Ophichthus longicorpus, Vo & Ho, 2021

Vo, Van Quang & Ho, Hsuan-Ching, 2021, A new species of the snake eel genus Ophichthus from Vietnam, with a new record of Echelus polyspondylus McCosker & Ho, 2015, Raffles Bulletin of Zoology 69, pp. 71-79 : 72-77

Ophichthus longicorpus , new species Long-body snake eel

( Figs. 1A‒C , 2A‒C ; Table 1)

Holotype. OIM-E. 55806, 625 mm TL, ripe female, field no. Q.00328, ca. 12°19′N, 109°20′E, LƯƠng SƠn, Nha Trang , Khánh Hòa, southeast coast of Vietnam, South China Sea, bottom trawl, ca. 150‒200 m, 14 April 2019.

Paratypes. Twelve specimens, 423‒580 mm TL, all collected from near type locality: NMMB-P34715 , 4 specimens, 423‒562 mm TL, 20 May 2019 ; OIM-E. 55805, 449 mm TL, 13 April 2019; OIM-E.55807, 4 specimens, 512‒580 mm TL, 14 April 2019; OIM-E.55808, 2 specimens, 433‒494 mm TL, 20 May 2019; OIM-E. 55809, 531 mm TL, cleared & stained, 10 May 2020.

Non-type. OIM-E.55807.2, 1 specimen, 450 mm TL, collected from near type locality, 20 May 2019.

Diagnosis. A species of Ophichthus that can be separated from its congeners by having DFO situated at 1.1‒1.5 head lengths behind head and anus situated at about middle of total length; trunk very long, 4.1‒4.9 times HL; HL 10.5‒12.2 in TL; posterior nostril above upper lip, covered by a flap that extends well below edge of mouth gape; pectoral fin small, pointed posteriorly; no barbel on upper lip; head pores: SO 1+4, POM 6+2; teeth moderately large and conical, biserial anteriorly and uniserial posteriorly on both jaws and vomer; uniformly yellowish-brown dorsally and grayish-brown ventrally; dorsal fin light grayish with an indistinct white margin; anal fin white with a slightly blackish base; total vertebrae 156–164, mean vertebral formula 27-68-159.

Description. Counts and measurements of the holotype (in mm): total length 624, head 55.2, trunk 256.8, tail 312, predorsal length 128, pectoral-fin length 13.5, body depth at gill opening 20.6, body width at gill opening 15.8, body depth at anus 21.2, body width at anus 19.7, snout 12.0, upper jaw 18.4, snout overhang beyond tip of lower jaw 2.7, eye diameter 7.1, interorbital width 8.9, gill opening height 7.2, isthmus width 11.6.

Body moderately elongate ( Fig. 1A ), subcircular to posterior portion of tail, then becoming slightly compressed, its depth at gill openings 30 (28–38, mean 31) times in TL. Branchial basket slightly expanded and deeper than trunk. The skin slightly wrinkled on nape and lateral side of tail tip; skin folds present on surface of branchial basket. Head 11.3 (10.5–12.2) in TL. Anus situated at about middle of total length, head and trunk 2.0 (2.0‒2.1) in TL. Snout moderately long and blunt, narrowly rounded when viewed from above and side; underside of snout not bisected by a groove. Lower jaw included, its tip extending slightly beyond anterior margin of anterior nostril tube. Upper jaw moderately long, rictus at a vertical through posterior margin of eye (or slightly before the margin in some paratypes, i.e., posterior margin of eye slightly behind rictus).

Eye large, at posterior third of upper jaw, its diameter 2.6 (2.4‒3.2) in upper jaw and 7.8 (6.9–9.2) in HL. Anterior nostril tubular, extending ventrolaterally from snout, reaching below upper lip and chin when directed downward. Posterior nostril a hole above upper lip, covered by a large flap that extends well below edge of mouth gape. No barbel on upper lip. Dorsal-fin origin far behind head, 4.4 (3.4‒5.7) times PF behind fin tip and 1.3 (1.1‒1.5) times HL behind head. Median fins low but obvious, ending approximately an eye diameter before the broadly pointed tail tip. Pectoral fin with a narrow base, its length less than three times its base, broad at middle and slightly lanceolate posteriorly, the longest rays at mid-fin.

Head pores small but apparent ( Fig. 2A ). SO 1 (ethmoid) + 4 on dorsal surface of snout and interorbital space; IO 4 + 2, 1 between nostrils, 3 below eye, and 2 behind eye; M 6 (1 paratype with 7 on right side), the last pore slightly behind rictus; POP 2, 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 (7–8), in an arching sequence, PDLL 27 (25‒29; see Remarks below); PALL 69 (67‒70); TLL 157 (154‒161), the last at about twice eye diameter in advance of tail tip. Teeth ( Fig. 2B, C ) moderately large, conical, and closely spaced. Intermaxillary with 16 (13–18) teeth arranged in an oval circle, no teeth at centre (some paratypes with a few additional teeth at centre), followed by 26 (22–31) teeth on vomer, biserial anteriorly (some paratypes with 3 irregular rows anteriorly) and uniserial posteriorly, which decrease slightly in size posteriorly. Maxilla with 28 (right side) or 30 (left side) (24–30) teeth, mostly biserial and becoming uniserial at posterior portion. Mandible with 35 (right side) or 34 (left side) (28‒36) teeth arranged in 2 well-separated rows anteriorly, gradually becoming 1 row posteriorly, some paratypes with additional teeth anteriorly forming 3 or 4 irregular rows and gradually becoming 2 rows at middle and then 1 row posteriorly ( Fig. 2C ) .

