The specimens were collected from the fish market at Nha Trang, a coastal city in central Vietnam, at depths of roughly 180 to 220 meters off the Vĩnh Lương fishing port. Rather than trawling remote seafloor habitats, the researchers turned to a surprisingly productive source: commercial catches brought ashore by local fishers. This approach has increasingly proven valuable for documenting deep-water species that live beyond the reach of most conventional surveys, particularly in regions where dedicated deep-sea expeditions are rare.

Neobythites is the most species-rich genus within the cusk eel family Ophidiidae, with 61 currently recognized species distributed across tropical and subtropical continental shelves in all three major oceans. These fish are characterized by a single well-developed opercular spine, two pelvic-fin rays, two medial basibranchial tooth patches, and a distinctive sulcus on their sagittal otoliths — the calcium carbonate structures in the inner ear that are often used as diagnostic characters in fish taxonomy. Despite 31 species being known from the western Pacific Ocean, only two had previously been recorded from Vietnamese waters.

N. nanhaiensis was described in 2026 based on 14 specimens collected from the southern South China Sea, near Nan’an Reef at a depth of 400 meters. It belongs to the non-ocellus group of the genus, meaning it lacks the prominent eyespot-like markings seen in many relatives, and can be distinguished from other species in that group by having two blotches on the lateral side of the body and by specific counts of dorsal-fin rays, anal-fin rays, and vertebrae. The new Vietnamese specimens — three individuals measuring between 119.1 and 122.5 millimeters in standard length — represent the northernmost record of the species to date.

The researchers provided detailed morphological descriptions of the specimens, including meristic counts determined from X-radiographs, osteological features, and comprehensive otolith descriptions. Dorsal-fin rays numbered 106 to 111, anal-fin rays 90 to 94, and total vertebrae 62 to 63. The body is compressed and elongated, with a small head and two black blotches on the upper half of the body when fresh. The sagittal otoliths are thick and oblong, with a long, shallow sulcus divided into an ostium and cauda, and no sexual dimorphism was observed.

Molecular analyses using two DNA barcode segments — partial mitochondrial cytochrome c oxidase subunit I (COI) and ribosomal 12S rRNA — confirmed the identification. The COI sequences showed percent identity values ranging from 99.12 to 100.0 percent with N. nanhaiensis sequences on GenBank, while the 12S sequences shared 100 percent identity with reference material. The newly generated sequences have been deposited in GenBank under accession numbers PX647189 through PX647191 and PX649349 through PX649351.

However, the study’s most consequential finding concerns the original description of N. nanhaiensis itself. Upon re-examination of the figures and data published in the species’ naming paper, the researchers identified several errors. Two of the specimens depicted in the original description appear to have damaged tails that had been regenerated — a condition that fundamentally alters fin-ray and vertebral counts. This interpretation is supported by the unusual variation in caudal-fin ray counts reported in the original description, ranging from 6 to 10, whereas related species consistently exhibit 8 rays. The reported counts of dorsal-fin rays, anal-fin rays, vertebrae, and associated morphometric measurements may therefore be unreliable, and the true range of variation in this species remains uncertain.

The second species documented, Grammonus robustus, is a deep-water bythitid cusk eel originally described in 1913 from a single specimen collected between Cebu and Leyte in the Philippines at a depth of 345.6 meters. It has subsequently been recorded from scattered localities across the Indo-West Pacific, including Japan, Taiwan, Queensland in Australia, western and southern India, the Red Sea, and South Africa. The Vietnamese specimen — a 73.4 millimeter female — fills the distribution gap between Queensland and the Philippines, providing a more complete picture of the species’ range across the region.

G. robustus can be distinguished from its ten congeners by a combination of characters including the numbers of dorsal-, pectoral-, and anal-fin rays, its vertebral formula, predorsal length, interorbital width, and its tendency to reach a larger size than related species. The genus Grammonus is a small group of cusk eels distributed worldwide that mainly inhabit sea caves or reef areas, with few species occurring in deep water. Only one species, G. minutus, had previously been recorded from Vietnamese waters. The new specimen’s meristic and morphometric characters all fall within the range of specimens collected from other localities, confirming its identification.

The molecular results for G. robustus were less straightforward. The most closely related bythitid species to the query COI sequence on GenBank was Diplacanthopoma brachysoma, with an identity value of only 86.21 percent, while the BOLD system identified Grammonus diagrammus as the closest species at 85.05 percent. The 12S sequence, however, blasted to G. robustus with 99.41 percent identity. The researchers attribute these inconsistent results to the scarcity of comprehensive reference sequences for the genus in public databases, as well as the potential misidentification of voucher specimens. Notably, a COI sequence identified as Liparis tessellatus — a fish from an entirely different family — showed 99.49 percent identity with the Vietnamese specimen, which the researchers suggest is clearly a misidentification and may indicate that G. robustus also occurs in Korean waters.

The study also provides a more comprehensive morphological comparison of N. nanhaiensis with its congeners than the original description offered. Among Indo-West Pacific species lacking ocelli, N. nanhaiensis is most similar to N. analis, N. solomonensis, and N. somaliensis in having dark distal margins on the dorsal and anal fins and two preopercular spines. It differs from these species in having higher counts of dorsal-fin rays, anal-fin rays, and total vertebrae, a shorter pelvic fin, and — critically — the presence of two black blotches on the lateral side of the body, which the other three species lack. The Vietnamese specimens also possess a straight posterior margin on the vomerine tooth patch, contrary to the concave margin described in the original species description, raising questions about whether this discrepancy represents intraspecific variation or a more fundamental taxonomic problem.

The broader context of this work highlights a persistent challenge in marine taxonomy: public genetic databases currently contain COI sequences for only 17 of the 61 recognized Neobythites species and 12S sequences for merely six. This scarcity of reference material likely accounts for the relatively low identity values observed when comparing query sequences to databases, and underscores the importance of morphology-based comparisons alongside molecular approaches. As deep-water fish markets continue to yield undocumented species and range extensions, researchers emphasize that careful re-examination of recently described species — including the condition of the specimens on which they were based — remains essential to ensuring the accuracy of the taxonomic record.

Subject of Research: Marine Biology

Article Title: Records of cusk eels from Vietnam, with comments on the original description of Neobythites nanhaiensis ( Actinopterygii , Ophidiiformes , Ophidiidae )

Article References: Su, Y., Chang, C.-H., Ho, H.-C., Vo, V. Q., & Lin, C.-H. (2026). Records of cusk eels from Vietnam, with comments on the original description of Neobythites nanhaiensis (Actinopterygii, Ophidiiformes, Ophidiidae). Acta Ichthyologica et Piscatoria, 56, 287-298. https://doi.org/10.3897/aiep.56.197399

Image Credits: AI Generated

DOI: 10.3897/aiep.56.197399

Keywords: Records, cusk, eels, Vietnam, comments, original, description, Neobythites, nanhaiensis, Actinopterygii, Ophidiiformes, Ophidiidae