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Five Natural Hybrid Combinations in Minnows (cyprinidae) · Frank B. Cross — chapter 2 of 4 · ~3,118 words · public domain

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This hybrid is intermediate between the two species in number of scales and pharyngeal teeth, and has a composite of the pigmentation found in the parental fishes (Table 2). For diagnostic purposes, greater importance is attached to the characters mentioned above than to proportional measurements, which are subject to considerable error because of the small size of the specimens. The few measurements that were taken indicate that this hybrid, like C. erythrogaster × N. c. frontalis, has a larger head than do specimens of like size of either parental species. The enlarged head affects measurements obtained for other structures that are parts of the head; only the length of the upper jaw, which is greatly different in the parental species, is actually intermediate in KU 2947.

Mill Creek is a clear stream, similar to Deep and Bluff creeks but somewhat larger. Mill Creek had an exceptionally large population of fishes at the time the hybrid was found, but Chrosomus and Semotilus were neither unusually common nor rare.

Two other crosses, both of which have been described in the literature, also have been found in Mill Creek. These are N. c. frontalis × S. atromaculatus, and N. c. frontalis × Notropis rubellus (Agassiz).

TABLE 2. COMPARISON OF ONE SPECIMEN OF CHROSOMUS ERYTHROGASTER × SEMOTILUS ATROMACULATUS WITH SPECIMENS OF THE PARENTAL SPECIES (MEANS ARE ABOVE, RANGES IN PARENTHESES BELOW)

=Campostoma anomalum plumbeum × Semotilus atromaculatus=: KU 4013 (three males, 86.0 to 96.0 mm. in standard length, mean 89.5 mm.) from Timber Creek, Scott Co., Kansas, Sec. 2, T. 16S, R. 33W, June 19, 1958. Compared in Table 3 with five specimens of C. a. plumbeum, KU 4034 (85.7 to 93.1 mm., mean 90.2 mm.) from the Smoky Hill River, Wallace Co., Kansas, Sec. 26, T. 13S, R. 39W, June 20, 1958; and with five specimens of S. atromaculatus, KU 4012 and 4047 (85.0 to 97.5 mm., mean 91.7 mm.) from the same locality and of the same date as KU 4013 (above), and Sappa Creek, Decatur Co., Kansas, Sec. 29, T. 2S, R. 28W, June 23, 1958, respectively. This hybrid combination has previously been recorded by Johnson (1945).

TABLE 3. COMPARISONS OF THREE SPECIMENS OF CAMPOSTOMA ANOMALUM PLUMBEUM × SEMOTILUS ATROMACULATUS WITH SPECIMENS OF THE PARENTAL SPECIES (MEANS ARE ABOVE, RANGES IN PARENTHESES BELOW)

=====================+=================+===============+================= | Campostoma a. | KU 4013 | Semotilus | plumbeum | (three spec.) | atromaculatus ---------------------+-----------------+---------------+----------------- Standard lengths | 90.2 | 89.5 | 91.7 | (85.7-93.1) | (85.7-96.2) | (85.0-97.5) | | | Predorsal length / | 511 | 533 | 557 Standard length | (505-517) | (523-542) | (547-564) | | | Head-length / | 251 | 276 | 289 Standard length | (244-258) | (273-278) | (280-299) | | | Snout-length / | 090 | 088 | 085 Standard length | (086-096) | (087-091) | (082-087) | | | Orbital length / | 044 | 048 | 049 Standard length | (043-045) | (047-049) | (048-050) | | | Interorbital width / | 075 | 094 | 110 Standard length | (073-078) | (091-099) | (104-113) | | | Distance from tip of | | | mandible to tip of | | | maxillary / | 057 | 076 | 098 Standard length | (053-063) | (072-078) | (095-104) | | | Gill rakers | 30 | 17 | 9 (1st arch) | (29-31) | (16-18) | (8-10) | | | Number scales in | 54 | 54 | 56 lateral line | (53-55) | (54-55) | (52-64) | | | Predorsal scale-rows | 25 | 27 | 35 | (23-27) | (27-28) | (34-36) | | | Anal rays | 7 | 7.3 | 8 | (6-7) | (7-8) | 8 | | | Vertebrae | 40 | 42-44 | 42-43 ---------------------+-----------------+---------------+-----------------

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.

