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Of all pigment characters, the most diag- nostic for the later larvae and early juveniles of bluehead and mountain suckers is the ex- tent of peritoneal pigmentation (Table 4). In the ventro-Iateral region of the peritoneum, sparse to patchy pigment is present in some postflexion mesolarvae as early as 14 or 15 mm SL and uniformly dark pigmentation is present in all metalarvae by 20 to 22 mm SL (Figs. 45-49 and 74-77). In contrast, uniform pigmentation (light or dark) in this region was rare for other species. It was not obser- ved at all for Utah sucker (Figs. 17-21) or in other species less than 35 mm SL, except for rare observations of uniform light pigmenta- tion in razorback juveniles as small as 26 mm SL. Uniform peritoneal pigmentation also is present in the ventral region of some razor- back juveniles as small as 29 mm SL but not apparent in other razorback specimens as large as 40 mm SL (observations through sur- face tissues; specimens were not dissected). Once melanophore pigmentation is suffi- ciently established, one of the more useful surface pigment characters is the extent of pigmentation on the ventral midline between the heart region and the vent (Table 5). White and mountain suckers typically have a continuous line of midventral pigment with over 20 me1anophores (Fig. 30-32, 34, and 72- 73), at least through the larval period. Exten- sion of this pigment line into the branchial region anterior to the heart is common in white sucker but rare in mountain sucker. Among the others, only bluehead sucker occasional have as many me1anophores along the ventral midline, but the line is either shorter or distinctly discontinuous (Figs. 44- 45). Complete absence of me1anophores along the ventral midline is rare among Utah, bluehead, and flannelmouth larvae but com- mon for razorback sucker larvae. Unlike the others, razorback larvae have not been obser- ved to have more than 6 melanophores along the ventral midline (Figs. 85-89). Table 4. Comparison of size (rom SL) relative to pigmental state (melanin) of eyes and bodies for proto- lalVae and lateral to ventral regions of the peritoneum for postflexion mesolalVae (P), metalalVae (M), and early juveniles (J, :S40 rom SL) of Upper Colorado River Basin catostomids. For peritoneal pigmen- tation, size is preceded by initials for the applicable developmental intelVals. The letter "r" indicates that the condition is rare. Catostomus Catostomus Catostomus Catostomus Catostomus Xyrauchen Character ardens commersoni discobolus latipinnis platyrhynchus texanus Eye pigmentation, protolarvaeB Unpigmented :s10 7 :s10 :s10 :s8 :s9 Light to moderate 9-11 7-9 9-11 9-11 8-9 7-10 Dark ~10 ~8 ~1O ~11 ~8 ~9 Body pigmentation, protolarvaeB Unpigmented :s11 :s9 :s10 :s10 :s8 :s10 1-12 spots on dorsum 9-12 7-9 9-10 9-11 8-9 8-10 ~13 spots on dorsum ~11 ~8 ~10 ~11 ~8 ~9 Peritoneal pigmentationb Lateral, P and M onl{ Absent PM all PM :s18 P :s17 PM :s22 P :s14 PM:s24 Sparse or patchy PM ~15 PM ~14 PM :s17 PM ~19 PM:s22 PM ~14 Uniformly light M 17-19 M~1 Uniformly dark M~17 M~1 Ventro-lateral surfaces Absent (or obscured in 1) PMl all PMl all P :S17 PMl all PM :s16 PMl all Sparse or patchy J ~19 PMl 16-37 PM 15-17 Ml ~23 PM 14-18 Ml 20-37 Uniformly light r-J 35-37 M 17-19 r-J ~35 M 19-21 r-J 26-37 Uniformly dark Ml 17-23 r-J~ Ml 20-22 r-J 35-37 Ventral surface Absent PMl all PMl all PM :S17 PMl all PM :s21 PMl all Sparse or patchy J 22-37 Ml 17-25 Ml ~22 Ml 17-34 J 23-37 Uniformly light r-J 35-37 MJ 18-25 r-J ~38 J26-34 J ~29 Uniformly dark MJ ~18 r-J ~38 J ~26 J<82 B Some to most specimens of each species will hatch with eyes or eyes and body well pigmented. b Pigmentation of the peritoneum is subsurface and should not be confused with surface or cutaneous pigmentation. Also, pigment might be apparent in the dorsal and dorso-lateral portions of the peritoneum of smaller larvae and should.not be interpreted as pigment in the lateral region. C In juveniles, lateral pigmentation of the peritoneum usually is obscured by muscle. 28