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.
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. How the genus managed to cross into the Atlantic and diversify to such an extent suggests a long history dating back to early divergence times. The presence of members in the Eastern Pacific, Indo-West Pacific and the Atlantic is consistent with the dispersal to the Atlantic either through the Panamanian isthmus prior to its closure, or around the Cape of Good Hope, at the southern tip of continental Africa.
Molecular rates resulted in the estimate that Prognathodes split away from Chaetodon approximately 24 mya (Fessler & Wesneat, 2007). The closure of the Panamanian isthmus occurred 12-15 mya, which would allow more than enough time for the members to cross into the Eastern Pacific. However, based on the biogeographic patterns of Prognathodes, it seems likely that the hub of diversification originated in the Atlantic, before radiating outwards and eventually forming species pairs that now reside in the Pacific and Indian Ocean. This is a similar pattern to the multiple lineages of labrid fishes that have Atlantic and Caribbean representatives (Wesneat and Alfaro, 2005). However, to fully prove this hypothesis, a larger and more complete sampling of Prognathodes has to be first procured.
As mentioned previously, the lack of specimens available for genetic analysis have resulted in the intractable nature of its phylogeny. To recreate a tree with such little information is impossible, and this article makes no attempt at doing so with any definitive outcome. In time to come, Prognathodes might even reveal itself to be polyphyletic. However, with the biogeography and shared morphological characteristics amongst the species, a rudimentary hypothetical tree can be postulated. In this article, we take a look at the various species and their biogeography.
Prognathodes marcellae, P. aya and P. guyanensis.
The three species in the aya clade are very similar in terms of appearance and biology. All are strictly Atlantic, distributed on the eastern and western coasts in moderate to fairly deep waters. The clade members are characterized by having an oblique stripe passing through the eye, and a laterally inverted band of similar shape on the posterior body. Hybridization is rare in Prognathodes, with only one naturally occurring hybrid documented to occur between P. aya and P. guyanensis.
It can be postulated that the evolution and speciation of the aya clade originated from the Eastern Atlantic along West Africa’s coast, moving west into the Caribbean. This pattern is thought to be similar to the angelfish genera Holacanthus, with H. africanus being P. marcellae’s counterpart here. Could the invasion and subsequent isolation into the Western Atlantic set the stage for the evolution of what we know today as P. aya and P. guyanensis?
In this handsome species, the ground color is silvery-white, heavily suffused with gold. This feature is quite variable, and specimens across the board range from scarcely yellow to highly xanthic. The eye is concealed in an oblique black stripe running from the first and second dorsal spines down to the base of the upper jaw. A similar stripe is present on the posterior, but in P. marcellae, this is vertical, and runs from the posterior dorsal fin to the anal fin. The fish is otherwise unmarked.
Prognathodes marcellae is common, but restricted to the West African coast, Cape Verde islands and São Tomé and Príncipe. It inhabits moderate to deep waters at depths ranging from 12 – 140m (33 – 300ft), where it is frequently observed swimming upside down or vertically oriented against drop offs. The species is replaced by the very similar P. aya in the Western Atlantic, and the two are presumably sister species, with the latter having evolved through the movement of Prognathodes westwards into the Caribbean.
P. marcellae is rare in the aquarium trade, and reasonably expensive when offered. It is, however, more frequently available in Asia, where demand for West African exports are high. The species is sensitive, but not overly difficult to keep when presented with suitable aquarium conditions.
The closely related Prognathodes aya is essentially similar in appearance to the former species with a few minor modifications. In P. aya, the body is always silvery-white with nary a hint of the usual gold suffusion. The yellow coloration is restricted only to the pelvic, dorsal and anal fins. In addition, the posterior body band is skewed in an oblique fashion, much unlike the vertical placement in P. marcellae. This band is a salient feature in Prognathodes, and will serve as a recurring theme throughout this article. It is very likely an important phenotypic characteristic when considering the evolution of Prognathodes.
