Stylomesus weddellensis Brandt & Andres, 2008, sp. nov.
Creators: name: Brandt, Angelika
name: Andres, Hans Georg
Publisher: name: Zenodo
Year: 2008
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Isopoda
Ischnomesidae
Stylomesus
Stylomesus weddellensis

description: Stylomesus weddellensis sp. nov. (Figs 1–7)
Material examined
Holotype: subadult ♂ (8.2 mm), ZMH K­ 41523, Weddell Sea; Station 81­8­E, (70° 31.08 S­ 70° 32.23 S ­ 14° 34.82 W­ 14° 34.90 W); depth 4382 m; 24 February 2005, RV Polarstern.
Allotype: non­ovigerous Ψ (8.5 mm), ZMH K­ 41524, Weddell Sea; Station 81­8­E, (70° 31.08 S­ 70° 32.23 S ­ 14° 34.82 W­ 14° 34.90 W); depth 4382 m; 24 February 2005, RV Polarstern.
Paratypes: 1 subadult ♂ (described and illustrated), ZMH K­41525 (slides), Weddell Sea; station 80­9­E, (70° 38.45 S­ 70° 39.18 S –14° 42.86 W­ 14° 43.43 W); depth 3100 m, 23 February 2005, RV Polarstern; 1 nonovigerous Ψ, ZMH K­41526 (slides), Weddell Sea; station 81­8­E, (70° 31.08 S­ 70° 32.23 S ­ 14° 34.82 W­ 14° 34.90 W); depth 4382 m; 24 February 2005, RV Polarstern.
Additional material examined: 7Ψ, 3 Mancas, Weddell Sea; station 80­9­E, (70° 38.45 S­ 70° 39.18 S – 14° 42.86 W­ 14° 43.43 W); depth 3100 m, 23 February 2005, RV Polarstern. 1 ♂, 1Ψ, 7 Mancas, Weddell Sea; station 81­8­E, (70° 31.08 S­ 70° 32.23 S ­ 14° 34.82 W­ 14° 34.90 W); depth 4382 m; 24 February 2005, RV Polarstern.
Etymology
The name refers to the Weddell Sea.
Diagnosis
Stylomesus without any spines on head, but with mediolateral to caudolateral blunt spines on pereonites 1 –3; antenna 1 consisting of 5 articles, article 2 with 4 sensory setae of varying lengths; pereopod 7 present; pereonite 7 not fused to pleotelson.
Description
Description of subadult holo­ and paratype males (Figs 1–5): Body 3.9 times longer than wide (Fig. 1 A). Head trapezoidal, 4 times as broad as long, anterolateral margins with strong frontolaterally acute projections; lateral margins of pereonites 2 and 3 rounded; pereonites 1–3 with 2 dorsomedial blunt short spines; pereonite 4 about as long as wide, narrowing at half length, broadest part 1.7 times as broad as narrowest part; pereonite 5 about 3.2 times as long as wide, 0.4 times as long as body, distally broadening, broadest part about twice as broad as narrowest part; pereonites 5–7 not fused; pleonite 1 fused but well distinguishable, about half as long as pereonite 7 and slightly narrower. Pleotelson 0.8 as long as wide, with faint elevation mediodorsally, lateral margins convex, terminal margin somewhat produced, rounded, distance between branchial chamber and anal valves 0.3 times as long as pleotelson. Body surface smooth except for spines on pereonites 1–3 (Fig. 1 A, B).
Antenna 1 (Fig. 2 A) about 0.12 as long as body, with 5 articles; article 1 almost quadrangular, 0.9 times as long as wide, with 1 distal feather­like seta and 2 lateral simple setae; article 2 slender, 2.2 times as long as article 1, inserted in distomedial half of article 1, only half as wide as article 1, with 1 distal feather­like seta and a small lateral one, 7 lateral simple setae of varying lengths, and 4 stout sensory setae, 2 being longer than article; articles 3–5 short, last article with 3 simple setae.
