Dimorphostylis elegans Gamô, 1960

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 : 46-49

Dimorphostylis elegans Gamô, 1960

( Figs. 1–3 )

Dimorphostylis elegans Gamô, 1960: 102 –104, figs.3, 4; Gamô, 1968: 184 –185.

Dimorphostylis latifrons Harada, 1960: 220 –222, figs. 10, 11; Bacescu, 1992: 333.

Material examined. 1 damaged ovigerous female (dissected), 6 preparatory females, 4.56–5.07 mm (NSMT-Cr 21686) southeast of Izu Peninsula, Honshu Isl., 34˚41.01´N, 139˚00.42´E, 74–77 m depth (KT-02-5, St. IZE-4), 29 May 2002; 5 preparatory females, 4.6–4.9 mm (NSMT-Cr 21687), west of Izu Peninsula, 34˚50.88´N, 138˚44.78´E, 130–134 m (KT-02-3, St. IZW-1), 29 May 2002; 1 ovigerous female, Cape Ashizuri Misaki, 32˚41.9´N, 132˚15.9´E, 260–261 m (KT-99-18, St. DG-5), 17 December 1999; 5 preparatory females, 5.0– 5.7 mm (NSMT-Cr 21688), Enshu Nada, 34˚35.32´N, 137˚58.55´E, 168–194 m (KT-04-6, St. EN-2), 2 May 2004; 1 preparatory female, 1 subadult male, 34˚17.26´N, 138˚15.05´E, 338–354 m (KT-04-6, KS-1), 30 April 2004; 1 preparatory female, dissected (NSMT-Cr 21689), 34˚17.55´N, 138˚14.46´E, 443 m (KT-04-6, St. KS-2-1), 30 April 2004; 1 preparatory female, Off Okinoshima, Kochi, 32˚43.95´N, 134˚41.99´E, 131–136 m (KT-05-30, St. OS-1-2), 22 November 2005; 1 preparatory female, 32˚38.18´N, 132˚39.91´E, 312–316 m (KT-05-30, St. OS-3), 22 November 2005; 1 preparatory female (dissected), 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, 27˚10.76´N, 125˚32.44´E, 122 m (N258, St. TR), 16 May 2008; 1 subadult male, 29˚25.09´N, 127˚17.97´E, 183–184 m, (N295, St. Dr.1), 21 November 2009.

Diagnosis. Carapace without transverse ridge on frontal lobe; 3 lateral ridges running in parallel; dorso-lateral ridge prominent, ending anteriorly with a spine projecting forward. Pleonite 6 with 2 spines on dorso-posterior margin. Antenna 1 peduncle article 3 with 2–5 thin setae. Uropod peduncle 2.4–2.7 times endopod length. Telson length less than 2 times width, as long as or shorter than pleonite 6 length.

Description. Preparatory females from east of Izu Peninsula, (KT-02-5, St. IZE-4, 5 specimens dissected, Fig. 1 , 2 ). Integument well calcified, surface covered with fine granules and small, shallow circular depressions. Carapace 0.38–0.40 times total body length, 1.3–1.4 times width and 1.65–1.8 times depth; 3 lateral ridges (dorsolateral ridge, median lateral ridge, ventrolateral ridge) running in parallel on each side, joined by a ridge running near hind margin of carapace; dorsolateral ridge prominent, ending anteriorly with a spine projecting forward, slightly behind eye lobe, joined posteriorly with transverse ridge; weak lateral ridge with small tubercles (frontal ridge) running between the pseudorostrum and dorsolateral ridge; anterior portion of carapace surrounded by frontal ridge, dorsolateral ridge and transverse ridge rather flat, with weak median carina with some small spines; dorsal surface surrounded by transverse ridge, hind ridge and hind margin of carapace depressed near median line; median lateral ridge bend downward anteriorly, meeting with ventrolateral ridge near serrated ventral margin of carapace. Pseudorostrum 0.21–0.23 times length of carapace, with a pair of small spines on tip. Width of rounded eye lobe 0.16– 0.21 times greatest width of left and right dorsolateral ridges, with a pair of small spines. Anterolateral angle with a tooth, succeeded by a series of small teeth on anterior half of ventral margin. Antennal notch widely open. Pereon 0.37–0.47 times as long as carapace. Side plate of pereonite 1 usually not observable. Pleon 0.44–0.47 times total body length; pleonite 6 with 2 spines on dorso-posterior margin.

Antenna 1 ( Fig. 1 C), peduncle of 3 articles, 1st article 1.1–1.2 times remaining 2 articles combined; 3rd article 1.8–2.2 times 2nd, with 2–5 thin setae on inner margin facing forward; main flagellum of 3 articles, 0.51–0.55 times 3rd article of peduncle; accessory flagellum bi-articulated, 0.6–0.9 times main flagellum, 1.3–1.6 times basal article of main flagellum. Antenna 2 ( Fig. 1 D) of 4 articles, with 4 plumose setae. Labium ( Fig. 1 E) with small process on tip. Left and right mandibles with 12 and 11 setae ( Fig. 1 F). Maxilla 1 ( Fig. 1 G), outer lobe with 9 stout setae. inner lobe with 4 setae; palp rather short, with 2 filaments. Maxilla 2 ( Fig. 1 H), 2 narrow endites with 5 and 3 setae respectively; protopod with a row of stiff plumose setae on upper margin and a row of fine setae on lower margin. Maxilliped 1 ( Fig. 1 I), with 2–6 branchial lobules; basis with no seta on inner margin; carpus with comblike setae on inner margin ( Fig. 1 I). Maxilliped 2 ( Fig. 1 J), basis 0.8–0.95 times combined length of following 4 articles, with a few long plumose setae on distal margin; propodus with 3 robust, serrated plumose setae. Maxilliped 3 ( Fig. 1 K), basis 1.3–1.5 times remaining distal articles, with 3–4 plumose setae on inner margin and 5 long plumose setae on outer distal corner.

