Tenuicephalus silus ( Carter & Sulak, 1984 )

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 : 80-81

Tenuicephalus silus ( Carter & Sulak, 1984)

Figs. 41 , 48 , 51 , Tab. 1–7

Porogadus silus Carter & Sulak, 1984: 361 , figs. 1–5, 6a, 7–8, 10–17; Nielsen et al. 1999: 86.

Material examined ( 5 specimens): Holotype USNM 238801, 139 mm SL, 24°16’N 76°16’W, 1962 m, R / V Columbus Iselin, Cruise 7511, Sta. 387, collected by Robins, Staiger and Sulak, 29 August 1975,; paratypes: USNM 238802 , 112– 141 mm SL (two specimens), 24°21’N 76°04’W, 1761 m, R / V Columbus Iselin, Cruise 7511, Sta. 378, collected by Robins, Staiger and Sulak, 29 August 1975; ZMUC P77729-77730 , 95– 130 mm SL (two specimens), Same data as holotype.

Diagnosis (new). Precaudal vertebrae 16–17, 8–10 loin vertebrae (last precaudal vertebrae without ribs); long gill rakers on first gill arch 15–18; pectoral-fin rays 17–19; HL:HD 1.35–1.45 (head depth 65–70% of head length in Carter & Sulak); no head spines; opercular spine flat, extruding; opercular flap small; vomer narrow, anteriorly naked, with few teeth posteriorly; palatines narrow with 1–2 rows of teeth; basibranchial tooth patch very small or absent. Addition from Carter & Sulak: lateral line scales to level of vent 14–20; first dorsal fin pterygiophore originating between second and third neural spine.

Description. Meristics: precaudal vertebrae 16–17, last 8–10 vertebrae without ribs; pectoral-fin rays 18 (17– 19); D/V = 5 (3–4); D/A = 23 (22–25); V/A = 18 (17–19); long gill rakers on lower gill arch 17 (15–18). Gill rakers in a paratype (ZMUC P77730) on lower first gill arch with five relatively short rakers, followed by a series of 12 long rakers. The lower ten of those intercepted by narrow plate shaped rakers with a few spines pointing inward. Upper gill arch with five short rakers intercepted by three semi long rakers.

Morphometrics: in % of SL: HL 14.8–17.6; maximal HD 10.3–12.8; HD through center of eye 7.4–8.6; bony interorbital width 3.5–3.7; snout length 4.6–5.6; upper jaw length 10.5–11.6; predorsal 16.0–17.1; preanal 28.3– 32.3; prepelvic 11.8–12.0; prepectoral 16.1–17.7; pectoral length not measurable. Relations: HL:HD = 1.35–1.45; maximal HD:HD through center of eye 1.40–1.54; HL to snout length 2.88–3.30; preanal to predorsal 1.75–1.88; predorsal to prepectoral 0.93–1.04.

Overall slender fish with long tapering tail, but stout head and snout. Maximal size of fishes investigated 141 mm SL (holotype 139 mm). Head with flat or slightly concave dorsal profile, without head spines except opercle with small, flat, extruding spine. Eye moderately small located in strongly asymmetric orbit. Maxilla extending far beyond eye, strongly widened posteriorly; supramaxilla small. Infra-/postorbital and mandibular-preopercular pores wide, usually no single pores discernable. Scales on head absent or not discernable. Opercular flap small. Lateral line not discernable.

Dentition. All teeth tiny and cone-shaped. Vomer naked anteriorly and with 2–5 fang-like teeth posteriorly; palatines with a narrow dentition patch with 1–2 rows of teeth. Premaxilla tooth patches, broad, not fused anteriorly, 4–10 rows of teeth in middle part. Dentary tooth patches broad, not fused anteriorly, 5– 10 rows in middle part. No basibranchial tooth patch or single very small median tooth patch.

Otoliths not known.

Coloration. Live coloration not known. Color of preserved dark brown to uniformly black.

Discussion. Tenuicephalus silus shows the probably most advanced morphology characterized by the complete absence of head spines except for a flat opercular spine, a short head (HL:HD = 1.35–1.45), the last 8–10 precaudal vertebrae without ribs and a much reduced dentition on vomer, palatines and basibranchial, the latter often without tooth patch. Carter & Sulak (1984) have undertaken a detailed osteological study of T. silus , the only one of its kind in the Porogadus lineage.

Distribution. Tenuicephalus silus is the only species of the genus occurring in the Atlantic Ocean, in a relatively restricted region within the Caribbean at depth of about 1750 to 2000 m and not venturing far from the shelf break.

Figures

FIGURE 41. Tenuicephalus silus (Carter & Sulak, 1984); 41A–B, USNM 238802, paratype, 141 mm SL, 24°21’N, 76°04’W, 1761 m; 41C–E, USNM 238801, holotype, 139 mm SL, 24°16’N, 76°16’W, 1962 m, C–D head drawings, E radiograph.

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.