Prognathodes basabei, a new species of butterflyfish (Perciformes, Chaetodontidae) from the Hawaiian Archipelago

Abstract A new species of the butterflyfish genus Prognathodes is described from specimens collected at a depth of 55–61 m off Pearl and Hermes Atoll, Northwestern Hawaiian Islands. This species has been observed by mixed-gas divers and from submersibles at depths ranging from 45–187 m throughout the Hawaiian Archipelago, with shallower sightings in the Northwestern Hawaiian Islands and deeper in the Main Hawaiian Islands. It is similar to Prognathodes guezei (Maugé and Bauchot 1976) from the western Indian Ocean, and at least one other undescribed species of Prognathodes from Palau, differing from these species in the number of soft dorsal-fin rays, size of head, and body depth. There are also differences in the life color, and a substantial genetic difference from the Palauan species (d » .08 in mtDNA cytochrome oxidase I).


Introduction
The genus Prognathodes Gill, 1862 (type species Chelmo pelta Günther, 1860= Chaetodon aculeatus Poey, 1860 currently includes eleven valid species: seven from the Atlantic, two from the tropical eastern Pacific, and two from the Indian Ocean and western Pacific. Pyle and Chave (1994) first reported the presence of a species of this genus in the Hawaiian Islands at depths of 106-187 m, based on video and observations from research submersibles operated by the Hawaii Undersea Research Laboratory (HURL). They noted its similarity to P. guezei (Mauge and Bauchot 1976), a species then known only from the two type specimens collected at a depth of 80 m off Réunion Island in the western Indian Ocean.
While conducting an exploratory dive using a mixed-gas closed-circuit rebreather off the south shore of O'ahu (Main Hawaiian Islands) on 17 May 1998, the senior author (RLP) observed (but was unable to collect) a group of three Prognathodes near an undercut limestone ledge at a depth of 114 m. Two weeks later (30 May 1998), with the assistance of Peter K. Basabe, RLP collected the first specimen of this species at a depth of 120 m near Kealakekua Bay on the Kona coast of the island of Hawai'i (TenBruggencate 1998). The following day he collected several more individuals at a depth of 115 m near the site of the observation of 17 May (Allen et al. 1998). All of the collected individuals were brought to the surface alive and maintained in captivity. Unfortunately, when they eventually died, only one was preserved, and it was too badly deteriorated to serve as a type specimen.
In the years that followed, several more individuals of this species were collected from depths of 115-125 m around O'ahu by mixed-gas rebreather divers. All were maintained in aquaria until their deaths, but none were properly preserved as suitable type specimens. On 27 April 2007, RLP collected two individuals of a similar species of Prognathodes at a depth of 116 m at Palau (Republic of Belau), in the western Pacific. Although some color differences between the Palauan and Hawaiian fishes were noted, the authors felt it was necessary to obtain specimens of the Hawaiian population for comparison of morphological and genetic characters before determining whether they are the same species.
In August of 2009, after the establishment of the Papahānaumokuākea Marine National Monument in the Northwestern Hawaiian Islands, the U.S. National Oceanic and Atmospheric Administration (NOAA) began a series of annual surveys of Mesophotic Coral Ecosystems (MCEs) within the Monument using mixed-gas diving technology. During the first of these surveys, the authors collected a group of three individuals of the unidentified Prognathodes at a depth of 61 m off the SW side of Pearl and Hermes Atoll. Unfortunately, tissue samples taken from these specimens were misplaced, so it was not possible to make genetic comparisons with the Palauan population.
Finally, in September of 2015, the authors were able to collect three more specimens at the same site off Pearl and Hermes Atoll where the three specimens had been collected in 2009, and obtain additional tissue samples for genetic analyses. Based on an examination of both morphological and genetic characters of the six Hawaiian specimens, as well as comparisons with the two specimens from Palau and Prognathodes guezei, we can now confirm that the Hawaiian population represents a new species, distinct from both P. guezei and the undescribed Palauan species. We herein describe the Hawaiian population as the new species, Prognathodes basabei.

Methods
Specimens were collected with hand nets during deep dives using mixed-gas, closedcircuit rebreathers. Additional observations, videos and images of this species were made from the two Pisces submersibles operated by the Hawaii Undersea Research Laboratory (HURL, at the University of Hawai'i), and by mixed-gas rebreather divers in the Main Hawaiian Islands and Northwestern Hawaiian Islands.
