Atractodenchelys brevitrunca Vo & Ho 2020, sp. nov.

Atractodenchelys brevitrunca sp. nov.

urn:lsid:zoobank.org:act: 88AE7CDF-C8A9-4880-B061-D5CF77F4887D

English name: Short-bodied arrowtooth eel

Figs.1A, B; Table 1

Holotype. OIM-E. 55743, 509 mm TL, off Nha Trang City, southern Vietnam, western South China Sea, bottom trawl at depths around 280–350 meters, 30 March 2019

Paratypes. Thirty specimens, 254–565 mm TL. OIM-E. 55742, 452 mm TL, off Quy Nhon City, southern Viet- nam, western South China Sea, bottom trawl at depths around 250–300 meters, 23 Aug. 2018. OIM-E.55744–9, 6 specimens, 382–501 mm TL; NMMB-P30953, 5 specimens, 337–495 mm TL, collected together with the holotype. OIM-E.55750–62, 13 specimens, 254–525 mm TL, off Nha Trang City, southern Vietnam, western South China Sea, bottom trawl at depths around 250–300 meters, 13 Apr. 2019. OIM-E.55763–7, 5 specimens, 337–565 mm TL, off Nha Trang City, southern Vietnam, western South China Sea, bottom trawl at depths around 250–350 meters, 14 Apr. 2019.

Diagnosis. A species of Atractodenchelys with 155–158 vertebrae; relatively short trunk (11–13 in TL), clearly shorter than the head; and 5 or 6 large compound teeth on vomer.

Description. Morphometric data of the holotype, in mm (%TL in parentheses): 509 mm TL; head length 62.7 (12.3% TL); predorsal length 68.2 (13.4); preanal length 110 (21.6); trunk length 47.3 (9.3); tail length 399 (78.4); depth at gill opening 19.1 (3.8); depth at anus 20.0 (3.9); width at anus 11.8 (2.3); eye diameter 7.8 (1.5); interorbital width 6.8 (1.3); snout length 19.8 (3.9); upper jaw 31.5 (6.2); gill opening 11.5 (2.3); interbranchial width 5.4 (1.1).

Body relatively slender, head and trunk slightly compressed, becoming more compressed posteriorly; body depth relatively uniform, narrowing gradually to caudal fin. Head relatively slender, its length 8.8 (8.1–9.9) in TL; origin of dorsal fin slightly behind the gill opening, predorsal length 8.0 (7.4–9.0) in TL; trunk short, its length 11.8 (10.3–13.8) in HL; anus well behind the tip of pectoral fin by about 3 times of the pectoral-fin length; origin of anal fin immediately behind anus, preanal length 5.0 (4.6–5.4) in TL; tail long, tail length 1.2 (1.2–1.3) in TL. Dorsal and anal fins low and fleshy, continuous with a small but well-developed caudal fin. Pectoral fin well developed, its base above the upper corner of gill opening.

Head slender in profile; snout relatively long, bluntly pointed anteriorly and narrow dorsally, covered by many small papillae, snout length 3.1 (2.8–3.3) in HL; tip of snout projecting well beyond the lower jaw; eye large, cov- ered by a thick and semitransparent membrane, eye diameter 8.4 (7.1–9.3) in HL; interorbital space narrow, slightly elevated, its width 8.8 (7.3–10.0) in HL; postorbital space relatively short, about 1.7 (1.4–2.0) times snout length. Anterior nostrils tubular, directed anteroventrally. Posterior nostril large and rounded, situated at the anterior margin of lower half of the eye, bearing a fleshy rim along most of its inner margin. Lower jaw shorter than upper, its tip reaching first pore of infraorbital series. Rictus is about one eye diameter behind posterior margin of eye, upper jaw length 2.0 in HL.

Gill opening a long narrow slit, not attached to each other. Head and lateral-line pores large (Figs. 1, 2 A–B). Supraorbital pores 4, with anterior 3 restricted to anterior portion of snout, and fourth above anterior portion of eye; infraorbital pores 5, the first below the posterior corner of the anterior nostril, the second at the middle of space between nostrils, the third below the posterior corner of the posterior nostril, the fourth below middle of the eye, and the fifth behind the eye; mandibular pores 7, along the lower jaw, the last pore well behind the penultimate one; preopercular pores 2, close together; adnasal 1; supratemporal commissure 0; frontal 1. Lateral line nearly com- plete, extending to close to caudal-fin base; prepectoral 6 (4–6), predorsal 9 (7–9), preanal 24 (20–24), and total 149 (146–151). Dorsal-fin rays anterior to anal-fin origin 36–43 (n=3).

