Porogadus abyssalis Nybelin, 1957

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 : 60-62

Porogadus abyssalis Nybelin, 1957

Figs. 29 , 46 , 49 , Tab. 1–7

Porogadus abyssalis Nybelin, 1957: 288 , figs. 19–20, pl.6 figs. 2–4; Nielsen et al. 1999: 86.

Material examined (3 specimens): MNHN 1979-0239 , 105 mm SL, 10°58’N 43°13’W, 5100 m, Biovema Expedition, Sta. a6/cp01, collected by J. Charcot, 13 November 1977 ; MNHN 1979-0240 , 160 mm SL, 10°58’N 43°13’W, 5100 m, Biovema Expedition, Sta. a6/cp01, collected by J. Charcot, 13 November 1977 ; MNHN 1992-1231, 158 mm SL, 10°24’N 46°43’W, 4830 m, Demeraby Expedition, Sta. cp14, collected by J. Charcot, 28 September 1980 .

Diagnosis. Precaudal vertebrae 18–19; long gill rakers on lower part of first gill arch 13–14; pectoral-fin rays 19–23; HL:HD 1.85–2.09; most head spines moderately strong or weak, present on ethmoidal, prefrontal, interorbital, supraorbital, sphenotic, inner preopercular rim; absent on lacrimal, 5th infraorbital, supratemporal, inner and outer posttemporal, outer preopercular rim; opercular spine sharp, extruding; opercular flap small; lower lateral line pores until beginning of anal fin 23; vomer with moderately broad dentition patch with 2–3 rows of teeth; palatines with moderately broad dentition patch 2–4 rows of teeth; otolith with single colliculum; OL:OH = 1.17; OL:TCL = 2.3.

Description (based on non-types and holotype data from Nybelin 1957). Meristics: precaudal vertebrae 18–19, 1 last vertebrae without ribs; pectoral-fin rays 19 (19–23); D/V = 6; D/A = 24–27; V/A = 19–21; long gill rakers on lower gill arch 14 (13–14). Gill rakers in a typical specimen (MNHN 1979-0240) on lower first gill arch with six short rakers, followed by a series of 16 long rakers. The lower 8 of those intercept by narrow plate shaped rakers. These are smaller than in most species. Upper gill raker with a series of three semi long rakers and three plate like rakers intercepted, followed by two very short rakers.

Morphometrics: in % of SL: HL 16.5 (16.5–17.0); maximal HD 8.2–9.0; HD through center of eye 5.2–6.1; bony interorbital width 3.4–3.6; snout length 6.1; upper jaw length 9.7–10.1; predorsal 17.4–18.4; preanal 33.9 (29.3–33.9); prepelvic 12.2–13.4; prepectoral 16.6–17.4; pectoral length 10.4. Relations: HL:HD = 1.85–2.09; HL to snout length 2.72–2.77; preanal to predorsal 1.68–1.69; predorsal to prepectoral 1.05–1.06.

Slender fish with long tapering tail and long, pointed snout. Maximal size of fishes investigated 209 mm SL (holotype). Head long and very slender, with flat dorsal profile, with moderate to weak spines as follows: ethmoidal (1), prefrontal (1 weak), interorbital (3), supraorbital (2 weak), sphenotic (2–3), inner preopercular rim (3–4); no spines on lacrimal, 5th infraorbital, supratemporal, inner and outer posttemporal, outer preopercular rim. Opercle with sharp, short spine. Eye 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, 1 pore on occiput in front of nape. Head squamation on opercle, cheeks and occiput; absent on frontal, around eyes, and on maxilla. Opercle with two distinct large neuromasts behind preopercular edge; opercular flap small, with few radial ridges along lower margin. Lateral line rows rarely well visible and therefore number of pores countable only in rare instances. Lower lateral line row with 23 pores until beginning of anal fin in one specimen.

Dentition. All teeth tiny and needle like. Vomer with a narrow dentition patch with 2–3 rows of teeth anteriorly; palatines with a narrow dentition patch with 2–4 rows of teeth in middle part. Premaxilla tooth patches not fused anteriorly; ca. 4 teeth rows anteriorly and 2–3 rows posteriorly. Dentary tooth patches not fused anteriorly; ca. 4 teeth rows anteriorly and a single row posteriorly. Median basibranchial tooth patch very short.

Otolith morphology (n = 1). Size 1.8 mm in length (MNHN 1979-0240); OL:OH = 1.17; OH:OT 2.8. Thin, small, compressed, roundish otolith. All rims regularly rounded and smooth. Inner face flat, smooth, with short, centrally positioned sulcus; OL:TCL = 2.3. Sulcus with shallow, undivided, uniform, oval, wide colliculum. Dorsal field with indistinct depression; ventral field with faint ventral furrow. Outer face smooth, without umbo.

Coloration. Color of preserved specimens light brown; belly slightly darker, opercle distinctly darker. Live coloration somewhat darker according to Nybelin (1957).

Discussion. Porogadus abyssalis was described based on a unique holotype by Nybelin (1957) caught at 5250– 5300 m in the central Atlantic, which then was the deepest catch of any Porogadus specimen which hitherto were caught shallower than 3300 m. The weak head ossification and armature and the small, morphologically simple otolith may reflect adaptation to a deep sea habitat below the CCD. Within the Porogadus gracilis group, Porogadus abyssalis is best recognized by its unusual high number of pectoral-fin rays (19–23 vs 15–18) and some relatively strong spines on the head. With its slender and elongate head and the high number of lower lateral line pores until the beginning of the anal fin P. abyssalis resembles several species of the Porogadus miles group, but differs in the generally much weaker head armature, weaker head ossification and the very small and compressed otoliths.

Distribution. The specimens described here are the only other specimens beside the unique holotype described by Nybelin (1957). They confirm the deep sea dwelling of the species in the tropical Atlantic and far away from any shelf break (700–1000 km, Fig. 49 ) on the abyssal plane near to the middle Atlantic ridge.

Figures

FIGURE 29. Porogadus abyssalis Nybelin, 1957; 29A–B, D–G, MNHN 1979-0240, 160 mm SL, 10°58’N, 43°13’W, 5100 m, (A–B photographs, D–E head drawings, F otolith inner face, G otolith ventral view); 29C, MNHN 1992–1231, 158 mm SL, 10°24’N, 46°43’W, 4830 m.

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).