Stephonyx perexcavatus Narahara & Tomikawa & Torigoe 2012, sp. nov.

(Japanese name: Hekomi-tsumamisokoebi, new)

(Figures 2–6)

Material examined

Holotype. NSMT-Cr 22011, 1 male (22.9 mm), Okinawa Trough (East China Sea), 28 ◦ 29.5 ′ N, 127 ◦ 59.2 ′ E, baited trap, 1073–1080 m, bottom at 10:08 on 29 July 2010, R / V Soyo-maru, Soyo-2010, St-trap 7 (Okinawa Trough), 30 July 2010, coll. K. Kakui.

Paratypes. NSMT-Cr 22012–22015, four males (20.4–22.9 mm), NSMT-Cr 22016– 22020, five females (21.9–23.2 mm). Data same as for the holotype.

Description of male (holotype, NSMT-Cr 22011)

Head (Figure 2A, B) deeper than long, lateral cephalic lobe rounded; rostrum absent; eyes L-shaped, faded in alcohol. Epimeral plates 1–3 (Figure 5G–I) with anteroventral corners rounded, with setae; posterior margin of plate 1 convex; posteroventral corner of plate 1 not pointed, corner of plate 2 with tooth, and corner of plate 3 weakly pointed.

Antenna 1 (Figure 2C, D) 0.2 times body length, peduncular articles 1–3 length ratio of 1.0: 0.5: 0.5; accessory flagellum nine-articulate; primary flagellum 28-articulate, calceoli present on articles 6, 7, 9, 12, 14–19 and 22–24; callynophore welldeveloped, 43 rows, two fields. Antenna 2 (Figure 2E) 1.9 times antenna 1; anterior margins of articles 4 and 5 with brush setae, peduncular articles 4 and 5 not swollen; flagellum 62-articulate; calceoli present on articles 3, 4, 7–9, 11, 21–25, 28, 31, 37, 39, 41, 43 and 49.

Mouthpart bundle, subquadrate. Epistome and upper lip (Figure 2F) separated by notch, subequal. Mandible (Figure 2H, I) incisor smooth, with left lacinia mobilis four-denticulate, right lacinia mobilis absent; accessory setal row present; molar setose, not triturative; palp attached midway, length of article 1 2.4 times width, article 2 with setae along distal half of lateral margin, article 3 with setae along most of posterior margin, articles 1–3 in length ratio of 1.0: 2.6: 1.8. Lower lip (Figure 2G) outer lobe broad, lateral lobe long, inner lobe indistinct. Maxilla 1 (Figure 2J–L) with ovate inner plate, bearing three plumose setae apically; outer plate narrow, with 11 spine-teeth in a 7/4 crown arrangement; palp two-articulate, terminal article with eight broad setae and slender seta. Maxilla 2 (Figure 2M) plates narrow and setose, inner plate shorter than outer plate. Maxilliped (Figure 3A, B) with subrectangular inner plate, bearing two nodular robust setae on inner distal corner and one on outer distal corner, oblique setal row well developed; outer plate large, subovate, with apical robust setae and plumose setae; palp four-articulate, dactylus with unguis.

Gnathopod 1 (Figure 3C–E) with reduced coxa, subquadrate; anterior margin of basis straight with slender setae, posterior margin bare; ischium elongate, 5.6 times width, 0.6 times basis; merus short; carpus linear, elongate, 7.3 times width and subequal to propodus in length, margins subparallel, anterior margin without setae, straight; palmar margin of propodus serrate, slightly convex, distal corner with one lateral and one medial tooth; dactylus 0.3 times propodus, distally bifid with medial seta. Gnathopod 2 (Figure 3F–H) with large coxa, slightly smaller than coxa 3, anterior margin convex; basis elongate, posterior margin weakly curved, anterior margin with minute setae; ischium elongate, 3.9 times width; posterior margin of merus rounded; carpus linear, elongate, 5.1 times width, margins subparallel; propodus oval, expanding distally, length 1.6 times width; palmar margin of propodus deeply concave, posterodistal corner 93 ◦, with six medial stout setae; dactylus not reaching palmar corner. Pereopod 3 (Figure 3I, J) subequal in size to pereopod 4; coxa subrectangular; propodus slender; dactylus simple, with posteroproximal crenulations. Pereopod 4 (Figure 3K) coxa exceeding distal part of basis, with well-developed posteroventral lobe; basis length 4.1 times width; merus short, 0.2 times basis; propodus slender; dactylus simple. Pereopod 5 (Figure 4A) with bilobed coxa; posteroventral lobe of basis developed, posterior margins of basis and merus expanded. Pereopod 6 (Figure 4B) with subrectangular coxa, weakly bilobed; posteroventral lobe of basis developed; posterior margin of merus broadly expanded. Pereopod 7 (Figure 4C) with subquadrate coxa; posterior margin of basis expanded, weakly convex, posteroventral lobe developed; posterior margin of merus broadly expanded.