The etymology behind this species is somewhat of an interesting story. Prognathodes aya was first known from the regurgitated stomach contents of a snapper, Lutjanus aya. The then undescribed butterflyfish was subsequently named aya as well, after the same lutjanid that spat it out. However, presently, Lutjanus aya is no longer a valid name, and is now a junior synonym for Lutjanus campechanus. The butterflyfish species, however, still keeps its original name, serving as a grim reminder of its unorthodox discovery.
Prognathodes aya is distributed in the Caribbean, north to the Gulf of Mexico and Southern Florida. It can be found as far north as North Carolina as stragglers, and possibly, although exceedingly rarely, further up the gulf stream as tropicals. This species inhabits deeper waters than P. marcellae, and can be found at depths from 50-200m (165-665 ft). It overlaps with P. guyanensis in the northern Caribbean, forming the only known hybrids in this genus. As mentioned in the previous segment, P. aya’s closeness to P. marcellae could be due to its movement westward from the African coast, subsequently diverging in the Americas.
P. aya is rare in the aquarium trade, where it commands prices in the low thousands. Its care requirement is similar to that of P. marcellae, and is generally hardy, provided they are presented with optimum water quality and care. A single specimen is on public display in the Steinhart Aquarium, California Academy of Sciences.
Prognathodes guyanensis is a magnificent species far more beautiful than the preceding two. The ground coloration is steely-silver, adorned with light brown striae arranged in horizontal stacks. The eye is concealed in the usual black stripe, and the posterior, in the same oblique band as with P. aya. However, an additional band is present at the edge of the dorsal fin. All its bands are margined in white, and they encroach significantly into the dorsal spines and anal fin. The pelvic fins are dark brownish-black.
P. guyanensis is presumably named after Guyana, or the French Guiana, both regions located on South America’s Atlantic coast. It is quite likely named after the latter, in accordance to its common name of French Butterflyfish. The species is distributed extensively throughout the Caribbean and much of South America’s tropical coastline. It is the deepest dwelling member of the aya clade, and can be found at depths exceeding 600ft along desolate looking slopes and rubble.
Despite P. guyanensis occupying a more southern distribution compared to P. aya, the two overlap marginally, producing very rare hybrids. The hybrids display intermediate phenotypes, but because both parental species are fairly similar in appearance, these can be hard to pick out. For example, the hybrid depicted below displays the yellow pelvic fin reminiscent of P. aya, as well as the horizontal body striae and faintly visible second posterior band of P. guyanensis.
P. guyanensis is very rare and very expensive in the trade. Its price is attributed to the infrequent collection of this species in Curaçao by means of an underwater submersible. The species is fairly easy to keep, but requires cooler water and preferably dim aquarium lights.
Prognathodes falcifer and P. carlhubbsi
This clade is home to two sister representatives, Prognathodes falcifer and the very similar P. carlhubbsi. Both species occupy the extreme Eastern Pacific, bordering the coastline of North and South America respectively. Phenotypically, these sisters greatly resemble the aya clade members, but with few modifications. Most apparent is the oblique posterior band making a mirrored ninety-degree extension at the base of the dorsal fin to form a distinct scythe mark. For this reason, these butterflyfishes are very aptly known as the Scythe Butterflyfishes.
The similarities and close geographical approximation to the aya clade members raises some important flags. Could these two clades represent a paired sister relationship? It would seem so, especially since they are clustered on the immediate sides of the Panamanian isthmus. Prior to its closure some 12-15 mya, these Prognathodes precursors more than likely swam together. The big question is, in which direction did the flow of species occurred?
Taking Holacanthus for example, H. passer, H. clarionensis and H. limbaughi form an Eastern Pacific clade originating from the Caribbean, and so Holacanthus displays a general westward movement across the isthmus before the closure. Did Prognathodes follow suit? Or did they assume the reverse.
Apart from that, the Scythe Butterflies are otherwise quintessential book fish, with less than five specimens of P. falcifer in private and public collections cumulatively, and zero specimens of P. carlhubbsi anywhere.
