A collage of Prognathodes species offered for the aquarium trade. Photo credit: Lemon TYK.
The genus Prognathodes is perhaps the most phylogenetically intractable of the long-nosed butterflyfishes. Unlike Chelmon and Forcipiger, Prognathodes is highly speciose, with a majority of its members having a great affinity for deep waters, far removed from the usual habitat of the former two genera. Its members are loosely scattered along the equatorial zone of the three major oceans, but attain their maximum development and diversity in the waters of the Atlantic. Despite their wide-ranging occurrence and morphologically handsome features, Prognathodes, on a whole, remains poorly known. With no formal systematic review, the genus is essentially a phylogenetic cipher. In addition, their predilection for deep-waters have resulted in a handful of scientifically undescribed species, of which, some will likely remain without a name for the foreseeable future due to the great difficulty in obtaining specimens. More than half the members adopt a lifestyle confined strictly to the dreary depths. Only few other chaetodontids display this habitat preference, and they are mainly from the genus Roa, in which Prognathodes is most closely related to. The remarkableness of this behavior can be appreciated through deep-sea submersibles and trawler fishing. Prognathodes guyanensis, for example, has been observed and filmed at depths exceeding 600ft, while P. guyotensis has been obtained from trawler nets in excess of 1000ft. These represent some of the deepest known records of any butterflyfish species.
A CO1 tree showing the strong genetic support of Prognathodes alongside Roa, sister to all Chaetodon. This tree was generated using genbank, courtesy of Joe Rowlett.
Morphologically, Prognathodes recalls that of Roa, and, indeed, molecular studies (Fessler & Wesneat, 2007) have shown the two genera to be sister clades, sister to all Chaetodon. Phylogenetically, Prognathodes still remains unresolved, and will likely remain as such until extensive samplings of the various species are carried out. In this regard, this article is in no way an attempt at resolving Prognathodes, for which, at this time (and with our current limitations), is impossible to say the least. It does, however, postulate the distribution and occurrence of this genus based on biogeography and shared morphological characteristics. Prognathodes is the only other genera apart from Chaetodon to display a fully circumtropical distribution. However, unlike Forcipiger, the larval duration of Prognathodes is not nearly as long, and the widespread nature of the genus is therefore not represented by a single species.… More:
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