Ophichthus megalops Asano, 1987

(English name: Large Eye Snake Eel)

(Taiwanese common name: ƛēķḋ)

Ophichthus megalops Asano, 1987:135 (type locality: Kumano-nada, off Owase, Mie Prefecture, Japan); Hatooka 2000:223 (off Owase, Mie); Hatooka 2013:223 (off Owase, Mie); McCosker 2010:28 (off Owase, Mie, Japan); McCosker et al. 2012:7 (Japan, key only); McCosker & Ho 2015:72 (Taiwan, key only); Ho et al. 2015:173 (in part; Nan-fang-ao, Taiwan).

Specimens examined. FAK 19057, holotype, 325 mm TL, sexuality unknown, Kumano-nada coast, off Owase, Mie Prefecture, Japan, 360 m depth, bottom trawl, collected by H. Asano; KAUM–I. 125145, 330 mm TL, sexuality unknown, Dong-gang, Taiwan, depth unknown, bottom trawl, 26 Dec. 2018, collected by K. Koeda; NMMB-P12006, 7 specimens, 288–500 mm TL, sexuality unknown, Nan-fang-ao, Taiwan, depth unknown, bottom trawl, 11 Mar. 2011, collected by H.-C. Ho; NMMB-P 12193, 408 mm TL, sexuality unknown, Nan-fang-ao, Taiwan, 300–400 m depth, bottom trawl, 12 Jan. 2011, collected by H.-C. Ho; TOU-AE 6871–6878, 8 specimens, 299–442 mm TL, 7 mature females and 1 immature male, Nan-fang-ao, Taiwan, 300–400 m depth, bottom trawl, 15 Apr. 2013, collected by Y.-C. Chiu; TOU-AE 7153, 375 mm TL, sexuality unknown, Nan-fang-ao, Taiwan, 300–400 m depth, bottom trawl, 24 Jun. 2013, collected by Y.-C. Chiu.

Diagnosis. A species of Ophichthus with a dark-tipped anal fin, and the following combination of characters: head 10.1–11.5% TL; tail 54.5–57.4% TL; snout blunt, 19.6–22.9% HL; eye large, 92.4–110.6% of snout length and 14.0–22.8% HL; dorsal-fin origin well behind pectoral-fin tip by about three times pectoral-fin length; pectoral fin short, pedal-shaped, tip not filamentous; anterior-nostril tube whitish; posterior nostril a hole above upper lip with a tiny flap anteriorly; teeth on maxilla biserial; SO 1+4; POM 6+3; total vertebrae 162–168 and MVF 31-61-164.

Description. Counts and measurements are shown in Table 1. Body long, subcylindrical, its depth at gill openings 22.2–30.3 in TL; tail slightly compressed posteriorly, its depth reduced gradually, tip elongate (Fig. 1A).

Head relatively large, branchial basket moderately expanded; head 8.7– 9.9 in TL; head and trunk 2.2–2.3 in TL; snout short, weakly acute, its length generally slightly longer than eye diameter; no ventral groove on snout; lower jaw included in upper jaw, its tip slightly beyond anterior base of anterior-nostril tube; mouth large, rictus well behind a vertical from posterior margin of eye, with a short, additional fold below rictus; eye large, 2.1–2.2 in upper jaw and 4.4–7.1 in head; anterior nostril a simple tube with a shallow notch on rim anteriorly; posterior nostril a hole opening outside mouth, above upper lip, slit-like with a short slim flap anteriorly; upper lip smooth, without protrusions, numerous small papillae on inner upper and lower lips; interorbital region smooth and mostly flat; gill opening constricted, located ventrolaterally (Fig. 2A).

Dorsal and anal fins relatively low, ending with a ridge almost equal to half of snout length; dorsal-fin origin behind pectoral-fin tip, 2.5–4.3 times pectoral-fin length (2.5 in holotype), behind level of gill opening and 1.2–1.5 times in HL (1.2 in holotype); caudal fin absent; pectoral fin short, pedal-shaped, its tip pointed but not filamentous.

Head pores small but obvious; arrangement of sensory pores on head as follows (Fig. 2A): one + four on supraorbital, one pore above the posterior rim of posterior nostril; three + three on infraorbital, one between anterior and posterior nostrils; six on mandible and three on preopercle; midtemporal and interorbital pores present. Lateral-line pores almost complete, 8–10 (10 in holotype) in branchial basket, 28–35 (32) anterior to dorsal-fin origin, 59–64 (61) anterior to anus, and 137–146 (141) in total, canal and pores absent in 0.6–0.9 times (0.7) of HL.

Teeth relatively small, conical, pointed, weakly recurved posteriorly; teeth on maxilla biserial, those on outer row smaller and arranged close-set; mandibular teeth arrangement X-shaped: biserial on site near snout (less, 2–3) and rictus site (more, ca. 20), others uniserial (Fig. 2B, C); vomerine teeth biserial anteriorly and uniserial posteriorly; intermaxillary teeth arranged in a broken circular pattern, fused with row on vomer.

Coloration when fresh (Fig. 1): head and body bluish to deep brownish dorsally and whitish on belly; peritoneum silver gray, with numerous minute, black dots along myomeres (Fig. 1B); anus generally without surrounding, darker marking or rarely less faded melanophores (only KAUM–I. 125145); tube of anterior nostril whitish; sensory pores not marginated. Dorsal fin white without darker margin; anal fin pale except distinct black end about 1/2 HL from tail tip (Fig. 1C); pectoral fin white. After preservation in 10% formalin and transferring in 75% ethanol, body coloration becomes monotonous.

Etymology. The scientific name megalops refers to the remarkably large eye (Asano 1987).

Distribution. Known from off Owase, Mie Prefecture, Japan (type locality); Nan-fang-ao, and Dong-gang, Taiwan, 300–400 m depth (Asano 1987; this study).