This dazzling species is one of the lesser known members of the genus. The ground coloration is pale cream, intensifying to yellow on the edges of its body and median fins. The usual eye stripe is present, but below the lower orbital limit, the stripe is nascent and very inconspicuous. The usual posterior oblique band is present, but this is modified such that is makes a sharp bend just shy of the dorsal fin base, before continuing downwards in a sickle, scythe like marking. The margins of the dorsal (with the exception of spines 3 and 4) and anal fins are black, and the pelvic fins are yellow.
P. falcifer is distributed along the west coast of tropical North America, along Southern California and the Baja California peninsular. It also reaches the islands of Catalina, Guadalupe and Revillagigedo. Like most Prognathodes species, it is primarily deepwater, and can be found at depths up to 150m (450ft). In Catalina Island, however, juveniles can be seen in waters as shallow as 10m (30ft).
Prognathodes falcifer is exceedingly rare in the aquarium trade, with no more than five in the entire world it seems. The Denver Aquarium in Colorado is the only place that houses a pair of this species on public display. It is replaced by P. carlhubbsi in the southern portion of the Eastern Pacific.
Prognathodes carlhubbsi is essentially identical to the preceding with the exception of three minor differences. 1) The eye stripe is complete and black below the lower orbital limit. In P. falcifer, this snout portion is yellowish-grey and inconspicuous. 2) The posterior back below the dorsal fin is grayish in P. carlhubbsi, but yellow in P. falcifer. 3) The black scythe mark touches the base of dorsal spines 3 and 4, but in P. falcifer, this reaches well beneath the spines. With such minor differences in coloration, genetic analysis would be needed to dispel all doubts of P. carlhubbsi’s specific status, although it would most likely keep it, seeing as the two species occupy allopatric distributions.
P. carlhubbsi is found on the South American coast of the Eastern Pacific, primarily in the Galapagos Islands (Ecuador). There are records of this species from Malpelo Island (Colombia) and Cocos Island (Costa Rica). It can be found at depths ranging from 12 to 270m (40-885 ft).
P. carlhubbsi is even rarer than P. falcifer, with zero specimens offered for private and/or public displays. The species will more than likely retain its book fish status for the foreseeable future.
Prognathodes aculeatus and P. brasiliensis
The aculeatus clade is remarkably unusual with regards to two salient features. 1) The members completely lack any of the usual bands associated with the Atlantic Ocean members, and 2) they have entirely overlapping distributions with the aya clade. How these evolve and speciated in close sympatry is unknown. Perhaps they diversified based on depth preferences. Indeed, the aculeatus clade features the shallowest members of the genus, and both can be found at depths of 15m in warm, tropical waters. And, although also noted to occur in the lower depth limits of about 100m (300ft), they are rare, and remain commonest between 15-55m (30-90 ft).
This clade is home to two members, one with a wide distribution in the Caribbean, while the other is restricted to the Brazilian coastline. Unlike the other fancier species, the aculeatus clade members are rather somber, where, coupled with their predilection for shallow waters, have allowed for their proliferation in the aquarium scene.
Prognathodes aculeatus is a modest, fairly unappealing species. The fish is light tan, fading to a silver wash ventrally. It darkens to a burnt orangey-brown towards the back, particularly along the dorsal fin spines and rays. The usual eye stripe is present, but is variable, but usually weakly expressed (although this can intensify depending on specimens) in orange. The rest of its body is relatively unmarked, and the anal and pelvic fins are light chartreuse.
Prognathodes aculeatus is curious with regards to its etymology. The specific epithet “aculeatus” translates to “long-snout”, but this is in no way a defining characteristic for the species, as all members of the genus have fairly elongated rostrums. It is widely dispersed in the Caribbean and southern United States, where it can be found in Southern Florida, the Gulf of Mexico, south to the Bahamas and the northern reaches of South America. It completely overlaps with Prognathodes aya and P. guyanensis, but fails to form any hybrids.
