The cusk eel genus Neobythites Goode et Bean, 1885 is the most diverse genus of the family Ophidiidae, and its representatives are distributed in tropical and subtropical regions of all three major Oceans and inhabits at continental shelves (Nielsen et al. 1999; Nielsen 2002; Uiblein and Nielsen 2023). It is characterized by having: operculum spine single, and well developed; pelvic-fin rays 2; medial basibranchial tooth patch 2; preoperclular spines 0 to 3 and not serrated; length of the snout equal to or longer than the width of eye window; sulcus of elongated sagittal otolith long and wide, with ostium and cauda equally wide; and ostial and caudal collicula usually separated (Nolf 1980; Schwarzhans 1981, 1994; Nielsen et al. 1999; Nielsen 2002; Nielsen and Machida 2006). Currently, 61 species are recognized as valid and can be divided into five species groups and one undefined group containing ten species (Nielsen 2002; Nielsen et al. 2009; Ohashi et al. 2012; Uiblein and Nielsen 2018, 2021, 2023; Chen et al. 2026). Although 31 species of Neobythites are distributed in the western Pacific Ocean (Nielsen 2002; Ohashi et al. 2012; Uiblein and Nielsen 2023; Chen et al. 2026), only two species, namely Neobythites sivicola (Jordan et Snyder, 1901) and Neobythites unimaculatus Smith et Radcliffe in Radcliffe, 1913 have so far been recorded from Vietnamese waters (Nguyen and Nguyen 1994).

Recently, Chen et al. (2026) described a new species, Neobythites nanhaiensis Chen, Zhao, Yang, Zhang, He, Zhang, Wang et Zhong, 2026 based on specimens collected from the southern part of the South China Sea. It belongs to the non-ocellus group (sensu Uiblein and Nielsen 2023) and can be distinguished from other species in the same group in having: two blotches (rarely three) on lateral side of body, and the counts of dorsal- and anal-fin rays and vertebrae (Chen et al. 2026).

On the other hand, the bythitid genus Grammonus Gill, 1896 is a small group of cuskeel that are distributed worldwide and mainly inhabits sea caves or reef area, with few species occurring in the deep sea (Nielsen et al. 1999; Nielsen 2007; Nielsen and Prokofiev 2010). It can be distinguished from other bythitid genera in having: pelvic-fin ray 1; palatine edentate; rear portion of maxilla expanded; dorsal and anal fins connected to caudal fin; pectoral radials short, not elongated (Nielsen et al. 1999; Randall and Hughes 2008). Currently, eleven species are recognized as valid, with only one species, Grammonus minutus Nielsen et Prokofiev, 2010 recorded from the waters of Vietnam (Nielsen and Prokofiev 2010).

Grammonus robustus Smith et Radcliffe in Radcliffe, 1913 is a deep-water bythitid originally described based on a single specimen collected from the Philippines (Radcliffe 1913). It was subsequently recorded from scattered localities in the Indo-West Pacific, including Japan, Taiwan, Queensland, Australia, western and southern India, Red Sea, and South Africa (e.g., Machida 1984; Nielsen et al. 1999; Nielsen 2007; Kannan et al. 2014; Ho and Moritz 2017; Bray 2025). It can be distinguished from the ten congeners in having a combination of characteristics such as the numbers of dorsal-, pectoral-, and anal-fin rays, vertebral formula, predorsal length, interorbital width, and reaching a larger size (see below for detail; Nielsen et al. 1999; Nielsen 2007; Nielsen and Prokofiev 2010).

A recent collection in the fish market of Nha Trang, Vietnam resulted two species of cuskeels, Neobythites nanhaiensis and Grammonus robustus. Detailed descriptions of the specimens and their sagittal otoliths are provided and compared to similar congeners. Additionally, a discussion on the original description of N. nanhaiensis is provided.

Morphological analyses. Whole specimens were fixed with 4% formaldehyde and subsequently transferred to 70% ethanol for long-term preservation. Terminology and methodology follow that of Nielsen (2002) and Ohashi et al. (2012). Counts of vertebrae follow Nielsen et al. (1999). Paired-fin characters are expressed as left/right when available. Counts of medial fins and vertebrae were determined by X-radiograph taken by a digital X-ray machine in the National Museum of Marine Biology and Aquarium, Taiwan. Measurements were taken using 150 mm digital calipers rounding to the nearest 0.1 mm. Measurements are expressed as standard length (SL) and/or head length (HL). Description of otoliths follow Lin and Chang (2012) and Nolf (2013). Specimens examined in this study are deposited at the Biodiversity Research Center, Academia Sinica, Taipei, Taiwan (ASIZP); the Pisces Collection, National Museum of Marine Biology and Aquarium, Pingtung, Taiwan (NMMB-P); and the Museum of Oceanography, Institute of Oceanography, Viet Nam Academy of Science and Technology (OIM); and otoliths (sagittae) are deposited at the marine paleontology lab, Biodiversity Research Center (CHLOL), with otolith length (OL), otolith height (OH), otolith thickness (OT), sulcus length (SuL), ostium length (OsL), and cauda length (CaL) being measured.