Antenna 2 (Fig. 2 B) about 0.7 times as long as body, peduncle 1.5 times as long as flagellum, peduncular article 1 triangular in ventral view, short and narrow (see Fig. 1 C), article 2 slightly longer than article 1, with stout spine­like sensory seta on small distoventral conical projection; article 3 0.9 times as long as article 5, with 2 simple setae, 4 spine­like sensory setae, 1 of which inserts on ventral conical projection, 1 distoventrally, and 2 shorter dorsally; article 4 about 0.3 times as long as article 3, without setae; article 5 more slender than article 3, with 7 simple setae; article 6 about 1.2 times as long as article 5, with several simple setae. Flagellum with 21 articles, article 1 about 1.7 as long as following articles, each article with 2–5 simple distal setae.
Mandibles (Fig. 2 C, D) with smooth outer margin. Incisor with 5 blunt teeth, spine row of 4/5 serrated spines on right/left mandibles; lacinia mobilis of left mandible with 4 blunt teeth, right mandible with short serrated spine instead. Molar with smooth grinding surface and 7 setulated subapical setae, distal part of molar with several short rows of small bristles, left and right molars with 3 lateral simple setae.
Hypopharynx (Fig. 3 F) consisting of 2 lobes. Outer lobes much larger than inner lobes, distomedially with 6 blunt serrated setae and few short setules below; inner lobes distally only with short thin setules.
Maxilla 1 (Fig. 3 A) outer lobe with 13 large serrated spine­like apical setae; medial margin with several rows of simple setae, lateral margin with rows of fine bristles, longest proximally. Inner lobe (see detail) with many slender apical simple setae and a subapical spine­like distally serrated seta, lateral margin with numerous fine bristles.
Maxilla 2 (Fig. 3 B) basis with numerous rows of short stout setae; outer and middle lobes each with 4 apical setulated spine­like setae and several rows of fine setae on lateral margin; outer lobe with several simple apical setae; inner lobe 3.5 times as broad as median or outer lobes, distal margin bearing 3 stout strongly serrated setae, 8 serrated setae, 4 of these swollen, spoon­like in distal half, many simple setae and numerous fine bristles; medial margin with 2 setulated prominent long setae, 1 stout simple seta proximally of these and several slender setae; surface with several rows of setae.
Maxillipeds (Fig. 3 C, D, E) with 1–5 retinaculae and 3 cuticular tooth­like setae on medial margin, one distally serrated. Palp articles with 1–6 long simple setae each, article 3 with additional fine bristles proximomedially, article 1 0.4 as long as wide, articles 2–3 of almost equal width, 0.6 times as broad as endite. Palpal article 4 about half as broad as article 3, with 2 apical simple setae, article 5 narrowest, with 4 long and slender simple setae and 2 stout spine­like setae (see detail in Fig. 3 C, D). Endite almost twice as long as broad, vaulted medial margin with numerous simple setae, most of these distally; apical margin with 4 fan setae, medial seta broadest, 2 short, blunt simple spine­like setae and some simple setae, 1 coupling hook. Epipod (not illustrated) sickle­shaped, as long as endite.
All pereopods (Fig. 4 A–G) without larger teeth.
Pereopod 1 (Fig. 4 A) basis 5.3 times as long as broad, ischium 2.4 times as long as broad; merus 1.3 times as long as broad, with 2 distal sensory spine­like setae of different lengths; carpus about twice as long as wide, slightly tapering distally after one third of length, ventral margin with 2 long sensory spine­like setae in proximal half and 2 shorter ones in distal half, 1 long simple seta distodorsally; propodus about 2.4 times as long as wide, 0.8 times as long as carpus, with 2 spine­like sensory setae and 1 simple seta ventrally, 1 simple seta laterally and 3 simple setae of varying lengths dorsally, dactylus with 4 lateral setae near unguis insertion.