Standard length (SL) was measured from the tip of the snout to the caudal-fin base. Total length (TL) was measured from the tip of the snout to the posterior edge of the caudal fin. Head length was measured from the tip of the snout to the posteriormost edge of the fleshy flap near the upper end of the gill opening. Body depth is the greatest depth of body measured as a vertical from the ventral edge of the abdomen to the upper edge of scaled fleshy sheath of the dorsal fin (typically from about fourth or fifth dorsal spine). Width of the body is the maximum width. Snout length is the distance from the tip of the snout to the closest point on the bony orbit. Predorsal length is the distance from the tip of the snout to the angle formed by the scaled fleshy sheath at the insertion point of the first dorsal-fin spine, when erected. Preanal length is the distance from the tip of the snout to angle formed by the scaled fleshy sheath at the insertion point of the first anal-fin spine, when erected. The base of the dorsal fin is measured from the extreme base of the first dorsal-fin spine to the extreme base of the last dorsal-fin soft ray. The base of the anal fin is measured from the extreme base of the first anal-fin spine to the extreme base of the last anal-fin soft ray. Orbit diameter is the maximum diameter of the bony orbit. Interorbital width is the width of the bony interorbital space. Depth of the caudal peduncle is the least depth. Pelvic-fin spine length was measured from the extreme base of the pelvic-fin spine to its distal tip. Pelvic fin length was only measured on specimens with intact filamentous extensions of the first pelvic-fin soft ray, and represents the length of that ray from its extreme base to the distal tip of the filamentous extension. Length of spines and soft rays of dorsal and anal fins were measured from the extreme base to the most distal tip. Caudal fin length was defined as the difference between TL and SL. Pectoral fin length was measured as the longest fin ray, from its extreme base to its distal tip.
The last dorsal-and anal-fin soft rays are branched to the base and were counted as a single ray. Caudal-fin ray counts include small unsegmented and rudimentary rays. Pectoral-fin ray counts include first and last unsegmented rays. Pored lateral-line scale counts include only those scales with pores. Scale-row counts above and below lateral line to origins of dorsal and anal fins (respectively) include small truncate scales at bases of respective fins. Vertebral counts include the first vertebra fused to the skull, and the last vertebra fused to the hypural plate.
All counts and measurements except vertebrae were made directly from specimens. Measurements were made using dial calipers with +/-0.05 mm precision. Lengths of dorsal-and anal-fin spines and soft rays were made with the aid of a bright light transmitted from behind the fins to reveal the position of their extreme bases. Vertebral counts were made from x-radiographs.
Head length, depth of body, width of body, snout length, predorsal length, preanal length, length of dorsal-fin and anal-fin bases, orbit diameter, interorbital width, caudal peduncle depth, and lengths of fin spines and rays are expressed as percent of SL. Counts and measurements for paratypes, if different from the holotype, are presented in parentheses after the value for the holotype.
The holotype and three paratypes have been deposited at the Bernice Pauahi Bishop Museum fish collection, Honolulu (BPBM), and paratypes have been deposited at the California Academy of Sciences fish collection, San Francisco (CAS), and the U.S. National Museum of Natural History, Washington, D.C. (USNM).
Tissue samples were obtained from the holotype and two paratypes (CAS 242132 and USNM 440272). DNA barcodes (cytochrome c oxidase I; COI) were sequenced following the protocol described in Copus et al. (2015). GenBank accession numbers and Barcode of Life Database (BOLD) identifiers for DNA sequences are presented along with museum catalog numbers for type material and non-type specimens.  Smith et al. 2003) distinguished by the following combination of characters: dorsal-fin soft rays 21 or 22; anal-fin soft rays 16 or 17; head 2.63-2.80 in SL; body depth 1.58-1.69 in SL; pelvic-fin spine length 3.63-4.07 in SL; color in life pale yellow dorsally fading to white ventrally (sometimes entirely white) with three black bands with narrow white margins on each side of the body, the first band originating at and including the first dorsal-fin spine, extending diagonally to the eye and continuing horizontally as an orangish brown stripe from the eye to the tip of the snout, the second band originating at and including the fourth to sixth dorsal-fin spines, extending vertically at a slightly posterior angle to the ventral surface of the abdomen just anterior to the anus, tapering slightly and curving slightly posteriorly below the pectoral fin, and the third band originating at and including the last four to five dorsal-fin spines and first four to five dorsal-fin soft rays, extending vertically at a slightly posterior angle to and including the first several anal-fin soft rays, a narrow orange band on the dorso-posterior margin of the operculum, extending ventrally the posterior angle of the operculum, an oblong orange spot with some dark pigmentation on the upper one-third of the pectoral-fin axis, pelvic fins white on the spine and anterior one-third of fin, and bright orange on the posterior two-thirds of fin, a bright orange submarginal band with narrow white posterior margin extending along the posterior margins of the soft portions of the dorsal and anal fins, and continuing across the caudal peduncle.