Genetic analyses. A piece of lateral muscle tissue was taken from each specimen and preserved in 100% ethanol prior to the fixation in formaldehyde. DNA extraction kit (Cat. No. GS100, Geneaid) was used and followed the kit protocol. The two DNA barcode segments, partial mitochondrial cytochrome c oxidase subunit I (COI) gene (~650 bp) and ribosomal 12S rRNA (12S) gene (~170 bp), were amplified by polymerase chain reaction (PCR), which was performed in a final reaction volume of 25 μL containing 20 ng DNA, 1 μL (10 μM) each of the forward and reverse primers (COI: FishF1+2 (5'-TCR ACY AAY CAY AAA GAY ATY GGC AC-3'); reverse: FishR1 (5'-TAG ACT TCT GGG TGG CCA AAG AAT CA-3') or FishR2 (5'- ACT TCA GGG TGA CCG AAG AAT CAG AA-3') (Chang et al. 2016), and 12S: MiFish-U-F: 5'-GTC GGT AAA ACT CGT GCC AGC-3'; reverse: MiFish-U-R: 5'-CAT AGT GGG GTA TCT AAT CCC AGT TTG-3' (Miya et al. 2015)), 12.5 μL of Fast-Run^TM Advanced Taq Master Mix (ProTech, Taipei, Taiwan), and distilled water. The thermal cycling protocol was as follows: one cycle at 94°C for 4 min; 35 cycles of denaturation at 94°C for 30 s, 50–55 °C for 30 s, and 72°C for 1 min.; and a final single extension step at 72°C for 5 min. Sequencing was performed using the COI forward primer and both of 12S forward and reverse primers by Genomics Biotech Inc., Taipei, Taiwan. The ends of amplified sequences were trimmed and two directional sequencing 12S segments of specimens are constructed contigs using CodonCode Aligner 12.0.3 software.

The COI and 12S sequences generated for N. nanhaiensis have been deposited in GenBank (Benson et al. 2012) under accession numbers PX647189–PX647191 and PX649349–PX649351, respectively. Additionally, the COI and 12S sequences of G. robustus available in GenBank are registered under accession numbers PX643086 and PX643100, respectively. These newly generated COI sequences were then compared against the animal species-level library database using the exhaustive search mode of the Barcode of Life Database (BOLD) system (Ratnasingham and Hebert 2007; https://boldsystems.org/) and also compared (using BLAST) to sequences in the GenBank database. The 12S sequences were also compared (using BLAST) to sequences in the GenBank database.

Ethical statements. All specimens used in this study were collected from fish markets and were dead on receiving them. No living animals were used in this study.

Comments on the original description. Neobythites nanhaiensis was recently described based on 14 specimens collected from the southern part of the South China Sea (Chen et al. 2026). It can be distinguished from congeners by having two (rarely three) ocelli on the body and several meristic characters. However, after re-examination of the figures and data, we found several errors in the original description.

The figures 2B and 2D in Chen et al. (2026) appear to depict damaged specimens, with tails that seem to have been regenerated. This interpretation is supported by the reported unusual variation in caudal-fin ray counts (6–10), whereas congeners consistently exhibit 8 rays (e.g., Nielsen 2002), as well as by the unusually broad ranges in dorsal- and anal-fin rays. Accordingly, we suggest that the reported counts of dorsal- and anal-fin rays, vertebrae, and associated morphometric measurements may be unreliable, and that the true range of variation in this species remains uncertain.

Furthermore, although Chen et al. (2026) stated that the presence of two (rarely three) blotches on body can distinguish N. nanhaiensis from its congeners, they restricted their detailed comparisons to only the 12 species included in their molecular phylogeny. Given that only a fraction of described species is represented in public genetic databases, it is essential to include comparisons with the remaining species based on morphology.

