Dimorphostylis bathyelegans, Akiyama, Tadashi, 2011

Akiyama, Tadashi, 2011, The Cumacean Dimorphostylis elegans Gamô, 1960 (Crustacea), D. bathyelegans n. sp. and D. brevicarpus n. sp. from Japan, Zootaxa 2994, pp. 45-59 : 50-54

Dimorphostylis bathyelegans n. sp.

( Figs. 5–7 )

Material examined. Holotype preparatory female, 5.7 mm (NSMT-Cr 21692), Shima Spur, 34˚00.72´N, 136˚53.28´E – 34˚01.55´N, 136˚52.98´E, 781–789 m (KT-08-3, St. SM-1-2), 4 March 2008. Paratypes: 2 subadult male, 6.2mm (dissected), 6.3 mm, 1 preparatory female, 6.0 mm (dissected) (NSMT-Cr 21693), Shima Spur, 33˚59.09´N, 136˚52.26´E – 33˚59.88´N, 136˚51.68´E, 839–846 m, brown mud (KT-05-30, St. KB-1-3), 24 November 2005; 2 preparatory female, 6.0, 6.0 mm, dissected (NSMT-Cr 21694), Shima Spur, 33˚59.33´N, 136˚51.25´E – 33˚58.29´N, 136˚51.43´E, 857–918 m (KT-05-30, St. KB-1-4), 24 November 2005; 1 preparatory female, 5.9 mm, dissected (NSMT-Cr 21695), Shima Spur, 34˚00.83´N, 136˚53.79´E – 34˚01.42´N, 136˚51.80´E, 805–852 m (KT- 08-3, St. SM-1-1), 4 March 2008; 4 preparatory females (5.1–5.2 mm, 3 specimens dissected), 1 subadult male (5.5 mm, dissected) (NSMT-Cr 21696), east of Hiraji-sone, East China Sea, 32˚09.32´N, 129˚30.00´E, 495 m (N282, St. B-2), 10 June 2009.

Other material. 3 subadult males (5.2–5.3 mm, dissected), east of Hiraji-Sone, 32˚09.11´N, 129˚31.25´E, 495 m (N282, St. B-1), 7 June 2009; 1 preparatory female, 5.86 mm, east of Okinawa Isl., Okinawa Trough, 26˚15.10´N, 125˚17.22´E – 26˚13.85´N, 125˚18.43´E, 955–991 m (KT-02-3, St. E2), 26 April 2002; 1 preparatory female, 4.62 mm, Okinawa Trough, 28˚03.79´N, 128˚56.31´N – 28˚04.71´N, 128˚57.59´E, 533–583 m (KH-05-1, St. OT-10), 14 May 2005.

Diagnosis. Carapace without transverse ridge on frontal lobe; 3 lateral ridges running in parallel; dorso-lateral ridge prominent, ending in a spine projecting forward. Pleonite 6 with 2 spines on dorso-posterior margin. Antenna 1 peduncle article 3 with 9–15 setae on inner margin. Uropod peduncle 3 times endopod length. Telson length more than 2 times width, longer than pleonite 6 length.

Description. Preparatory females. Integument moderately calcified, surface covered with minute granules. Carapace ( Fig. 5 A, B) 0.36–0.41 times total length, 1.2–1.35 times width and 1.7–1.9 times depth; 3 lateral ridges running in parallel on each side, meeting via transverse ridge near hind margin of carapace; dorsolateral carina marked, ending anteriorly with a spine directed forward; mid ridge meets lower ridge near antero-lateral angle; pseudorostrum 0.22–0.25 times length of carapace; dorsal surface of frontal lobe with weak mid ridge meeting a ridge connecting to hind end of dorsolateral ridge. Pereon 0.33–0.44 times as long as carapace. Pleon 0.44–0.48 times total body length; pleonite 6 with 2 spines on dorso-posterior margin.

Antenna 1 ( Fig. 5 C), basal article of peduncle 1.0–1.05 times as long as remaining 2 articles together; 3rd article 2.3–2.9 times as long as 2nd, with 9–15 thin setae on inner margin; main flagellum tri-articulated, basal article 0.9–1.1 times combined length of 2nd and 3rd articles; bi-articulated accessory flagellum 0.6–0.65 times main flagellum, 1.2–1.5 times basal article of main flagellum. Antenna 2 of 4 articles, 3 plumose and 2 simple setae ( Fig. 5 D). Left and right mandibles ( Fig. 5 E) with 13 and 10 setae respectively; pars incisiva bi-dentate and lacinia mobilis bi-dentate. Maxilla 1 ( Fig. 5 F), outer endite with 10 stout setae; inner endite with 4 setae; palp with 2 filaments. Maxilla 2 ( Fig. 5 G) with a row of thin simple setae on lower margin; 2 narrow endites with 5 and 3 setae respectively. Maxilliped 1 ( Fig. 5 H, I) with 3–5 branchial lobules; carpus and propodus with comb-like setae on inner margin. Maxilliped 2 ( Fig. 5 J), basis subequal in length to combined length of ischium, merus, carpus and propodus, with 3 long, plumose setae on distal margin. Maxilliped 3 ( Fig. 5 K), basis 1.3–1.6 times remaining distal articles, with 3–8 plumose setae on inner margin and 5 long plumose setae on outer distal corner; ischium with plumose seta on inner margin; merus with 1–2 plumose setae on inner margin and plumose seta on outer diatal corner; carpus with several plumose setae on inner margin.

