Echelus polyspondylus McCosker & Ho, 2015

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 : 77-78

Echelus polyspondylus McCosker & Ho, 2015

( Fig. 3A‒C , Table 1)

Echelus polyspondylus McCosker & Ho, 2015: 73 , figs. 1‒3 (type locality: Dong-gang, Pingtung, southwestern Taiwan, South China Sea, depth ca. 200 metres). Ho et al., 2015a: 8. Ho et al., 2015b: 168.

Specimens examined. All collected by bottom trawl, ca. 150–200 m, from off LƯƠng SƠn, Nha Trang , Khánh Hòa, southeast coast of Vietnam, South China Sea. NMMB-P34716 , 2 specimens, 455‒468 mm TL, 24 August 2019 ; NMMB-P34717 , 3 specimens, 368‒445 mm TL, 3 August 2019 ; OIM-E.55810, 11 specimens, 354‒480 mm TL, 1 cleared & stained, 24 August 2019; OIM-E.55811, 2 specimens, 336‒364 mm TL, 1 cleared & stained, 3 August 2019.

Diagnosis. A species of Echelus with tail 64–69%, head 7.6–9.3%, and body depth at gill opening 1.8–2.9% TL; tail tip flexible with clear caudal‒fin rays; DFO 0.7‒1 HL behind head; pectoral fin small, pointed, not elongate; posterior nostril within upper lip, covered by a large flap; head pores: SO 1+4, IO 4+2, POM 6+3; teeth small, conical, biserial anteriorly and uniserial posteriorly on jaws, uniserial on vomer; colouration light brown dorsally, pale ventrally, fins pale except anal-fin membrane which is black before tail tip; total vertebrae 172–183 ( McCosker & Ho, 2015; this study). Description. Body rather elongate ( Fig. 3A ), slightly compressed, gradually becoming more compressed to tail, its depth at gill openings 35‒45 (mean 41) times in TL. Branchial basket not especially expanded. The skin smooth through entire body. Head moderately large, its length 10.7–13.1 in TL. Anus situated well before middle of total length, head and trunk 2.5‒3.1 in TL. Snout moderately long and blunt, broadly pointed when viewed from above; underside of snout not bisected by a groove. Lower jaw included, its tip extends to posterior margin of anterior nostril tube. Upper jaw moderately long, rictus well behind a vertical through posterior margin of eye.

Eye large, its anterior margin at about middle of upper jaw, its diameter 1.9‒2.7 in upper jaw and 8.2‒10.5 in HL. Anterior nostril tubular, extending anteroventrally from snout, not reaching lower jaw when directed downward. Posterior nostril within upper lip, covered by a large flap that extends well below edge of mouth gape. No barbel on upper lip. Dorsal-fin origin at about anterior 1/3 of trunk length, 0.7‒1 HL behind head. Median fins low but obvious, continuous to a small caudal fin, the fin with short rays. Pectoral fin small with a narrow base, slightly elongate and slightly pointed posteriorly, the longest rays at mid-fin.

Head pores apparent ( Fig. 3B ). SO 1 (ethmoid) + 4 on dorsal surface of snout and interorbital space; IO 4 + 2, 2 between nostrils, 2 below the eye, and 2 behind the eye; M 6, the last pore slightly behind rictus; POP 2; ST 3; F 0. Lateralline pores large, HLL 9–10, in an arching sequence, PDLL 20‒25; PALL 50‒56, TLL 165‒171, the last at about two eye diameters before tail tip.

Teeth ( Fig. 3C ) moderately large, conical, and closely spaced. Intermaxillary with a long oval patch of teeth arranged in 3 or 4 rows; single row of vomerine teeth. Maxillary and mandibular teeth biserial mostly, except for some additional small teeth on anterior portions forming 3 rows. Some specimens with single row of teeth in posterior portion of mandibular.

Colouration. When fresh ( Fig. 3A, B ), body light greenish brown dorsally with lower portion of head and trunk pale; fins pale, except for posterior portion of anal-fin, about 2/3 head length in advance of tail tip, and lower margin of caudal fin darkened. When preserved, body pale yellow dorsally, gradually paler ventrally. Fine light brown pigments on dorsal surface of head and trunk and chin, gradually extending lower on tail; underside of head and trunk pale without pigments. Posterior portion of anal fin about 2/3 head length in advance of tail tip, and lower margin of caudal fin darkened. Mouth cavity pale. Peritoneum pale with many black pigments on upper half; stomach and intestine pale.

Distribution. Described from Taiwan and newly recorded from off Nha Trang, Vietnam.

Remarks. Our specimens have branchiostegal rays largely overlapped and short but clearly visible fin rays on caudal fin. The morphology and meristic data of Vietnamese specimens agree well with the type series and additional specimens collected from Taiwan, except there are relatively few pores in PALL 50‒56 (vs. 56‒60 in type series) and TLL 165‒171 (vs. 171‒178). These differences may be attributed to geographic and population variations.

Comparative materials. Ophichthus aphotistos: ASIZP 63657 (1, 657 mm TL), Daxi , Yilan, NE Taiwan, 10 April 2000. ASIZP64287 (1, 469 mm TL), Daxi, 6 July 2004. CAS 209192 (1, 580 mm TL), holotype, Dong-gang, Pingtung, SW Taiwan, 20 February 1999. USNM 356862 (1, 577 mm TL), paratype, Dong-gang, 20 February 1999. Echelus polyspondylus : type series listed in McCosker & Ho (2015).

Figures

Fig. 3. Echelus polyspondylus McCosker & Ho, 2015, OIM-E.55810, 482 mm TL. A, fresh specimen; B, close-up of head; C, teeth on jaw. Pin is inserted at dorsal-fin origin.