Pereopods 2–7 present, all similar (Fig. 4 B–G), walking legs, increasing in lengths from pereopod 2–6, pereopod 7 slightly shorter than 6. Bases longest articles, with few scattered dorsal and/or ventral small cuticular teeth, basis of P3–P7 ventrally with proximal spine­like sensory setae, basis and ischium with up to 16 short simple setae; ischium of P3–P6 with few small cuticular teeth; merus of pereopods with 3 to 4 ventral setae and 2–3 dorsal setae; carpus of pereopods 2–6 with 6–7 ventral sensory setae, second distal seta longest and strongest, 2–5 dorsal simple setae, pereopods 3, 4 and 6 with distodorsal feather­like seta (possibly broken off in the other legs); propodus about 0.75 times as long as carpus, with 5–8 ventral sensory setae, distal one longest, 3–6 dorsal simple setae and 1 dorsal subapical feather­like seta; dactyli of all pereopods with 4 lateral setae and 2 setae inserting ventrally near base of unguis. Propodi and dactyli of P5 and P7 of paratype are broken off, but are similar to those of the other walking legs.
Pleopod 1 (Fig. 5 A) 2.3 times as long as wide proximally, only slightly bent ventrally, narrowest part in distal third, 0.7 times as wide as proximal part, with longitudinal keels distally of narrowest part, distal expansion 1.2 times as wide as narrowest part. Distal tip broken off in paratype on one side, distal margin of inner lobes rounded, outer lobes projected, narrow, 0.25 width of inner one.
Pleopod 2 (Fig. 5 B) sympod 2.4 times as long as wide, margins slightly rounded, with row of 11 plumose setae and 2 simple setae proximally; endopod inserting in distal half of sympod, stylet 0.5 times as long as sympod, with broad rounded tip; exopod oval, not surpassing sympod, thick.
Pleopod 3 (Fig. 5 C) endopod, almost semicircular, 1.8 times as long as broad, with 3 distal plumose setae; exopod 0.7 times as long and 0.3 times as wide as endopod, with apical plumose seta and a fringe of fine bristles on outer margin.
Pleopod 4 (Fig. 5 D) uniramous, 1.9 times as long as wide, without setation.
Pleopod 5 absent.
Uropod (Fig. 5 E) 2­articulated, 0.6 times as long as pleotelson, inserting posterolaterally, exceeding terminal margin of pleotelson, sympod about 1.6 times longer than ramus, with 8 setae, terminal ramus narrowing distally, with 1 lateral seta and bifid tip.
Description of non­ovigerous allo­ and paratype females (only differences are illustrated and described, Figs 6, 7): Body surface smooth, except for the small blunt projections of pereonites 1–3 (Fig. 6 A, B). Body 5.6 times as long as wide. Head trapezoidal, 2.0 times as broad as long, with antennae inserting on large dorsolateral projections. Pereonite 1 slightly broader than head, anterolateral and lateral margins with small blunt projections, lateral margin of pereonite 2 also with small blunt spines, of pereonite 3 almost rounded; pereonites 1–3 with 2 caudomedial blunt projections; pereonite 4 0.8 times as long as wide, narrowing in posterior third, narrowest part 0.7 times as broad as broadest part; pereonite 5 about 3.1 times as long as wide, 0.3 times as long as body, caudally broadening more gradually than in male, broadest part about 1.5 times as broad as narrowest part, pereonite 7 not fused with pleotelson, pereopod 7 present; pleonite 1 distinguishable but completely fused with pleotelson. Pleotelson slightly longer than wide, without ornamentation, lateral margins slightly convex, terminal margin slightly concavely rounded, anal valves visible in dorsal view, distance between branchial chamber and anal valves 0.2 times as long as pleotelson.
