Psednos argyrogaster Stein, 2012, n. sp.

Psednos argyrogaster n. sp.

Figs. 25, 26

Holotype. NMNZ P.028668, male, 63 mm SL, 74 mm TL, 42°42.97' S, 178°06.15' E, Chatham Rise, R/V Tangaroa, Stn. TAN 9206/089, 21 June 1992, 1015– 1037 m.

Diagnosis. Vertebrae 56 (12+44), dorsal fin rays 49, anal fin rays 43, caudal fin rays 6 (3/3), pectoral fin rays 15 (7+3+5), pyloric caeca 6. Mouth angle about 30°. Teeth arranged in about 30 oblique rows forming a moderately wide band anteriorly. Strong opercle curved posteroventrally, almost horizontal at its tip. Coronal pore absent, mandibular symphyseal pores moderately large, round, distant from each other by 4–5 pore diameters. Abdomen relatively short and deep, distance from pectoral symphysis to abdomen end slightly shorter than or equal to head. Peritoneum externally silvery when viewed at an angle, whitish black in direct side view, internally black.

Description. Counts: V 56, D 49, A 43, C 6, P 15, radials unknown, gr 6/6, pc 6, pore formula unknown. Ratios: HL 26.2% SL, HW 13.5, sn 7.1, E 5.1, orbit 6.5, uj 11.1, go 9.2, UPL 16.5, LPL 10.5, bd 30.2, bdA 30.0, preD 32.7, preA 40.2, sna 21.7, ma 21.9, aAf 24.1, mabd 43.8, pabd ~25.2. In % HL: HW 51.5, sn 27.2, E 19.4, orbit 24.8, uj 42.4, go 35.2, UPL 63.0, LPL 40.0, bd 115.1, bdA 114.6, preD 124.8, preA 153.3, sna 83.0, ma 83.6, aAf 92.1, mabd 167.3, pabd ~96.4.

Head about 1/4 body length, its anterodorsal profile almost flat, rising evenly and abruptly to just behind vertical through posterior margin of orbit, then rising more slowly to deepest point of body, above opercle. Snout short, its tip about even with lower margin of orbit. Nostrils single, close to anterior margin of orbit, directly in front of pupil of eye. Eye about 1/20 SL, 1/5 HL, black, pupil large. Mouth oblique, its angle about 30°. Upper jaw included, mandibular symphysis anterior to tip of snout. Angle of retroarticular about 90°. Teeth long, slender; outer teeth smallest, progressively larger towards inside of mouth. Teeth arranged in about 30 oblique rows forming a moderately wide band anteriorly. Narrow gap at premaxillary symphysis, obscured by teeth crossing over it; mandible with a narrow symphyseal gap. Gill opening about 1/3 HL, long and obliquely angled anteriorly, apparently above pectoral base and in front of 1–2 rays, divided about in half by opercular spine. Gill rakers of first arch 6/6, roughly alternating. Spinules on each raker relatively few, forming a narrow acute triangle, usually composed of two rows of outward pointing spinules. Opercular flap small, its margin broadly angled posteriorly and sharp tipped; strong, clearly visible opercle curved posteroventrally, almost horizontal at its tip. Coronal pore absent, mandibular symphyseal pores moderately large, round, distant from each other by 4–5 pore diameters, well spaced on either side of mandibular symphysis. Suprabranchial pore single.

Pectoral fin base below opercular flap, dorsalmost ray on horizontal slightly below lower margin of orbit. Upper lobe reaching to below and slightly behind dorsal origin but not reaching end of abdominal cavity; lower lobe reaching below tip of opercle. Pectoral rays 7+3+5, upper and lower lobe rays closely spaced; rays of notch distinctly more widely spaced, easily distinguished from those in lobes, rudimentary rays absent. Notch moderately deep, about half length of upper lobe. Pectoral girdle not examined.

Body with distinct but not exaggerated hump over branchial cavity, tapering rapidly and then more evenly to caudal. Vertebrae 12+44. Neural spines single except the first is forked. Dorsal origin well behind head, at about 1/3 body length, its origin between vertebrae 8–9. Anal fin origin between vertebrae 13–14, preanal fin distance about 2/5 SL. Caudal fin rays broken, six, (3/3). Hypural fused, a single plate lacking a slit. Anus below anterior of gill cavity, between ventral rays of lower pectoral fin lobes. Genital papilla present, distant from anus about half eye diameter. Abdomen relatively short and deep. Pyloric caeca digitate and thick. Skin thin, fragile.

Fresh color whitish, muscles, dark branchial cavity, and peritoneum visible through skin. Color in alcohol: skin translucent white, body musculature unpigmented. Dorsal, anal, caudal and pectoral fins of body color; Orobranchial cavity brownish, tongue dusky, making head darker than remainder of body. Peritoneum visible through body wall, externally silver when viewed at an angle but black internally; when seen directly from the side, whitish black. Stomach brown, pyloric caeca white.

Distribution. Known from a single specimen taken at depths slightly below 1000 m on the east Chatham Rise, east of New Zealand.

Etymology. The name arygyrogaster from the Greek silver and gaster, stomach, in reference to the silvery outer lining of the abdomen.

Comparisons. Psednos argyrogaster is most likely to be confused with P. n e m n e z i (see below), but differs not only in color, but also in the distance from mandible to abdomen end (44 vs 33% SL, 167 vs 141–153% HL), upper jaw length (42 vs 49–52% HL), interorbital width (36 vs 50–52% HL), preanal fin length (153 vs 170% HL). It could also be confused with P. c h a t h a m i, from which it differs in eye diameter (19 vs 26% HL), snout length (27 vs 17% HL), upper jaw length (42 vs 51% HL), and distance from mandible to end of abdomen (44 vs 36% SL).

Psednos argyrogaster is similar to P. nataliae Stein and Andriashev in Stein et al. (2001) and P. whitleyi Stein et al. 2001 from Australia. It differs from both in its shallower mouth angle (30° vs 40–45°), body color (white with brownish head vs white or pale entirely), upper jaw diastema width (narrow but distinct vs wide), greater body depth at anus (30 vs ~20% SL), greater distance from mandible to anus (22 vs 17–19% SL), narrower interorbital width (36 vs 40–43% HL), shorter upper jaw (42 vs 50–53% HL), and lower pectoral fin lobe length (40 vs 63–76% HL).