Colouration. When fresh ( Fig. 1A ), body uniformly dark gray; pectoral fin darker; anal fin broadly pale with dark base. When preserved, body uniformly yellowish-brown dorsally and grayish-brown ventrally. Dorsal fin light grayish with indistinct white margin; anal fin white with slightly blackish base. Pectoral fin mostly pale with scattered grayish-brown pigments. Mouth cavity pale with gray pepper dots. Peritoneum pale with gray pepper dots on upper half; stomach and intestine pale. Tail tip white.

Size. The two largest specimens (580, 625 mm TL) are both ripe females with loose eggs. Other specimens are either unripe females or males which cannot be determined.

Etymology. The specific name is derived from the Latin longus (long) and corpus (body), in referring to the very long trunk, one of the key characters of the new species.

Distribution. Only known from the type series collected from off LƯƠng SƠn, Nha Trang, Khánh Hòa, southeast coast of Vietnam (ca. 12°19′N, 109°20′E) by bottom trawls. The depth range is estimated to be 150‒ 200 m.

Comparisons. Ophichthus longicorpus , new species, is similar to a number of its congeners with their DFO more than two pectoral-fin lengths behind the head and no blackened anal-fin base in advance of the tail tip. Selected characters for comparing these species are listed in Table 2. Compared to those species, Ophichthus longicorpus has a distinctly posterior position of DFO which is 1‒1.5 head lengths or 4.4‒6.7 pectoral-fin lengths behind the head, whereas the other congeners have DFO 0.5‒1 head length or 2‒3.5 pectoral-fin length behind the head. Directly related to this character, the new species has 25‒29 predorsal vertebrae, whereas the others have 15‒20 (mean value) predorsal vertebrae. Ophichthus longicorpus , new species, is most similar to O. kusanagi , both sharing the DFO in a far more posterior position, a long trunk, and a similar number of total vertebrae. It can be separated from O. kusanagi in having a different MVF 27-68-159 (vs. 19-60-160), tail length 50‒53% TL (vs. 58‒60% TL), no barbel on its upper lip (vs. a single knob); PDLL 25‒29 (vs. 19‒23); and PALL 68‒72 (vs. 61‒65) (comparative data from Hibino et al., 2019b).

Ophichthus longicorpus is also similar to O. humanni in having a relatively long trunk and its anus situated at the middle of total length, but it can be separated by its MVF 27-68-159 (vs. 19-70-168), fewer total vertebrae (156‒164, vs. 168) and its DFO 1‒1.5 HL behind the head (vs. 0.9 HL) ( McCosker, 2010).

Although O. naga also has its anus situated at the middle of its total length, it has a rather forward position of DFO (0.9 HL behind the head), predorsal length 1.9 HL, 15 PDV, relatively short trunk (3.3 HL), and relatively few TV (153), and can be separated from our new species easily ( McCosker & Psomadakis, 2018).

The colour of O. longicorpus , new species, is uniformly dark brown which is similar to O. aphotistos McCosker & Chen, 2000 , co-occurring in the South China Sea. However, O. aphotistos differs from O. longicorpus in having its DFO situated at 2 PF behind the head, a relatively long snout (19‒22% HL) and upper jaw (32‒35% HL), fewer PDV (16‒20) and PAV (57‒61), and a different MVF (18-59-160). Moreover, the body of O. aphotistos is relatively slender (BD 34‒50 in TL) compared to similarly sized individual of O. longicorpus (BD 28‒38 in TL) ( Hibino et al., 2016).

At first sight, our specimens are very similar to Ophichthus urolophus ( Temminck & Schlegel, 1846) which also occurred along the coast of Vietnam ( Vo et al., 2019). Both species have a stout head and a blunt snout. Ophichthus urolophus can be separated from O. longicorpus , new species, by having its DFO above or slightly behind the tip of pectoral fin (less than 1 fin length), predorsal vertebrae 13‒16, preanal vertebrae 51‒56, and total vertebrae 134‒140, one barbel on the upper lip, and a fresh body colour of yellow brown ( Vo et al., 2019).

Remarks. It is notable that one specimen (OIM-E.55807.2, 450 mm TL) had only 24 predorsal vertebrae and a relatively short predorsal length (18.9% TL). However, the rest of its characters are identical to the other specimens and it is thus recognised as a non-type specimen of the new species.

The snout length and eye diameter of the new species is somewhat variable relative to the HL, but the snout is short (3.8‒4.8 in HL, mean 4.6) and the eye is large (6.9‒9.2 in HL, mean 7.8) in general, and the ratio of SNL/ED is 1.5‒2.1 (mean 1.9). The pectoral-fin length is also somewhat variable (3.5‒4.7 in HL, mean 4.1) but short in general. Although some specimens have a somewhat truncate pectoral fin, most specimens, including the holotype, have the fin clearly pointed posteriorly. These specimens with truncated pectoral fins are likely due to artefacts of preservation.

Figures

Fig 1. Ophichthus longicorpus, new species, holotype, OIM-E.55806, 624 mm TL. A, fresh specimen, arrows point to the DFO and AFO, respectively; B, close-up of head, arrow points to the DFO; C, close-up of posterior end of tail.

Fig. 2. Drawing demonstrating the head pores (A) and teeth on both jaw (B, C) of Ophichthus longicorpus, new species. A, B, from the holotype; C, from NMMB-P34715, 562 mm TL.