Porogadus melanocephalus ( Alcock, 1891 )

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 : 64-67

Porogadus melanocephalus ( Alcock, 1891)

Figs. 32–33 , 46 , 49 , Tab. 1–7

Dermatorus melanocephalus Alcock, 1891: 32 ; Alcock 1898: pl. 21 fig. 4.

Porogadus melanocephalus: Nielsen et al. 1999: 86 .

Material examined (48 specimens): Syntype BMNH 1896.9 .11.6, 183 mm SL, 11°58’N 88°52’E, 3197 m ; BSKU 65559, 175 mm SL, 13°05’N 90°39’E to 13°03’N 90°37’E, 2967–2978 m , R / V Hakuho-maru, 3 m beam trawl, 9 Feb. 1990 ; ZMUC P 2397161, 161 mm SL, Galathea 2 expedition, Sta. 282, 32 m wide herring otter trawl, 05°32’N 78°41’E, 4040 m, 11 April 1951 ; ZMUC P2397162-206 , (45 specimens), 86–172 mm SL, Galathea 2 expedition, Sta. 299, 32 m wide herring otter trawl, Bay of Bengal , 17°10’N 84°30’E, 2820 m, 24 April 1951 .

Diagnosis. Precaudal vertebrae 16–17; long gill rakers on first gill arch 12–15; pectoral-fin rays 15–18; HL: HD 1.71–1.88; most head spines weak, present on ethmoidal, prefrontal, interorbital, sphenotic, outer posttemporal; absent on lacrimal, supraorbital, 5th infraorbital, supratemporal, inner posttemporal, inner and outer preopercular rim; opercular spine weak, flat; opercular flap small; lower lateral line pores until beginning of anal fin 17; vomer with narrow dentition patch with 1–2 rows of teeth; palatines with broad dentition patch with 5–8 rows of teeth; otolith with single colliculum; OL:OH = 1.15; OL:TCL = 3.25.

Description (based on non-types and syntype with dissected head top). Meristics: precaudal vertebrae 16 (16– 17), 1 (1–2) last vertebrae without ribs; pectoral-fin rays 18 (15–18); D/V = 5 (4–6); D/A = 24 (23–26); V/A = 17 (17–18); long gill rakers on first lower gill arch 12 (12–15), blade like. Gill rakers in a syntype (BMNH 1896.9.11.6) on lower first gill arch with seven short rakers, followed by a series of 12 long rakers. The lower four of those intercept by narrow plate shaped rakers. These are smaller than in most species and placed toward the inner margin. Upper gill raker with a serie of three semi long rakers and three plate like rakers intercepted, followed by two very short rakers.

Morphometrics: in % of SL: HL 17.3 (15.4–17.3); maximal HD 8.3–9.8; HD through center of eye 5.4–6.5; bony interorbital width 2.4–3.1; snout length 5.1–6.1; upper jaw length 10.5 (9.1–10.6); predorsal 17.2–18.5; preanal 33.2 (29.9–33.2); prepelvic 13.8 (12.9–13.9); prepectoral 16.0–17.7; pectoral length 9.9–11.4. Relations: HL: HD = 1.70–1.84; HL to snout length 2.76 (2.64–3.07); preanal to predorsal 1.74 (1.72–1.87); predorsal to prepectoral 1.01 (0.99–1.07).

Slender fish with long tapering tail and moderately long snout. Maximal size of fishes investigated 183 mm SL (syntype). Head moderately long and slender, with flat to slightly concave dorsal profile, with weak spines as follows: ethmoidal (1 broad), prefrontal (1), interorbital (1–2), sphenotic (2–3), supratemporal (2), outer posttemporal (1–2); no spines on lacrimal, supraorbital, 5th infraorbital, inner posttemporal, inner and outer preopercular rim. Opercle with weak, flat spine. Eye moderately small located in strongly asymmetric orbit. Maxilla extending far beyond eye, strongly widened posteriorly and with distinct supramaxilla. Infra-/postorbital and mandibular-preopercular pores wide. Head squamation only discernable on opercle on any of the specimens. Opercle with one moderately sized neuromast behind preopercular edge; opercular flap small. Lateral line rows rarely well visible and therefore number of pores countable only in rare instances. Upper lateral line row with 8 pores and lower lateral line row with 17 pores until beginning of anal fin in one specimen.

Dentition All teeth tiny and cone-shaped. Vomer dentition almost absent, with 1–2 rows of tiny teeth anteriorly; palatines with very broad dentition patch with 5–8 rows of teeth in the middle part. Premaxilla tooth patches very narrow and not fused anteriorly; ca. 1–2 rows of very tiny teeth in middle part. Dentary tooth patches not fused anteriorly; ca. 4 rows anteriorly and 3 rows posteriorly. Median basibranchial tooth patch short, moderately wide.

Otolith morphology (n = 1). Size 2.2 mm in length (BSKU 65559); OL:OH = 1.15; OH:OT = 2.35. Moderately thick, small, compressed, roundish otolith. All rims regularly rounded and smooth; dorsal rim somewhat expanded middorsally. Inner face flat, smooth, with short, centrally positioned sulcus; OL:TCL = 3.25. Sulcus with shallow, undivided, uniform, very small, oval colliculum. Dorsal field with indistinct depression; ventral field with broad, indistinct ventral furrow. Outer face smooth, with central umbo.

Coloration. Live coloration not known. Color of preserved specimens light to pinkish brown; entire head distinctly darker, almost black.

Discussion. Porogadus melanocephalus differs from the other two species in the Porogadus gracilis group in the weak nature of all head spines, the very dark, nearly black head (which however may fade in old specimens), the broad palatines with 5–8 teeth rows (vs 2–4 rows of teeth), the weak, flat opercular spine (vs sharp and pointed), and the very small colliculum in the sulcus of the otolith (OL:TCL = 3.25 vs 2.3–2.5).

Distribution. Porogadus melanocephalus belongs to the relatively deep living species of the genus having been caught between 2820–4040 m. It appears to be endemic to the Bay of Bengal in the northeastern Indian Ocean. Alcock (1891) based his description of the species on two syntypes, one kept at BMNH and unfortunately not suitable as holotype and one at ZSI, which was not studied. A trawl in the Bay of Bengal during the Galatea 2 expedition (station 299, 17°10’N 84°30’E, 2820 m) has yielded 45 specimens of P. melanocephalus , which represents the single largest catch of Porogadus specimens and indicates that P. melanocephalus (and possible other species) may not be as rare as could be perceived from the relatively few specimens in institutional collections.

Figures

FIGURE 32. Porogadus melanocephalus (Alcock, 1891); 33A, BSKU 65559, 175 mm SL, 13°05'N, 90°39'E, 2967–2978 m; 32B–C, ZMUC P2397166, 168 mm SL, 17°10'N, 84°30'E, 2820 m; 32D, ZMUC P2397162, 152 mm SL, 17°10'N, 84°30'E, 2820 m.

FIGURE 33. Porogadus melanocephalus (Alcock, 1891); 33A–B, BSKU 65559, 175 mm SL, 13°05'N, 90°39'E, 2967–2978 m, head drawing; 33C–D, ZMUC P2397174, 166 mm SL, 17°10'N, 84°30'E, 2820 m, head drawing; 33E–F, BSKU 65559, 175 mm SL, 13°05’N, 90°39’E, 2967–2978 m, otolith (E inner face, F ventral view).

FIGURE 46. Geographic distribution of the species of the Porogadus gracilis group.

FIGURE 49. Depth versus distance from shelf plots for specimens of the Porogadus miles group (upper panel) and the Porogadus gracilis group (lower panel).