Tenuicephalus melampeplus ( Alcock, 1896 )

Schwarzhans, Werner W. & Møller, Peter R., 2021, Revision of the ‘ dragon-head’ cusk eels of the genus Porogadus (Teleostei: Ophidiidae), with description of eight new species and one new genus, Zootaxa 5029 (1), pp. 1-96 : 75-76

Tenuicephalus melampeplus ( Alcock, 1896)

Figs. 38 , 48 , 51 , Tab. 1–7

Dermatorus melampeplus Alcock, 1896: 65 ; Alcock 1897: pl. 17 fig. 3.

Porogadus melampeplus: Nielsen et al. 1999: 86 ; non Shcherbachev 1980: 92.

Porogadus sp. : Nielsen & Møller 2007: 33 (pars, not figured).

Material examined ( 2 specimens and photograph and selected data of holotype): Holotype ZSI F60/ 1, 218 mm SL (calculated from photograph), 15°11’N 72°28’E, 1702 m; ZMMGU 15103 , 177 + mm SL, R / V Fiolent, Cruse 9, trawl 51, 01 September 1977, 09°48’S 60°42’E; ZMUC P 771580, 135 mm SL, 13°45’S 156°41’E, 2255–2283 m.

Diagnosis. Precaudal vertebrae 14–16, 3 loin vertebrae (last precaudal vertebrae without ribs); long gill rakers on first gill arch 14–16; pectoral-fin rays 16–17; HL:HD 1.40–1.48; no head spines; opercular spine sharp, extruding; opercular flap small; vomer narrow, naked or with few teeth; palatines narrow with 2–3 rows of teeth; basibranchial tooth patch very small with 1–2 rows of teeth; otolith with single colliculum; OL:OH = 0.95; OL:TCL = 1.87.

Description. Meristics: precaudal vertebrae 14–16, 3 last vertebrae without ribs; pectoral-fin rays 16 (16–17); D/V not discernable; D/A not discernable; V/A = 15; long gill rakers on lower gill arch 16 (14–16). Gill rakers in a non-type (ZMMGU 15103) on lower first gill arch with nine short rakers, followed by a series of 15 long rakers. The lower nine of those intercepted by short spiny rakers directed inwards. Upper gill arch with five semi-long rakers intercepted by five short rakers.

Morphometrics: in % of SL (values for holotype calculated from photograph): HL 16.4 (16.4–17.2); maximal HD 11.6 (11.6–12.1); HD through center of eye 7.4; bony interorbital width not measurable; snout length 4.6 (4.6–5.8); upper jaw length 9.8 (9.8–11.9); predorsal 16.3 (16.0–16.3); preanal 32.2 (30.0–32.6); prepelvic and prepectoral not measurable; pectoral length not measurable. Relations: HL:HD = 1.42 (1.40–1.48); maximal HD: HD through center of eye 1.50–1.63; HL to snout length 3,58 (2.95–3.58); preanal to predorsal 1.97 (1.97–2.04); predorsal to prepectoral not measurable.

Overall slender fish with long tapering tail, relatively stout head and moderately long snout. Maximal size of fishes investigated 218 mm SL (holotype, calculated from photograph). Head with flat or slightly concave dorsal profile, without spines except opercle with small but sharp, extruding spine. Eye moderately small located in strongly asymmetric orbit. Maxilla extending far beyond eye, strongly widened posteriorly; supramaxilla not discernable. Infra-/postorbital and mandibular-preopercular pores wide. No head squamation discernable. Opercular flap small, without ridges. Lateral line not discernable.

Dentition.All teeth tiny and cone-shaped. Vomer naked or with few sharp teeth posteriorly (in ZMUC P771580, Fig. 38D ); palatines with a narrow dentition patch with 1–3 rows of teeth only anteriorly. Premaxilla tooth patches not fused anteriorly; 5 rows of teeth in middle part. Dentary tooth patches not fused anteriorly; ca. 8 rows anteriorly and 2 rows posteriorly. Median basibranchial tooth patch very small and narrow with 1–2 rows of teeth.

Otolith morphology (n = 1). Size 2.65 mm in length (ZMMGU 15103); OL:OH = 0.95; OH:OT = 2.9. Otolith distinctly higher than long, moderately thick. All rims regularly curved and smooth, preventral and postdorsal regions expanded. Inner face more or less flat, with short, distinctly inframedian sulcus; OL:TCL = 1.85. Sulcus with shallow, undivided, uniform, oval, moderately narrow colliculum. Dorsal field very wide, without distinct depression; ventral field smooth. Outer face smooth, with low relief inferior umbo.

Coloration. Live coloration not known. Color of preserved holotype medium brown throughout (from photograph) but described by Alcock (1896) as uniform purple-black. The color change may be due to the extended period of preservation. The two more recently caught specimens are indeed nearly black and thus supporting Alcock’s description.

Discussion. Alcock (1896) described Dermatorus melampeplus based on a single specimen which he noted as too much damaged for description but noted that it would be “at once distinguished from Dermatorus trichiurus and melanocephalus (1) by the complete absence of teeth on the vomer, and (2) by its uniform purple-black color.” His drawing ( Alcock 1897) is consistent with the photograph thankfully provided by ZSI. The characters mentioned by Alcock as diagnostic for T. melampeplus , however, are now consider prime diagnostic features for the new genus Tenuicephalus .

Tenuicephalus melampeplus is recognized by a relatively large head (HL in % of SL = 16.4–17.2) and a low number of long gill rakers (14–16), while the co-occurring species T. multitrabs n. sp. and T. squamilabrus n. sp. have smaller heads (HL in % of SL = 14.4–1.6.2) and more long gill rakers (17–21). Also, both species exhibit a few weak head spines. Tenuicephalus melampeplus shares the absence of head spines with T. silus from the Caribbean, and the head size and number of gill rakers overlap as well, but it differs in having 3 loin vertebrae (vs 8–10) and a sharp opercular spine (vs flat). Tenuicephalus melampeplus further is unique for its otoliths which is higher than long, a unique observation in the entire Porogadus lineage, but however otoliths are not known from T. silus .

We were not able to investigate the unique holotype, but thankfully received photographs from ZSI (Kolkata, see acknowledgements) and also certain counts such as long gill rakers and pectoral fin-rays. Most of the specimens of the records of Porogadus melampeplus by Shcherbachev (1980) from the Indian Ocean held at ZMMGU were investigated and except for one are now interpreted to represent T. multitrabs n. sp. The two newly recognized specimens of T. melampeplus unfortunately are poorly preserved and therefore a number of deemed important characters could not be sufficiently investigated or verified.

Distribution. Tenuicephalus melampeplus is based on only three verified specimens, two of which, including the holotype, stem from the western Indian Ocean and one from the Solomon Sea in the western Pacific.

Figures

FIGURE 38. Tenuicephalus melampeplus (Alcock, 1896); 38A, ZSI F60/1, holotype, 218 mm SL, 15°11’N, 72°28’E, 1702 m, (no scale given, calculated from photograph); 38B–D, ZMUC P771580, 135 mm SL, 13°45’S, 156°41’E, 2255–2283 m, B–C head drawings, D drawing of vomer and tooth bearing anterior part of palatines; 38E–G, ZMMGU 15103, 177+ mm SL, 09°48’S, 60°42’E, otolith (E inner face, F ventral view, G anterior view).

FIGURE 48. Geographic distribution of the species of the genus Tenuicephalus.

FIGURE 51. Depth versus distance from shelf plots for specimens of the genus Tenuicephalus.