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10 LARVAL CHARACTERIS1. ~ at the genus or species levels. Specific identifica- tion relies largely on meristics such as myomere and fin ray counts, morphometrics such as snout to vent lengths, and melanistic (brown or black) pigment patterns. In addition, the size at which certain developmental events occur and the form of various structures can be useful. There is often a notice- able amount of intra- as well as inter-regional variability in many of the characters to be discussed. Awareness of this variability or its possible presence, and use of several diagnostic characters, if possible, will increase confidence in identifica- tion while reducing the probability of error. Myomeres: Myomeres, because they are obvious morphological features and relatively consistent in number and position, are one of the most useful characters available for identification of larvae above (and sometimes at) the specific 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 beginning of the metalarval phase they evolve to their typical three-angled adult form. Fish (1932) and many subsequent authors indicated that there is a nearly direct, one-to-one correla- tion between total myomeres and total vertebrae. Snyder (1979), including or assuming inclusion of the .Weberian ossicles in total vertebra counts, provided considerable data on cypriniform fishes in support of this generalization. The most anterior and posterior myomeres are frequently difficult to distinguish. The most anterior myomeres are apparent only in the epiaxial or dorsal half of the body; the first is often deltoid in shape and is located immediately behind the occiput. The most posterior myomere is defined as lying anterior to the most posterior complete myoseptum. Siefert (l969) describes a "false (partial) myoseptum" posterior to the last complete myoseptum which adds to the difficulty of discern- ing the last myomere. Early in the larval period, myomeres are most readily observed using transmitted light. Polarizing filters, depending on the thick- ness and certain other qualities of the preserved tissues, can often be used to dramatically increase the contrast between the muscle tissue of the myo- meres and the myosepta that separate them. Surface staining (i .e., dipping momentarily in Alizarin Red, or perhaps another dye, then rinsing) or submergence in glycerin might also be useful in helping to distinguish individual myomeres. The myomeres of some metalarvae and juveniles are often difficult to observe; reflected light at a low angle from one side and higher magnification some- times facilitates observation. Typical counts used in taxonomic work include total, preanal, and postanal counts. Partial counts are frequently used to reference the location of various structures in addition to the vent. The most generally accepted method of making partial counts is that described by Siefert (1969) for distinguishing preanal and postanal myomeres: "postanal myomeres include all [entire] myomeres posterior to an imaginary vertical line drawn through the body at the posterior end of the anus . . , Remaining myomeres, inclUding those bisected by the line, are considered preanal." The technique is equally applicable to other struc- tures or points of reference such as the origins of various fins or finfolds. Another approach used by Snyder et al. (1977), Snyder and Douglas (1978), Loos and Fuiman (1977) and, according to the latter authors, Fish (1932) is essentially the opposite; only entire myomeres are included in the count anterior to the structure of reference. As counts resulting from Siefert's methods are expected to more nearly approximate the number of vertebrae to the point of interest, that approach will be accepted as the standard for this guide and future Identification Circulars. Snyder (1979) reported: "The range of total vertebra and/or myomere counts for 70 cyprinid species, 28 to 51, is larger and essentially includes that for 27 catostomids, 32 to 52. Preanal and postanal myomere counts ranged from 19 to 31 and 10(9?} to 18, respectively, for cyprinids and 25 to 42 and 5(3?} to 12(14?) for catostomids. The two families can be readily distinguished by the proportion of postanal to preanal myomeres, about 1/2 or greater for cyprinids and 1/3 or less for catostomids; or preanal to total myomeres, about 2/3 or less for cyprinids and 3/4 or more for catostomids. The genera of each family are charac- terized by distinctive ranges of total myomeres or vertebrae Which can be used to help determine the identity of unknown cypriniform larvae." Fins and Finfolds: Fin ray meristics and fin positions, usually determined from older juveniles and adults or gleaned from published descriptions of adults, are among the most useful characters for later mesolarvae and metalarvae, especially among the cyprinids. The sequence and timing (relative to larval length) of fin development as well as fin lengths and basal lengths of the dorsal and anal fins are also useful. The median finfold, one of the most obvious of larval structures in protolarvae and early mesolarvae, is a continuous structure originating on the dorsal surface, usually well behind the head, and extending posteriorly to and around the end of the notochord then anteriorly along the ventral surface to the posterior margin of the vent. During the mesolarval phase, this finfold differentiates at the sites of the future median fins then, as the fins develop, it recedes or diminishes before and between the fins until it is no longer apparent, usually at or near the end of the metalarval phase. The p!.eanal finfold. a second median finfold, mayor may not be present upon hatching, depending upon the size and shape of the yolk sac. In the burbot (Lata 7.ota) and its marine relatives (Gadidae), the preanal finfold is initially continuous with the ventral portion of the median finfold, the vent opening to one side of the finfold; they later separate. In cypriniforms, the preanal finfold is typically absent or barely apparent upon hatching. As yolk is consumed and the yolk sac is reduced in size, either during the late embryonic phase or the protolarval phase, a small finfold appears just anterior to the vent. As more yolk is consumed and the larva grows, the preanal finfold enlarges and extends anteriorly, usually well in advance of the origin of the dorsal finfold. The preanal finfold remains prominent throughout the mesolarval phase and slowly diminiShes in a posterior direction during the metalarval phase. It is typically the last of the finfolds to completely disappear.