Dimorphostylis brevicarpus, 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 : 54-58

Dimorphostylis brevicarpus n. sp.

( Figs 8–10 )

Material examined. Holotype preparatory female, 4.9 mm (NSMT-Cr 21697), paratypes: 4 preparatory females, 4.5–5.1 mm, 1 subadult male, 4.9 mm, dissected (NSMT-Cr 21698), Mekura-sone, Okinawa Trough, the East China Sea, 30˚22.66´N, 129˚02.59´E – 30˚22.95´N, 129˚05.82´E, 171–236 m, 23 May 2008; subadult male, length 5.2 mm (dissected), 1 preparatory female (damaged during collection, dissected) (NSMT-Cr 21699), Mekura-sone, 30˚22.79´N, 129˚03.77´E, 195 m, 12 May, 2008; 1 subadult male, 4.3 mm, dissected (NSMT-Cr 21700), near Hiraji-sone, west of Kyusyu Isl., the East China Sea, 32˚14.17´N, 129˚39.56´E – 32˚14.62´N, 129˚ 38.00 E, 165– 180 m (N237, St. C), 8 May 2007; 1 subadult male, length 4.4 mm, dissected (NSMT-Cr 21701), west of Okinawa Isl., 26˚49.50´N, 127˚42.00´E, 199–202 m (TY03-9), 22 May 2003.

Diagnosis. Carapace without transverse ridge, 3 lateral ridges running in parallel, dorso-lateral ridge prominent, ending anteriorly in 2-3 spines, carapace covered with small round pits with a seta in each. Antenna 1 peduncle article 3 with 12–17 thin setae on inner margin. Maxilliped 1 with setules on ventral surface. Pereopods 3–5 with carpus very short. Uropod peduncle 2.1–2.5 times endopod length. Telson length 1.1-1.5 times width, shorter than pleonite 6 length.

Description. Preparatory females ( Figs 8 , 9 ). Body heavily calcified. Carapace with numerous small round pits, each with a short seta, projecting in a similar direction with setae located nearby ( Fig. 8 C). Carapace 0.35– 0.38 times total body length, 1.2–1.3 times width and 1.6–1.8 times depth, lateral surface with 3 lateral ridges running in parallel, dorsolateral ridge marked, ending anteriorly in 2–3 spines; middle ridge weak and short, with free end anteriorly and posteriorly; pseudorostrum rather short, 0.150–0.160 times length of carapace; width of rounded eye lobe 0.17–0.19 times greatest width of left and right dorsolateral ridges, width a pair of marked spines on tip; dorsal surface of frontal lobe with weak mid ridge meeting a ridge connecting to hind end of dorsolateral ridge. Pereon 0.39–0.52 times as long as carapace. Pleon 0.48–0.49 times total body length; pleonite 6 with 2 spines on dorso-posterior margin.

Antenna 1 ( Fig. 8 D), 1st article of peduncle 1.05–1.2 times combined length of 2nd and 3rd articles; 3rd article 2.4–3.0 times 2nd, with 12–17 setae on inner margin; main flagellum of 3 articles, 0.75–0.8 times 3rd article of peduncle; bi-articulated accessory flagellum 0.5–0.6 times main flagellum, 1.2–1.3 times basal article of main flagellum. Antenna 2 tri-articulated, with 3 plumose setae ( Fig. 8 E). Left and right mandibles ( Fig. 8 F) with 13 and 11 setae respectively; pars incisiva and lacinia mobilis bi-dentate. Palp of maxilla 1 with 2 filaments ( Fig. 8 G). Maxilla 2 ( Fig. 8 H). Maxilliped 1 with 4–6 branchial lobules ( Fig. 8 J); basal article with small sharp setules on ventral surface ( Fig. 8 I). Maxilliped 2 ( Fig. 8 K), basis 0.7–0.9 times combined length of ischium, merus, carpus and propodus. Maxilliped 3 ( Fig. 8 L), basis 1.4–1.5 times remaining distal articles, with 4–5 plumose setae on inner margin.

Pereopod 1 ( Fig. 9 A), basis 0.4 times combined length of distal articles, 1.2 times carpus length, 0.97–0.98 times propodus length; propodus 1.21 times carpus length and 2.9–3.1 times dactylus length. Pereopod 2 ( Fig. 9 B), basis 0.7–0.8 times distal articles together, with 2 plumose setae on distal outer corner; carpus 0.85–0.9 times propodus and dactylus together. Pereopod 3 ( Fig. 9 C), basis 0.9–1.0 times distal articles together; merus 2.1–2.3 times carpus; terminal robust seta on dactylus fused with the article. Pereopod 4 ( Fig. 9 D), basis 0.7–0.8 times distal articles together; merus 2.0–2.5 times carpus. Pereopod 5 ( Fig. 9 E), basis 0.5–0.6 times distal articles together; merus 1.5–1.9 times carpus.

Uropod ( Fig. 9 F) peduncle 2.3–2.9 times pleonite 6 length, 2.0–2.6 times exopod length, 2.1–2.5 times endopod length, with 7–9 setae on inner margin; exopod 0.9–1.1 times endopod length, with 2 simple setae on inner margin and terminal seta; endopod with 2 spiniform setae on inner distal corner and inner margin of each article, 1st article 0.5–0.6 times combined length of 2nd and 3rd, 2nd article 0.9–1.1 times the 3rd. Telson 0.71–0.80 times as long as pleonite 6 and 1.1–1.35 times its width ( Fig. 9 F), with 2 minute setae on apical end.

