University of Kansas Publications
Museum of Natural History

Volume 13, No. 7, pp. 323-348, pls. 21-24, 2 figs.
February 10, 1961

Geographic Variation
In the North American Cyprinid Fish,
Hybopsis gracilis

BY
LEONARD J. OLUND AND FRANK B. CROSS

University of Kansas
Lawrence
1961

University of Kansas Publications, Museum of Natural History

Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
Robert W. Wilson

Volume 13, No. 7, pp. 323-348, pls. 21-24, 2 figs.
Published February 10, 1961

University of Kansas
Lawrence, Kansas

PRINTED IN
THE STATE PRINTING PLANT
TOPEKA, KANSAS
1961

28-5871

Geographic Variation In the North American Cyprinid Fish, Hybopsis gracilis

BY
LEONARD J. OLUND AND FRANK B. CROSS

CONTENTS

•   PAGE
• Introduction 325
• Methods, Materials, and Acknowledgments 326
• Description of the Species Hybopsis gracilis 327
• Hybopsis gracilis gracilis 328
• Hybopsis gracilis gulonella 330
• Intraspecific Variation 333
• Natural History 334
• Habitat 334
• Associated Species 336
• Food 338
• Spawning Season 338
• Discussion 340
• Literature Cited 343

INTRODUCTION

The flathead chub, Hybopsis gracilis (Richardson), occurs in the Plains Region of Canada and the United States, in four major drainage systems: Mackenzie River, which discharges into the Arctic Ocean; Saskatchewan River, which discharges into Hudson Bay via Nelson River; and Missouri-Mississippi System and Rio Grande, both draining into the Gulf of Mexico. Each of these systems is occupied in part only. In the Mackenzie Basin, H. gracilis has been reported as far north as Fort Good Hope (Walters, 1955:347). Flathead chubs occur in the Saskatchewan Basin from Alberta eastward to Lake Winnipeg, Manitoba, but have not been found in other streams that flow into Lake Winnipeg (Red River, Brokenhead River and Whitemouth River) nor in Nelson River downstream from Lake Winnipeg. 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. The species prob[pg 326]ably attains its greatest abundance in the Missouri Basin, but it is scarce or absent in tributaries north and east of the Missouri mainstream, in the South Platte Basin, and in the central part of the Platte River in Nebraska. The flathead chub is unknown in the Mississippi Basin above the mouth of the Missouri River, and in the Ohio River Basin above its mouth. In the Arkansas River Basin, records are restricted to (1) the headwaters and tributaries of the Arkansas River from eastern Colorado downstream as far as Garden City, Kansas, (2) the Cimarron River at Kenton, Cimarron County, Oklahoma, and (3) the South Canadian River and tributaries from northeastern New Mexico eastward as far as Norman, McClain County, Oklahoma, but rarely there. Thus, the range in the Arkansas Basin seems to consist of three isolated segments. Likewise, isolated populations exist in the Rio Grande System, where flathead chubs are confined to the upper parts of the Rio Grande and Pecos basins, above the confluence of the Rio Grande and Pecos Rivers. Records resulting from introductions have been reported for the Gila River by Koster (1957:62) and from the Snake River, Wyoming, by Simon (1946:72).

Six names apply to the flathead chub, the earliest of which is Cyprinus gracilis Richardson (1836:120). Other names have sometimes been accepted as applicable to valid species and/or subspecies, but usage, diagnoses, and stated ranges have been confusingly inconsistent. For most of the past 100 years, Platygobio Gill has been recognized as the appropriate generic name for the flathead chub, but Bailey (1951:192) places Platygobio and other nominal genera of barbeled minnows having short guts, protractile premaxillae, and four teeth (primary row) in the single genus Hybopsis (Agassiz, 1854). Strangely, the orthotype of Hybopsis, H. gracilis Agassiz, is a junior synonym of H. amblops (Rafinesque) (Hubbs and Ortenburger, 1929b:66) and is a younger name than C. gracilis Richardson.

The purpose of this paper is to redescribe the species and to make known its pattern of geographic variation. Natural history will also be considered, as will habitat, food habits, and breeding season.

METHODS, MATERIALS AND ACKNOWLEDGMENTS

DESCRIPTION OF THE SPECIES

Hybopsis gracilis (Richardson)

Flathead Chub

(Synonymy under accounts of subspecies)

Hybopsis gracilis gracilis (Richardson)

(Plate 22)

Hybopsis gracilis gulonella (Cope)

(Plate 23)

INTRASPECIFIC VARIATION

Two subspecies of H. gracilis are recognized by us: one northern and eastern, characteristically inhabiting large rivers (H. g. gracilis), and one southern and western, characteristically inhabiting small streams (H. g. gulonella). Other scientific names that have been applied to this fish in the past are listed in the synonymy.

