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RESULTS AND CONCLUSIONS Results are divided into three interrelated sections -- Comparative Summary, Keys, and Species Accounts. Although 445 specimens were analyzed in detail for morphometries and meristics and hundreds more were docu- mented for size, developmental state, skeletal characters, and pigmentation patterns, there are undoubtedly rare specimens with charac- ter extremes beyond those we observed. For example, Kevin Bestgen, an associate studying larvae in the Lower Colorado River Basin, measured reared razorback sucker flexion and postflexion mesolarvae as small as 10 and 11 mm SL, respectively. Those measures are a millimeter less than the smallest specimens we measured and are included in the results that follow. Because of the similarity among larvae of Upper Colorado River Basin suc- kers, the specific identity of some larvae will remain inconclusive or questionable after application of the keys and diagnostic criteria provided herein. The identity of such speci- mens must be considered tentative and should be designated as such by use of a question mark ("?") after the more probable taxon name (preferably with a footnote on other possibilities), or leaving the identity at the family level (e.g., "unidentified Catosto- midae"). Some inconclusive specimens may be hybrids. Hybridization among Colorado River System catostomids is well documented (e.g., Holden and Stalnaker 1975, Hubbs et al. 1943, Hubbs and Hubbs 1947, Hubbs and Miller 1953, McAda 1977, McAda and Wydoski 1980, Prewitt 1977, and Smith 1966). Intermediacy of characters for white X blue- head sucker hybrids as small as 25 mm SL and flannelmouth X bluehead sucker hybrids as small as 34 mm SL were documented by Hubbs et al. (1943) and Hubbs and Hubbs (1947) respectively. Using diagnostic charac- ters and descriptive data that follow, hybrid metalarvae and early juveniles might be at least tentatively identified as such; hybrid protolarvae and mesolarvae will likely be identified as the parental species they most closely resemble or remain questionable. In the species account sections on reproduction, all six suckers are classified according to Balon's (1975a, 1981) reproduc- tive guilds as non-guarding, open-substrate, lithophils. Lithophils prefer to spawn over predominately rock or gravel substrates. Their recently hatched larvae are photo- phobic and usually hide or remain in the substrate for at least a few days before emerging and drifting with the current. Although considered broad-cast spawners, razorback sucker produce discrete, identi- fiable redds in reservoirs (Bozek et al. 1984). This may suggest tendency towards a brood- hiding guild. Comparative Summary The diagnostic criteria that follow are provided to help confirm identities determin- ed through the keys or, for biologists familiar with the larvae of concern, to serve as a par- tial alternative to the keys. Since extremes in character states beyond those reported here are likely to occur, identifications should be based on as many criteria as possible. Size relative to state of development: Flannelmouth sucker eggs are the largest of Upper Colorado River System suckers (3.8- 3.9 mm diameter versus 3.3-3.5 for blue-head sucker and 2.3-3.3 for the others) and larvae hatching from them are usually much larger as well. This relative size difference is characteristic of flannelmouth sucker through- out its early development (Table 1). In con- trast, razorback, mountain, and some white sucker eggs are notably smaller (2.3-2.8 mm diameter) than other species and their recently hatched protolarvae and recently transformed mesolarvae tend to be corres- pondingly small. These species also complete yolk absorption at a much smaller size, usually by 12 mm SL; flannelmouth larvae finish their yolk at 15 mm SL. Size relative to state of development for all species but flannelmouth sucker is nearly the same by the beginning of the metalarval phase. Fin development tends to proceed fastest (at smaller sizes) for white sucker and slowest (at larger sizes) for flannelmouth sucker. White and Utah suckers acquire the adult complement of all fin rays, lose their preanal fmfold, and become juveniles at the smallest sizes (19-20 mm SL) while transfor- mation to the juvenile period for some razor- back sucker occurs at sizes nearly as large as for flannelmouth sucker (22-23 and 23-24 mm SL, respectively). 23