Stephonyx perexcavatus, Narahara & Tomikawa & Torigoe, 2012

Narahara, Yukiko, Tomikawa, Ko & Torigoe, Kenji, 2012, Four species of the genus Stephonyx (Crustacea: Amphipoda: Uristidae) from Japan, with description of a new species, Journal of Natural History (J. Nat. Hist.) 46 (23 - 24), pp. 1477-1507 : 1479-1486

publication ID	https://doi.org/ 10.1080/00222933.2012.675598
publication LSID	lsid:zoobank.org:pub:B4D66BC0-1866-4F68-95C0-2A1584B6141B
DOI	https://doi.org/10.5281/zenodo.10536839
persistent identifier	https://treatment.plazi.org/id/3F55985F-FFB0-FF88-DE47-AFA826A0FD1E
treatment provided by	Felipe
scientific name	Stephonyx perexcavatus
status	sp. nov.

Treatment

Stephonyx perexcavatus View in CoL sp. nov.

(Japanese name: Hekomi-tsumamisokoebi, new)

( Figures 2–6 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 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. GoogleMaps

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 View Figure 2 ) deeper than long, lateral cephalic lobe rounded; rostrum absent; eyes L-shaped, faded in alcohol. Epimeral plates 1–3 ( Figure 5G–I View Figure 5 ) 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 View Figure 2 ) 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 View Figure 2 ) 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 View Figure 2 ) separated by notch, subequal. Mandible ( Figure 2H, I View Figure 2 ) 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 View Figure 2 ) outer lobe broad, lateral lobe long, inner lobe indistinct. Maxilla 1 ( Figure 2J–L View Figure 2 ) 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 View Figure 2 ) plates narrow and setose, inner plate shorter than outer plate. Maxilliped ( Figure 3A, B View Figure 3 ) 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 View Figure 3 ) 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 View Figure 3 ) 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 View Figure 3 ) subequal in size to pereopod 4; coxa subrectangular; propodus slender; dactylus simple, with posteroproximal crenulations. Pereopod 4 ( Figure 3K View Figure 3 ) 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 View Figure 4 ) with bilobed coxa; posteroventral lobe of basis developed, posterior margins of basis and merus expanded. Pereopod 6 ( Figure 4B View Figure 4 ) with subrectangular coxa, weakly bilobed; posteroventral lobe of basis developed; posterior margin of merus broadly expanded. Pereopod 7 ( Figure 4C View Figure 4 ) with subquadrate coxa; posterior margin of basis expanded, weakly convex, posteroventral lobe developed; posterior margin of merus broadly expanded.

Coxal gills ( Figures 4D–F View Figure 4 , 5A–C View Figure 5 ) 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 View Figure 5 ) with paired retinacula ( Figure 5E View Figure 5 ) 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 View Figure 5 ); 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 View Figure 5 ) 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 View Figure 5 ) 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 View Figure 5 ) 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.

Telson ( Figure 5M View Figure 5 ) 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 View Figure 6 ) with primary flagellum 22-articulate, calceoli absent; callynophore much smaller in size, 17 rows. Antenna 2 ( Figure 6C View Figure 6 ) 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 View Figure 6 ) bearing fewer plumose setae. Oostegites ( Figure 6D View Figure 6 ) 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 View Figure 5 ) or without ( Figure 5M View Figure 5 ) 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).