Mouth small, the upper jaw 2.77 in head, slightly diagonal, the gape forming an angle of about 20° to the horizontal, the upper jaw slightly protruding; teeth in jaws densely setiform, the longest 7.8 in orbit diameter; nostrils anterior to the eye horizontally in line with the top of the iris, the anterior in a short membranous tube with a well-developed posterior flap, the posterior slightly larger, ovate, with a low fleshy rim. Lower edge of lacrimal smooth; margin of preopercle finely serrate; margins of other opercular bones smooth.
Lateral line forming a broad arc, ending below the base of the third to fifth soft dorsal ray and within the second black band on the body. Scales ctenoid, moderately large on body except for chest and near origins of dorsal and anal fins, where small; head fully scaled except anterior portions of both jaws and around nostrils, the scales on the head small; scales on fleshy sheath surrounding base of dorsal and anal fins moderately large anteriorly and proximally, reducing in size posteriorly and distally; scales on caudal peduncle and covering base of caudal fin small.
Origin of dorsal fin slightly posterior to upper end of gill opening, its base 1.45 (1.43-1.52) in SL; first dorsal-fin spine the shortest, its length 3.09 (2.49-3.68) in head; second dorsal-fin spine 1.27 (1.42-1.98, broken in one paratype) in head; third dorsal-fin spine the longest, its length 0.93 (0.94-1.11, broken in one paratype) in head; fourth dorsal-fin spine nearly as long as the third, its length 1.04 (1.03-1.13, broken in one paratype, deformed in one paratype) in head; fifth dorsal-fin spine shorter, its length 1.20 (1.14-1.23, broken in one paratype) in head; dorsal-fin spines progressively shorter posteriorly, the last 1.80 (1.80-2.06) in head; membranes between anterior dorsal-fin spines deeply incised, progressively less so posteriorly; first dorsal-fin soft ray the longest, approximately the same length as the last dorsal-fin spine, 1.80 (1.70-1.88) in head, dorsal-fin soft rays progressively shorter posteriorly; first anal-fin spine the shortest, its length 2.54 (2.44-3.10) in head; second anal-fin spine the longest, its length 1.31 (1.16-1.31) in head; third anal-fin spine 1.52 (1.39-1.62) in head; first anal-fin soft ray the longest, its length 1.35 (1.39-1.62) in head, anal-fin soft rays progressively shorter posteriorly; caudal fin slightly convex with a slight concavity at the mid-line, its length 1.79 (1.83-2.00) in head; pectoral fins 1.37 (1.26-1.43) in head; pelvic spine 1.38 (1.36-1.51) in head; first soft ray of pelvic fin with a filamentous extension, its length 1.08 (1.19, broken in all but one paratype) in head.
Color in life as in Figures 1-5. Body pale yellow dorsally fading to white ventrally and on the thorax and lower head (color of body sometimes lacking pale yellow coloration); three prominent black bands on each side of the body with narrow white margins, the first band originating at and including the first dorsal-fin spine, extending diagonally to the eye and continuing horizontally as an orangish brown stripe from the eye to the tip of the snout, the second band originating at and including the fourth to sixth dorsal-fin spines, extending vertically at a slightly posterior angle to the ventral surface of the abdomen just anterior to the anus, tapering slightly and curving slightly posteriorly below the pectoral fin, and the third band originating at and including last four to five dorsal-fin spines and first four to five soft dorsal rays, extending vertically at a slightly posterior angle to and including the first several anal soft rays, the bands becoming dark orangish brown distally on the dorsal fin; a narrow orangish brown stripe extending from the dorsal side of the snout broadening dorsally on the nape to a point just above the interorbital space, becoming darker dorsally; a narrow orange band on the dorso-posterior margin of the operculum, extending ventrally to the posterior angle of the operculum, an oblong orange spot with some dark pigmentation on the upper one-third of the pectoral-fin axis; pelvic fins white on the spine and anterior one-third of fin, and bright orange on the posterior two-thirds of fin; a bright orange submarginal band with narrow white posterior margin extending along the posterior margins of the soft portions of the dorsal and anal fins and continuing across the caudal peduncle; caudal fin and pectoral fins translucent.
