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r?C-tL( GY ?? ? -lltyz r'Et <br />COMPARATIVE MORPHOLOGY OF THE INTERNEURAL BONES OF METALARVAL AND / <br />EARLY-JUVENILE RAZORBACK SUCKER (XRAUCHEN TEXANUS),BLUEHEAD <br />SUCKER (CATOSTOMUS DISCO_BOLUS), AND FLANNELMOUTH SUCKER <br />(CATOSTOMUS LATIPINNIS), FROM THE UPPER COLORADO RIVER BASIN <br />Robert T. Muth and Steven P. Platania <br />Larval Fish Laboratory <br />Colorado State University <br />Fort Collins, Colorado <br />ABSTRACT <br />The distribution of reproducing populations of the rare, endemic <br />razorback sucker, X?rauehen tex_anus, is virtually unknown in the Upper <br />Colorado River Basin. A major problem has been the taxonomic <br />identification of it's early life--history stages. Adult razorback <br />sucker are characterized by a distinct bony, dorsal keel and easilly <br />distinguished from other Coloraado River Basin catost.omids. In general, <br />dorsal keel formation is due to the extensive development and <br />modification of the interneural bones at the nape which directly <br />correlate with size and age. Prior to any external evidence of the <br />dorsal keel, no reliable taxonomic criteria exist for differentiating <br />young razorback sucker from the sympatric Catostomus discobolus and C. <br />Iatipinnis. Comparative morphology studies of the interneural bones of <br />metalarval and early-juvenile specimens have revealed discriminating <br />features for these three Upper Colorado River Basin fish species. We <br />believe that specimens as small as ZO mm TL can now be identified with <br />confidence using specific osteological techniques. <br />at- <br />170