Epigonus okamotoi (Perciformes: Epigonidae), a junior synonym of E. draco , with new distributional records for E. atherinoides and E. lifouensis in the West Pacific

Epigonus okamotoi Fricke, 2017 was originally described on the basis of a single specimen collected from New Britain, Papua New Guinea during one of the exploratory cruises (campaign: MADEEP) in 2014 organized under the Tropical Deep-Sea Benthos program. However, there are no clear differences in the meristic and morphometric characters between the holotype of the new species and specimens of E. draco Okamoto, 2015, including two additional specimens of the species found in the ichthyological collections in the NTUM. The genetic distance ( p- distance) between the two “species” at the COI locus was negligible. Accordingly, the holotype of E. okamotoi is considered to be a specimen of E. draco , and the former nominal species is reduced to a junior synonym of E. draco . In addition, we rediagnose and report new distributional records for E. atherinoides (Gilbert, 1905) and E. lifouensis Okamoto & Motomura, 2013 in the West Pacific.


Introduction
Epigonus okamotoi Fricke 2017 was originally described from a single specimen collected in New Britain, Papua New Guinea during one of the exploratory cruises (campaign: MADEEP) in 2014 organized under the Tropical Deep-Sea Benthos program (TDSB). This newly described species appears to belong to the "E. constanciae group" (sensu Okamoto 2012) by having a pungent opercular spine and more than 40 pored lateral-line scales (Fricke 2017). According to the original description, although this species is similar to Epigonus atherinoides (Gilbert 1905) and Epigonus draco Okamoto 2016, it can be distinguished from these species in some meristic and morphometric characters (Fricke 2017). However, re-examination of the type specimen of E. okamotoi revealed that the original description did not accurately report several important diagnostic characters. We compared the type specimen of E. okamotoi with type specimens and other specimens of E. draco with morphological and molecular data. In addition to the re-examination of the holotype of E. okamotoi, we report new distributional records of E. atherinoides and Epigonus lifouensis Okamoto & Motomura 2013 in the western Pacific based on the specimens collected through the TDSB program and the cooperation project between Taiwan and France entitled "Taiwan France Marine Diversity Exploration and Evolution of Deep-sea Fauna".

Materials and methods
Meristic and morphometric methods followed Mayer (1974) and Okamoto (2011). The number of missing lateral-line scales was estimated by counting scale pockets. The number of pored lateral-line scales on the caudal fin is represented as "+ n". The first caudal vertebra is defined as the first vertebra bearing a distinct hemal spine. Measurements were made with calipers to the nearest 0.1 mm. Counts of supraneurals, vertebrae, and ribs were taken from radiographs. The term "maxillary mustache-like process" is used for a lateral process on the maxillary head (see Okamoto 2012: fig. 2). The number of pyloric caeca and sex were determined by dissection of the right side of the abdomen. Standard length is abbreviated as SL. Institutional abbreviations for the depositories of the material examined are: CAS (California Academy of Sciences, San Francisco), MNHN (Muséum National d'Histoire Naturelle, Paris), NSMT (National Museum of Nature and Science, Tsukuba), and NTUM (National Taiwan University, University Museums, Taipei).
Small pieces of muscle-tissue were excised from the specimens, preserved in 95% ethanol, and stored at −20•C in the Marine Biodiversity and Phylogenomics Laboratory at the Institute of Oceanography, National Taiwan University, Taipei. Genomic DNA was extracted using an automated DNA-extractor (LabTurbo 48 Compact System with LGD 480-220 kits: Taigene Bioscience Corporation, Taipei) following the manufacturer's protocol. The standard barcoding marker of a mitochondrial cytochrome c oxidase I (COI) gene was used to provide the molecular data for the sequence comparison among the specimens examined. PAUP* (Swofford 2002) was used to compute pairwise p-distances of the sequences to evaluate the genetic divergences between species and within species. Protocols for COI gene amplification and sequencing follow those outlined in Ward et al. (2005) and in Lo et al. (2017). The sequences obtained were deposited in GenBank (http://www.ncbi.nlm.nih. gov/) as a genetic reference for future DNA-identification and research on deepwater cardinalfishes.

