Frank B. Cross

“ The color of the peritoneum in the hybrids is the glossy, jet-black of C. erythrogaster in two specimens, and the dusky-black of N. c. frontalis in one.
Chrosomus and Notropis differ greatly in the length and convolution of the intestine. Chrosomus has a long, coiled gut, which is crossed by the mid-ventral line eight or nine times; in N. c. frontalis, the intestine forms a flat, S-shaped loop that does not cross the mid-ventral line. In the two largest hybrids (KU 4170 and 4185) , the gut is intermediate, crossing the mid-ventral line four times. ”

“ The second subspecies, for which H. g. gulonella is the oldest applicable name, is small, and has 1) a deep, chubby body, 2) head convex in dorsal contour (less depressed than in H. g. gracilis) , 3) fins less falcate than in the latter subspecies, with the dorsal and pelvic fins originating more posteriorly, and 4) fewer [pg 341] scales, vertebrae, and pectoral fin-rays than H. g. gracilis. These differences are consistently expressed throughout the size-ranges of the subspecies, and in series collected at the same or nearby localities in several different years. ”

“ 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. ”

“ It has already been noted that H. g. gulonella seems to be the ecological equivalent of Semotilus atromaculatus in streams in which S. atromaculatus is not found. S. atromaculatus occurs in creeks of eastern Kansas and Missouri, and may provide interspecific competition that prevents establishment of the flathead chub in these creeks. Regardless of cause, the gaps in distribution of H. gracilis tend to limit gene flow.
Many characters used in the separation of the two subspecies are known to be influenced by environmental conditions, especially temperature. ”

“ Both extreme values for the hybrid seem to result from the cumulative influence of characters in which the parental species differ slightly in mean value (especially head-length, in which the hybrid is like whipplei, and caudal peduncle-length, in which the hybrid approaches venustus, despite the 9-rayed anal fin of the hybrid) . The basicaudal spot of the hybrid is like that of N. v. venustus except for being less intense. ”

“ We think it likely that temperature does influence the expression of characters in H. gracilis, directly in individual development, and indirectly as a selective mechanism in the evolutionary process. The extent to which each kind of influence exists can be proved only by experimental work with both subspecies, which we hope to undertake at a later date. ”

“ Hubbs and Miller (1943:373-374) reported that hybrids of Gila orcutti × Siphateles mohavensis have larger, more robust heads than either of the parental species, perhaps because of heterosis. The enlarged heads in hybrids of C. erythrogaster and N. c. frontalis result primarily from elongation of the postorbital region, with lesser elongation of the snout and orbit. The enlarged head affects measurements obtained for other structures that are parts of the head (and expressed as proportions of standard length or head-length) , causing a tendency toward N. c. ”

“ In the Missouri Basin the species occurs more or less continuously from the high plains adjacent to the Rocky Mountains in Montana and Wyoming down the mainstream of the Missouri River to its mouth, and down the mainstream of the Mississippi River as far as Barfield, Arkansas, but not to the Gulf. ”

“ Many rivers occupied by H. g. gulonella (and by intergrades) are intermittent, and in some years their sand-filled channels become wholly dry for many miles. These factors probably promote mixing of the two subspecies, and may account, over long periods of time, for the wide dispersal of H. g. gulonella in the Missouri Basin. Flathead chubs are known from Pleistocene beds at Doby Springs, Oklahoma (the Doby Springs local fauna) (Smith, 1958:177) . ”

“ 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. ”

“ Variation in H. gracilis, as shown in the graphic analysis (Figs. 1 and 2) and distribution map (Plate 21) , presents two clines: a north-south cline and a large-river to small-river (mainly east-west) cline. The absence of H. gracilis from certain portions of river systems is a matter of concern. The species has not been found in the lower Arkansas River and the Rio Grande, nor in sandy tributary creeks in eastern Kansas and Missouri that appear to provide suitable habitat. ”

“ Specimen No. 4253 is mostly pallid, resembling Gila much more than Rhinichthys in pigmentation. A mid-dorsal dark streak is conspicuous in the hybrid, especially anteriorly, but is less intense than in Gila. Rhinichthys lacks a well-developed dorsal stripe. The preorbital and suborbital areas are more heavily pigmented in the hybrid than in Gila, but not nearly so dark as in Rhinichthys. The hybrid has a faint dark basicaudal spot that is variably developed in Rhinichthys but absent in Gila. ”

