7146

• UCREFRP Catalog Number: 7146
• Author: Hubbs, C. L. and R. R. Miller
• Title: Hybridization in Nature Between the Fish Genera
• USFW Year: 1953
• Copyright Material: YES

218

Hubbs and J.ll iller

Hybridization between Catostomus and Xyrauchen

219

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vanishes, and the hybrids are not very markedly different from either
parental species and tend to transgress the averages of both.
In the length of the base of the dorsal fin the parental species
?verlap t? some extent, and aU but two of the hybrids are definitely
mtermedIate, much more so than in the number of dorsal rays.
In the interpelvic width t~e hybrids are extremely variable and
even on the average are not intermediate.
. Measurements were also taken of head length, head depth, head
wIdth, snout length, eye length, longest dorsal ray" the lengths of
the ~nal, pectoral, and pelvic fins, and the width of the pelvic base,
but III aU these respects the proportions for the two parent species
overlap so very widely and the difference between them is so smaU
(less than 5 per cent) as to render the measurement.s too uncertain
for u.se (and presentation) in determining the relationships of the
hybrIds to the parental species.
. Unquantified features.-Even more striking is the intermediacy
dIsplayed by the hybrids in various characters, particularly of struc-
ture, that have not been quantified.
Like the cyprinid hybrids between Gila orcutti and S.iphateles
mohavensis (Hubbs and Miller, 1943, p. 360, pI. 1, figs. 1-3), the
Catostomus latipinnis X Xyrauchen texanus hybrids are intermediate
in fundamental scale structure. In C. latipinnis the scales are
broadly suboval and bear radii on the lateral (dorsal and ventral) .
as well as on the anterior and posterior fields, whereas in Xyrauchen
the scales are relatively longer and more rectangular, the lateral
fields are largely to wholly devoid of radii, and the anterior border
of t~e sc~le is di.stinctively lobate (Ellis, 1914, fig. 59). The hybrids
are mterJacent III all these respects. They are also intermediate in
the poor imbrications of the scales anteriorly, part.icularly on the
belly and the breast, for these scales are rather weU imbricated in C.
latipinnis but are more or less isolated from one another in X.
texanus .
The gillrakers are' intermediate in size and structure as well as in
number-a point that was determined after we had already felt
~ome co~fiden.ce in the hybrid interpretation. They are definitely
Illterm~dJate In degree of fimbriation, as they also are, conspicu-
ously, m Catostomus X Chasmistes hybrids. The rakers in Xyr-
a~chen approach those of the presumably plankton-eating lacus-
trme suckers comprising the genus Chasmistes not only in number

and length but also in fuzziness-apparently adaptations to permit
eating minute organisms (in some of the preserved specimens of
X yrauchen a sheet of such organisms has been retained between
the rakers and the enlarged pharyngeal pad).
Intermediacy is particularly marked in respect to the sharpness,
form, and elevation of the nuchal keel, for hardly a trace of thi.s
highly diagnostic character of Xyrauchen is evident externaUy in
Catostomus (PIs. I and III). In the hybrids the keel is distinctly
formed, though it is much less strikingly developed than in Xyr-
auchen. The crest of the hump is much more evenly rounded and
is located behind rather than before the vertical through the pectoral
insertion. Though subject to some variation, the hump of Xyr-
auchen rapidly assumes with age a striking keel 'and an abrupt far-
forward angle (PI. I, Fig. 3; see also Jordan, 1891, pI. 4, fig. 11; Ellis,
1914, pI. 1, fig. 7; and Simon, 1946, fig. 37). As a result of the de-
velopment of the keel the occipital region becomes very concave and
the nape abruptly convex in Xyrauchen. Catostomus latipinnis has
a weakly and regularly curved anterodorsal profile. The hybrids are
intermediate in this respect.
The intermediacy of the hybrids in the development of the nuchal
hump is evident in the neural spines of the anterior vertebrae and
the anterior interneurals, as well as in the external form. As shown
by X-ray photographs', reproduced in part as Plate III, and as in-
dicated by Snyder (1915, pp. 579-80, pIs. 76-77), these structures
are much more expanded in Xyrauchen than in Catostomus. Ac-
cording to X-ray pictures of four of the six adult hybrids and ex-
ploration by touch, the bones are definitely more expanded in the
hybrids than in C.latipinnis, but are less enlarged than in Xyrauchen.
It is noted, however, that the same elements that are greatly ex-
panded in Xyrauchen are slightly enlarged in C. latipinnis. The
degree of expansion of the bones in the hybrids is rather variable.
The hybrids are also intermediate in the flatness of the ventral
surfaces between the pectoral fins and between the pelvic fins, re-
gions which are notably flat in Xyrauchen and somewhat rounded
in Catostomus latipinnis, and in the correlated angulation of the
bony shoulder girdle in front of the pectoral-fin base.
Particularly striking is the intermediacy in all of the many
features that differentiate the lip structures of the parental species
(PI. II). Differences in measurements have aiready been treated