| Campostoma a. plumbeum | KU 4013 (three spec.) | Semotilus atromaculatus | |||
| Standard lengths | 90.2 (85.7-93.1) | 89.5 (85.7-96.2) | 91.7 (85.0-97.5) | ||
| 511 (505-517) | 533 (523-542) | 557 (547-564) | ||
| 251 (244-258) | 276 (273-278) | 289 (280-299) | ||
| 090 (086-096) | 088 (087-091) | 085 (082-087) | ||
| 044 (043-045) | 048 (047-049) | 049 (048-050) | ||
| 075 (073-078) | 094 (091-099) | 110 (104-113) | ||
| 057 (053-063) | 076 (072-078) | 098 (095-104) | ||
| Gill rakers (1st arch) | 30 (29-31) | 17 (16-18) | 9 (8-10) | ||
| Number scales in lateral line | 54 (53-55) | 54 (54-55) | 56 (52-64) | ||
| Predorsal scale-rows | 25 (23-27) | 27 (27-28) | 35 (34-36) | ||
| Anal rays | 7 (6-7) | 7.3 (7-8) | 8 8 | ||
| Vertebrae | 40 | 42-44[A] | 42-43 |
[A] Three deformed vertebrae in one specimen with 44; other two specimens have 42 vertebrae.
The hybrids seem uniformly intermediate between the parental species. Application of the hybrid index to the characters listed in Table 3 results in a value of 55.7 when C. a. plumbeum is assigned the value 0.
The pharyngeal arches of the hybrids are peculiarly deformed. Expressed in terms of the one-or two-rowed arrangement common to all North American cyprinids, tooth-counts of 0,5-4,1; 1,3(?)-4,0; and 2,5-4,1 best fit the three fish. However, one arch bears only three teeth, all deformed and badly aligned, plus a pit that presumably represents a lost fourth tooth. At the other extreme, one arch bears eight teeth, some of which are attached to the arch between and behind others that are countable as part of the basic main row. Supernumerary teeth and other deformities may have resulted from abnormalities in the replacement process. In some cases, replacement teeth probably failed to develop; in others, replacement teeth seemingly developed, but attached to the arch in abnormal positions, with or without loss of previous teeth, causing irregularity in alignment. Hubbs (1951) described an irregular (seemingly three-rowed) alignment in a fish that Hay (1888:249) reported from western Kansas as Squalius elongatus. However, Hubbs considered the specimen to be an aberrant example of S. atromaculatus, and the characteristics that he lists for it do not correspond closely with those of the hybrid specimens that we have. Evans and Deubler (1955:32) found three rows of teeth in two of 150 specimens of Semotilus, and attributed the abnormality to failure of old teeth to fall out after formation of new teeth. The teeth of Campostoma usually number 0,4-4,0, and those of Semotilus 2,5-4,2. The pharyngeal arches are much smaller in Campostoma than in Semotilus.
The peritoneum is mottled dark and silvery in the hybrids; it has a composite of the coloration in the parental species rather than a blended shade. The intestine has two diagonal loops crossing the ventral part of the body cavity, and the hindgut lies high in the cavity, along the left side of the air bladder. In Campostoma, the long gut is transversely coiled around the air bladder, whereas in Semotilus the gut forms a longitudinal, flattened, S-shaped loop, ventral to the air bladder.
In the hybrids, the mouth is slightly oblique and nearly terminal. The lower lip is thick and fleshy, but has only a suggestion of the projecting mandibular shelf that is unique in Campostoma. The upper lip is uniform in width, not medially expanded as in S. atromaculatus. One of the hybrids lacks barbels, one has a Semotilus-like barbel on the right side only, and one has a vestigial barbel on the right side and an anomalous barbel that is nearly terminal on the left upper lip.
In coloration, the hybrids lack the spot in the anterior base of the dorsal fin that is characteristic of Semotilus, but each has a poorly-developed dark lateral band, and a weak basicaudal spot. This band and spot are usually prominently developed in S. atromaculatus and usually are absent in adults of C. a. plumbeum.
In the position and obliquity of the mouth, basic color pattern (diffuse lateral band and basicaudal spot), and the presence in one specimen of a nearly terminal, barbel-like structure, the hybrids somewhat resemble Hybopsis biguttata (Kirtland), which occurs rarely in the Kansas River Basin. These partial similarities are coincidental, because other characters of the hybrids make relationship with H. biguttata implausible. The high number of gill rakers (Table 3) and the length and position of the gut indicate strongly that the three specimens are hybrids with C. anomalum as one parent; the pharyngeal arches, though deformed, indicate that the other parental species has two rows of teeth, with five teeth in the main row. Only S. atromaculatus, among species in the Kansas River Basin, usually has such a dental formula, and other characters of our three specimens fit expectations in a hybrid between that species and C. a. plumbeum.
Timber Creek, where the three hybrids were collected, is a small, spring-fed, sandy-bottomed tributary to Scott County State Lake in the extreme southwestern part of the Kansas River Basin. The stream was less than 10 feet wide and six inches deep, except in three pools near road crossings. The hybrids were found in two of these pools, along with numerous S. atromaculatus and one adult C. a. plumbeum.
Another specimen of C. a. plumbeum × S. atromaculatus (KU 4841, 39.3 mm. in standard length) was taken in the North Platte River at Lisco, Garden County, Nebraska, on September 11, 1959. That specimen has 7 anal rays and 52 scales in the lateral line; otherwise, it is similar to the three hybrids described above.