Porogadus gracilis ( Günther, 1878 )

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 : 62-64

Porogadus gracilis ( Günther, 1878)

Figs. 30–31 , 46 , 49 , Tab. 1–7

Bathynectes gracilis Günther, 1878: 21 ; Günther 1887: 112, pl. 16 fig. B.

Porogadus gracilis: Nybelin 1957: 288 , pl. 6 figs. 5–6; Nielsen et al. 1999: 86; Fricke et al. 2014: 33.

Material examined ( 5 specimens): Holotype BMNH 1887.12 .7.52, 185+ mm SL, 12°08’S 145°10’E, 2560 m, H.M.S. Challenger Expedition, trawl, 29 August 1874; ZMUC P 2397227, 147 mm SL, R / V Vityaz 2615, 15°07’S 42°10’E, 3000–3130 m, 17 November 1988; ZMUC P 2397158, 165 mm SL, Galathea 2 expedition, St. 575, 30 m wide shrimp otter trawl, Tasman Sea, 40°11’S 163°35’E, 3710 m, 19 December 1951; tentatively assigned specimens: ZMUC P2397159-60 , 181 and 195 mm SL, Galathea 2 expedition, Sta. 235 (2 specimens), 32 m wide herring otter trawl, 04°47’S 46°19’E, 4810 m, 11 March 1951.

Diagnosis. Precaudal vertebrae 16–18; long gill rakers on first gill arch 13–17; pectoral-fin rays 16–18; HL:HD 1.73–2.01; most head spines weak, present on ethmoidal, lacrimal (ridge), prefrontal, interorbital, sphenotic, outer posttemporal, inner preopercular rim; absent on supraorbital, 5th infraorbital, supratemporal, inner 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 2–5 rows of teeth; palatines with moderately broad dentition patch 3–5 rows of teeth; otolith with single colliculum; OL:OH = 1.3; OL:TCL = 2.45.

Description. Meristics: precaudal vertebrae 16 (16–18), 1 (1) last vertebrae without ribs; pectoral-fin rays 16 (16–18); D/V = 5 (5–6); D/A = 27 (26–27); V/A = 18 (18–19); long gill rakers on lower gill arch 17 (13–17). Gill rakers in holotype on lower first gill arch with seven plate-shaped rakers, followed by a serie of 17 long rakers. The lower seven of those intercept by single plate shaped rakers. Upper gill arch with three short rakers intercepted by four plate shaped rakers.

Morphometrics: in % of SL: HL 15.0 (15.0–16.6); maximal HD 8.7 (7.8–9.0); HD through center of eye 5.6 (5.3–6.6); bony interorbital width 2.7 (2.2–3.4); snout length 4.7 (4.7–5.6); upper jaw length 9.0 (8.4–9.8); predorsal 18.2 (17.0–18.3); preanal 30.4 (28.2–30.4); prepelvic 11.8 (11.8–14.2); prepectoral 15.1 (15.1–17.3); pectoral length 9.8–10.8. Relations: HL:HD = 1.73 (1.73–2.01); HL to snout length 3.14 (2.93–3.14); preanal to predorsal 1.67 (1.64–1.78); predorsal to prepectoral 1.21 (0.99–1.21).

Slender fish with long tapering tail and moderately long snout. Maximal size of fishes investigated 195 mm SL (holotype 185+ mm). Head long and slender, with flat dorsal profile, with weak spines as follows: ethmoidal (1), lacrimal (ridge), prefrontal (1), interorbital (1), sphenotic (2), supratemporal (1), outer posttemporal (1–3), inner preopercular rim (3–4); no spines on supraorbital, 5th infraorbital, inner posttemporal, outer preopercular rim. Opercle with sharp, extruding 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, 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 one or two distinct large neuromasts behind preopercular edge; opercular flap small, without ridges. 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 cone-shaped. Vomer with a moderately broad dentition patch with 2–5 rows of teeth anteriorly; palatines with a narrow dentition patch with 3–5 rows of teeth in the middle part, tapering to one row posteriorly and three rows anteriorly. Premaxilla tooth patches not fused anteriorly; ca. 10 rows of very tiny teeth in middle part. Dentary tooth patches fused anteriorly; ca. 5 rows anteriorly and 3 rows posteriorly. Median basibranchial tooth patch long.

Otolith morphology (n = 1). Size 2.0 mm in length (ZMUC P2397227,); OL:OH = 1.3; 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 = 2.45. Sulcus with shallow, undivided, uniform, oval, relatively narrow colliculum. Dorsal field with indistinct small depression; ventral field smooth. Outer face smooth, with low relief central umbo.

Coloration. Live coloration not known. Color of preserved specimens light brown throughout; opercle somewhat darker.

Discussion. Two of the five specimens studied of P. gracilis are only tentatively allocated because of the relatively low number of long gill rakers (13–14 vs 15–17). Porogadus gracilis closely resembles P. abyssalis in the slender head and the weak head armature, but differs in the lower number of pectoral-fin rays (16–18 vs 19–23), the lower number of precaudal vertebrae (16–17, possibly 18 vs 18–19), and the even lesser and weaker head spines. Porogadus gracilis and P. abyssalis likely represent an vicariant species pair.

Distribution. Porogadus gracilis was originally described based on a single specimen from the deep water of the Coral Sea, off Raine Island, Queensland, Australia. A second specimen from off Madang, Papua New Guinea, has been listed in Fricke et al. (2014) but was not reviewed here. We have now also identified P. gracilis from the Tasman Sea and from off East Africa indicating that it is a widely distributed species in the southern Indian and West-Pacific oceans at depth of 2560 to 4810 m. It is found offshore at about 100 to 700 km off the shelf break.

Figures

FIGURE 30. Porogadus gracilis (Günther, 1878); 30A–B, BMNH 1887.12.7-52, holotype, 185+ mm SL, 12°08’S, 145°10’E, 2560 m; 30C–D, ZMUC P2397158, 165 mm SL, 40°11’S, 163°35’E, 3710 m.

FIGURE 31. Porogadus gracilis (Günther, 1878); 31A–B, BMNH 1887.12.7-52, holotype, 185+ mm SL, 12°08’S, 145°10’E, 2560 m, head drawings; 31C–D, tentatively assigned, ZMUC P2397160, 281 mm SL, 04°47’S, 46°19’E, 4810 m, head drawing; 31E–G, ZMUC P2397227, 147 mm SL, 15°07’S, 42°10’E, 3000–3130 m, otolith (E inner face, F anterior view, G 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).