|
99
<br />on the hump, and leading ventro-posteriorly directly to the upper part of
<br />the gill openings, allows water to irrigate the gills when the mouth and
<br />body are pressed downward. The ventral mouth would therefore be
<br />usable, when protected from the onrushing water by the snout (in a manner
<br />similar to that occurring in some swift-water suckers). The length of the
<br />caudal peduncle of G. cypher, shorter than that of the bonytail chub, and
<br />a reduction in the number of dorsal fin-rays, indica#e that it is not a further
<br />extreme in the trend from chubby, quiet-water forms to the exceedingly
<br />specialized, swift-water forms of the "robusta series" of chubs (Miller,
<br />1946c), but that it is specialized into some other niche, which is yet to
<br />Eje determined with accuracy.
<br />Almost nothing is known of the biology of this fish, principally be-
<br />cause of the difficulties in collecting in its presumed habitat, and its re-
<br />sulting rarity in collections. Specimens that were recently taken from below
<br />Glen Canyon Dam had fed mostly on planktonic crustaceans and algae
<br />that must have been carried into the river from the upstream reservoir.
<br />Colorado River (roundtail) chub, Gila robusta Baird and Girard
<br />As noted in footnote 8, p. 95, chubs of the "robusta series" have a compli-
<br />•Gated taxonomic history. Many workers have referred to the various subspecies
<br />(and/or species, as currently recognized) simply as Gila robusta. Literature records
<br />for Arizona that combined the various subspecies of the true G. robusta, at least in
<br />part, are given here, with synonymies and records for the different subspecies pro-
<br />vided under the individual accounts.
<br />Gila robusta, Girard, 1856: 205. Evermann and Rutter, 1895: 482.. Jordan
<br />and Evermann, 1896: 227. Gilbert and Scofield, 1898: 493. Snyder, 1915: 581.
<br />Jordan, et al., 1930: 114. Miller, 1945c: 104/ 19616: 376/ 1963a: 5/ 1964a: 7.
<br />Rostland, 1952: 270. Winn and Miller, 1954: 273. Koster, }957: 58. Kimsey and
<br />Fisk, 1960: 469. Lowe, 1960: 172. Miller and Lowe, 1464: 140/ 1967: 140. Koehn,
<br />1965a: 115. Minckley, 1965a: 49/ 1971: 185. Bradley and Deacon, 1967: 229.
<br />Minckley and Deacon, 1968: 1427. Moore, 1968: 60. Eddy, }969: 87. Holden and
<br />Stahlnaker, 1970: 409. Rinne and Minckley, 1970: 16.
<br />Body thick, moderately streamlined. Scales small, thin, and only slightly
<br />imbricated, sometimes locally absent. Basal radii absent to weak on scales. Lateral
<br />line with more Than 80 scales. Dorsal fin-rays nine or more. Anal fin-rays 8 to ] 0,
<br />most often 9 or more. Length of head divided by depth of caudal peduncle usually
<br />3.0 or more. Total number of vertebrae 43 to 49. Pharyngeal arch with lower
<br />ramus elongated, approaching that of Ptychocheilus, teeth 2, 5-4, 2.
<br />This species is represented by four nominal subspecies in the Colo-
<br />rado River system (including Pluvial White River, Nevada; Hubbs &
<br />Miller, 1948), three of which occur in Arizona waters. The following key
<br />will serve to characterize the various forms. Rinne (1969) deserves credit
<br />for working out the problems associated with these chubs. It should be
<br />remembered that this key is for subspecies, or geographic races, of a
<br />species, and therefore is based even more upon the mean (average) con-
<br />ditions within a population; individual specimens may or may not be
<br />identifiable to subspecies.
<br />3
<br />r
<br />s
<br />~. .
<br />i~ .
<br />r:
<br />a
<br />t
<br />€.
<br />~~~
<br />€_
<br />i
<br />
|