P. aculeatus is fond of shallow waters, although it (rarely) penetrates depths exceeding 100ft. It’s preference for shallow water could be a reason for its evolution away from the aya clade members, explaining the close sympatric nature of the various species here. This is the commonest member of the genus to be offered in the aquarium hobby, and likewise also the most affordable. P. aculeatus is fairly easy to maintain in captivity, and because of its habitat preference, is more tolerant of warm water and bright artificial lighting.
This is another lesser-known species that was only described in 2001. Prognathodes brasiliensis follows a similar appearance to P. aculeatus, but is bright chartreuse instead of tan. It also lacks the darkening on the posterior body and dorsal fin. Unlike P. aculeatus, however, P. brasiliensis is shaded rather extensively in a fuliginous grayish-black on the nape. Its usual eye stripe, although present, is subsequently concealed behind this quasi-mask like pattern.
P. brasiliensis is the southern counterpart of P. aculeatus, and, as its name suggests, is found primarily in Brazil. It appears to be endemic, and has a restricted distribution along the Brazilian coastline. Its habitat and depth preference remains otherwise similar to the preceding species.
P. brasiliensis used to be fairly uncommon and affordable in the trade, but has fallen off the radar in recent years with regards to its availability. Like many Brazilian endemics, it has since become very rare and difficult to obtain. Despite its narrow distribution, the species is reportedly common in the wild. It is curious to note that this species bears an uncanny resemblance to Forcipiger, although this is likely nothing more than a happy coincidence.
Prognathodes obliquus and P. dichrous
Prognathodes obliquus and P. dichrous make up the only two members here, where both are equally curious with respects to their highly bizarre and aberrant phenotypes. This, coupled with their extremely restricted range, suggests some kind of genetic bottleneck that resulted in their remarkably atypical form today. This of course, begs the question. Where and how did these two butterflyfishes originate?
The obliquus clades reside in the Central Atlantic, approximately equal distances between the Eastern and Western Atlantic of the Western African coast and the Americas respectively. It is, therefore, difficult to say which direction is took to reach its current distribution range.
In closer examination of P. obliquus, certain unifying characteristics from the aya clade can be seen. 1) The body scales are rather prominently defined, giving the appearance of horizontal rows. This is comparable to those seen in P. guyanensis and P. aya. 2) The oblique posterior dorsal saddle is essentially a modified version of the usual band seen in the aya group members. Closer inspection of P. obliquus’ posterior dorsum saddle reveals a linear delimitation, but, because of the extensive white shading, is often poorly recognized.
P. obliquus, although highly restricted in its range, is found not far off the Brazilian coast. Could this be close enough for P. guyanensis to reach? Possibly. Is this plausible? While we postulate the movement of the aya clade to follow a general westward direction across the Atlantic, the presence of a central Atlantic gyre could indicate movement in a reverse direction, seeding a small population in the Saint Paul archipelago where P. obliquus currently resides.
Its extremely restricted range, low population density and severely aberrated phenotype are in line with the effects of genetic bottlenecking. This is seen in the local and sympatric population of Holacanthus ciliaris, where specimens often display bizarre koi-like coloration. Another example of genetic bottlenecking can be seen in the Rowley Shoal endemic Conniella apterygia, a species superficially similar to Cirrhilabrus, but lacking pelvic fins and all associated structures completely.
Even more bizarre still is Prognathodes dichrous, which, unlike P. obliquus, completely lacks all discernible traits of Prognathodes. The oblique back is now further distorted to form a rectangular saddle. Like P. obliquus, however, it is also equally isolated and far flung, being found in the St. Helena and Ascension Island groups. While we question the origin of this clade as a whole, there is little doubt that both members are immediately related to each other. P. dichrous could likely be a result of further isolation and genetic discontinuation of P. obliquus, resulting in an almost unrecognizable species within the genus Prognathodes.
This, of course, is entirely speculative, and until extensive molecular studies can be carried out, it will likely remain as such.
