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
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).
Figure 1. Graphic analysis of lateral line scales, pectoral rays and post-Weberian vertebrae in Hybopsis gracilis. In each symbol, horizontal line = range, vertical line = mean, open rectangle = one standard deviation on each side of mean, black rectangle = twice the standard error on each side of mean. Numbers to left of symbols = number of specimens examined from that locality; combined collections indicated by brackets. The dash-lines represent drainage patterns of rivers in which this species occurs.
H. g. gracilis is found in large rivers throughout its range, occasionally 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 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.