Subadult males ( Fig. 10 ). Carapace similar to females ( Fig. 10 A, B). Pereon 0.36–0.48 times length of carapace. Pleon 0.45–0.49 times total body length; pleonite 1 and 2 each with pair of immature pleopods.

Antenna 1 ( Fig. 10 C) 3rd article of peduncle thicker than in female; main flagellum of 4 articles, 1.0–1.1 times length of 3rd article of peduncle; accessory flagellum 0.6–0.65 times main flagellum length. Antenna 2 ( Fig. 10 D) with 3 plumose setae on peduncle. Maxilliped 3 ( Fig. 10 E), basis 1.45–1.65 times remaining distal articles, 4 plumose setae on inner margin.

Pereopod 1 ( Fig. 10 F), basis 0.4–0.5 times distal articles together, 1.3–1.6 times carpus, 1.0–1.15 times propodus; propodus 1.19–1.39 times carpus and 2.4–3.1 times dactylus. Pereopod 2 ( Fig. 10 G), basis 0.9–1.0 times remaining distal articles; carpus 0.85–0.9 times combined length of propodus and dactylus. Pereopods 3 ( Fig.

10H), basis strongly constructed, 1.1–1.3 times distal articles together; merus 1.95–2.1 times carpus. Pereopod 4 ( Fig. 10 I), basis 0.8–0.9 times remaining distal articles; merus 1.8–2.1 times carpus. Peropod 5 ( Fig. 10 J), basis 0.6–0.7 times remaining distal articles; merus 1.5–1.8 times carpus. Pereopods 1–4 with developed exopods. Pleopods undeveloped, with a few minute setae on tips of rami ( Fig. 10 K).

Uropod ( Fig. 10 L) peduncle 2.4–2.7 times pleonite 6, 2.2–2.5 times exopod length, 2.4–2.5 times endopod length, with 7–9 spiniform setae on inner margin; exopod 1.0–1.1 times endopod length; endopod tri-articulated, with 2 or 1 spiniform setae on inner margin of each article. Telson 0.77–0.85 times pleonite 6 length, 1.3–1.5 times pleonite 6 width ( Fig. 10 L).

Etymology. The species name refers to the short carpus of pereopods 3–5.

Remarks. The present new species resembles Dimorphostylis elegans Gamô, 1960 , but can be distinguished from the latter by (1) carpus of pereopods 3–5 short, (2) anterior end of dorso-lateral ridge with 2–3 spines (1 spine in D. elegans ), and (3) maxilliped 1 with a group of small, sharp setules on the ventral surface of basis (no setules in D. elegans ).

In D. brevicarpus , the integument of the carapace is covered in numerous small round pits with a short seta in each of them, which is currently unique in cumaceans. The pits were also observed on some other regions including the pereon, pleon, and ventral surface of pereopods 1 and 2, although rather sparsely distributed compared to those on the carapace. Although the carapace of D. elegans is also covered with shallow round depressions which are similar to D. brevicarpus in appearance, no setae were observed in the depressions. The tiny setae arranged somewhat regularly suggest that the pits work as mechano-receptors.

Dimorphostylis brevicarpus also resembles D. cornigera Harada, 1960 , from the Izu Peninsula, but can be distinguished from the latter by (1) dorsolateral carina ending anteriorly in 2–3 prominent teeth (ending of the carina not well defined in D. cornigera ), (2) pereopod 2 carpus shorter than combined length of propodus and dactylus (longer than combined length of propodus and dactylus in D. cornigera ), (3) uropod peduncle less than 3 times pleonite 6 length (3 times as long as pleonite 6 in D. cornigera ), (4) uropod endopod 2nd and 3rd articles with 2 spiniform setae on inner margin (1 spiniform seta in D. cornigera ), and (5) telson shorter than pleonite 6 (subequal to pleonite 6 in D. cornigera ).

Most specimens were collected from about 200 m depth, at the plateau of Mekura-sone, a small sea mount where the Kuroshio Current flows. Sediment collected by the biological dredge was very coarse sand. Strongly constructed walking legs of the new species may have developed in response to the habitat of coarse sediments and considerable water current.

Distribution. Off Kyushu and Nansei Islands, East China Sea, 165– 236 m.

Figures

FIGURE 8. Dimorphostylis brevicarpus n. sp., A, B, holotype preparatory female; C – L, paratype preparatory female. A, lateral view; B, anterior part of body, dorsal view; C, surface of carapace; D, antenna 1; E, antenna 2; F, mandibles; G, maxilla 1; H, maxilla 2; I, J, maxilliped 1; K, L, maxillipeds 2, 3.

FIGURE 9. Dimorphostylis brevicarpus n. sp., paratype preparatory female. A – E, pereopods 1 – 5; F, uropod with pleonite 6.

FIGURE 10. Dimorphostylis brevicarpus n. sp., paratype subadult male. A, lateral view; B, anterior part of body, dorsal view; C, antenna 1; D, antenna 2; E, maxilliped 3; F – J, pereopods 1 – 5; K, pleopod 1; L, uropod with pleonite 6.