Prognathodes obliquus is immediately recognizable for its bizarre and unique appearance. The ground coloration is chocolate brown, with scales edged prominently in distinct transverse rows. The usual eye stripe is completely absent, and the posterior oblique band is modified to form an extensive saddle in silvery-white. This dorsal saddle is delimited at the base in a thick, poorly distinguishable band, much like those seen in P. aya and P. guyanensis. In addition to these odd features, P. obliquus has a highly reflective and prominent lateral line. The tubular lateral line scales are iridescent silver, and, together with the similarly colored eye, give the fish a very mercuric, extra-terrestrial appearance.
P. obliquus is restricted to a single archipelago in the Central Atlantic. The St. Paul’s archipelago is home to a bizarre collection of fish species, and how P. obliquus might have reached here and subsequently evolved is discussed in the aforementioned introduction to this clade.
The availability of this species in the aquarium trade predates the modern reef-keeping era. This species was rarely available back in the late 20th century, but has since disappeared with the discontinuation of exports from St. Paul’s archipelago. Prognathodes obliquus has since attained book-fish status once again, and it is unlikely to relinquish its title anytime soon.
Prognathodes dichrous is a true oddity in the genus. It shares a similar ground coloration and general pattern to the preceding species, but with few minor modifications. Instead of the oblique saddle, P. dichrous has, on its posterior dorsum, a roughly rectangular wedge of silvery-white scales. In very old specimens, this tarnishes to a light tan. The iris and lateral lines scales are quicksilver, and in the correct incidence of light, may appear completely lustrous. The specific epithet “dichrous” is latin for “bicolor”, and while although not inaccurate, is quite a lacklustre description for a fish that exudes so many other unique qualities.
Like P. obliquus, P. dichrous has a restricted range in the Central Atlantic, although with a slightly larger distribution. It can be found in Saint Helena and Ascension Island, with scattered populations in-between the two archipelagos. The latter was only recently documented, and prior to this, its disjunct biogeography was somewhat of a mystery. P. dichrous is common but very localized in St. Helena and Ascension, where it inhabits ledges and vertical walls replete with overhangs and cracks in shallow water. Between these two major landmasses, P. dichrous assumes the more typical deepwater behavior, especially in Grattan Seamount, as well as the other guyots and seamounts that connect the two regions. It is perhaps for this reason that P. dichrous was only discovered to occur between St. Helena and Ascension at a much later date. Its biogeography is now no longer disjunct.
The lack of any noticeable traits pertaining to Prognathodes is highly evident in P. dichrous. The usual eye stripe, oblique or even vertical bands are absent in this species. While the bizarreness of this fish is perhaps equivocal to P. obliquus, the resemblance to the other Atlantic Prognathodes is still very weakly evident in the latter. It’s quite likely therefore that P. dichrous arose from the further isolation and bottlenecking of its ancestral lineage in the St. Helena and Ascension Island groups. As mentioned previously, a thorough molecular analysis is absolutely needed to put these hypotheses to the test.
Like P. obliquus, Prognathodes dichrous is now no longer available to the aquarium trade. This species is another icon that predates the modern reef-keeping era. It made its appearance quite regularly with Ascension Island exports, along with the likes of Centropyge resplendens and Chaetodon sanctaehelenae. Because the region now no longer offers legal export of fish, this species is now completely unobtainable to the hobbyists of the current day.
It joins the likes of Centropyge hotumatua, Centropyge resplendens, Chaetodon smithi, Chaetodon sanctaehelenae and Prognathodes obliquus in a special collection of unavailable species that were once fairly abundant in the trade. Somewhere between two to five (perhaps even less) individuals persists in private collections today, and these are fish that were collected from the pioneering batch at Ascension, making them more than two decades old.
Prognathodes guezei, Prognathodes sp. and Prognathodes guyotensis.
Members occupying the Indo-Pacific represent this clade, and, unlike their Atlantic and Eastern Pacific counterparts, are shrouded in a suffocating miasma of uncertainty and mystery. This clade is home to some of the least known and most enigmatic of all the chaetodontids, where, despite their immense biogeography, are practically unknown outside of their initial discovery. The members here are noteworthy for being adorned with vertical bands, instead of the usual oblique ones that are ever so familiar with the Atlantic representatives. The only exception here lies with Prognathodes guyotensis, a remarkable species that is so poorly known and so seldom seen, that it truly is one of the last great butterflyfish awaiting rediscovery.