=Gila nigrescens × Rhinichthys cataractae=: KU 4253 (a male, 60.6 mm. in standard length), from New Mexico, Bernalillo County, Rio Grande 12 mi. S Bernalillo on U. S. Highway 85 (Corraleo Bridge). Compared in Table 4 with six specimens of G. nigrescens: KU 4251, 4254, and 4262 (63.1-72.4 mm. in standard length, mean 66.4 mm.); and with five specimens of R. cataractae: KU 4248, 4258, and 4264 (55.6-65.0 mm. standard length, mean 59.5 mm.). Comparative material was taken at the same locality as KU 4253 and at nearby localities in the Rio Grande.

The hybrid is intermediate in almost all of the features in which the parental species differ from each other. For six of the characters included in Table 4, the hybrid index is 49.7 per cent, when Gila is assigned the value 0 (height of dorsal fin and numbers of fin rays and teeth excluded). There is no enlargement of the head in KU 4253, such as was found in Gila orcutti × Siphateles mohavensis (Hubbs and Miller, 1943:373), Chrosomus erythrogaster × Notropis cornutus frontalis, and C. erythrogaster × Semotilus atromaculatus. The height of the dorsal fin, which Hubbs and Miller (loc. cit.) found to be extreme in G. orcutti × S. mohavensis, exceeds the average for the parental species in G. nigrescens × R. cataractae also; but, dorsal fins as high as that of the hybrid were found in some individuals of both parental species. In R. cataractae, all fins are more rounded and more expansive than in G. nigrescens, and fins other than the dorsal have an intermediate size in the hybrid. This intermediacy has doubtful significance, because fin-size in Rhinichthys varies greatly with body-size, sex, and probably with the state of sexual development. Rhinichthys matures at smaller size than Gila, and never becomes so large as that species.

Gila nigrescens and R. cataractae differ strikingly in features involving the snout and mouth, and these differences provide the most conclusive evidence that KU 4253 is a hybrid of these species. The projecting, fleshy snout of R. cataractae is bridged to the ventral mouth by a frenum that is approximately 3 mm. wide in specimens 60 mm. in standard length. In Gila, the snout does not project beyond the mouth, which is oblique, lacks a frenum, and is larger than in Rhinichthys. The snout of the hybrid projects less than in R. cataractae and is bridged to the upper lip by a frenum 1.7 mm. wide. The mouth of the hybrid is intermediate in size, obliquity, and thickness of the lips. Rhinichthys has barbels, Gila lacks them, and the hybrid has one vestigial barbel, on the right side. The lower surface of the head of Rhinichthys is broad and flattened, with pronounced rugosity on the gular area and isthmus. In Gila the underside of the head is convex, with comparatively smooth membranes; the hybrid is intermediate, but tends toward Gila.

TABLE 4. COMPARISONS OF ONE SPECIMEN OF GILA NIGRESCENS × RHINICHTHYS CATARACTAE WITH SPECIMENS OF THE PARENTAL SPECIES (MEANS ARE ABOVE, RANGES IN PARENTHESES BELOW)

======================+==============+=================+================= | Gila | | Rhinichthys | nigrescens | KU 4253 | cataractae ----------------------+--------------+-----------------+----------------- Standard lengths | 66.4 | 60.6 | 59.5 | (63.1-72.4) | | (55.6-65.0) | | | Head-length / | 282 | 281 | 281 Standard length | (277-290) | | (273-293) | | | Orbital length / | 063 | 054 | 044 Standard length | (063-065) | | (041-047) | | | Snout-length / | 083 | 092 | 106 Standard length | (081-085) | | (099-113) | | | Dorsal fin-height / | 225 | 234 | 221 Standard length | (212-238) | | (206-234) | | | Postorbital length / | 140 | 135 | 131 Standard length | (134-142) | | (127-136) | | | Distance from tip of | | | mandible to tip of | | | maxillary / | 081 | 076 | 066 Standard length | (079-085) | | (064-069) | | | Length of | | | infralabial groove / | 060 | 045 | 036 Standard length | (058-064) | | (034-038) | | | Upper jaw | protractile | non-protractile | non-protractile | | | Number scales in | | | lateral line | 60 | 63 | 65 | (58-63) | | (63-67) | | | Anal fin-rays | 8 | 7 | 7 | (7-8) | | (7) | | | Pelvic fin-rays | 9 | 8 | 8 | (9) | | (8-9) | | | Pectoral fin-rays | 16 | 16-15 | 13 | (16-18) | | (13-14) | | | Pharyngeal teeth | 2,5-4,2 | 2,5-4,2 | 2,4-4,2 ----------------------+--------------+-----------------+-----------------

