7723 - Laserfiche WebLink

Home Browse Search

the larval forms of closely related species look alike. At the same time, larvae of dis- tantly related forms may be closely similar in gross appearance." Cypriniform larvae as a group are distinctive and generally easy to distinguish from larvae of other families. Beginning workers should become familiar with the general larval characteristics of each family likely to be encountered. The guides and keys cited in Snyder (1983b) are most useful in this respect. Auer (1982) is parti- cularly recommended since it covers all fami- lies and some species in the Upper Colorado River Basin. Discussions of taxonomic char- acters by Berry and Richards (1973) and Kendall et al. (1984) are also recommended. In the Upper Colorado River System, cypriniform larvae are readily categorized as cyprinids or catostomids. But elsewhere, if members of the cyprinid subfamily Cyprini- nae ( carps) and the catostomid subfamily Ictiobinae (carpsuckers and buffalofishes) or tribe Erimyzontini (chubsuckers) are present, identification at the family level can be more difficult. Within their respective families, and especially at the subfamily level, cypriniform larvae are very homogeneous in gross struc- ture and appearance. Accordingly, they may be especially difficult to discriminate at genus or species levels. This is particularly true of Colorado River System suckers. For the latter, specific identification relies on size at which certain developmental events occur, form of the gut, melanistic (brown or black) pigment patterns, osteological characters, and to a limited extent, morphometrics and meris- tics (especially dorsal fin ray counts for metalarvae and juveniles). There is often a noticeable amount of intra- as well as inter-regional variability in many of the characters to be discussed. This variability necessitates confirmation of iden- tity based on as many diagnostic characters as possible. Myomeres Myomeres, because they are obvious morphological features and relatively con- sistent in number and position, are one of the most useful characters available for identifica- tion of larvae above (and sometimes at) the species level, especially for protolarvae and mesolarvae. They begin as part of the embryonic somites and are usually formed in their full complement prior to hatching. Throughout the protolarval and much of the mesolarval 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 myo- meres and total vertebrae (including Weber- ian ossicles in cypriniforms). Snyder (1979) and Conner et al. (1980) summarized myo- mere and vertebral counts for many cyprin- iform fishes. The most anterior and most posterior myomeres are frequently difficult to dis- tinguish. 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 immediately behind the occi- put. The most posterior myomere is defmed as lying anterior to the most posterior com- plete 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. Polarizing filters, depending on thickness and certain other qualities of the preserved tissues, can dramatically increase contrast between the muscle tissue of myomeres and the myosepta that separate 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 sometimes facilitates observation. Typical counts used in taxonomic work include total, preanal, and postanal myo- meres. Partial counts are frequently used to reference location of various structures in addition to the vent. The most generally accepted method of making partial counts was described by Siefert (1969) for distin- guishing preanal and postanal myomeres: "postanal myomeres include all [entire] myo- meres posterior to an imaginary vertical line drawn through the body at the posterior end of the anus. . . Remaining myomeres, includ- ing those bisected by the line, are considered preanal." The technique is equally applicable with other structures or points of reference such as origins of fms or fmfolds. The 9