Color in alcohol similar to life color, except body a uniform dull yellow, bands dark brown, and orange areas pale brown.
A single juvenile, about 25 mm SL, was observed by RLP at a depth of 120 m during a dive off Pearl Harbor, O'ahu on 16 August 1998. The general body shape and color pattern were the same as for adults.
Morphometric data for selected characters of type specimens are provided in Table 1 (Randall et al. 1985;Wagner et al. 2014), nor has any similar fish been observed or collected anywhere in the central or eastern Pacific. Thus, it appears that P. basabei is endemic to the Hawaiian Archipelago (although further exploration of MCEs in nearby regions may yet reveal its presence elsewhere). This is consistent with the observation that fish assemblages on deep coral reefs have proportionally more endemic species than on shallow reefs (Pyle 1996, Kane et al. 2014, Kosaki et al. 2016. Habitat. Pyle and Chave (1994: 92) described the habitat for this species based on videotaped observations from submersibles as follows: Eighteen (56%) of the observed [fish] were in areas of basalt substrata (e.g., basalt talus, blocky lava, lava tubes and pillows, basalt boulders), 13 (41 %) were in limestone habitats (primarily limestone holes and ledges), and one fish was sighted on a large (2-m diam.) water pipe. Four of the fish were in the vicinity of an unidentified antipatharian coral, three near Cirrhipathes spiralis (Linnaeus), and one near Antipathes dichotoma Pallas.
Subsequent observations of this species by divers and submersible dives, totaling several dozen individuals mostly off O'ahu and various sites within the NWHI, were all found in association with limestone ledges and discontinuities representing ancient shorelines (Figures 3-5). In almost all cases, the fish were found underneath, inside of, or in close proximity to small undercut overhangs or caves, often swimming upsidedown in association with the roof of the overhangs and caves. There are no obvious associations with other species, such as antipathinarian corals, other corals and sessile invertebrates, or particular fish species; although certain other fish species, such as than anthias Odontanthias fuscipinnis (Jenkins, 1901) and the wrasse Bodianus sanguineus (Jordan & Evermann, 1903), tend to occupy the same depth and habitat.
Etymology. We take great pleasure in naming this species basabei, in honor of Peter K. Basabe, long-time diver, aquarium fish collector and resident of Kona, Hawai'i, both for his role in the collection of the first specimen of this new species in 1998, and more generally for his extensive contributions and assistance to many researchers (especially the authors) in the ichthyological community.
Morphological comparisons. Prognathodes basabei appears to be most similar in color and morphology to an undescribed Prognathodes species collected at a similar depth in Palau. These two species differ from each other in number of dorsal-fin soft rays (21-22 for basabei, compared to 17-19 for the Palau species) and anal-fin soft rays (16-17, compared to 15). P. basabei also has a smaller head (2.63-2.80 in SL, compared to 2.48-2.49 in SL), deeper body (1.58-1.69 in SL, compared to 1.71-1.76 in SL), and shorter pelvic-fin spine (3.63-4.07 in SL, compared to 4.18-4.46 in SL) than the Palau species. The two species also differ in certain aspects of life color. The anterior edge of the second black band of the Palau species originates at the third dorsal-fin spine, whereas this band originates on the fourth dorsal-fin spine in P. basabei. Moreover, both of the dark bands on the Palau species are proportionally broader dorsally, tapering more substantially ventrally than in P. basabei. Also, the orangish coloration on the pelvic fins and posterior margin of the soft dorsal and anal fins of the Palau species are much darker and brownish than in P. basabei. P. basabei is also similar in color and morphology to P. guezei from the western Indian Ocean. It differs from that species morphologically in number of dorsal-fin soft rays (21-22 for basabei, compared to 20 for guezei), head size (2.63-2.80 in SL, compared to 2.47-2.48 in SL), body depth (1.58-1.69 in SL, compared to 1.87-1.95 in SL), and shorter pelvic-fin spine (3.63-4.07 in SL, compared to 4.21-4.33 in SL). There are also   several differences in life color between the two species. In particular, P. guezei ( Figure  6) has more pronounced and discrete yellow bars on the body between the black bands, compared with more diffuse and paler yellow in P. basabei. As with the Palau species, the anterior edge of the second black band of the P. guezei originates at the third dorsalfin spine, whereas this band originates on the fourth dorsal-fin spine in P. basabei, and the two black bands on the body of P. guezei taper even more substantially than they do in the Palau species, with the dorsal end of the posterior band in P. guezei covering the last five dorsal-fin spines, compared with the last four dorsal-fin spines on P. basabei. Also, the orangish coloration on the pelvic fins and posterior margin of the soft dorsal and anal fins of P. guezei are much paler and yellowish than in P. basabei. Genetic comparisons. Genetic comparisons provide another compelling justification for regarding P. basabei as distinct from the Palau species. The vertebrate mtDNA barcode (cytochrome oxidase I) sequences obtained from the holotype and two paratypes of P. basabei, compared to specimens of Prognathodes sp. collected in Palau, reveal 8% uncorrected sequence divergence. This is consistent with species-level divergences in other fish taxa (Johns and Avise 1998, Bellwood et al. 2004, Fessler and Westneat 2007, Randall and Rocha 2009, Rocha 2004, Rocha et al. 2008. The accepted mtDNA clock rate of approximately 2% per million years in fishes (Bowen et al. 2001, Reece et al. 2010 indicates divergence between these species on the order of 4 million years.