The biogeography of this clade encompasses much of the Indian Ocean. On the Western Pacific side, records are sketchy with sparsely scattered reports, mostly centered around Palau and Indonesia. The Hawaiian archipelago is home to an endemic species, which, although known for over a decade now, is yet to be given an official name. Ironically, it is perhaps the most well known and well documented of the Indo-Pacific Prognathodes.
Strangely enough, the genus is absent from the Red Sea, Micronesia, Polynesia and much of Melanesia. This, undoubtedly, can only be attributed to inadequate sampling due to their penchant for extremely deepwater. Curiously, the genus Roa follows this puzzling absence in the Western Pacific. Again, this is almost surely due to the difficultly in reaching suitable habitats, and there is very little doubt that more discoveries will be made in the future.
Currently, this clade is represented by three “species”, although this is almost certainly an underestimate. It is quite likely that the Indian and Pacific Ocean representatives may be specifically distinct as well.
Prognathodes guezei is an enigmatic species that almost certainly represents a species complex throughout its range. The nominate representative is found in the Indian Ocean, specifically in South Africa and the immediate surroundings. This handsome fish is cream colored with two thick vertical black bands. These stripes are blockish, tapering in a gentle “S” shape toward the ventral portion of the fish. The eye is marked in the usual fashion, concealed behind an oblique stripe. The pelvic fins are yellowish, and the dorsal and anal fin margins are bordered in the same color. However, because few photos (all in low quality) exist, the subtle nuances in color should not be taken as a definitive representation.
P. guezei has only been recorded within the vicinity of South Africa in very deep waters, often in excess of 100m (300ft). It can also be found in Madagascar and Reunion. A single record in the form of an underwater photograph was taken at 82m in Bali. This, although identified as Prognathodes guezei, is quite likely a separate species.
Another similar banded Prognathodes was filmed and collected in the mesophotic twilight zone of Palau. The footage was released in a segment of the documentary series titled “Pacific Abyss”, where Richard Pyle, Brian Greene and other notable scientists surveyed and documented the ichthyological fauna of the twilight zone. The Prognathodes was seen swimming in a calcareous matrix, alongside Centropyge abei and Chromis abyssus. This was the first time any of these fishes have been filmed alive in situ. Again, based on the disparate biogeography and phenotype, it is unlikely that these are the same species as P. guezei. To elucidate this conundrum, more specimens from each location have to be first obtained – a task most onerous.
Based on these few photographic records, it would appear that there are at least two phenotypes masquerading under the Prognathodes guezei guise. Apart from Bali and Palau, exactly how far the distribution pattern stretches is currently unknown. The same representative should be looked out for in adjacent Micronesia. Until then, the Prognathodes guezei species complex is plagued with many unanswered questions.
Moving northeast to the Hawaiian Archipelago, another banded Prognathodes is to be found in the same habitats. The species is in the process of being described as new.
Prognathodes sp. “basabei”
This fish is marked in the same fashion as the other vertically banded Prognathodes, but with a few subtle differences. 1) The bands are curved only very slightly ventrally, and are otherwise uniform in width. 2) The edges of the dorsal, anal and pelvic fins are deep orange, lending the common name of Orange-margined Butterflyfish to be quite fitting. It possesses the usual black eye stripe, but the lower half touching the rostrum is orange.