H. g. gulonella is a chubby, deep-bodied fish, whereas H. g. gracilis is long and slender. The head of the creek subspecies is deeper and longer than that of H. g. gracilis, being rounded anteriorly when seen in sideview. The head of the large-river subspecies is acutely wedge-shaped in profile. H. g. gracilis has a larger orbit than H. g. gulonella. Fins of H. g. gracilis are more strongly falcate than those of the other subspecies. H. g. gracilis has a greater number of lateral line scales, pectoral rays and post-Weberian vertebrae than the creek subspecies. The large-river subspecies attains much larger size than does the creek subspecies (Plate 24). Except in areas of intergradation, complete separation of the two subspecies can be made on the basis of lateral line scales, pectoral rays, post-Weberian vertebrae and head-depth. The regressions of head-depth on standard length in H. g. gracilis from the Saskatchewan River (several localities) and in H. g. gulonella from Beaver Creek, Arkansas River Drainage (KU 4769) are shown in Plate 24. Although values for the largest specimens of H. g. gracilis are omitted from Plate 24, the regression remains essentially linear to standard lengths of approximately 250 mm. On the basis of head-depth alone, separation of the two subspecies is possible in specimens larger than 40 mm. Similar results were obtained by using the regression of postorbital length on standard length, and could have been obtained by using other proportional measurements.

NATURAL HISTORY

Habitat

The species inhabits alkaline streams with shifting sand bottoms where the waterlevel fluctuates considerably with heavy rains and melting snow. The flathead chub is found in silty water and often is the predominant species in streams that have high turbidity. The remarkable ability of this fish to withstand exceedingly high turbidity is illustrated by its predominance in the Little Missouri River, which has an average concentration of suspended silt two and one-half times that of the Missouri River at Kansas City (Personius and Eddy, 1955:42).

H. g. gracilis is found in large rivers throughout its range, occa[pg 335]sionally migrating into smaller streams, especially in the spawning season. It prefers the main channel of rivers in moderate to strong current. 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. No brush or other debris was near the pools. In each case the streams carried little water, although they undoubtedly carry greater volumes of water in spring and early summer after rains and spring thaws. The preferred bottom-type for this subspecies seems to be gently shifting sand.

H. g. gulonella is found in warm-water streams, whereas H. g. gracilis occurs in cooler water. The southwestern subspecies was taken in August in the Mora River drainage at Sapello (temperatures above 80° F.) but not at Mora (temperatures below 70° F.). In the Purgatoire River, a thriving population was found where the water temperature was 92° F., on September 6, 1959. In the Arkansas and Pecos rivers and the Rio Grande this subspecies is most abundant below the mountainous parts of the stream-courses, but at elevations higher than 4,000 feet on the plains.

Associated Species

In the Pecos and Arkansas basins, species commonly taken with H. g. gulonella are Catostomus commersonnii, Hybognathus placita, [pg 337] Notropis lutrensis lutrensis, Notropis stramineus missuriensis, Pimophales promelas, and Campostoma anomalum plumbeum. The only spiny-rayed fishes that we have found with H. g. gulonella are Lepomis cyanellus and L. humilis, both of which are scarce. Associates of H. g. gracilis include the same species, plus other ostariophysan fishes such as species of Carpiodes, Ictiobus, and silt-adapted species of Hybopsis and Notropis.

We failed to find the flathead chub at any of 11 localities in the South Platte drainage, where we collected in September, 1959. Dr. George Baxter, of the Department of Zoology, University of Wyoming, told us that he has never found H. gracilis in that drainage. The fauna of the South Platte includes Catostomus catostomus, Semotilus atromaculatus, Hybopsis biguttata, Hybognathus hankinsoni, Notropis cornutus frontalis, Etheostoma nigrum and E. exile--species rarely if ever found with H. gracilis.

Ecologically, H. g. gulonella seems to be the counterpart of Semotilus atromaculatus in streams where the latter species is absent. Observations of H. g. gulonella in the Purgatoire River indicated [pg 338] that loosely-organized groups of flathead chubs congregated one to four inches above the bottom of pools, and near or under protective cover such as roots of vegetation or debris lodged against shore. Individuals moved about independently within the group (rather than as schools), and occasionally rose to the surface, perhaps for food.

