Stephonyx mytilus ( Barnard and Ingram, 1990 )

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 : 1499-1506

publication ID	https://doi.org/ 10.1080/00222933.2012.675598
publication LSID	lsid:zoobank.org:pub:B4D66BC0-1866-4F68-95C0-2A1584B6141B
persistent identifier	https://treatment.plazi.org/id/3F55985F-FFA4-FF9C-DDE8-AF462170FDB3
treatment provided by	Felipe
scientific name	Stephonyx mytilus ( Barnard and Ingram, 1990 )
status

Treatment

Stephonyx mytilus ( Barnard and Ingram, 1990) View in CoL

(Japanese name: Igai-tsumamisokoebi, new)

( Figures 15–18 View Figure 15 View Figure 16 View Figure 17 View Figure 18 )

Euonyx mytilus Barnard and Ingram, 1990: 3 View in CoL , figs. 1–3.

Stephonyx mytilus: Diffenthal and Horton, 2007: 39 View in CoL (key).

Material examined

NSMT-Cr 22035–22039, five males (20.6–22.5 mm), NSMT-Cr 22040–22044, five females (20.3–24.0 mm), Okinawa Trough , 31 ◦ 28 ′ N, 128 ◦ 31 ′ E, baited trap, 534–546 m, bottom at 9:20 on 1 August 2008, R / V GoogleMaps Soyo-maru , Soyo-2008, St-trap 8, 2 August 2008, coll. M. Okanishi. NSMT-Cr 22045–22049, five males (20.3–23.2 mm), NSMT-Cr 22050–22054, five females (21.8–23.6 mm), East China Sea, 31 ◦ 29.8 ′ N, 128 ◦ 31.6 ′ E, baited trap, 546–551 m, bottom at 8:52 on 27 July 2010, R / V GoogleMaps Soyo-maru , Soyo-2010, St-trap 6 (East China Sea), 28 July 2010, coll. K. Kakui.

Description of male

Head ( Figure 15A View Figure 15 ) deeper than long, lateral cephalic lobe rounded; rostrum absent; eyes L-shaped. Epimeral plates 1–3 ( Figure 17G–I View Figure 17 ) with anteroventral corners rounded, with setae; posterior margin of plate 1 convex; posteroventral corner of plate 2 with tooth and corner of plate 3 weakly pointed.

Antenna 1 ( Figure 15B, C View Figure 15 ) 0.2–0.3 times body length, peduncular articles 1–3 length ratio of 1.0: 0.3–0.6: 0.5–0.6; accessory flagellum 7–11-articulate; primary flagellum 25–29-articulate; calceoli present on articles 6–24; callynophore welldeveloped, 38–45 rows, two fields. Antenna 2 ( Figure 15D View Figure 15 ) 1.8–2.2 times antenna 1; anterior margins of peduncular articles 4 and 5 with brush setae, peduncular articles 4 and 5 not swollen; flagellum 54–69-articulate, calceoli present on articles 1–60.

Mouthpart bundle, subquadrate. Epistome and upper lip ( Figure 15E View Figure 15 ) separated by notch, subequal. Mandible ( Figure 15G, H View Figure 15 ) incisor smooth, left lacinia mobilis five-denticulate, right lacinia mobilis absent, accessory setal row present; molar setose, not triturative; palp attached midway, 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: 2.6–3.3: 1.9–2.5. Lower lip ( Figure 15F View Figure 15 ) outer lobe broad, lateral lobe long, inner lobe indistinct. Maxilla 1 ( Figure 15I–K View Figure 15 ) with ovate inner plate, bearing two to four plumose setae apically; outer plate narrow, with 11 spine-teeth in a 7/4 crown arrangement; palp two-articulate, terminal article with five to eight robust setae and one or two slender setae. Maxilla 2 ( Figure 15L View Figure 15 ) plates setose, inner plate shorter than outer plate. Maxilliped ( Figure 15M, N View Figure 15 ) 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 16A–C View Figure 16 ) with reduced coxa, subquadrate; anterior margin of basis with slender setae, posterior margin bare; ischium elongate; merus short; carpus linear, elongate, margins subparallel, posterior margin straight; palmar margin of propodus serrate, slightly convex, distal corner with two lateral and one medial tooth; dactylus 0.3–0.4 times propodus, distally bifid with medial seta. Gnathopod 2 ( Figure 16D–F View Figure 16 ) with large coxa, slightly smaller than coxa 3, anterior margin convex; basis elongate, posterior margin weakly curved, anterior margin with minute setae; ischium elongate; posterior margin of merus rounded; carpus linear, elongate, margins subparallel; propodus oval, length 1.4–1.6 times width; palmar margin of propodus slightly convex, posterodistal corner with medial stout setae; dactylus not reaching palmar corner. Pereopod 3 ( Figure 16G, H View Figure 16 ) subequal to pereopod 4, coxa subrectangular, propodus slender, dactylus simple, with posteroproximal crenulations. Pereopod 4 ( Figure 16I View Figure 16 ) with well-developed posteroventral lobe of coxa, propodus slender, dactylus simple. Pereopod 5 ( Figure 17A View Figure 17 ) with bilobed coxa, length slightly shorter than wide; posteroventral lobe of basis well developed, posterior margins of basis and merus expanded. Pereopod 6 ( Figure 17B View Figure 17 ) with subrectangular coxa, weakly bilobed; posterior margin expanded, posteroventral lobe of basis well developed; posterior margin of merus broadly expanded. Pereopod 7 ( Figure 17C View Figure 17 ) with subquadrate coxa; posterior margin of basis expanded, weakly convex, posteroventral lobe well developed; posterior margin of merus broadly expanded.

Coxal gills 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; and gill 7 subquadrate, exceeding distal margin of coxa, but not reaching middle of basis.