Results and discussion
Epigonus draco Okamoto 2015 English name: Dragon Deepwater Cardinalfish ( Fig.1 pectoral-fin rays 19-20; total gill rakers 22-23; pyloric caeca 7-10; pored lateral-line scales 47-49 + 3-4; scales below lateral line 9; vertebrae 10 + 15; opercular spine present; maxillary mustache-like processes absent; ribs absent on last abdominal vertebra; uppermost margin of pectoral-fin base lower than horizontal line through center of eye; proximal radial of first anal-fin pterygiophore slender; and mouth cavity black. Distribution. Known from the Philippines (Iwamoto & McCosker 2014), Papua New Guinea (Fricke 2017; this study), Solomon Islands, Vanuatu (Okamoto 2015), and Society Islands (Okamoto 2016), at depths of 391-870 m (Fig. 2) Abramov 1986. Fricke (2017 stated that E. okamotoi is distinguishable from similar species of this group in having a combination of the following characters: dorsal-fin rays VII-I, 9; pectoral-fin rays 15; gill rakers 22; pyloric caeca 4; pored lateral-line scales 47 + 4; scales below lateral line 8; vertebrae 10 + 15; opercular spine present; maxillary mustache-like process absent; ribs absent on last abdominal vertebra; upper margin of pectoralfin base on level of upper margin of pupil; proximal radial of first anal-fin pterygiophore slender; mouth cavity light grey. However, re-examination of the holotype of E. okamotoi revealed that the holotype has dorsal-fin ray VII-I, 10, pectoral-fin rays 20, and scales below lateral line 9. Additionally, the position of the upper margin of pectoral-fin base of the holotype of E. okamotoi in fresh condition is lower than a horizontal line through the center of eye (Fricke 2017: fig. 3). The position of the pectoral fin of species of Epigonus slightly change after fixation owing to the shrinkage of the body. These characters and other meristic characters except for pyloric caeca (not confirmed to avoid damaging the specimen) of the holotype of E. okamotoi correspond to that of E. draco. Fricke (2017) noted that E. okamotoi differs from E. draco in the orbital diameter, postorbital length, snout length, preanus length, preanal-fin length, and length of third spine of first dorsal-fin. However, there are no clear differences in the morphometric characters between this holotype and the examined specimens of E. draco, including two additional specimens of the species from Papua New Guinea (Table 1). Further comparison of DNA sequences at the COI locus for the samples from E. draco and E. "okamotoi" demonstrated that the genetic distance (p-distance) between the two "species" was negligible (0 -0.00156) ( Table 2). The p-distances between E. draco and E. atherinoides, both of the E. constanciae species group, was substantially higher at 0.09390 and 0.09546 (Table 2). Therefore, the holotype of E. okamotoi is considered to be a specimen of E. draco, and the former nominal species is reduced to a junior synonym of E. draco.
Remarks. We found additional specimens of E. draco, collected in Papua New Guinea, at the National Taiwan University Museums (NTUM). These specimens represent the first record of this species from this locality. Epigonus draco is distributed on continental slopes in the western and central tropical Pacific, at depths greater than 315 m.    Epigonus atherinoides (Gilbert 1905) English name: Slender Deepwater Cardinalfish Japanese name: Hira-yasemutsu ( Fig. 3
Remarks. The Japanese specimen is a juvenile with dense pigmentation on the posterior half of the lateral side of the body. Epigonus megalops (Smith & Radcliffe in Radcliffe, 1912) had been regarded as a junior synonym of E. atherinoides; however, the former was recognized as a valid species by Okamoto (2016).
Remarks. Epigonus lifouensis was originally described on the basis of two specimens collected from the south of Lifou Island, Loyalty Islands, New Caledonia, at depths of 500-575 m and recognized as a member of the E. pandionis group (Okamoto & Motomura 2013). The present specimen represents the first record of the species from the South China Sea, Taiwan. Deepwater cardinalfishes of Taiwan are represented by two species including the first record of E. lifouensis here reported and E. denticulatus Dieuzeide, 1950. Although Gon (2000) listed E. macrops (Brauer 1906) in the checklist of the fishes of the South China Sea, it may be a misidentification (see Okamoto & Nakayama 2016). It should be noted that the specimens available to date were collected at a ridge nearby remote islands or a seamount rather than on the continental slope, suggesting a particular habitat of the species.