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<br />2~26
<br />
<br />Hubbs and Miller
<br />
<br />"\.gain, the hybrids are intermediate in coloration, and in this
<br />combination the intermediacy applies to more characters. In the
<br />seri,~s of young used for the comparison the large lateral blotches are
<br />dcfinitely evident ill nearly all of the Catostomus insignis specimens,
<br />are .nore or less evident in the hybrids, and are apparent in only the
<br />vcr,)' smallest of the humpback suckers. The spots on the scale
<br />pockets, one per scale, that characterize the adults of C. insignis
<br />are already developing in the young of this species, though in some-
<br />wha t irregular order and in incomplete series; they are detectable in
<br />the hybrids, but are not evident in Xyrauchen texanus. The finely
<br />penf:iled dark borders of the scale pockets are strongly developed
<br />over most of the body in the young of X. texanus, are weaker in
<br />the hybrids, though traceable as far forward as the dorsal base, and
<br />arc very indistinct in the C. insignis specimens. In the young of
<br />X. texanus large melanophores in single series rather consistently
<br />line the basal part of the caudal rays; in the young of C. insignis
<br />such pigment is usually lacking or is developed as minute melano-
<br />phol'es irregularly scattered; in the hybrids the development is
<br />intermediate. The black peritoneum shows distinctly through the
<br />ventral body wall in the smaller specimens of the X. texanus series,
<br />more weakly in the comparable hybrids, and very weakly or not at
<br />all ill the young of C. insignis.
<br />
<br />DISCUSSION AND CONCLUSIONS
<br />
<br />We regard as incontrovertible the circumstantial evidence that
<br />despite its huge size (commonly from twelve to fourteen pounds;
<br />occa ,ion ally to sixteen pounds), bizarre form, and general preference
<br />for I:Lrge waters, the humpback sucker Xyrauchen texanus hybridizes
<br />soml~what frequently in nature with two species of Catostomus,
<br />which, like it, are endemic to the Colorado River fauna. Similar
<br />evid'~nce, which we hope to present later, convinces us that the large
<br />lacu.;trine suckers comprising the genus Chasmistes also cross with
<br />Cato,ltomus. We thus extend beyond the limits of Catostomus and
<br />the closely related group known as Pantost~us the list of suckers that
<br />inter breed.
<br />Despite their distinctive physiognomy, Chasmistes and X yrauchen
<br />appE'ar to be phyletically akin to Catostomus. It may therefore still
<br />be concluded that only closely related genera of Catostomidae hy-
<br />bridize. Chasmistes seems to be merely a lacustrine offshoot of
<br />
<br />Hybridization between Catostomus and Xyrauchen
<br />
<br />227
<br />
<br />"
<br />. .
<br />
<br />
<br />Catostomus, evidently adapted by mouth and gillraker structures to
<br />feeding chiefly in midwater on plankton, which characteristically
<br />abounds in lakes and is poorly developed in streams, rather than on
<br />bottom organisms such as predominate in swift water. To some
<br />extent as indicated above, Xyrauchen is modified in a similar way.
<br />The chief point of its divergence from Catostomus lies in peculiarities
<br />of form-the flattened breast, the depressed head, and, most no-
<br />table, the abruptly high and sharp, skeletally supported nucha~
<br />hump (PIs. I and III). These peculiarities, which are mt,l:?h,rl~ss
<br />striking in the young than in the adult,.are,interpretableasadapta-
<br />. tions to provide stability and to permit upstreamprientation:in~-the;
<br />almost ceaselessly swift and often torrential flow thatchara~terIzed
<br />theColorado River and the main stems of its largertributarie~.he!?reil
<br />the recent damming and diversions. Though they inay.ha-vedropped
<br />'into quieter pools and recesses where these wereavail~bll~;,~emb:rs
<br />Of the species must have stemmed such currents durmg:mIgratlOn
<br />and during floods; ,In form, Xyrauchen is remarkably simulated by
<br />a cyprinid fish, Gila cypha, recently described from the Colorado
<br />River in Grand Canyon (Millcr, ID46b) and now known to occur
<br />elsewhere in the swift sections of the main river. Other fishes of
<br />large swift waters are similarly modified.
<br />There is no apparcnt phyletic significance in the indication by
<br />Snyder (1915, pp. 579-80, pIs. 76-77) t~at the osse~us crest. of
<br />Xyrauchen is not much unlike that of Carpwdes and Ictwbus, whiCh
<br />represent the subfamily Ictiobinae (buffalofishes and carpsuckers).
<br />As noted above, and as shown on Plate III, the elements of the crest
<br />are evident and incipiently modified in Catostomus latipinnis; they
<br />are similar in Catostomus insignis. Nor is much weight to be ac-
<br />corded Snyder's statement that the shape of the fontanel~e in
<br />Xyrauchen and "other peculiar cranial characters," not speCIfied,
<br />"indicate no very close relationship between it and Catostomus."
<br />Nelson's studies of the Weberian apparatus (1948, pp. 237, 241-42)
<br />and of the opercular series (1949, p. (63) have linked both Xyrauchen
<br />and Chasmistes with Catostomu8 in the subfamily Catostominae and
<br />the tribe Catostomini (which were erected by Hubbs, 1930, p. 9).
<br />The reference of Xyrauchen and Chasmistes as well as Catostomus
<br />and Pantosteus to a single tribe indicates that in the Catostomidae,
<br />as in the Poeciliidae and other families, hybridization does not trans-
<br />gress the recognizcd tribal limits (or the seemingly equivalent rela-
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