Comparison of N. nanhaiensis with congeners. Our specimens are identified as N. nanhaiensis based on having the lateral side of body with two blotches, developed gill rakers 3 + 1 + 7 = 11, preopercular spines 2, outer margins of dorsal and anal fins dusky, and pelvic fin short, not reaching the anus when adpressed (Chen et al. 2026). Nevertheless, our specimens possess a straight posterior margin of the vomerine tooth patch, contrary to the concave margin in the original description. As the shape of vomer was regarded as a diagnostic character (e.g., Nielsen 2002), further study is needed to determine whether this discrepancy represents intraspecific variation or not.

Additionally, we herein provide a more comprehensive comparison of N. nanhaiensis based on our specimens with similar congeners. Among species of Neobythites distributed in the Indo-West Pacific Ocean, 26 species are characterized by having no blotches or ocellus on both dorsal and anal fins, including 24 species listed in Uiblein and Nielsen (2005), Neobythites machidai Ohashi, Nielsen et Yabe, 2012, and Neobythites solomonensis Uiblein et Nielsen, 2021 (see Uiblein and Nielsen 2023). Among non-ocellus bearing congeners in the Indo-West Pacific Ocean, Neobythites nanhaiensis is most similar to Neobythites analis Barnard, 1927, N. solomonensis, and Neobythites somaliensis Nielsen, 1995 in having distal margin of dorsal and anal fins dark or black (vs. dusky or pale in the rest species; Nielsen 2002; Uiblein and Nielsen 2021) and preopercular spines 2 (vs. 0, rarely 1). It differs from the three aforementioned species in having dorsal-fin rays 106–111 (vs. 99–107 in N. analis; Table 1), anal-fin rays 90–94 (vs. 84–89 in N. analis), pseudobranchial filaments 3–4 (vs. 5–8 in N. somaliensis), total vertebrae 61–62 (vs. 58–61 in N. analis and N. solomonensis), anal-fin origin below 18^th–19^th dorsal-fin ray (vs. 20^th–23^rd in N. solomonensis and 20^th–22^nd in N. somaliensis), anal-fin origin below 14^th vertebra (vs. 16^th in N. solomonensis and 15^th–16^th in N. somaliensis), pelvic-fin length 9.2–10.4%SL (vs. 14.5%–17.5%SL in N. somaliensis; Table 2), longest gill filament on anterior gill arch 7.7%–8.8%HL (vs. 5.4%–7.7%HL in N. analis, 4.6%–4.9%HL in N. solomonensis and 11.0%–14.0%HL in N. somaliensis), posterior margin of vomerine tooth patch nearly straight (vs. concave in N. analis and N. somaliensis), and the presence of two black blotches on lateral side of body (vs. blotches absent; Nielsen 2002; Uiblein and Nielsen 2021).

Morphological comparison of specimens of Grammonus robustus. The Vietnamese specimen is identified as G. robustus based on having dorsal-fin rays 88, originating above 6^th vertebra, pectoral-fin rays 25, anal-fin rays 59, originating below 30^th dorsal-fin ray and 17^th vertebra, vertebrae 12 + 32 = 44, preopercle without spines, head canal with one pore above gill opening, posterior maxillary depth 5.8%SL, interorbital width 6.6%SL, and predorsal length 36.2%SL (Nielsen 2007; Nielsen and Prokofiev 2010). Compared to previous data (Tables 3 and 4), all of the meristic and morphometric characters of present specimen fall in the range of other specimens collected from Japan, Taiwan, the Philippines, India, and the Red Sea.

Molecular analyses. Currently, 61 species of Neobythites and eleven species of Grammonus are recognized as valid (Nielsen and Prokofiev 2010; Uiblein and Nielsen 2023; Chen et al. 2026). However, the BOLD system has only collected COI sequences from 17 species of Neobythites and one species of Grammonus, and GenBank has retrieved 12S sequences from merely six species of Neobythites and one species of Grammonus. The scarcity of comprehensive reference COI and 12S sequences for Neobythites spp. in the databases likely accounts for the relatively low identity values observed when comparing the query sequences to the database. Moreover, the inconsistent DNA barcoding results, derived from COI and 12S sequences of Grammonus are likely attributable to the limited availability of comprehensive reference sequences for Grammonus spp., as well as the potential misidentification of voucher specimens. Additionally, the COI sequence (MK182380) identified as Liparis tessellatus (Gilbert et Burke, 1912) (Family Liparidae) from Korea shows 99.49% Percent Identity value and 100% query coverage with our specimen of G. robustus. It is apparently a misidentification, as the two families are very different from each other, and we further suggest that G. robustus also occurs in Korea.