Pleopods 1–3 ( Figure 17D View Figure 17 ) with paired retinacula ( Figure 17E View Figure 17 ) and associated setae on inner distal corners of peduncle; inner basal margins of inner rami with five to nine bifid plumose setae (clothes-pin setae) ( Figure 17F View Figure 17 ); inner rami of pleopods 1–3 24–25-, 24–26- and 23–24-articulate, respectively; outer rami of pleopods 1–3 27–28-, 27–28- and 26–28-articulate, respectively.

Uropod 1 ( Figure 17J View Figure 17 ) peduncle with medial and lateral robust setae; outer ramus 0.8–0.9 times peduncle, bearing 8–12 lateral and four to nine medial marginal robust setae and terminal robust seta; inner ramus 1.0–1.1 times outer ramus, bearing 7–10 lateral and 12–15 medial marginal robust setae and terminal robust seta. Uropod 2 ( Figure 17K View Figure 17 ) peduncle with medial and lateral robust setae; outer ramus 1.1 times peduncle, lateral and medial margins each with seven to nine and three to six robust setae, with terminal robust seta; inner ramus 1.3 times outer ramus, outer and inner margins each with six or seven and 15–17 robust setae, with terminal robust seta. Uropod 3 ( Figure 17L View Figure 17 ) laterodistal and mediodistal corners of peduncle with robust setae; outer ramus 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 18A, B View Figure 18 ) cleft 90% of length, dorsal surface with ( Figure 18B View Figure 18 ) or without ( Figure 18A View Figure 18 ) marginal robust setae, distal margin incised, with one penicillate seta and two or three robust setae on each lobe.

view; (G) left mandible, medial view; (H) incisor of right mandible, medial view; (I) maxilla 1, dorsal view; (J) outer plate of maxilla 1, dorsal view; (K) terminal article of palp of maxilla 1, dorsal view; (L) maxilla 2, dorsal view; (M) maxilliped (inner plate twisted outward), dorsal view; (N) inner plate of maxilliped, ventral view.

Description of female

Similar to male, except the following. Antenna 1 ( Figure 18C, D View Figure 18 ) with primary flagellum 22–26-articulate, calceoli absent; callynophore smaller, with 16–20 rows. Antenna 2 ( Figure 18E View Figure 18 ) 1.4–1.8 times antenna 1, flagellum 43–49-articulate, calceoli absent. Inner margin of uropod 3 ( Figure 18G View Figure 18 ) outer ramus has few plumose setae.

Oostegites ( Figure 18F View Figure 18 ) on gnathopod 2 and pereopod 3–5, baculiform, margins irregular, without setae.

Remarks

The present specimens were identical to S. mytilus in having a round lateral cephalic lobe, 11 spine-teeth on the outer plate of maxilla 1, three nodular robust setae on inner plate of maxilliped, convex palmar margin of gnathopod 2, and dactylus not reaching the distal corner of palmar margin of gnathopod 2. The Japanese specimens differ from the original description of S. mytilus in the following aspects (information from the original description is given in parentheses): pereopod 5 of female with oostegite (lacking), pereopods 6–7 with broader article 4 (narrower), peduncles of pleopods 1–3 each with two (four) retinacula, and telson cleft 90 % (80).

This is the first record of this species from Japanese waters.

Distribution

North Pacific Ocean ( Barnard and Ingram 1990) and Okinawa Trough (East China Sea, this study); 534–2635 m.

Key to the species of Stephonyx View in CoL

1. Pleonites and urosomites dorsally carinate............................... 2 Pleonites and urosomites dorsally smooth............................... 3

2. Pereonites 5–7 and pleonites 1–3 with dorsal teeth; palm of gnathopod 2 straight; coxa 5 produced in a conical hump.................. S. scutatus Pereonites View in CoL 5–7 and pleonites 1–3 lacking dorsal teeth; palm of gnathopod 2 convex; coxa 5 without conical hump........................ S. carinatus View in CoL

3. Carpus of gnathopod 1 <0.5 times length of propodus......... S. normani Carpus View in CoL of gnathopod 1> 0.75 times length of propodus................. 4

4. Article 1 of mandibular palp> 0.5 times length of article 2; carpus of gnathopod 2 about 3.5 times propodus........................... S. pirloti View in CoL Article 1 of mandibular palp <0.5 x length of article 2; carpus of gnathopod 2 <2.5 times propodus................................................. 5

5. Palm of gnathopod 2 slightly concave (almost straight)........ S. arabiensis Palm View in CoL of gnathopod 2 deeply concave.................................... 6 Palm of gnathopod 2 convex............................................ 8

6. Lateral cephalic lobe with sharply pointed; posteroventral corner of epimeral plate single pointed........................................... S. talismani Lateral View in CoL cephalic lobe rounded; posteroventral corner of epimeral plate 1 rounded............................................................... 7

7. Palmar corner of gnathopod 2 acute angle, dactylus reaching almost distal corner of palmar margin....................................... S. laqueus View in CoL

Palmar corner of gnathopod 2 obtuse angle, dactylus not reaching distal corner of palmar margin..................................................... Stephonyx perexcavatus View in CoL sp. nov.

8. Distal margin of telson without spine......................... S. uncinatus View in CoL Distal margin of telson with spine....................................... 9

9. Outer plate of maxilla 1 with six spine teeth; posterodistal corner of epimeral plate 2 blunt................................................... S. incertus Outer View in CoL plate of maxilla 1 with 11 spine teeth; posterodistal corner of epimeral plate 2 with tooth..................................................... 10

10. Inner ramus of uropod 3 as long as proximal article of outer ramus..................................................................... S. biscayensis Inner View in CoL ramus of uropod 3 as long as outer ramus................. S. mytilus View in CoL