Mesosignum weddellensis Choudhury & Brandt, 2004, sp. nov.

Mesosignum weddellensis sp. nov. (Figs. 1–4)

Material examined: Holotype: 1 ɗ (2.0 mm), ZMH K40530, Weddell Sea; Station 135, 65°006’S, 43°119’W, depth 4677–4679 m, 11 March 2002, RV Polarstern.

Also examined: Mesosignum asperum Menzies & Frankenberg, 1968, holotype ɗ (1.6 mm), AMNH 12494, Station: V–15–48, 10°07’N, 89°50’W, 3718 m, East Pacific, off Costa Rica, 21 November 1958. Mesosignum brevispinis Birstein, 1963, holotype (no registration number available) Ψ (1.6 mm), 48°50.8’N, 160°01.0’E, 5670–5680 m, Vityaz Station 3114, Kurile Islands, 1954; Mesosignum usheri (Menzies, 1962), holotype ɗ (2 mm), AMNH 12049, Station: V–15–13, 11°3’N, 75°3’W, 2875–2944 m, continental rise northwest of Cartegena, Colombia, 8 November 1958.

Type locality: Weddell Sea, 65°006’S–43°119’W, 4677–4679 m.

Diagnosis: Pereonite 1 without lateral extensions, pereonites 2 and 3 with long lateral extensions, with two branches, pereonite 4–7 only with one lateral extension. Posterodorsal part of the body with few small spines. Pleotelson oval, apex with 8 medial spines, slightly longer than wide, with 1 pair of lateral extensions on each side.

Description (Fig.1 a): Body 1.4 times as long as wide; pereonite 1 as long as head and slightly broader, lateral margin smooth. Pereonites 2 and 3 with long lateral extensions, with 2 branches, anterior branch longer than posterior, pereonites 4–7 with only 1 lateral extension. Lateral extensions with spines. Anterior lateral extensions of pereonite 2–3 directed frontally. Pereonite 4 as long as pereonite 7, both longest segments.

Pereonites 5 and 7 dorsal surface with very few short spines. Pereonite 5 laterally directed, pereonite 6–7 more or less caudolaterally directed. Pereonite 6 widest. Pleotelson oval, 0.3 as long as body, slightly longer than wide, with 16 lateral spines; lateral margins each with 2 lateral extensions, both directed posteriorly, anterior extension longer than posterior, at 0.5 length and posterior extension at 0.8 length.

Antennula (Fig. 1 b): Peduncle with 3 articles, flagellum with 4 articles. First peduncular article broadest with 1 distomedial spine and 1 simple seta. Second peduncular article about twice as long as first, but narrower, with 1 long and 1 shorter feather­like seta. Third peduncular article half as long as first, with 1 simple seta. Flagellum article 1 slightly shorter than peduncular article 3, with 1 feather­like seta; flagellum article 2 about as long as article 1, with 1 long aesthetasc and 1 shorter simple seta; remaining articles becoming progressively shorter and narrower, each with 1 long aesthetasc, which is broken off at tip and 1 short simple seta, terminal article shortest and narrowest, with 1 aesthetasc, and 2 long simple setae.

Antenna (Fig. 1 c): Only the first 4 short peduncular articles present. First article small and rectangular, without setation; second article almost quadrangular with 1 proximolateral and 3 distolateral setae, which are broken off; third article half as long as second, with 1 medial and 2 lateral robust setae, with broken tips; fourth article as long as second, with 2 distolateral simple setae, 4 medial simple setae, 3 of these broken off.

Mandibles (Fig. 2 a, b): first article with 1 simple seta, third article longer and narrower than first, with 5 simple setae of varying lengths; incisor of right mandible with 3 cusps, incisor of left mandible broken off; lacinia mobilis of right mandible with 2 teeth; right spine row of 5 setae; molar very long and slender, right mandible with 3 setulated setae and left mandible with 4.

Maxillula (Fig. 2 c): Lateral endite with 10 strong spine­like setae, smaller inner endite with 6 dorsal and several medial simple setae.

Maxilla (Fig. 2 d): Inner endite with 4 distal strong, spine­like setae, distolaterally serrated; medial endite with 4 bristles of varying lengths; lateral endite with 8 setae in two rows.

Maxilliped (Fig. 2 e): Coxa broken off. Basis with 2 coupling hooks, lateral margin finely setulated. Endite with 5 indented spines, 1 setulated one and 4 small spines. Epipod twice as long as broad, 0.63 as long as second palp article, lateral margin fringed with short setules. First palp article shortest, rectangular, with 1 simple seta. Second article almost 3 times as long as first with 3 simple distomedial setae of varying lengths. Third article 1.5 times as long as first with 3 distolateral setae. Fourth article slightly shorter and narrower than third, with 3 distal simple setae. Last palp article narrowest, as long as fourth, with 3 apical setae, 1 setulated seta and 1 distolateral simple seta.

