Ophichthus nguyenorum Vo, Hibino & Ho, 2025

Vo, Quang Van, Hibino, Yusuke, Ho, Hsuan-Ching, Le, Thao Thu Thi & Seah, Ying Giat, 2025, Two new snake eels (Anguilliformes, Ophichthidae, Ophichthus) from Viet Nam, with redescriptions of O. macrochir (Bleeker) and O. rutidoderma (Bleeker), ZooKeys 1231, pp. 311-329 : 311-329

Ophichthus nguyenorum Vo, Hibino & Ho sp. nov.

Figs 3 , 4 ; Tables 1 , 2 English name: Darked Long-body Snake Eel Vietnamese name: Ca ́ li ̣ ch da ̀ i lưng đen

Type material.

Holotype • OIM-E.55827 , 887 mm TL, field no. Q. 01095-3, ca 12°19'N, 109°20'E, Đồng Hòa , Cần Giờ district, Hồ Chí Minh City, southeast coast of Vietnam, South China Sea, bottom trawl, ca 10–20 m, 10 Nov. 2023 . Paratypes: Seventeen specimens, 680–976 mm TL • NMMB-P 41235 , 5 specimens, 703–908 mm TL • KMNH VR 100623 , 3 specimens, 740–848 mm TL • OIM-E.55826 , 4 specimens, 784–976 mm TL, all collected with the holotype • OIM-E.55828 , 4 specimens, 697–852 mm TL • same location with holotype, bottom trawl, ca 10–20 m, 20 Jan. 2023; OIM-E.55829 , 680 mm TL • bottom trawl, ca 10–20 m, 20 Sep. 2023 .

Diagnosis.

An extremely elongate Ophichthus with the following combination of characters: snout rather pointed but occipital strongly convex (duck-shaped); body with numerous longitudinal wrinkles, weak on posterior abdomen; head 5.4–6.2 % TL; preanal length 31.4–33.5 % TL; tail 66.5–68.6 % TL; snout length 14.6–16.4 % HL; one protrusion along upper lip (rarely 2 on one side); dorsal-fin origin slightly behind pectoral-fin tip; SO 1 + 3, POM 5 or 6 + 2; all teeth small and sharp; teeth on maxilla in one row anteriorly but increasing posteriorly; teeth on mandible biserial; body dark, usually including abdomen; dorsal fin darker with dark margin; anal fin initially pale but darkening towards tip; total vertebrae 190–196, MVF 15-62 - 192.

Description.

Counts and measurements of the holotype (in mm): total length 887, head 47.8, trunk 236.2, tail 601.4, predorsal length 67.4, pectoral-fin length 14.9; body depth at gill opening 14.4; body width at gill opening 13.7; body depth at anus 13.5; body width at anus 12.5; snout 7.3; upper jaw 14.1; snout overhang beyond tip of lower jaw 3.5; eye diameter 2.8; interorbital width 6.2; gill opening height 8.4; isthmus width 7.3.

An extremely elongate snake eel (Fig. 3 a ), subcircular to posterior portion of tail, then becoming slightly compressed, tail tip pointed and strong, its depth at gill openings 60.9 (57.6–64.2) in TL, Branchial basket slightly expanded and deeper than trunk. The skin slightly longitudinally wrinkled on body; skin folds not deep dorsally on body and weaker on posterior abdomen. Anus situated approximately in the anterior part of total length, head, and trunk 3.2 (3.0–3.2) in TL, head short 17.5 (16.1–18.6) in TL, and tail elongate 1.5 in TL. Snout short 6.4 (6.1–7.0) in HL, tip narrowly conical at dorsal view, underside of snout not bisected by a groove. Lower jaw short, its tip not extending beyond anterior margin of nostril tube; its length 4.1 (3.8–4.4) in HL. Upper jaw moderately long, rictus well behind posterior margin of eye; its length 3.4 (3.2–3.6) in HL.

Eye moderate in size, positioned above upper jaw, its diameter 4.8 (4.5–5.1) in upper jaw and 16.3 (15.1–17.3) in HL. Anterior nostril tubular, extending ventrolaterally from snout, reaching below upper lip and chin when directed downward. Posterior nostril is a hole above upper lip, covered by a broad flap that extends well below edge of upper lip. One protrusion on upper lip, positioned just behind anterior-nostril tube, rarely another one present below eye but extremely tiny and only on one side. Dorsal-fin origin behind head, by less than one pectoral-fin length behind pectoral-fin tip and 1.5 (1.4–1.6) times head length behind head. Median fins low but obvious, ending approximately one upper-jaw length before the broadly pointed tail tip. Pectoral fin wedge-shaped with a narrow base, its length less than three times its base width, broad at middle and the longest rays at mid-fin.

Head pores small but apparent (Fig. 4 a ). SO 1 (ethmoid) + 3 on dorsal surface of snout and interorbital space; IO 3 + 3, 1 between nostrils, two below eye and three behind eye; POM 5 or 6 + 2 (5 + 2 in holotype), the last pore slightly before rictus; F 1, ST 3. Indistinct minute sensory papillae present along nape, anterior margin of orbit, and around base of anterior nostril. Lateral-line pores apparent; HLL 8 (8–9), in an arching sequence, PDLL 15 (13–16); PALL 62 (61–64); TLL 187 (185–190), the last at approximately one jaw length in front of tail tip.

