notochord length). Thereafter, SL is measured from anterior margin of the snout to most posterior margin of the hypural plates (usually the superior plate or hypurals). Use of notochord length for proto larvae and early meso larvae gives the appearance of greater allometric growth differences than may really exist, at least in comparison with subsequent measures based on the posterior margin of the hypural plates. This undesirable effect is a result of upward bending or flexing of the noto- chord and the switch from use of end of the notochord to posterior margin of the hypurals as the basis for length measurement. These factors must be taken into account when reviewing morphometric data herein. In contrast to procedures recommended by Hubbs and Lagler (1958) for larger juveniles and adults, measurements of body length and various parts thereof for fish larvae are generally taken along lines parallel to the horizontal axis of the fish. Exceptions are fin lengths which, in studies conducted for this manual, were mea- sured from origin of the fin base to most distal margin of the fin rays. Typical measures include total, standard, head, snout, eye, and fin lengths, as well as snout-to-vent and snout-to- origin-of-fin (dorsal, anal, and pelvic) lengths. Snout-to-vent length is measured to the pos- terior margin of the vent or anus. It is a primary diagnostic character for many species, especially at the family and sometimes subfamily level. In the Upper Colorado River System, most cyprin- id larvae are readily differentiated from catos- tomid larvae by snout-to-vent lengths less than 72% SL. Exceptions are most larvae of com- mon carp (Cyprinus carpio) and occasionally mesolarvae of Colorado pikeminnow (Ptycho- cheilus lucius). The term "preanal length" is often applied to this measure but might be misinterpreted as length to origin ofthe anal fin. For many fishes, including cypriniforms, the latter measure is approximately the same as snout-to-vent length since the anal fin begins at or near the posterior margin of the vent. Head length is typically measured to the posterior margin of the operculum in juveniles and adults, but the operculum may be absent or incomplete throughout much of the larval period. Accordingly, many biologists have rede- fined head length for larvae to be measured to the posterior end of the auditory vesicle or the anterior or posterior margin of the cleithrum, one of the first bones to ossify in fish larvae (Berry and Richards 1973). Unfortunately, the auditory vesicle and cleithrum are not always easy to observe, especially in postflexion meso- larvae and metalarvae. Also, resultant measures to the auditory vesicle are considerably anterior to the eventual posterior margin of the oper- culum. Snyder et al. (1977) and Snyder and Douglas (1978) measured larval head length to origin (anterior insertion) of the pectoral fin. This measure has distinct advantages over the alternatives-the base of the pectoral fin is read- ily observed throughout the larval period (except in the few species that hatch prior to pectoral bud formation), it somewhat approximates the 13