Ophichthus pratasensis, Ho & Ng & Lin, 2022

Ho, Hsuan-Ching, Ng, Shing-Lai & Lin, Tzu-Yung, 2022, Description of a new Ophichthus eel from Dongsha Atoll, South China Sea, and a range extension of Ophichthus kusanagi Hibino, McCosker & Tashiro, 2019, Raffles Bulletin of Zoology 70, pp. 312-319 : 313-318

Ophichthus pratasensis , new species

( Figs. 1‒3 ; Table 1)

Holotype. NMMB-P 35993, 585 mm TL, male, off Pratas Island , Dongsha Atoll , South China Sea (ca. 19°22′N, 116°42′E), bottom trawl, ca. 500 m, 17 September 2021.

Diagnosis. A species of Ophichthus with large eyes typical of the O. megalops species group, but lacking a blackened anal-fin base in advance of the tail tip. It is separable from all its Indo-West Pacific congeners in having the following features in combination: eyes large (15.9% HL); snout short and blunt (22.0% HL); rictus slightly anterior of vertical through posterior margin of eye; dorsal-fin origin 2.2 times pectoral-fin length behind tip of pectoral fin; knob present between nostrils; body bicoloured, grey-brown dorsally and uniformly pale ventrally; teeth biserial on upper jaw and uniserial on vomer and lower jaw; 1+4 supraorbital pores and 6+2 preoperculo-mandibular pores; and vertebral formula 20-59-177.

Description. Measurements of holotype in mm: total length 585, head length 51, trunk length 178, tail length 356, predorsal length 93, preanal length 229, pectoral-fin length 13.5, pectoral-fin base 5.8, body depth at gill opening 18.5, body width at gill opening 15.2, body depth at anus 17.3, body width at anus 14.6, snout length 11.2, upper jaw length 20, snout overhang beyond tip of lower jaw 1.8, eye diameter 8.1, interorbital width 9.9, gill opening height 6.5, interbranchial width 13.0.

Body rather slender ( Fig. 1A ), subcircular in cross-section to anterior part of tail, then becoming slightly compressed laterally, its depth at gill openings 32 times in TL. Branchial basket slightly expanded, not deeper than trunk. Skin entirely smooth except for slight wrinkling on branchial basket. Head length 11.5 in TL. Anus situated well before middle of total length, head and trunk combined length 2.6 in TL. Snout short and blunt, narrowly rounded when viewed from above and side, its length 4.6 in HL; underside of snout bisected by short groove. Eye large, over posterior half of upper jaw with posterior margin slightly behind rictus, its diameter 2.5 in upper jaw length and 6.3 in HL. Interorbital space broad, its width 1.2 times eye diameter and 5.2 in HL.

Anterior nostril tubular, extending ventrolaterally from snout, reaching chin when directed downward. Posterior nostril a hole above upper lip, covered by large flap extending slightly below edge of mouth gape. No barbel on upper lip, but broad knob present between nostrils. Tip of lower jaw extending very slightly beyond anterior margin of anterior nostril tube. Upper jaw moderately small, its length 2.6 in HL. Rictus situated slightly anterior to vertical through posterior margin of eye, followed by short fold at posterior end of mouth gape; rear end of this fold reaching to slightly behind same vertical.

Dorsal-fin origin far behind head, 2.2 times pectoral-fin length behind fin tip and 0.9 times HL behind head. Median fins low but obvious, ending slightly less than one eye diameter before tapering tail tip. Pectoral fin with narrow base, fin length 2.3 times basal height, broad at midlength, somewhat wedge-shaped and slightly pointed, with its longest rays at mid-fin.

Head and body pores small but well apparent ( Fig. 3A ). Supraorbital pores (SO) 1 (ethmoid) + 4 on dorsal surface of snout and interorbital space; infraorbital pores (IO) 3 + 3, comprising 1 between nostrils, 2 below eye, and 3 behind eye; mandibular pores (M) 6 (right) / 7 (left), the last pore being situated anterior to rictus; preopercular pores (POP) 2; frontal pore (F) 1; supratemporal pores 3 (ST; single median pore). No sensory papillae present on head. Lateralline pores small: cephalic pores 7 (right) / 7 (left), forming arch, predorsal lateral-line pores 21/21; preanal lateral-line pores 60/59; total pores 161/157, the last at about 1/2 head length in advance of tail tip.

Teeth ( Fig. 3B ) moderately large, conical and closely spaced. Intermaxillary with 14 irregularly sized teeth arranged in oval patch, followed by single row of 16 smaller teeth on vomer, these decreasing slightly in size posteriorly. Biserial tooth rows on maxilla comprising 25/25 teeth in outer row, these becoming gradually smaller posteriorly, and 12/13 teeth in inner row, these being flanked by about posterior 2/3 of outer row with inner teeth slightly larger than adjacent outer teeth. Mandible with 34/ca. 32 moderately small teeth arranged in single row and becoming slightly smaller posteriorly.

Colouration. In both fresh and preserved condition, body distinctly bicoloured, with dorsal half of head and body greyish-brown and ventral surface uniformly pale; dorsal, pectoral, and anal fins pale. Mouth cavity pale with several scattered irregular back dots. Peritoneum densely covered with black pepper dots, thereby appearing nearly uniformly black; stomach and intestine pale.

