Resolving Taxonomy and Historic Distribution for Conservation of Rare Great Plains Fishes: Hybognathus (Teleostei: Cyprinidae) in Eastern Colorado Basins

Similar morphology and confused historical taxonomy of Hybognathus hankinsoni (brassy minnow) and Hybognathus placitus (plains minnow) have made determination of their historic distributions and conservation status unclear in eastern Colorado basins. We developed logistic regression models from morphometric measurements to predict species identity of Hybognathus collections from Colorado and adjacent counties (n = 1154 specimens in 134 lots). A model based on orbit diameter, standard length, and eye position correctly predicted 98% of the specimens examined and 100% of the museum lots. Hybognathus hankinsoni have larger eyes centered on a horizontal line through the tip of the snout, whereas H. placitus have smaller eyes centered above the tip of the snout. The two species were historically sympatric in the Platte, Republican, and Smoky Hill River basins, whereas H. placitus was allopatric in the Arkansas River basin. The taxonomic characters defined here will allow accurate identification of future collections to determine the status of these native fishes.

Fig. 1.

Eye position characteristic for Hybognathus hankinsoni (A) and Hybognathus placitus (B). Hybognathus hankinsoni typically have larger eyes with the centers even with a horizontal line drawn back from the anteriormost tip of the snout. Hybognathus placitus have smaller eyes with the centers often above this line. Images are by R. E. Zuellig, with permission

Fig. 2.

Total number of scale radii (faint, incomplete, and complete) as a function of standard length for Hybognathus hankinsoni (filled squares) and Hybognathus placitus (open circles). Regression lines are shown for each species. Horizontal dashed lines indicate commonly used criteria for separating the two species. Hybognathus hankinsoni was reported to have 17 to 19 radii in the original full description (Bailey, 1954) and about 20 radii in subsequent keys. Hybognathus placitus was described as having 10 to 15 radii in most keys

Fig. 3.

Orbit diameter as a function of standard length for the 132 known specimens. Solid lines show regression lines for Hybognathus hankinsoni (r² = 0.88, P < 0.001) and Hybognathus placitus (r² = 0.86, P < 0.001). Dashed line is the 50% probability line (y = 0.0431x + 0.8951) predicted from logistic regression. Fish represented by points above the line are predicted to be H. hankinsoni, whereas those below are predicted to be H. placitus

Fig. 4.

Orbit diameter as a function of standard length for the 1154 unknown specimens with even eye position (A) and above eye position (B). Decision lines show where the probability of a specimen being Hybognathus hankinsoni is predicted to be 50%. In A, the thin line shows the prediction for the original two-variable model based on 132 known specimens (see Fig. 3), and the thick line shows the prediction for fish with the even eye position for the final three-variable model based on 1154 verified unknowns (y = 0.0431x + 0.6119). In B, the thick line shows the prediction from the final three-variable model for fish with the above eye position (y = 0.0431x + 0.9019), whereas the thin line shows the prediction for fish with the even eye position for comparison (same as thick line in Fig. 4A)

Fig. 5.

Historical distribution of Hybognathus hankinsoni and Hybognathus placitus in Colorado and adjacent counties in neighboring states based on verified museum collections (Scheurer, 2002). Closed circles indicate H. hankinsoni and open circles H. placitus

Received: 29 Jan 2002

Accepted: 03 Aug 2002