Food

The flathead chub is chiefly carnivorous, but its food includes some aquatic vegetation (Table 1). 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.)."

Spawning Season

Specimens of H. g. gulonella that have been examined reach sexual maturity at approximately 65 mm standard length. Most specimens of H. g. gracilis less than 85 mm in standard length are immature, but larger specimens probably are mature.

The spawning season is in late summer, beginning in July and extending into September. Specimens from the Peace River, collected on August 10, 1952, include females that were mostly spent and tuberculate males. Males and females in spawning condition were collected in the Milk River in August of 1955. A large prespawning female was obtained in Red Deer River in June of 1952. A male from Fort McMurray had fairly well developed tubercles on August 9, 1955. A prespawning female was taken from the Saskatchewan River at Clarkboro Ferry on June 7, 1957. Tuberculate males were collected in the Powder River on June 30, 1957. Specimens from the White River in South Dakota, collected on July 7, 1934, include tuberculate males. The specimens discussed above are H. g. gracilis or intergrades tending toward that subspecies.

Specimens of H. g. gulonella collected in the Arkansas River at Pueblo and Florence, Colorado, on September 7, 1959, include some tuberculate males, although most females are spent. On August 8, 1957, a series of flathead chubs that includes tuberculate males was collected in the Redwater River, Montana. In the Pecos River on [pg 340] August 25, 1958, spawning seemingly had been completed, although a few males still bore tubercles.

[pg 339] Table 1. 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. Underhill (1959) reports that this species is rare in the Vermillion River, a northeastern tributary of the Missouri River, except in autumn when large numbers occur near the mouth of the river. We suspect that this is associated with spawning.

PLATE 21

PLATE 22

PLATE 23

PLATE 24

DISCUSSION

Hybopsis gracilis is highly variable in several morphological characteristics, including size and shape of head, body, and fins, and number of scales, vertebrae, and fin-rays. The variations are correlated in a way that indicates the existence of two subspecies. One of these, H. g. gracilis, attains large size, and has 1) a slender, streamlined body, 2) a depressed head that is acutely wedge-shaped in profile, 3) strongly falcate fins with the dorsal and pelvic fins originating anteriorly, and 4) many scales, vertebrae, and pectoral fin-rays. 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. Considerable variability was found in features other than those mentioned above, but individual variation among specimens from the same locality and adjacent localities is so great that none is diagnostic of subspecies. For example, orbital size and length of fins (but not their falcate shape) are variables that have little diagnostic value, although both features seem to vary in clinal fashion, with the higher values in the north.

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. 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. Hubbs (1922, 1926, 1941), Schultz (1927), Vladykov (1934), Tåning (1952) and Weisel (1955), among others, have pointed out a correlation between temperature (or developmental rate of fish) and the number of vertebrae, scales, and fin-rays. 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. To our knowledge, the most extreme differences that have been induced by modification of temperature are those reported for Salmo trutta by Tåning (1952:181-182), who states: "Shock treatment produced by especially great changes in temperature (c. 10-14° C), especially during the super-[pg 342]sensitive period [of somatic differentiation that fixes vertebral number] may produce ... a difference of 3-4 vertebrae ... in offspring of the same parents." The difference cited approximates that which distinguishes natural populations of H. g. gracilis and H. g. gulonella. Although we cannot assume that the sensitivity of the brown trout is the same as that of the flathead chub, the causative conditions in Tåning's study could scarcely be expected in nature; furthermore, it seems significant that extremely high (as well as extremely low) mean numbers of scales and vertebrae were found at southern localities, and that low mean numbers of scales and vertebrae were found as far north as Wyoming and Montana. 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.

Other environmental factors that may have selective influence in this species are rate of current, volume of flow, and turbidity. Interaction of these environmental factors could result in genetic fixation of morphological characters through natural selection. The characters that distinguish H. g. gracilis from H. g. gulonella seem adaptive to life in large rivers and small streams. Evidence that these characters are under limited, direct environmental influence is found among populations in the Arkansas River System. Although populations in the Arkansas River have no continuity with populations of H. g. gracilis, upstream-downstream variations like those found in other river systems are apparent, but in lesser degree. The direction of variation in the Arkansas River is the reverse of that in the Platte and other tributaries of the Missouri River. For example, the populations farthest upstream (Florence, Pueblo) have slightly higher mean numbers of lateral line scales than do populations from Kansas, downstream.

