Arthrura monacantha Vanhoffen 1914

Arthrura monacantha (Vanhöffen, 1914)

Figures 9–10

Restricted synonymy and bibliography:

Paranarthura monacanthus Vanhöffen, 1914: 456, 479; Kusakin, 1966: 326, 348; Lang, 1971: 361. Libanius monacanthus Lang, 1971: 389, 395–400; Kudinova-Pasternak, 1975: 222; Sieg, 1986: 4 –5, 7, 33–34; Schmidt & Brandt, 2001: 422 –423, 425, 427.

Material examined: 2 females, 24 mancas, ZMH K- 42690, ANTXXII/3, PS67/074-6-S, 71º 18.35'– 71º 18.28' S, 13º 57.71'– 13º 57.31' W, depth 1030–1040 m, 20 Feb 2005; 61 females (one dissected on slides), 29 juvenile males, two mature males (one dissected on slides) and 133 mancas, ZMH K- 42691 (ZMH K- 42699 for dissected female, ZMH K- 42700 for dissected male), PS67/074-6-E, 71º 18.35'– 71º 18.28' S, 13º 57.71'– 13º 57.31' W, depth 1030– 1040 m, 20 Feb 2005; one female, SR-EBS-4-E, 53° 37.68'– 53° 37.59' S, 40° 54.46' – 40° 54.69' W, depth 201–203 m, 11 Apr 2006; one female, two mancas, SR-EBS-5-E, 53° 19.27'– 53° 19.27' S, 42° 14.05'– 42° 13.79' W, depth 501–505 m, 12 Apr 2006; one female, R/V Eltanin 22, sta. 1512, 54° 09'– 54° 07' S, 52° 08'– 52° 17' W, depth 419– 483 m, 30 Jan 1966; three females, R/V Eltanin 6, sta. 339, 53° 05'– 53° 08' S, 59° 31'– 59° 24' W, depth unknown, 0 3 Dec 1962.

Arthrura cf. monacantha: one manca and one juvenile male, ZMH K- 42692, ANTXXII/3, PS67/088-8-E, 68° 3.66'– 68° 3.61' S, 20° 27.90'– 20° 27.52' W, depth 4929–4931 m, 27 Feb 2005; one female and one juvenile male, ZMH K- 42693, ANT XXII/3, PS67/016-10, 41° 7.06'– 41° 6.99' S, 9° 54.88'– 9° 54.75' E, depth 4669–4687 m, 26 Jan 2005.

Female. Body length up to 4.7 mm (Figs 10 A, B), 6.5 times as long as wide.

Carapace about 20 % of total body length, with pair of simple setae proximally near antennule insertion.

Pereonites 1–4 rounded laterally; pereonites 5 and 6 trapezoidal; all pereonites subequal in width. Pereonite 1 shortest, about 0.3 times as long as carapace, with simple seta on lateral margin and with hyposphaenium. Pereonites 2, 3 and 6 subequal, 1.5 times as long as pereonite 1. Pereonite 4 as long as pereonite 6, 1.2 times as long as pereonite 3. Pleon (Fig. 10 D) 20% of body length. Pleonites slightly narrower than pereonites, each with small setae laterally, pleonite 5 also with long seta. Pleotelson as long as four pleonites, with distal margin pointed with four distal setae.

Male. Body (Fig. 10 C) of length 4.3 mm similar to female.

Antennule (Fig. 10 F) thicker than in females; article 1 almost twice as long as wide, with one long, simple seta and three combs of bipinnate setae subdistally. Article 2 little wider than long, about half as long as article 1, with one simple and four bipinnate setae subdistally; article 3 0.7 times as long as article 2, with one simple and one bipinnate seta distally. Article 4 twice as long as article 3, with one bipinnate seta proximally and one bipinnate and six simple setae terminally.

Pleopod (Fig. 10 H) basal article naked; endopod 1.6 times as long as basal article, with one seta subdistally and nine strong setae terminally; exopod about as long as endopod, with fourteen setae terminally.

Mouth parts, pereopods and uropods as in female (see Lang 1971: 397–399)

Juvenile male: body length 3–4.5 mm, pleopod lacking setation, except one ramus that bears a single seta.

Manca: body length up to 2.7 mm, similar to adults, except last pereonite and pereopod not fully developed.

Distribution: This species is known from Gauss Station at a depth of 385 m (type locality, Vanhöffen, 1914), as well as from the Scotia Sea and the Eastern Weddell Sea from a depth range of 201 to 4931 m and the vicinity of 41° S, 9° E from depth range 4669 to 4687 m.

Remarks: Lang (1971) used the presence/absence of pleopods to distinguish Arthrura (Libanius) monacantha from A. pulcher. The other diagnostic character pinpointed was the presence of a hyposphaenium on the first pereonite in A. monacantha. The abundant material of A. monacantha examined during this study reveals that the structure of the pleopods is related to sexual dimorphism, while the presence of the hyposphaenium is correlated with age. Thus, in all males and females below 4.7 mm body length, a hyposphaenium is present on pereonite 1, but it is absent in females larger than 5 mm. Intriguingly, none of 150 specimens studied by us had a brood pouch or even rudimentary oostegites, and we assumed that the specimens without pleopods are females (Figs 10 B, D), those with setose pleopods are males (Figs 10 C, H), while specimens with rudimentary pleopods (lacking setae) (Fig. 10 E) are juvenile males.