Ophichthus exourus McCosker, 1999

Tashiro, Fumihito, Hibino, Yusuke & Miyamoto, Kei, 2017, First Records of the Rare Snake Eel Ophichthus exourus (Pisces: Anguilliformes: Ophichthidae) from the Northern Hemisphere, Species Diversity 22, pp. 213-217 : 213-216

Ophichthus exourus McCosker, 1999

[New standard Japanese name: Gunbai-umihebi] ( Figs 1–3 , Table 1)

Ophichthus exourus McCosker, 1999: 575 , figs 1, 2A (type locality: New Caledonia, 18°56.30′S, 163°12.90′E, 520 m depth); McCosker, 2010: 15 ( New Caledonia).

Material examined. FAKU 103293, 685 mm TL, female, off Saipan , August 1967; OCF-P03211 , 634 mm TL, female, sea surface off western coast of Okinawa Island , Okinawa Prefecture , East China Sea, Japan, 26°34.43′N, 127°51.53′E, 16 March 2016 .

Diagnosis. A species of Ophichthus with following combination of characters: head 8.3–10.0% TL; upper jaw 36.1–38.1% HL; pectoral-fin rounded, length 19.2–24.1% HL; rear margin of eye above or slightly behind rictus; eye diameter 44.4–61.2% snout length; horizontal length of posterior nostril dermal flap much greater than diameter of base of anterior nostril; gill opening to dorsal-fin origin greater than twice pectoral-fin length; preopercular pores 2; predorsal and total vertebrae 20–21 and 173–177, respectively; lower-jaw teeth nearly uniserial; body without bands or spots; posteriormost portion of anal-fin membrane (much shorter than head length) indistinct darkish brown.

Description. Counts and proportional measurements are given in Table 1. Body moderately elongate, cylindrical anteriorly, gradually tapering and laterally compressed posteriorly; head considerably elongate; anus situated somewhat anterior to midpoint of body. Snout obtuse, conical; fleshly tip congruent with or slightly overhanging tip of mandible. Mouth moderate, subterminal; rictus nearly straight, its posterior end just below or slightly beyond posterior margin of eye; lips with minute papillae, barbels ab- sent on upper lip. Eye large, round, its rear margin behind midpoint of rictus. Anterior nostril short, tubular, its height 2.7–2.8 in eye diameter; tip distinctly notched. Posterior nostril a slit-like pore covered by a flap, situated on outer edge of upper lip slightly behind anterior nostril; horizontal length of posterior nostril dermal flap much greater than diameter of anterior nostril tube. Gill opening large, crescentic, situated low on side just anterior to pectoral-fin base. Gill opening to dorsal-fin origin greater than twice pectoral-fin length; anal-fin origin just behind anus; dorsal and anal fins moderately elevated, sharply reducing in height posteriorly, but posteriormost portion expanded ( Fig. 1 ); maximum heights of dorsal and anal fins approximately equal to eye diameter. Pectoral fin rounded, lacking elongate rays ( Fig. 2A ). All teeth small, simple and conical, with tips slightly curved posteriorly. Intermaxillary with rosette of 7–14 teeth, single row of 16–21 teeth on vomer ( Fig. 2B ). Maxillary dentition biserial, 24–32 teeth on outer and 15–18 on inner rows ( Fig. 2B ); inner teeth distinctly larger than outer. Lower jaw teeth mostly uniserial with 30–35 teeth, a few inner teeth scattered on inner edge ( Fig. 2B ). Lateralline and cephalic pores minute. Frontal pore 1; supratemporal pores 0−1+1+1=2–3; supraorbital pores 1+4=5; infraorbital pores 2+4=6; preoperculo-mandibular pores 6−7+2=8–9 ( Fig. 2A ).

Color in fresh condition (based on OCF-P03211) ( Fig. 1 ). Body brownish dorsally, whitish ventrally, but uniformly brown for a portion equal to head length just before tail tip; fine brownish mottling ventrally on trunk. Median and pectoral fins colorless, except for posteriormost portion of anal fin (length equal to ca. one third head length) indistinct darkish brown ( Fig. 3 ).

Color in alcohol. Body coloration of OCF-P03211 had scarcely changed from that in fresh condition. The older specimen, FAKU 103293, was entirely faded due to longterm preservation in 10% formalin solution.

Distribution. Widely distributed in the western Pacific, including New Caledonia, Fiji, Saipan and Japan. The type specimens were collected from the deep-sea floor (400– 520 m depth) off New Caledonia (holotype) and off Fiji (paratype), although two individuals were collected from midwater (exact depth unknown) and the sea surface, respectively ( McCosker 1999, 2010; this study).

