Comparison of a Pharyngeal Filtering Apparatus in Seven Species of the Herbivorous Cyprinid Genus, Hybognathus (Pisces: Cyprinidae)

In all seven species of the herbivorous cyprinid genus Hybognathus, pharyngeal taste buds or papillae are arranged in a pattern that suggests a filtering apparatus for trapping diatoms and other small food items. The dorsal pharynx has a patch of small papillae that grades smoothly into larger, posterior buds. Ventrally, prominent transverse ridges cross the first, second, and third gill arches, joining anterior gill rakers to those on the posterior border of the arch. Proximally, the ridges merge into V-shaped flaps covering the basibranchials. The largest of these, termed the major comb, is associated with the first gill arch, and typically bears marginal and surface papillae. The minor comb, associated with the second arch, is smaller and bears marginal papillae. Proximal gill rakers of the second arch, located between the major and minor combs, have rows of papillae, and merge into a central, random patch of papillae. None of the 40 other nearctic cyprinid species surveyed, including Dionda episcopa, D. diaboli, and D. erimyzonops, has comparable pharyngeal modifications. Six counted variables and one measurement, major comb width, were used to make intra- and interspecific comparisons. Hybognathus hankinsoni has consistently low values for counted variables. Hybognathus nuchalis has high values for counted variables and is most similar to H. argyritis. Hybognathus placitus is most similar to H. amarus. Hybognathus hayi is the most distinct species in the genus, but is similar to H. regius in two variables including major comb width. Discriminant analysis classified 92.02% of specimens correctly, indicating that the variation observed in these structures is probably species-specific.