“ Organisms Found in Stomachs of Hybopsis gracilis From Various Locations, Expressed as Percentage of Total Volume.
Spawning apparently occurs when river levels recede to the seasonal lows. In late summer, temperatures of these rivers probably are maximal, their turbidities are reduced, and their sandy bottoms are stable. ”

“ Likewise, Martin (1949) and Hart (1952) have shown that the proportions of some body-parts vary in response to temperature during early development. In H. gracilis, the general nature of the clines found in a majority of characters (but not all characters) suggests a temperature influence. However, temperature-dependent variability that has so far been demonstrated experimentally in fishes is generally of lesser magnitude than the differences distinguishing H. g. gracilis and H. g. gulonella. ”

“ 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. ”

“ All series examined are from elevations lower than 3,000 feet.
H. g. gulonella occupies small rivers and creeks, preferring pools with moderate currents. In fall, dense concentrations of this subspecies have been found in small pools, where brush, driftwood or [pg 336] other debris deflects the current and prevents filling with drifting sand. Hundreds of flathead chubs were collected in such pools in the Purgatoire and Arkansas rivers. Specimens were also collected with ease in Beaver Creek, Colorado, from pools with murky water and slight flow, over bottoms of gravel and bedrock. ”

“ KU 2947 (28.0 mm. in standard length) from Mill Creek, Wabaunsee Co., Kansas, Sec. 30, T. 12S, R. 9E, Mar. 22, 1953. Compared in Table 2 with five specimens of C. erythrogaster, KU 2836 (27.2 to 31.0 mm., mean 28.5) from the same locality and of the same date as KU 2947 (above) ; and with five specimens of S. atromaculatus, KU 1954, 2499, 2703, and 2838 (25.5 to 31.1 mm., mean 28.9 mm.) from streams in the same area.
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) . ”

“ Most organisms found in specimens (both subspecies) were terrestrial insects (Coleoptera, Diptera, Orthoptera) ; all insects were adult stages, except those designated as larvae in Table 1. Roundworms probably were parasites, rather than food.
Hubbs (1927:76) states that the food of young flathead chubs that were obtained from the Arkansas River System in New Mexico consisted "almost entirely of crustaceans (small ostracods and cladocerans to the exclusion of all else but an occasional larval or adult insect, etc.) ”

“ Some of these abnormalities presumably resulted from retention of larval characteristics of the fish, correlated with the degree of infestation by tapeworms. No teratological adults were found, indicating that severe infections prevent survival to maturity. ”

“ Nuptial tubercles of male minute and densely scattered over top of head and snout; usually present on pectoral rays 1-8, weak when present on rays beyond the eighth, never found beyond the eleventh ray; minute tubercles usually found on dorsal, pelvic and anal fins, rarely on lower scales of caudal peduncle; predorsal scales have a fine peripheral row of tubercles.
Hybopsis gracilis gracilis (Richardson)
(Plate 22)
Cyprinus (Leuciscus) gracilis Richardson, 1836:120 and Pl. 78 (original description; Saskatchewan R. at Carlton House) . ”

“ Hybopsis gracilis physignathus, Beckman, 1952:50 (characters; habitat) .
Hybopsis gracilis, Eddy, 1957:111 (in part; characters; key) ; Koster, 1957:61 (characters; habitat; spawning; food) ; Moore, 1957:110 (in part; key) ; Smith, 1958:177 (fossil record; Doby Springs, Okla.) .
Diagnosis.--Post-Weberian vertebrae 36-38, rarely 39; lateral line scales 42-54, usually less than 50; pectoral rays 14-19, usually fewer than 17; head-depth 13.5-18.0 per cent of standard length, usually 14.8 per cent or more. ”

“ Guide to the fishes of New Mexico. Univ. New Mexico Press, in cooperation with the New Mexico Dept. of Game and Fish, Albuquerque, New Mexico. 1-116.
Lindsey, C. C.
1956. Distribution and taxonomy of fishes in the Mackenzie drainage of British Columbia. Jour. Fish. Res. Bd. Canada. 13 (6) :759-789. 1957. Possible effects of water diversions on fish distribution in British Columbia. Jour. Fish. Res. Bd. Canada. 14 (4) :651-668.
Martin, W. R.
1949. The mechanics of environmental control of body form in fishes. Pub. Ontario Fish. Res. Lab., 70:1-91.
Meek, Seth E.
1892. ”

“ Variations in the number of vertebrae and other meristic characters of fishes correlated with the temperature of water during development. Amer. Nat., 61:360-372. 1926. The structural consequences of modifications of the developmental rate in fishes, considered in reference to certain problems of evolution. Amer. ”