Antenna 1 (Fig. 7 A, A') about 0.11 times as long as body, with 5 articles; article 1 almost globular, about as long as wide, with one distal feather­like seta and a proximal simple seta; article 2 3.6 times as long as article 1, with 2 distal feather­like setae, 5 lateral sensory setae of different lengths, 3 medial simple setae and 2 setules; articles 3–5 minute, decreasing in size, article 3 with 1 small seta, article 5 with 1 short and 3 longer setae (2 of these broken off, see detail in Fig. 7 A').
Antenna 2 (Fig. 7 B) about 0.7 times as long as body, peduncle 1.8 times as long as flagellum, peduncular article 1 triangular (Fig. 7 B); article 2 with stout spine­like sensory seta on distoventral conical projection and 2 simple setae on distal margin; article 3 about 2.9 times as long as articles 1 and 2 together, with several simple setae, 2 stout spine­like sensory setae on ventral conical projections; article 4 about 0.3 times as long as article 3, with 2 simple setae; article 5 about as long as article 3, more slender, with several short simple setae and 1 rather long distal seta; article 6 about 1.4 times as long as article 5, with several simple setae, 2 long distal setae and 3 feather­like setae in distal fifth of article. Flagellum with 18 articles, article 1 longest, each article with 1–5 simple distal setae, except for last article with 8 distal simple setae.
Maxilliped with up to 7 retinaculae.
Pereopod1 (Fig. 6 D) basis 6.3 times as long as broad, with 1 ventral sensory seta and a few setules; ischium 3 times as long as broad, with 1 long and 3 shorter setae; merus 1.1 times as long as broad, with 2 distoventral sensory setae of different lengths, 1 ventral setule and 1 long simple dorsal seta; carpus about 3.0 times as long as wide, slightly tapering distally after one third of length, 1 ventral simple seta and 2 long sensory setae in proximal half and 5 shorter sensory setae in distal half, 3 simple setae of different lengths dorsally; propodus about 2.7 times as long as wide, 0.7 times as long as carpus, with 2 long sensory setae and 2 simple setae ventrally, and 4 simple setae of varying lengths dorsally; dactylus with 1 simple dorsal seta and 4 lateral setae near unguis insertion.
Pleopod 2 (operculum) (Fig. 7 C) subcircular, almost round, 0.7 times as long as pleotelson, ventral surface smooth, with approximately 16 small and short plumose setae, medial ones broken off, and 2–3 proximolateral simple setae.
Pleopod 3 (Fig. 7 D) endopod teardrop­like, 1.3 times as long as broad, with 3 distal plumose setae; exopod 0.5 as long and 0.3 times as wide as endopod, with apical plumose seta and a fringe of fine bristles on lateral margin.
Remarks
Stylomesus weddellensis sp. nov. is most similar to Stylomesus natalensis Kensley, 1984 from South Africa, but can easily be distinguished by the mediolateral to caudolateral blunt spines on pereonites 1 – 3. Stylomesus natalensis has a pair of very small blunt spines mediocaudally of the head and frontomedially of pereonites 1 and 2. Moreover, contrary to S. wedellensis, antenna 1 of S. natalensis has only 2 articles and the second article bears only 3 long sensory setae and not 4 as in S. weddellensis. Stylomesus weddellensis is the third species of the genus described from Antarctica (Table 1). The other Antarctic species can easily be distinguished from S. weddellensis. Stylomesus hexapodus Brökeland & Brandt, 2004 can easily be distinguished by its neoteny; adult males bear only 6 pereopods. Besides, this species, like S. inermis (Vanhöffen, 1914) lack the caudomedial spines on pereonites 1–3 which are characteristic for S. weddellensis. All other species which are described by Menzies (1962) and are known from the Atlantic Ocean lack this character as well.
Distribution
Type locality and a station in close proximity, 3100–4382 m depth.
description: Published as part of Brandt, Angelika & Andres, Hans Georg, 2008, A new species of Ischnomesidae (Crustacea: Isopoda) from the Southern Ocean, Stylomesus weddellensis sp. nov., pp. 263-276 in Zootaxa 1866 on pages 264-275, DOI: 10.5281/zenodo.183926