Pereopod 1 ( Fig. 5 L), basis 0.4–0.55 times combined length of distal articles; propodus 1.2–1.3 times carpus and 2.3–2.7 times dactylus. Pereopod 2 ( Fig. 6 A), basis 0.8–1.0 times distal articles together; carpus 0.95–1.25 times combined length of propodus and dactylus. Pereopod 3 ( Fig. 6 C), basis 0.9–1.1 times distal articles together; merus 1.1–1.45 times carpus; terminal robust setae on dactylus. Pereopod 4 ( Fig. 6 D), basis 0.7–0.9 times distal articles together; merus 1.1–1.3 times carpus. Pereopod 5 ( Fig. 6 E), basis 0.5–0.6 times distal articles together; merus 0.8–0.95 times carpus.

Uropod ( Fig. 6 F) peduncle 2.4–2.7 times pleonite 6 length, 2.6–3.1 times exopod length, 2.9–3.3 times endopod length; peduncle with 5–10 spiniform setae on inner margin; exopod 1.04–1.18 times endopod length, with 2– 3 setae on outer margin and long terminal setae and subterminal seta; endopod tri-articulated, 1st article 0.65–0.9 times combined length of 2nd and 3rd, with 1–2 setae on inner margin, 2nd article 1.1–1.4 times 3rd, with spiniform seta on inner distal corner, 3rd article with spiniform setae on inner margin and long terminal seta. Telson 1.10–1.28 times as long as pleonite 6 ( Fig. 6 F), with 2 minute setae on apical end; length of telson 1.95–2.4 times its width.

Subadult males ( Fig. 7 ). Carapace similar to females. Pereon 0.34–0.46 times carapace. Pleon 0.45–0.46 times total body length; pleonite 1 and 2 with 2 pairs of immature pleopods.

Antenna 1 ( Fig. 7 C), 3rd article of peduncle thicker than in females; main flagellum of 4 articles. Antenna 2 ( Fig. 7 D) with 3 plumose setae on peduncle. Maxilliped 3 ( Fig. 7 E), basis 1.5–1.7 times remaining distal articles, with 3–7 plumose setae on frontal margin.

Pereopod 1 ( Fig. 7 F) basis 0.5–0.7 times distal articles together, 1.5–2.0 times carpus length, 1.2–1.7 times propodus length, propodus 1.2–1.3 times carpus length and 2.2–2.8 times dactylus length. Pereopod 2 ( Fig. 7 G) basis 0.9–1.2 times remaining distal articles, with 2 plumose setae on inner distal corner; carpus 1.0–1.1 times combined length of propodus and dactylus. Pereopods 3 and 4 ( Figs. 7 H, I) more strongly constructed than in females; basis 1.05–1.2, 0.8–0.9 times combined length of distal articles respectively. Basis of pereopod 5 ( Fig. 7 J) 0.58–0.64 times remaining distal articles. Pleopods undeveloped ( Fig. 7 K).

Uropod similar to females ( Fig. 7 L) peduncle 2.4–2.8 times pleonite 6, 2.5–2.8 times exopod length, 2.85–3.3 times endopod length; exopod 1.1–1.2 times endopod length. Telson ( Fig. 7 L) 1.15–1.26 times pleonite 6 length; length of telson 2.1–2.3 times its width.

Etymology. The species name refers to the similarity of this species to D. elegans and in combination with its bathyal habitat.

Remarks. The present new species is similar to Dimorphostylis elegans , but is distinguished from the latter by (1) telson longer than pleonite 6, 2 times as long as its width (shorter or subequal to pleonite 6, less than twice its width in D. elegans ) (2) antenna 1 peduncle 3rd article with 6–15 thin setae (2–5 setae in D. elegans ) (3) uropod peduncle elongate, 3 times as long as endopod (2.5–2.7 times in D. elegans ) (4) width of ocular lobe of the females (N = 9) was 0.11–0.14 times width of carapace (maximum width between left and right dorsolateral ridges), whereas the ratio was 0.14–0.21 (N = 14) in D. elegans . The carapaces of D. bathyelegans specimens were all very similar to each other, in contrast with D. elegans , which shows considerable morphological variation of the carapace.

Dimorphostylis elegans and D. bathyelegans live at 40–450 m and 495–918 m depth respectively, possibly showing habitat segregation according to depth. In addition, the habitat of D. bathyelegans at Shima Spur, south of Honshu Island, was covered with a thin muddy layer, indicating that this species prefers smaller sediment particles than D. elegans . The slightly smaller eye lobe of D bathyelegans than D. elegans may be related to different photic environment between bathyal and shallow waters.

Distribution. Pacific coats of southern Honshu, Shikoku Islands. Off Kyusyu and Nansei Islands, East China Sea, 495– 918 m.

Figures

FIGURE 5. Dimorphostylis bathyelegans n. sp., A, B, holotype preparatory female; C – L, paratype preparatory female. A, lateral view; B, anterior portion of body, from above; C, antenna 1; D, antenna 2; E, mandibles; F, maxilla 1; G, maxilla 2; H, I, maxilliped 1; J, K, maxillipeds 2, 3; L, pereopod 1.

FIGURE 6. Dimorphostylis bathylegans n. sp., paratype preparatory female. A, pereopod 2, B, ventral surface of basis of pereopod 2; C – E, pereopod 3 – 5; F, uropod with pleonite 6.

FIGURE 7. Dimorphostylis bathyelegans n. sp., paratype subadult male. A, lateral view; B, anterior part of body, from above; C, antenna 1; D, antenna 2; E, maxilliped 3; F – J, pereopods 1 – 5; K, pleopod 1; L, uropod with pleonite 6.