Etymology. The specific name, an adjective, is derived from Pratas Island, the only island of Dongsha Atoll, whose large coral reefs surrounded by deep sea are situated near the holotype’s collection site. With this name we also hope to call attention to the possible damage that frequent fishery activity in the area might be causing to the environment and natural resources of Dongsha Atoll.

Distribution. Known only from the holotype taken at a depth of ca. 500 m near Dongsha Atoll, but possibly having a broader range in the South China Sea.

Comparison. When first found, the present holotype was identified as Ophichthus megalops , but it lacks one of that species’ most distinct features, a blackened posterior anal-fin base. The species is still recognised as a member of the O. megalops species group (herein established) on account of its large eyes, the DFO’s location well behind the tip of the pectoral fin, and the bicoloured body (grey-brown dorsally and pale ventrally).

It can be separated from the three members of the species group, viz., O. brevidorsalis , O. megalops , and O. semilunatus , by the single row of teeth on the lower jaw, the position of the rictus slightly before a vertical through the posterior margin of the eye, the relatively few predorsal vertebrae (20 vs.>28) and predorsal lateral-line pores (21 vs.>28), the smaller head (8.7% TL vs.>9.4% TL), the shorter predorsal length (15.9% TL vs.>20.7% TL) and upper jaw (39.2% HL vs.>41.2% HL), and the smaller pectoral fins (26.5% HL vs.>30.0% HL). It can be further distinguished from them by the vertebral formula 20-59- 177, vs. 31-61-164 (mean) in O. megalops , 29-64- 176 in O. semilunatus , and 43-61- 164 in O. brevidorsalis (data from Hibino et al., 2019a).

Moreover, it differs from O. mccoskeri in having a single row of teeth on lower jaw (vs. irregularly biserial and ending in triserial), rictus slightly behind posterior margin of eye (vs. well behind the margin), a smaller head (8.7% TL vs. 13.0‒14.0% TL), a smaller eye (15.9% HL vs. 17.1‒19.4% HL), a shorter preanal length (39.1% TL vs. 44.2‒45.4% TL), a shorter predorsal length (15.9% TL vs. 18.4‒20.5% TL), a short and broad pectoral fin (26.5% HL in length vs. the fin slender and 29.1‒33.0% HL), 2 POP pores (vs. 3), and relatively more vertebrae (20-59-177 vs. 20-55-153) (data from Sumod et al., 2019).

Among the species of Ophichthus that have the DFO situated behind the head by two pectoral-fin lengths or more and no blackened anal-fin base in advance of the tail tip (see Vo & Ho, 2021: table 2), O. pratasensis is readily distinguished by having a different vertebral formula. The low knob on the upper lip in O. pratasensis is reminiscent of O. kusanagi , from which the new species differs in having a bicoloured body (vs. uniformly dark brown), more total vertebrae (177 vs. 158‒163), the DFO situated 2.2 times the pectoral-fin length behind the tip of the fin (vs. 3‒3.5 times) and HL 3.5 times in trunk length (vs. 4.1 times) (data from Hibino et al., 2019b, and this study).

Among the species of Ophichthus that have the DFO situated well behind the head and a blackened anal-fin base in advance of the tail tip (e.g., O. exourus McCosker, 1999 , O. brachynotopterus Karrer, 1983 , and O. mystacinus McCosker, 1999 ), O. pratasensis is most similar to O. exourus in having almost identical body proportions and vertebral formula (mean 19.7-60-177.3, total 176‒179; see McCosker, 1999, 2010). Ophichthus pratasensis differs from O. exourus in having uniserial teeth on lower law (vs. some additional teeth on inner row anteriorly and posteriorly), a somewhat pointed pectoral fin (vs. pectoral fin rounded and paddle-like), a shorter and stouter snout (22.0% HL vs. 25.2‒28.4% HL), a larger eye (15.9% HL vs. 11.2‒12.8% HL), a broader interorbital space (19.4% HL vs. 13.5‒14.1% HL), a broader interbranchial width (25.5% HL vs. 14.1% HL), and a smaller ratio of snout length/eye diameter (1.4 vs. 2.2) (data from McCosker, 1999, 2010).

Ophichthus pratasensis differs from O. brachynotopterus in having uniserial teeth on the lower jaw and vomer (vs. biserial), the rear margin of the eye slightly behind the rictus (vs. slightly in advance of the rictus) and the posterior nostril well covered by a large flap on the upper lip (vs. barely covered with a narrow flap), and from O. mystacinus in having the DFO situated at about one-third trunk length (vs. about mid-trunk), short pectoral fin (vs. elongate fins with threadlike central rays), the posterior margin of eye behind the rictus (vs. in front of the rictus), and uniserial teeth on the lower jaw and vomer (vs. biserial on the lower jaw and anterior part of the vomer) (data from McCosker, 2010).

Remarks. The holotype is a ripe male with moderately developed testes.

Figures

Fig. 1. Ophichthus pratasensis, new species, holotype, fresh, NMMB-P35993, 585 mm TL. A, lateral view; B, lateral view of right side of head; C, posterior portion of tail, left side, anterior to left, preserved. Not to scale.

Fig. 2. Ophichthus pratasensis, new species, holotype, fresh, NMMB-P35993. A, dorsal view; B, ventral view.

Fig. 3. Drawings of (A) head pores and (B) teeth on upper jaw (left) and lower jaw (right). Knob between nostrils is indicated by arrow. Missing teeth are represented by dotted lines.