A remarkable effect of extreme parasitism in H. gracilis has been described by Hubbs (1927). Very young chubs that harbored numerous tapeworms (Proteocephalus) had unusually large numbers of lateral-line scales, large eyes, short snouts, small fins, small mouths lacking barbels, and coalescent nares (internarial bridge weak or absent). 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.

[pg 343] H. g. gracilis occurs in three separate river systems (Mackenzie, Saskatchewan, Missouri-Mississippi) from latitude 36° N to 66° N, and longitude 89° W to 123° W. H. g. gulonella exists as several seemingly-isolated populations in the upper parts of the Rio Grande, Pecos, South Canadian, Cimarron, Arkansas, Platte, and upper Missouri basins, from latitude 35° N to 48° N, and longitude 97° W to 100° W.

There is evidence of high mobility on the part of both subspecies, based on irregularity of their occurrence in certain localities. Many collections have been made in the Cimarron River in the vicinity of Kenton, Oklahoma, from 1925 to the present, but only one of these (in 1957) contained flathead chubs. Bait dealers who seine the South Canadian River in Dewey County, Oklahoma, have taken flathead chubs in abundance in some seasons, but not at all in others. Seasonal variation in abundance in the lower Vermillion River, South Dakota (Underhill, 1959:100) has been cited, and the number collected in the lower Kansas River near Lawrence has varied similarly. 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). Drainage connections between the Arkansas, Kansas and Platte river systems existed in Kansan and Nebraskan times (Frye and Leonard, 1952:189-190). Populations that have subsequently become isolated in those rivers could be accounted for in this way. Flathead chubs could have entered the Rio Grande-Pecos system by stream-capture from the Arkansas System, in northeastern New Mexico or southern Colorado. H. g. gracilis undoubtedly entered the Saskatchewan and Mackenzie basins from the upper Missouri Basin, following glacial retreat (Walters, 1955:347).

LITERATURE CITED

Agassiz, L.

Bailey, Reeve M.

Beckman, William C.

Bryant, Francis T.

Churchill, Edward P., and William H. Over.

Cleary, Robert E.

Clemens, W. A., A. MacDonald, H. McAllister, A. Mansfield and D. Rawson.

Cockerell, Theodora D. A.

Cope, E. D.

Cope, E. D., and H. C. Yarrow.

Cragin, F. W.

Cross, Frank B., Walter W. Dalquest and Leo Lewis.

Cuvier, M. le B^on and M. A. Valenciennes.

Dymond, J. R.

[pg 345] Eddy, Samuel.

Eigenmann, Carl H.

Ellis, Max M.

Evermann, Barton W., and Ulysses O. Cox.

Evermann, Barton Warren, and Edmund Lee Goldsborough.

Forbes, Stephen Alfred, and Robert Earl Richardson.

Frye, J. C., and A. B. Leonard.

Gilbert, Chas. H.

Gill, Theodore.

Gerard, Charles.

Graham, I. D.

Hankinson, Thomas L.

Harlan, James R., and Everett B. Speaker.

Harrison, Harry M., and Everett B. Speaker.

Hart, J. S.

[pg 346] Hinks, David.

Hubbs, Carl L.

Hubbs, Carl L., and Karl F. Lagler.

Hubbs, Carl L., and A. I. Ortenburger.

Jordan, David Starr.

Jordan, David Starr, and Barton Warren Evermann.

Jordan, David Starr, Barton Warren Evermann, and Howard Walton Clark.

Jordan, David Starr, and Charles H. Gilbert.

Jordan, David Starr, and Seth E. Meek.

[pg 347] Keleher, J. J.

Keleher, J. J., and B. Kooyman.

Koster, William J.

Lindsey, C. C.

Martin, W. R.

Meek, Seth E.

Miller, R. B., and M. J. Paetz.

Moore, George A.

O'Donnel, D. John.

Ortenburger, A. I., and Carl L. Hubbs.

Personius, Robert G., and Samuel Eddy.

Rawson, D. S.

Richardson, John.

[pg 348] Schultz, Leonard P.

Scott, W. B.

Shoemaker, Hurst H., Quentin H. Pickering and Leonard Durham.

Simon, James R.

Slastenenko, E. P.

Smith, C. Lavett.

Suckley, George.

Tåning, A. Vedel.

Thompson, Ernest Seton.

Underhill, James C.

Vladykov, Vadim D.

Walters, Vladimir.

Weisel, George F.

Wynne-Edwards, V. C.

Transmitted November 8, 1960.

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