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<br />Coin.. 1993(2). pp. 334-343
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<br />Analysis of Sexual Dimorphism in an Endangered Cyprinid Fish
<br />(Gila c)'pha Miller) Using Video Image Technology
<br />
<br />MICHAEL E. DOUGLAS
<br />
<br />Univariate and multivariate techniques were used to evaluate sexual dimorphism
<br />in 53 morphometric measures taken from 63 adult specimens of an endangered
<br />cyprinid, the humpback chub (Gila c)'pha). Specimens were filmed and released
<br />unharmed after their capture in the Colorado and Little Colorado rivers of the
<br />Grand Canyon (Arizona). Morphometric data were later extracted from film
<br />using a microcomputer and image analysis software. Because of the unique mor.
<br />phology of this fish, analyses emphasized its anterodorsal hump. Only two of 53
<br />characters (3.8%) revealed significant sexual dimorphism in an analysis of co-
<br />variance; approximately what one would expect from chance alone (i.e., one in
<br />20, or 5%). A discriminant analysis correctly classified specimens by sex only
<br />60% of the time, which is not significantly different from random expectation.
<br />Multiple group principal component analysis (MGPCA) and sheared PCA using
<br />all 53 characters also failed to delineate significant sexual dimorphism. Mor-
<br />phological shape differences among individuals, regardless of sex, were clearly
<br />apparent from character loadings onto both principal component 2 and sheared
<br />principal component 2. These differences were due to extent and development
<br />of the nuchal hump and to concomitant changes in concavity and length of the
<br />head which accompany its development. These differences appear ontogeneti-
<br />cally based and are unrelated to either sexual or seasonal variation.
<br />
<br />NATIVE fishes inhabiting large rivers of
<br />western North America are unique, be-
<br />cause their phenotypes have been molded over
<br />evolutionary time by the unusually harsh con-
<br />ditions of a turbulent and sediment-rich habitat
<br />(Miller, 1961; see Carlson and Muth, 1989, for
<br />a description of the mainstream Colorado Riv-
<br />er). As a result, many of these fishes evolved a
<br />characteristic morphology: scalloped crania,
<br />humped or crested dorsums, thin but rigid cau-
<br />dal peduncles, and large, falcate fins. These
<br />characteristics are believed to have evolved in
<br />response to intense selective pressure for sta-
<br />bility and maneuverability in swift currents and
<br />for explosive propulsion through turbulent,
<br />boulder-strewn rapids. A reduction in relative
<br />eye size, coupled with an imbedding of scales
<br />into epidermis, are suspected to be adaptive re-
<br />sponses to heavy (preimpoundment) sediment
<br />loads in the rivers (Miller, 1946; Minckler,
<br />1973). These fishes frequently attain relatively
<br />large body sizes (to 40 cm TL, or more) and
<br />are either known, or suspected, to achieve con-
<br />siderable longevity (Rinne et aI., 1986; McCar-
<br />thy and Minckley, 1987). Adult size and lon-
<br />gevity both suggest an adaptive strategy (Wylbur
<br />et aI., 1974) evolved in response to the tremen-
<br />dous environmental variabilitv inherent in
<br />preimpoundment western river~, such that the
<br />species maintains itself in spite of successive
<br />failed breeding seasons or lost year-classes. The
<br />
<br />majority of these fishes are now listed as en-
<br />dangered (or are candidates for such listing; see
<br />Minckley and Douglas, 1991) due to numerous
<br />recent modifications of the riverine habitat by
<br />modern humans.
<br />The humpback chub (Gila cypha, Family Cy-
<br />prinidae; Fig. 1 B). described from Bright Angel
<br />Creek (Miller, 1946) in Grand Canyon National
<br />Park (GCNP), is one of the most morphologi-
<br />cally bizarre fishes on this continent (Rolston,
<br />1991 :94). According to Miller (1946:415), "Gila
<br />[)jihn . . . judged from its large, falcate fins, spe-
<br />cialized nuchal hump, inferior mouth, and dor-
<br />so-ventrally flattened head. . . is well adapted
<br />for life in the swift current very near or on the
<br />bottom. The action of the current against the
<br />prominent nape tends to force the fish down
<br />towards the bottom or the sides, where the flow
<br />is not so torrential as in mid-water. The small
<br />eye may represent a degeneration correlated
<br />with reduced light due to excessive silt, or it
<br />may be a response to the direct effect of the
<br />scouring action of the suspended matter, or
<br />both." Minckley (1973:98-99) elaborated fur-
<br />ther, stating that". . . the belly and lower part
<br />of the head are flattened, the mouth is essen-
<br />tially horizontal and overhung by a produced,
<br />fleshy snout. It would seem that the hump acts
<br />as a barrier to passing water, forcing the fish's
<br />body against the bottom. Perhaps the pro-
<br />nounced grooves laterally on the hump, and
<br />
<br />(Q 1993 by lhe American SOciC:IY of Irhrhyolol;islS and H(~rp(,lOloKisIS
<br />
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