Although still undescribed, the fish has been known for quite some time now as Prognathodes “basabei”. The specific epithet is named after Pete Basabe who assisted in collecting the first specimens. Prognathodes sp. “basabei” is endemic to the Hawaiian Archipelago, west to Pearl and Hermes atoll. In all but the latter location of Northwestern Hawaii, it is to be found only in depths exceeding 300ft, often in pairs, and in the company of the Hawaiian endemic Roa excelsa. The Hawaiian region is well known for harboring a rich collection of endemic fish, and so based on this biogeographic pattern, it is quite justifiable to assign this phenotype as a new species. In the cooler waters of Northwestern Hawaii, it can be found in significantly shallower water, often at around 200-250ft. This phenomenon can be observed with other deepwater species there as well, such as Genicanthus personatus, Bodianus sanguineus, Odontanthias fuscipinnis and Chromis struhsakeri.
Because of the extensive rebreather diving and survey in Hawaii, P. sp. “basabei” is very well documented. It has even been collected with some regularity for the aquarium trade. This is the sole representative of the genus this side of the globe to be offered to hobbyists, but, unlike the Atlantic Ocean counterparts, it is reported to be extremely difficult to keep.
The occurrence of these banded Prognathodes in the Western Pacific and Hawaii is strongly suggestive of an undiscovered population in Micronesia and the rest of the Central Pacific. In comparison of these banded species in the Indo-Pacific to their Atlantic counterparts, it seems remarkable that they are even closely related. Like Centropyge, could Prognathodes be polyphyletic instead? It wouldn’t seem like a farfetched idea, but without extensive molecular analysis, we wouldn’t know.
And finally, we have Prognathodes guyotensis. This is perhaps the most enigmatic and elusive member of the entire butterflyfish family, with no more than five specimens known only from deepwater trawls and submersible videos. This species was first known in 1982 on the basis of three specimens collected from a trawling operation on the top of a guyot at 342m (>1000ft) located in the Kyushu-Palau Ridge, on the Philippine plate (Nalbant, 1991). It was subsequently described by Yamamoto and Tameka as Chaetodon guyotensis, the specific epithet alluding to the guyot from which it came.
Six years later in 1988, Randall and DeBruin obtained a fourth specimen off of Maldives, and subsequently moved the species into the genus Prognathodes. Nalbant, on the basis of morphology and meristics, disagreed with this generic placement, and erected a new genus Peterscottia for guyotensis. He also considered the Maldivian representative to be allopatric and specifically distinct from the Pacific Ocean representative, and alluded the Indian Ocean specimen to be of a separate species. Today, Peterscottia is a junior synonym, and guyotensis remains in Prognathodes; although this is quite likely subjected to future changes.
The appearance of this fish is highly unusual. The body is yellowish-white with a pair of parallel oblique bands running on the posterior dorsum. The eye stripe is very unique, being completely vertical instead of the usual slanted nature originating from the first and second dorsal spines in many Prognathodes. This, coupled with its significantly shorter snout, reduced lateral line scales and general morphology, has led Nalbant to erect a new generic placement for guyotensis. He also placed Peterscottia phyletically alongside Roaops and Prognathodes; in this regard, the morphology and color pattern is highly reminiscent of the former.
With so little specimens and no genetic information on P. guyotensis, it is not possible to draw any useful comparisons to the rest of the species. Its placement within Prognathodes is contentious at best. How extensively it spreads throughout the Indo-Pacific is also not entirely known. With no more than five specimens from both Ocean basins, this is clearly one of the most elusive member of the family yet.
And so Prognathodes, in all its charm, dapper and handsome qualities, remains as much of a mystery. There is certainly a need for extensive molecular and morphological comparisons, no doubt eventually shedding some light on this phylogenetically puzzling genus. Perhaps in time to come, and with more deepwater explorations being carried out, the mysteries of the genus will finally be elucidated.
Nalbant T. T., 1991. Studies on chaetodont fishes IV. A new deep-water butterflyfish genus from the Indo-West-Pacific (Pisces: Perciformes, Chaetodontidae). Trav. Mus. Hist. nat. << Grigore Antipa >>, 31: 421-426.
Fessler J. L., Westneat M. W., 2007. Molecular phylogenetics of the butterflyfishes (Chaetodontidae): Taxonomy and biogeography of a global coral reef fish family. Molecular phylogenetics and evolution 45 (2007) 50-68.