Pereopod 1 (Fig. 3 a): (broken into two pieces after ischium): Shorter and more slen­ der than pereopods 2–7; basis 3 times as long as ischium, with 3 simple setae and 4 dorsal spines of different lengths; ischium with 1 robust and 1 small dorsal simple spine; merus shorter than ischium, with 2 distodorsal, 1 distoventral and 1 proximodorsal simple setae; carpus 0.7 as long as basis, with 1 ventral long simple seta and 1 medioventral robust flagellate seta, distodorsally with 1 simple seta; propodus 0.7 as long as basis, with 2 ventral robust flagellate setae and 1 long simple seta, distodorsally with 1 simple seta, ventral margin with 2 small setal combs; dactylus 0.4 as long as propodus, 1 simple seta on ventral side.

Pereopods 2–4 (Fig. 3 b, c; Fig. 4 b): Basis 3.0 times as long as ischium, with different numbers of simple setae, 1 robust seta, 1 small spine in pereopod 2 and 1 feather­like seta on P4; ischium with few setae, in pereopod 3 with 1 smaller and 1 bigger spine; merus shorter than ischium, with few simple setae and 1 distodorsal small spine in pereopod 3; carpus 0.7 as long as basis, with 2–3 ventral robust flagellate setae, distodorsally with 1 robust flagellate seta and one feather­like seta; propodus slightly more than 0.7 as long as carpus, ventrally 4–5 robust flagellate setae and varying numbers of setal combs, distodorsally 1–4 simple setae and 1 feather­like seta; 2 distodorsal setae.

Pereopods 5–7 (Fig. 3 d, e; Fig. 4 a): basis, 2.2–3.0 times as long as ischium, distodorsally with few simple and 1 feather­like setae, ventrally with few simple seta, pereopod 7 ventrally with 4 tiny spines, distodorsally with 7 small spines; ischium broad, 1 ventral simple seta and 1 distodorsal robust setae, in pereopod 6 also with 1 small spine; merus 0.5–0.6 times as long as ischium, with 1–3 distodorsal simple setae and 1 short ventral seta; carpus 0.7–0.8 times as long as basis, distodorsally with 1 feather­like seta, 1–2 simple and 1 robust flagellate seta, ventrally with 2 robust flagellate setae and 1 simple seta in pereopod 7; propodus 0.7–0.9 times as long as carpus, with 3–4 ventral robust flagellate setae and 3 setal combs, distodorsally with 1–2 simple setae.

Pleopod 1 (Fig. 4 c): proximally broad, narrow midlength, distal margin each with 7 simple setae, most of these broken off.

Pleopod 2 (Fig. 4 d): sympod 2.4 times as long as wide, stylet 0.7 times as long as sympod; exopod short, rounded and distally with few setae.

Uropod (Fig. 4 e): Uniramous, article 1 with 1 simple long seta on medioventral margin. article 2 3.5 times as long as article 1, with 4 feather­like distal setae (2 broken off) and 3 lateral simple setae of different lengths.

Remarks: Mesosignum weddellensis sp. nov. is most similar to M. brevispinis Birstein, 1963. Birstein illustrated only the dorsal view, antennula, proximal articles of antenna, mandible, maxilliped, and pereopod 1, other appendages not being illustrated. As the holotype is now incomplete, and the posterior part of the animals missing, we could not provide the necessary revision.

The following comparison is based on the illustration of Birstein (1963) and the holotype of M. brevispinis. M. brevispinis differs from M. weddellensis in the following characters: the fourth pereonite of M. brevispinis has no long and acute spines on the lateral border as in M. weddellensis, instead it has rounded edges with a few tiny spines. Furthermore, the first pereonite is not as broad as in Birstein’s illustration. The lateral extensions of the seventh pereonite of M. brevispinis are short, nearly half as long as those of the equivalent pereonite of M. weddellensis. The apex of the pleotelson of M. brevispinis is not equipped with medial spines in contrast to the apex of the pleotelson of M. weddellensis which bears eight medial spines. The first peduncular article of the antennula of M. brevispinis does not bear any setae, that of M. weddellensis possesses one distomedial spine and one simple seta. The epipod of the maxilliped of M. brevispinis is broader than in M. weddellensis, reaching to mid of fourth palp article, besides, the lateral margin of M. brevispinis does not have the short setules of M. weddellensis. The maxilliped of M. brevispinis only has one coupling hook, whereas that of M. weddellensis has two, the second palp article has only one simple distomedial seta in M. brevispinis, it has three in M. weddellensis.

Three other species are somewhat similar to M. weddellensis sp. nov.: M. asperum Menzies & Frankenberg, 1968, M. usheri Menzies, 1962 and M. elegantulum Birstein, 1963. However, they can be easily distinguished from M. weddellensis by the following characters: the pleotelson of M. asperum and M. usheri bear only one lateral spine and this is more slender, while the pleotelson of M. weddellensis bears two lateral spines; the seventh pereonite does not have lateral extensions; each extension of M. asperum bears a long seta, these being absent in M. weddellensis. The habitus of M. elegantulum is similar to that of M. weddellensis but more slender, and the lateral extensions more acute; moreover the antennula is different, in M. elegantulum it has no spines and only one simple seta at the second article, while in M. weddellensis the second article of the antennula bears a small distal spine.

Distribution: Known only from type locality.

Etymology: The epithet is adapted from that of the type locality.