Teeth (Fig. 4 b, c ) small, conical, sharp. Intermaxillary with 4–6 teeth arranged in two rows (some paratypes with one row with 3 or 4 teeth) followed by 22 (18–25) teeth on vomer, with biserial rows anteriorly (some paratypes with triserial irregular rows in middle) and uniserial posteriorly, which decrease slightly in size posteriorly. Maxilla with 22 (right side) (20–24) or 23 (left side) (21–25) teeth, mostly uniserial anteriorly and becoming biserial at posterior portion with irregular rows. Mandible with 27 (right side) (24–31) or 29 (left side) (27–34) teeth arrange in one irregular row anteriorly, gradually becoming two irregular rows posteriorly, some paratypes with additional teeth anteriorly forming two irregular rows and gradually becoming one row at middle and then two irregular rows posteriorly (Fig. 4 c ). Predorsal vertebrae 15 (13–17), pre-anal vertebrae 63 (61–64), and total vertebrae 192 (190–196) (one paratype specimen with a broken tail, total vertebrae only 185).

Coloration. When fresh (Fig. 3 a ) body uniformly dark; pectoral fin pale brown with peppered dots; anal fin pale anterior and change to darker toward the ending in approximately three times head length before the tail tip. After preservation, body uniformly dark dorsally and pale brown ventrally. Dorsal fin dark with dark margin; anal fin pale white anteriorly with slightly blackish pigment toward end of tail tip. Pectoral fin mostly dark grayish with scattered pigments. Mouth cavity dark except pale tooth ridges. Peritoneum pale with gray peppered dots on upper half; stomach and intestine pale. Tail tip blackish.

Size.

The two largest specimens (976, 908 mm TL) are both ripe females with loose eggs.

Etymology.

The specific name of the new species is derived to honor three doctors with the last name Nguyen: Dr. Phung Huu Nguyen, Huong Khac Nguyen, and Thi Nhat Nguyen for their contributions to marine fish taxonomy in Viet Nam.

Distribution.

Only known from the type series collected from Mekong coastal region, southeast coast of Vietnam by bottom trawls. The depth range is estimated to be 10– 20 m.

Comparisons.

Ophichthus nguyenorum sp. nov. is different from most congeners belonging to the species group with elongate and extremely elongate bodies. Selected characters for comparing these species are listed in Table 2 . Compared to those species, it has a distinct total vertebral count range 190–196, which differs from Ophichthus cuulongensis sp. nov. (199–207), O. congroides (204–208), O. microcephalus (214), O. rotundus (178–184). Moreover, the new species differs from O. congroides , O. chilkensis , and O. macrochir in having fewer supraorbital pores (4 = 1 + 3, vs 5 = 1 + 4).

Although Ophichthus nguyenorum sp. nov. is most similar to O. rutidoderma , both sharing a short head, relatively long tail, body depth at anus, and pores on head and lateral line; it can be distinguished from the latter species by its body uniformly dark and less body depth (61.8–68.8 in TL, vs 39.2–56.2 in TL), count of protrusions (generally 1 vs 2) and fewer total vertebrae (190–196 vs 195–199).

Ophichthus nguyenorum sp. nov. is also similar to O. chilkensis and O. macrochir , sharing a short head, relatively long tail, and its anus situated at the front of total length, preopercular mandibular pores and numerous longitudinal wrinkles on the body, but it can be separated from the two species by its MVF 15-62 - 192 (vs 11-69 - 210 and 12-69 - 214, respectively), fewer total vertebrae (190–196, vs 206–214 and 207–221, respectively), shorter upper-jaw length (3.8–4.3 in HL, vs 3.2–3.8 and 2.4–3.6 in HL, respectively), and DFO behind the pectoral-fin tip (vs above tip of the fin). Ophichthus nguyenorum sp. nov. also can be separated from O. congroides in having fewer lateral-line pores before anus (62–64 vs 78) and a shorter head length (5.4–6.2 % TL vs 8.3–8.4 % TL), a shorter snout length (6.1–6.8 in HL, vs 4.9–5.1 in HL), a shorter upper-jaw length (3.2–3.6 in HL, vs 2.3 in HL), and different MVF (15-62 - 192 vs 21-76 - 206).

Remarks.

Ophichthus nguyenorum sp. nov. has some characteristics, such as body coloration, tooth arrangement, and the shape of protrusions, which may be caused by ontogenetical changes. In small sizes, the body is dark grey dorsally and pale brown ventrally; the anal fin is pale brown anteriorly in a larger paratype. The vomerine teeth are arranged in biserial rows, and the upper jaw also has biserial rows posteriorly in some specimens. The protrusions are usually two; however, the posterior one can degenerate or become very small on the side lip.

Figures

Figure 3. Ophichthus nguyenorum sp. nov. a, b fresh specimen of the holotype, OIM-E. 55827, 887 mm TL, arrows point to the DFO and AFO, respectively c close-up of head view from lateral side of the paratype, OIM-E. 55827, 703 mm TL, arrow points to the DFO d close-up of tail view from ventral side of the holotype.

Figure 4. Drawing demonstrating the head pores (a) and teeth on both jaws (b, c) of Ophichthus nguyenorum sp. nov. a, b from the holotype c from OIM-E. 55828, paratype, 708 mm TL.