TABLE 5. COMPARISONS OF ONE SPECIMEN OF NOTROPIS V. VENUSTUS × NOTROPIS WHIPPLEI WITH SPECIMENS OF THE PARENTAL SPECIES, AND WITH N. LUTRENSIS × N. V. VENUSTUS. MEASUREMENTS (LENGTHS AND DEPTHS) ARE EXPRESSED AS THOUSANDTHS OF STANDARD LENGTH (MEANS ABOVE, RANGES IN PARENTHESES BELOW)

Column headings:

A: Notropis whipplei B: KU 3516 C: Notropis venustus, KU 3510 D: Notropis venustus, from Gibbs (1957a) E: Notropis lutrensis × N. venustus

=================+=============+=========+=============+=========+============= | A | B | C | D | E -----------------+-------------+---------+-------------+---------+------------- Standard length | 50.6 | 47.8 | 47.3 | | 44.7 | (45.0-54.0) | | (44.5-49.6) | | (43.3-47.3) | | | | | Predorsal length | 525 | 523 | 534 | 523 | 532 | (513-535) | | (519-547) | | (528-538) | | | | | Dorsal origin to | | | | | caudal base | 497 | 508 | 497 | 496 | 508 | (493-502) | | (478-504) | | (502-514) | | | | | Prepelvic length | 505 | 492 | 505 | | 499 | (498-518) | | (500-510) | | (486-517) | | | | | Head-length | 257 | 255 | 261 | 260 | 263 | (250-262) | | (256-267) | | (261-267) | | | | | Caudal | | | | | peduncle-length | 217 | 221 | 224 | | 224 | (211-220) | | (213-230) | | (214-231) | | | | | Caudal | | | | | peduncle-depth | 110 | 119 | 127 | 125 | 126 | (106-116) | | (124-133) | | (122-131) | | | | | Head-depth | 170 | 182 | 186 | | 190 | (167-173) | | (182-190) | | (189-192) | | | | | Snout-length | 079 | 079 | 080 | | 081 | (076-083) | | (072-083) | | (078-082) | | | | | Eye-diameter | 069 | 069 | 070 | 073 | 070 | (063-078) | | (066-072) | | (068-074) | | | | | Postorbital | | | | | length, head | 112 | 115 | 116 | | 117 | (108-115) | | (112-120) | | (115-120) | | | | | Upper jaw, | | | | | length | 078 | 077 | 081 | 079 | 077 | (076-081) | | (076-082) | | (076-081) | | | | | Body depth | 239 | 253 | 278 | 274 | 282 | (233-248) | | (261-288) | | (275-294) | | | | | Lateral-line | | | | | scales | 36-37 | 36 | 36-38 | 36.5 | | | | | (34-39) | | | | | | Scales above | | | | | lateral-line | 13 | 14 | 15 | 15 | | | | | (13-16) | | | | | | Anal fin-rays | 9 | 9 | 8 | 8 | | | | | (7-8) | | | | | | Pectoral | | | | | fin-rays | 14 | 14-14 | 15 | 14.2 | | (14-15) | | (14-16) | (12-17) | | | | | | Caudal spot | Absent | Present | Present | Present | Present | | | | | Vertebrae | 37-38 | 38 | 37 | | -----------------+-------------+---------+-------------+---------+-------------

Orbital diameter.

The air bladder of KU 4253 is nearly as large as in Gila, and much larger than the degenerate air bladder of R. cataractae. Although the hybrid appears to be male, the gonads (especially the right one) are poorly developed. The hybrid is intermediate in curvature of the lateral line, which is nearly straight in Rhinichthys and strongly decurved in Gila.

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