Chapter 2: Genus and Species Description and Distinguishing Characteristics
The genus Hypophthalmichthys Bleeker 1860 first appeared in a key without any included species. A type for the genus was established by subsequent designation (Bleeker 1863). The genus is valid as Hypophthalmichthys Bleeker 1860 in the family Cyprinidae (Eschmeyer 2003).
Two species of the genus Hypophthalmichthys—bighead and silver carps—were originally described as species of the genus Leuciscus. They were subsequently placed in the genus Hypophthalmichthys where they remained until Oshima (1919) described the genus Aristichthys for the bighead carp. Morphological characters used by Oshima (1919) to distinguish Aristichthys from Hypophthalmichthys included differences in gill raker morphology, position of the abdominal keel, and pharyngeal dentition. Recognition of the genus Aristichthys was not universal, which resulted in the bighead carp being variously placed in one of the two genera, Aristichthys or Hypophthalmichthys, until the late 1970s. Gosline (1978) reported that differences in the gas bladders between the two taxa (gas bladder being bi-lobed in Hypophthalmichthys and appearing tri-lobed in Aristichthys) were evidence of a deviation from common lineage, thus supporting separate genera. However, the gas bladder of Aristichthys and Hypophthalmichthys typically consists of two chambers. The confusion in this characteristic state is because of a constriction of the gas bladder of bighead carp, which is variously developed and has been erroneously interpreted as a third chamber (Howes 1981). The number of chambers in the gas bladder varies widely among unrelated groups of cyprinids and has no value in indicating a common ancestry (Howes 1981). A phylogenetic analysis conducted by Howes (1981) concluded that the species of bigheaded carp share unique derived morphological characteristics and consequently belong to the same genus, Hypophthalmichthys. A third species, the largescale silver carp H. harmandi was later described as a species of Hypophthalmichthys.