No tissue samples or DNA sequences have been reported for P. guezei, but given the geographic distributions of P. guezei in the western Indian Ocean, the Palau species, and P. basabei, we anticipate that the genetic divergence between P. basabei and P. guezei will prove to be even deeper than that between P. basabei and the Palau species. Discussion. Prognathodes basabei is an example of the conspicuous new fish species that have been discovered on deep coral reefs over the past two decades, mostly involving the use of modern mixed-gas closed-circuit rebreather diving technology (Pyle 1996(Pyle , 2000. There has been increased attention focused on mesophotic coral ecosystems (MCEs), coral-reef habitat at depths of approximately 30-150 m in tropical regions worldwide (Hinderstein et al. 2010, Baker et al. 2016.
One particularly unusual characteristic of this species is the tendency for it to be found in groups of three individuals. Although Pyle and Chave (1994) reported that most videotaped observations from submersibles involved apparent pairs or solitary individuals, in most cases these observations were incidental to the research focus on the submersible dives, so no concerted effort was made to determine the total number of individuals at each sighting. Every observation of adults of this species by the authors during mixed-gas dives in both the main Hawaiian Islands and NWHI (nearly two dozen instances), as well as observations by RLP during several submersible dives off south O'ahu in 2011, involved groups of three individuals; the only exception was the solitary juvenile observed by RLP in 1998. Only two individuals of the Palau species were observed together, and none of the approximately ten individuals of P. guezei observed by RLP at depths of 115-120 m off Sodwana Bay in 2011 were found in a group of three. Butterflyfishes (Chaetodontidae) in general are known to display a variety of social and mating systems, including monogamous pairs, harems, and schools (Reese 1975, Yabuta and. Territoriality and the distribution of food resources are important determinants of these social systems (Hourigan 1988, Pratchett et al. 2013. Groups of three individuals as a primary social grouping have not been noted in other chaetodontid species. The six type specimens of P. basabei were found as two groups of three individuals. In both cases, the groupings included a single female and two males. More samples are necessary to determine whether such associations represent loose social groupings, territorial behavior, a mating system, or coincidence.
Another interesting aspect of this new species is the strikingly similar color pattern it shares with both the Palau species, and with P. guezei, in contrast to the deep genetic divergence that exists between the Hawaiian and Palauan specimens. It will be interesting to compare the genetics of P. guezei once tissue samples can be obtained, and as previously noted, we expect the genetic divergence to be similarly deep. A more thorough analysis and discussion of genetic comparisons between P. basabei and the Palauan species will be included in the forthcoming description of the latter species.
Prognathodes basabei is the twelfth recognized member of the genus, a group generally inhabiting deeper habitats than most other chaetodontid species. Nalbant (1995) suggested that the group may have an antitropical distribution, which would apply to P. basabei in Hawai'i and P. guezei in the southwest Indian Ocean, but less so in light of the undescribed species in Palau. One potential explanation for such disjunct distributions is that these are relics of a once more widely distributed genus (paleoendemics; Bellwood and Meyer 2009). Another, perhaps more likely explanation is that the dearth of mesophotic exploration across the tropical central and western Pacific and Indian Oceans has left significant gaps in our understanding of Prognathodes distribution, and that additional populations and species await discovery.