7723 - Laserfiche WebLink

Home Browse Search

opposite approach was used by Snyder et al. (1977), Snyder and Douglas (1978), Laos and Fuiman (1977) and, according to the latter authors, Fish (1932) -- only entire myomeres were included in counts anterior to points of reference. Siefert's method is recommended as standard procedure since resulting counts are expected to more nearly approximate the number of vertebrae to points of reference. In the United States and Canada, the range of total myomere (and vertebral) counts for cyprinids, 28 to 52, is slightly larger find nearly includes that for catostomids, 32 to 53. Ranges for preanal and postanal myo- mere counts also overlap with 19 to 35 and 9 to 22, respectively, for cyprinids and 25 to 42 and 5 (possibly 3) to 14, respectively, for catostomids. Despite the magnitude of over- lap in these ranges, proportions of postanal to preanal and preanal to total myomeres will distinguish most cyprinids from catostomids (Snyder 1979). The preanal to postanal myo- mere proportion is at least 2/5 (often greater than 1/2) for cyprinids (exclusive of subfamily Cyprininae, the carps) and less (often less than 1/3) for catostomids. Also, the propor- tion of preanal to total myomeres is 5/7 or less (often less than 2/3) for cyprinids and greater (often greater than 3/4) for catos- tomids. For cypriniform fishes in the Upper Colorado River System the degree of overlap in total and preanal myomere counts is less and larvae with fewer than 42 total or 32 preanal myomeres can be cyprinids only. Fins and Finfolds Fin ray meristics and fin positions are among the most useful characters for later mesolarvae and metalarvae, especially among the cyprinids. These data can be determined from older juveniles and adults or gleaned from published descriptions of adults. The sequence and timing of fm development, fm lengths, and basal lengths of the dorsal and anal fms are also useful. The median tinfold, one of the most obvious structures in protolarvae and meso- larvae, is a thin, erect, medial fold of tissue that originates on the dorsal surface usually well behind head. It extends 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, the soft-rayed portions of the median fms (dorsal, anal and caudal) differentiate from this fmfold. As the median fins develop, the fmfold diminishes and recedes before and between the fins until it is no longer apparent during or near the end of the metalarval phase. The preanal tinfold is a second median fold of tissue that extends forward from the vent. In most fishes the preanal finfold is completely separated from the ventral portion of the median finfold by the vent. But in burbot (Lota Iota), and its marine relatives (Gadidae, codfishes), the preanal finfold is initially continuous with the median fmfold and only later are the finfolds entirely separ- ated by the vent (vent initially opens through right side of finfold). The preanal finfold mayor may not be present upon hatching, depending upon size and shape of the yolk sac. In cypriniform fishes, it is typically absent or barely apparent upon hatching. As yolk is consumed and the yolk sac decreases in size prior to hatching or during the proto- larval phase, a small preanal finfold appears just anterior to the vent. As more yolk is consumed and the larva grows, the preanal finfold enlarges and extends anteriorly. Ultimately, its origin lies anterior to that of the dorsal portion of the median finfold. The preanal finfold remains prominent throughout the mesolarval phase, then slowly diminishes and recedes in a posterior direction during the metalarval phase. It is typically the last fmfold to be absorbed or lost. The caudal tin is the first fm to differ- entiate from the median fmfold in cyprini- form and most other fishes with homocercal tails. The portion of the fmfold involved first thickens along the ventral side of the poster- ior end of the notochord and begins to differ- entiate into the hypural elements of the cau- dal skeleton. Immediately thereafter, the first caudal fm rays appear (beginning of flexion mesolarval phase) and the posterior portion of the notochord begins to bend or flex upward. Be careful not to confuse striations or folds in the fmfold with developing fm rays. As the fm develops and the notochord continues to flex upward, the hypuraIs and developing caudal fm rays, all ventral to the notochord, move to a posterior or terminal position. The first principal rays are medial and subsequent principal rays form and pro- 10