Pereopod 1 ( Fig. 1 L), basis 0.4–0.45 times combined length of distal articles, 1.2–1.5 times carpus, 1.1–1.2 times propodus; propodus 1.1–1.3 times carpus and 1.7–1.9 times dactylus. Pereopod 2 ( Fig. 1 M), basis 0.9–1.0 times distal articles together, with 3–5 plumose setae on inner margin; carpus 0.8–0.9 times combined length of propodus and dactylus. Pereopod 3 ( Fig. 1 N), basis 0.8–1.0 times distal articles together; merus 1.3–1.5 times carpus; terminal robust setae on dactylus defined at base. Pereopod 4 ( Fig. 1 O), basis 0.7–0.8 times distal articles together; merus 1.2–1.4 times carpus. Exopods absent on pereopods 3 and 4, except for 1 specimen with rudimentary, uniarticulated exopods on pereopod 3 and 4. Pereopod 5 ( Fig. 1 P), basis 0.5–0.6 times distal articles together; merus 1.0–1.3 times carpus.

Uropod ( Fig. 1 Q), peduncle 2.0–2.2 times pleonite 6 length, 2.40–2.67 times exopod length, 2.49–2.70 times endopod length; peduncle with 6–8 spiniform setae on inner margin; exopod 0.93–1.06 times endopod length, with 2 setae on outer margin and long terminal seta and subterminal seta; endopod triarticulated, with spiniform seta on inner distal corner of each article and long terminal seta; article 1 0.78–0.89 times combined length of articles 2 and 3; article 2 0.97–1.36 times length of article 3. Telson 0.77–0.87 times as long as pleonite 6 ( Fig. 1 Q), with 2 minute spiniform setae on apical end; length of telson 1.41–1.57 times width.

Remarks. Morphological characters of the specimens from east of Izu Peninsula agree well with the original description of Dimorphostylis elegans , including the telson being shorter than pleonite 6. Although the telson of some specimens from other localities was subequal in length to pleonite 6, in such cases, the telson protruded somewhat and the arthrodial membrane connecting the telson and pleonite 6 was exposed, which was also shown in Harada’s illustration of D. latifrons . Thus the relative length of the telson to pleonite 6 is largely dependent on the state of the specimens when they were fixed. With the new description, it is now clear that D. latifrons is a junior synonym of D. elegans , and the telson of D. elegans is shorter or subequal to pleonite 6.

Some morphological variations of the carapace between individuals were observed:

(1) The median lateral ridge on carapace occasionally does not reach the hind ridge, with a free end posteriorly or anteriorly ( Fig. 2 D).

(2) Ridges on carapace occasionally have numerous tubercles or blunt spines. ( Figs. 2 A, B). Although specimens from the eastern coast of the Izu Peninsula and those described by Gamô (1960) and Harada (1960) do not have such tubercles or spines on the ridges, specimens from the western coast of the peninsula were characterized by the tubercles and blunt spines. Specimens from Enshu Nada are characterized by presence or absence of the tubercles.

(3) In a few specimens from Enshu Nada, southern coast of Honshu Island, the anterior oblique ridge is directed slightly outward anteriorly, ending in large terminal teeth directed forward ( Figs. 2 C, D). All specimens from the eastern coast of the Izu Peninsula, including Harada’s specimens, are characterized by the anterior oblique ridge directed slightly inward anteriorly.

According to Harada (1960), the color of living D. elegans specimen from Izu Peninsula was "milk white". In the present study, color of living specimens was not examined, except for a specimen collected from the East China Sea (N295, St. Dr1) characterized by broad, irregular reddish stripes ( Fig. 3 ), which were lost during preservation.

Dimorphostylis elegans is also closely allied to D. gibbosa Harada, 1960 . According to Gamô (1968), the former can be distinguished from the latter, in that the middle ridge of the carapace does not reach near the hind margin. Considerable morphological variation of this ridge in D. elegans suggests that D. gibbosa may be another synonym of D. elegans . However, in D. gibbosa , the anterior end of the anterior oblique ridge does not bear a spine, which is prominent in D. elegans . To clarify the taxonomic status of D. gibbosa , further studies on additional specimens are needed.

Distribution. Pacific Coast of southern Japan, the East China Sea, 40– 443 m.

Figures

FIGURE 1. Dimorphostylis elegans Gamô, 1960, preparatory female. A, lateral view; B, carapace, dorsal view; C, antenna 1; D, antenna 2; E, labium; F, mandibles; G, maxilla 1; H, maxilla 2; I – K, maxillipeds 1 – 3; L – P, pereopod 1 – 5, Q, uropod with pleoite 6.

FIGURE 2. Dimorphostylis elegans Gamô, 1960. Carapace of 2 preparatory female specimens (A, B and C, D) collected from Enshu Nada (KT- 04 - 6, EN- 2) showing morphological variation between individuals. A, C, dorsal view; B, D, lateral view; scale bar, 1 mm.

FIGURE 3. Dimorphostylis elegans Gamô, 1960, subadult male collected in the East China Sea (N 295, St. Dr. 1). Body length, 4.4 mm.