Remarks. Morphological features of the present specimens were almost identical with those of O. exourus as given in the original description, except for the number of infraorbital pores; i.e., 5 (2+3) pores in the holotype of O. exourous vs. 6 (2+4) in the former ( McCosker 1999). However, examination of the holotype showed that the infraorbital pores in fact numbered 6 (2+4), not 5 (2+3) as shown in McCosker (1999).

Ophichthus exourus shares a similar general appearance with Ophichthus brachynopterus Karrer, 1983 and Ophichthus mystacinus McCosker, 1999 , the three species being distinguishable from other congeners by the following combination of characters: preopercular pores 2; gill opening to dorsal-fin origin greater than twice pectoral-fin length; body without bands or spots; and anal-fin end darkened ( McCosker 2010). Ophichthus exourus is further distinguishable from O. brachynopterus by head length (8.3–10.0 vs. 10.2–10.8% TL), pectoral-fin shape and length (rounded and 19.2–24.1 vs. wedge-shaped and 25.0–27.8% HL), position of rear margin of eye (above or slightly behind rictus vs. slightly in advance of rictus), eye diameter (44.4–61.2 vs. 75.1–85.1% of snout length), horizontal length of dermal flap of posterior nostril (much greater than diameter of anterior nostril tube vs. almost equal to diameter of anterior nostril tube), upper jaw length (36.1–38.1 vs. 38.5–45.5% HL), predorsal vertebrae (20–21 vs. 27–31) and teeth arrangement in lower jaw (almost uniserial vs. almost biserial); and from O. mystacinus by pectoral-fin shape and length (rounded and 19.2–24.1 vs. filamentous and 38.5–50.0% HL), position of rear margin of eye (above or slightly behind rictus vs. slightly in advance of rictus), upper jaw length (36.1–38.1 vs. 50.0–55.6% HL), counts of predorsal and total vertebrae (20–21 predorsal and 173–177 total vs. 31–35 and 169–174), teeth arrangement in lower jaw (almost uniserial vs. completely biserial), and coloration and length of posterior-most darkened portion of anal-fin (indistinct darkish brown and much shorter than head vs. distinctly black and longer than head length) ( Karrer 1983; McCosker 1999, 2005, 2010; this study).

The type specimens of O. exourus were collected from the deep-sea floor at 400–520 m depth off New Caledonia (holotype) and Fiji (paratype). An additional specimen (MNHN 2001-1062), captured by pelagic trawl in 515–585 m depth off New Caledonia, was listed in McCosker (2010). The present two specimens from Saipan and Okinawa extend the distributional range of the species to the Northern Hemisphere. A new standard Japanese name, “Gunbai-umihebi”, is proposed for O. exourus , in reference to a species of Ophichthus (= “Umihebi”) having a strange tail-end shape, viz., expansion of the dorsal and anal fins in a manner reminiscent of a Japanese traditional gourd-shaped fan (=“Gunbai”). Although it is generally known that ophichthid eels are bottom-inhabiting fishes, the two O. exourus individuals were collected in midwater (MNHN 2001-1062) and on the surface (OCF-P03211). Some similar cases have been reported for other species of Ophichthidae (e.g., McCosker 1977; Ho et al. 2013), and the occurrence of bottom-inhabiting species on the surface has been occasionally noted during field surveys by the first two authors. However, reasons for this phenomenon remain unclear.

Comparative materials. Ophichthus brachynotopterus: MNHN 1979 -21, 435 mm TL, holotype, Madagascar, 12°40′S, 48°13.8′E, 18 January 1972; MNHN 1979-22, 441 mm TL, paratype, Madagascar, 12°52′S, 48°10.3′E, 4 March 1971; MNHN 1979-23, 411 mm TL, paratype, Madagascar, 12°43′S, 48°15′E, 14 April 1971. Ophichthus exourus: MNHN 1995 -425, 590 mm TL, holotype, New Caledonia, 18°56.30′S, 163°12.90′E, 18 September 1985; MNHN 2001- 1062, 549 mm TL, New Caledonia, 21°13.12′S, 165°55.12′E, 12 March 1993 . Ophichthus mystacinus: MNHN 1998 -46, 426 mm TL, holotype, MNHN 1998-47, 383 mm TL, paratype, 18°55.80′S, 163°13.80′E, 20 September 1985 .

Figures

Fig. 1. Fresh specimen of Ophichthus exourus, OCF-P03211, 634mm TL, off Okinawa Island, Okinawa Prefecture, East China Sea, Japan.

Fig. 2. Lateral view of the head (A), and schematic diagram of dentition on upper jaw (left) and lower jaw (right) (B), of Ophichthus exourus, OCF-P03211, 634mm TL.Dotted circles in (B) represent missing teeth.

Fig. 3. Enlarged lateral view of posterior end of Ophichthus exourus, OCF-P03211, 634mm TL.