Coxal gills (Figures 4D–F, 5A–C) on gnathopod 2 and pereopods 3–7. Coxal gills 2–6 with accessory lobe; gill 2 subovate, exceeding distal part of basis; gills 3–5 pyriform, exceeding distal part of basis; gill 6 horizontally long, reaching middle of basis, proximal part of accessory lobe curled; and gill 7 subquadrate, exceeding distal margin of coxa, but not reaching middle of basis.

Pleopods 1–3 (Figure 5D) with paired retinacula (Figure 5E) and associated setae on inner distal corners of peduncles; inner basal margins of inner rami with 7–11 bifid plumose setae (clothes-pin setae) (Figure 5F); inner rami of pleopods 1–3 24, 25 and 24-articulate, respectively; outer rami of pleopods 1–3 each 27-articulate.

Uropod 1 (Figure 5J) peduncle with medial and lateral robust setae; outer ramus 0.9 times peduncle, bearing nine lateral and seven medial marginal robust setae and terminal robust seta; inner ramus 1.1 times outer ramus, bearing eight lateral and 13 medial marginal robust setae and terminal robust seta. Uropod 2 (Figure 5K) peduncle with medial and lateral robust setae; outer ramus 1.0 times peduncle, lateral and medial margins each with eight and three robust setae, with terminal robust seta; inner ramus 1.3 times outer ramus, lateral and medial margins each with five and 16 robust setae, with terminal robust seta. Uropod 3 (Figure 5L) with peduncle length 1.6 times width, laterodistal and mediodorsal corners with robust setae; outer ramus 1.6 times peduncle, two-articulate, medial margin of proximal article with plumose setae; inner ramus subequal in length to outer ramus, medial margin with numerous plumose setae.

(F) epistome and upper lip, anterior view; (G) lower lip, ventral view; (H) left mandible, medial view; (I) incisor of right mandible, medial view; (J) maxilla 1, dorsal view; (K) outer plate of maxilla 1, dorsal view; (L) terminal article of palp of maxilla 1, dorsal view; (M) maxilla 2, dorsal view.

Telson (Figure 5M) length 1.7 times width, cleft 88% of length, distal margins slightly incised, with one penicillate setae and two or three robust setae on each lobe.

Description of female (paratype, NSMT-Cr 22018)

Similar to male, except the following. Antenna 1 (Figure 6A, B) with primary flagellum 22-articulate, calceoli absent; callynophore much smaller in size, 17 rows. Antenna 2 (Figure 6C) 1.7 times antenna 1, anterior margins of articles 4 and 5 lacking brush setae, flagellum 39-articulate, calceoli absent. Medial margin of outer ramus of uropod 3 (Figure 6E) bearing fewer plumose setae. Oostegites (Figure 6D) on gnathopod 2 and pereopods 3–5, baculiform, margins irregular, without setae.

Variation

Posterodistal corner of propodus of gnathopod 2 91–102 ◦ (n = 10). Dorsal surface of telson with (Figure 5N) or without (Figure 5M) robust setae.

Distribution

Okinawa Trough, East China Sea.

Etymology

The specific name “ perexcavatus ”, an adjective, is derived from per (Latin; very) and excavatus (Latin; concave) referring to the deeply concave palmar margin of gnathopod 2.

Remarks

Stephonyx perexcavatus sp. nov. is similar to S. arabiensis, S. laqueus and S. talismani in having article 1 of mandibular palp shorter than 0.5 times article 2, carpus of gnathopod 1 longer than 0.75 times propodus, concave palmar margin of gnathopod 2 and truncated distal margin of telson. However, the present new species is distinguished from the above species in the following features: from S. arabiensis (features of S. arabiensis in parentheses), lateral cephalic lobe rounded (sharply pointed), and palm of gnathopod 2 deeply concave (slightly concave); from S. laqueus (features of S. laqueus in parentheses), posterodistal corner of gnathopod 2 obtuse angle (acute angle), and dactylus of gnathopod 2 not reaching distal margin of propodus (almost reaching); and from S. talismani (features of S. talismani in parentheses), eyes L-shaped (reniform), lateral cephalic lobe rounded (pointed) and posteroventral corner of epimeral plate 1 rounded (pointed).

Stephonyx perexcavatus sp. nov. differs from S. biscayensis and S. mytilus in the following features: from S. biscayensis (features of S. biscayensis in parentheses), palmar margin of gnathopod 2 deeply concave (convex), and inner ramus of uropod 3 as long as outer ramus (shorter than outer ramus); from S. mytilus (features of S. mytilus in parentheses), palmar margin of gnathopod 2 deeply concave (convex).