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<br />
<br />JAMES E. DEACON AND W. L MINCKLEY
<br />
<br />Minckley, 1970). These fishes have been variously interpreted, over the
<br />years, as ecological "types of one species or as different genera, but have
<br />recently been defined as full species (Rinne, 1969; Holden and Stalnaker,
<br />1970). Gila cypha still is in doubt (Holden and Stalnaker, 1970), princi-
<br />pally because of collection of numbers of putative hybrids between it and
<br />other species from a disturbed part of the river basin (the Lake Powell
<br />area). Gila cypha is rare in collections, and its ecology may only be
<br />inferred.
<br />Vanicek et al. (1970) found G. elegans and the less streamlined G.
<br />robusta living sympatrically in pools and eddies of the Green River, Utah,
<br />sometimes adjacent to, but never in, swift currents. Swift-water forms of
<br />thebluehead mountain-sucker (Pantosteus discobolus), a species with
<br />morphotypes differentiated similar to those of G. elegans and G. robusta
<br />(in the mainstream and in smaller tributaries, respectively), were con-
<br />sistently in current, and usually in the swiftest parts. The highly variable
<br />Rhinichthys osculus also has numerous morphotypes that similarly reflect
<br />the impact of current on its body form within the Colorado River system.
<br />.The extreme body shape of the bony tail chub (G. elegans) perhaps
<br />correlates most closely with their potential for movement through swift
<br />waters in periods of low or modal flow, and within current in times of flood.
<br />The general lack of development of organs of attachment in Colorado
<br />River fishes must reflect the instability of bottoms and channels in that
<br />remarkably erosive stream. Instead, a highly modified body shape permits
<br />the fish to be held against the bottom under conditions of turbulence
<br />(through pressure of water on the curved dorsal surfaces and expanded
<br />fins). Shape thus substitutes for boIdfast organs. It is notable that
<br />bony tail chub maintaining reproducing populations in larger impound-
<br />ments of the Colorado system retain their streamlined body shapes, ap-
<br />parently moving about in an active, limnetic manner (Minckley, 1971)
<br />as do some oceanic fishes of similar morphology.
<br />Basic modifications of body shape seem to have occurred in paralIel in
<br />many groups of fishes, and these modifications may relate to sediment loads
<br />and to river currents which carry them. The woundfin (Plagopterus argentis-
<br />simus) is a species characteristic of the mainstream of the relatively swi(t
<br />and silt-laden Virgin River, Utah-Arizona-Nevada (Deacon et al., 1971),
<br />Many of its traits are similar to features of some bottom-dwelling, swift-
<br />water, silty-stream minnows of eastern United States, and elsewhere.
<br />Barbels are present on the eastern fishes now referred to the genus
<br />Hybopsis (Moore, 1968) and also are weII developed on Plagopterus.
<br />Dermal keels similar to those of the woundfin are present on the antero-
<br />dorsal scales of Hybopsis gelida (Cross,. 1967). Coarse, close-set papilJae
<br />on the intermandibular (gular) region of the wound fin (Snyder, 1915) are
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