Teeth (Fig. 3 A–C) pointed, small to large. Intermaxillary with about 26 (23–26) fang-like teeth forming an oval cluster; 6 (5–6) large, widely-spaced compound vomerine teeth in a row. Maxillary with 4 (3–5) regular rows of sharp teeth, those in inner row and anterior portion of the jaw clearly larger than the rest and each with a blade-like tip, gradually smaller laterally and posteriorly. Lower jaw with 5 (4–6) regular rows of teeth, in similar arrangement of those on the upper jaw.

Mean vertebral formula 10-23-156; predorsal vertebrae 10 (9–10), preanal vertebrae 25 (23–25), and total ver- tebrae 155 (155–158) (n=13).

Coloration. Specimens examined have most of the skin surface rubbed off the posterior half of tail region, but the species assumed to be uniformly dark brownish. Peritoneum and mouth cavity pale.

Distribution. Known from the type series collected off Nha Trang and Quy Nhon, southern Vietnam, western South China Sea, by bottom trawl at depths around 250–350 meters.

Size. The largest known specimen is 565 mm TL.

Etymology. The specific name is derived from the Latin brevis (short) and truncus (trunk), refer to the short trunk of this species.

Remarks. There are a total of three species of Atractodenchelys, including A. phrix from the western Atlantic Ocean, A. robinsorum from Chesterfield and Vanuatu Islands, southern Pacific Ocean, and the new species described herein. The new species differs in having 155–158 total vertebrae (vs. 168–172 in A. phrix and 186–199 in A. robinsorum); head length 10.1–12.3% TL (vs. 9.1–9.9% TL in A. robinsorum); snout length 30.0–35.8% HL (vs. 37.5–39.5% HL A. robinsorum); head length 1.8 (1.6–1.9) times in preanal length (vs. 1.8–2.1 in A. phrix and 2.0– 2.1 in A. robinsorum, both calculated from the original descriptions); 4 supraorbital pores (vs. 5 in A. robinsorum); 5 infraorbital pores (vs. 8 in A. robinsorum); 5 or 6 compound teeth on vomer (vs. 7–8 in A. robinsorum).

It is notable that the original drawing of the holotype of A. phrix (Robins & Robins, 1970:fig. 3) showed 4 supraorbital pores, 5 infraorbital pores (instead of 6 in the description), 8 mandibular pores, and 2 preopercular pores. In another drawing of A. phrix (Robins & Robins, 1989: figs. 227), it shows 5 infraorbital pores and 7 mandibular pores. The original description stating 6 infraorbital pores may be a mistake and Robins & Robins (1970) mentioned their specimens had 9 pores in the preopercular-mandibular canal, except for 1 with 10 (only 10 was adopted by Karmovskaya, 2003). In their first description and drawing (Robins & Robins, 1970: fig. 9), there were 5 compound teeth on vomer, however, in another drawing (Robins & Robins, 1989: fig. 228), they showed 6 compound teeth on vomer. This suggests that slight variation in the number of compound vomerine teeth may present in A. phrix. There are 7 lateral-line pores before pectoral-fin origin, 8 before dorsal-fin origin and 22 lateral-line pores before anal-fin origin, as shown in drawing of the holotype.

Although Robins & Robins (1970) noted that “ anus moderately far forward, slightly more than one head length behind gill opening ” in their diagnosis of Atractodenchelys, the data provided by them showed that the trunk lengths of the holotype and 2 paratypes of A. phrix are shorter than head length, and 1 paratype is slightly longer than the head length. Atractodenchelys brevitrunca exhibits an overlapping trunk length with that of A. phrix and the diagnostic character of Atractodenchelys is modified accordingly.

Comparative material. MNHN 1995-0386, 683 mm TL, holotype of A. robinsorum, Chesterfield Islands, 710 m, 19 Oct. 1986.