Thylakogaster namibiensis, Brenke & Buschmann, 2009

Brenke, Nils & Buschmann, Anika, 2009, Thylakogaster namibiensis sp. nov. (Isopoda: Asellota: Janiroidea), a new species of Haplomunnidae from the southeast Atlantic deep sea *, Zootaxa 2096 (1), pp. 381-394 : 384-390

publication ID	https://doi.org/ 10.11646/zootaxa.2096.1.23
DOI	https://doi.org/10.5281/zenodo.5334998
persistent identifier	https://treatment.plazi.org/id/03C687CD-F038-FFC2-FF16-FF4BE749FAF7
treatment provided by	Felipe
scientific name	Thylakogaster namibiensis
status	sp. nov.

Treatment

Thylakogaster namibiensis View in CoL sp. nov. ( Figs. 1–5 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 )

Material: 14 specimens of Thylakogaster namibiensis sp. nov. were found at four stations during the Diva 1 expedition ( Tab. 1).

Holotype: ♂, 1.9 mm, Area 4, Station #340 ( EBS 09 ): 18°18.3’S 004°41.3’E to 18°19.4’S 004°41.9’E, 5395 m depth; ZMH K-40840 A–N, 14 slides. (Diva 1 Id No.: D1- HM9 ) GoogleMaps

Paratype (Allotype): 1 ♀, 1.6 mm, Area 4, Station #340 ( EBS 09 ): 18°18.3’S 004°41.3’E to 18°19.4’S 004°41.9’E, 5395 m depth; ZMH GoogleMaps K-40842. (D1- HM8 )

Paratypes: 1 ♂, 1.2mm, Area 5, Station #344 ( EBS 10 ): 17° 06.2' S 004° 41.7' E to 17° 07.5' S 004° 42.3' E, 5415 m depth; ZMH GoogleMaps K-40841 A–L, 12 slides. (D1- HM1 ) . 1 juvenile (stage IV) 1.1mm (D1- HM3 ) and 1 juvenile (stage II) 0.7mm (D1- HM2 ) both from Area 6, Station #348 ( EBS 11 ): 16° 18.1' S 005° 27.2' E to 16° 19.3' S 005° 27.2' E, 5387m depth; ZMH GoogleMaps K-40843, K-40844.

Distribution. Known only from the Angola Basin, Atlantic Ocean, 850 km west of Namibia. Depth range: 5387–5415 m.

Etymology. Thylakogaster namibiensis sp. nov. is named after the sample locality: the new species was sampled 850 km westwards of the African coast off Namibia, in the Atlantic Ocean.

Diagnosis. Cephalothorax-pereon length 3 times longer than wide. Antenna 1 of copulatory male consisting of 3 articles in peduncle and 12 in flagellum, aesthetascs present on articles 9–13 (formula: P3–F12[A8–12]). Article 3 length 1.1 article 2 length. Antenna 2 with 6 articles in peduncle and 11 in flagellum. Pleotelson length 0.66 cephalothorax-pereon length, length 1.34 width. Pleotelson with 30–40 simple spines developed on each lateral half. Uropods mace-like, broadened, inserting beneath broad keel close to rounded tip of pleotelson. Pleotelson nearly as long as broad, with rounded tip and small disto-medial projection, laterally expanded into 2 huge bumps on either side of frontal indentation.

Description of male (description of pereopods from juvenile paratypes): Body ( Fig. 1a–d View FIGURE 1 ) compact, cephalothorax-pereon length width 1.51, dorsoventrally flat and convex, unpigmented.

Cephalothorax ( Fig. 1a, b, e View FIGURE 1 ) rounded, length 0.27 cephalothorax-pereon length. Clearly visible fronsclypeal ridge developed anteriorly. Cephalon fused completely to first pereonite. Antennal insertion located on small projection on dorso-lateral surface.

Pereon ( Fig. 1a, b View FIGURE 1 ) with 1–2 spines on lateral margins of pereonites 1–7 next to coxae. Pereonites 1–3 of subequal proportions. Pereonite 3 widest. Pereonites 4–7 decreasing in length, with one or 2 small simple spines on ventral margins.

Pleotelson ( Fig. 1a, d, e, g View FIGURE 1 ) nearly as long as wide, length 0.66 cephalothorax-pereon length (measured basis of Plt to apex), with rounded tip; located dorsally above pereon and covering more than last 6 pereonites; laterally expanded into 2 huge bumps on either side of a frontal indentation. Pleotelson with 30–40 simple spines on each lateral side, often broken off at ends. Frontal indentation spineless. Uropods tiny, positioned on projecting lobes on ventral side close to pleotelson tip. Comb-like row of thin, unequally bifid setae below insertion of uropods ( Fig. 1c View FIGURE 1 )on both posterior margins ( Fig. 1d View FIGURE 1 ). Setae more closely together than lateral cuticular spines on pleotelson. Both setae and spines increasing in length from tip to basis of pleotelson.

Pleopods 1 and 2 in a posterio-dorsal position due to rounded form and position of pleotelson. Pleotelson covered entirely with spines in both sexes. External surfaces of male pleopods 1 and 2 equipped with single rows of spines. Surface of female operculum completely covered with strong spines.

Antenna 1 ( Fig. 2c, d View FIGURE 2 ): about as long as cephalothorax-pereon length, inserting dorso-medially at basis of A2. 3 articles in peduncle: article 1 with 1 broom seta and 1 short simple seta on disto-frontal margin ( Fig. 2f View FIGURE 2 ). Article 2 with 1 plumose seta on disto-ventral margin. Article 3 without setae. Length ratio of peduncle articles: 1:0.7:0.8. Flagellum composed of 12 articles. Article 1 short, second article elongated, article 12 tiny ( Fig. 2e View FIGURE 2 ). Length ratio of flagellar articles: 1:5.5:2:2.2:2:2.5:2.5:2.8:2:1.5:2.2:0.25. Flagellar articles with 1–2 fine simple seta. Flagellar articles 8–12 with 1 single aesthetasc each.

Antenna 2 ( Fig. 2b View FIGURE 2 ): length 2.0 cephalothorax-pereon length. Peduncle with 6 articles (Articles 1 broken off, article 2 damaged during dissection). Articles 1–4 short (only articles 2–4 are illustrated). Article 3 with 1 small seta on ventral side. Article 4 with 2 unequally bifid setae on dorsal side. Articles 5 and 6 elongated, bearing unequally bifid setae at irregular intervals (Articles 5 partially fractured in illustrated specimen). Setae insert with slight elevation, most setae broken off. Length ratio of peduncle articles: 1:1.1:0.5:1:8:14. Flagellum with 11 articles, first and second elongated, remaining articles smaller, subequal. Flagellar articles 1, 3 and 5 without setae, 8 with 1, articles 2, 4, 7, 9 and 10 with 2 setae. Article 6 with 3 small and thin simple setae. Article 11 with 5 long, thin setae on distal end ( Fig. 2a View FIGURE 2 ).

Mandibles ( Fig. 2g, h View FIGURE 2 ): proximally broad, tapering distally, without palp. Left mandible with long, slender, simple, medial seta. Right mandible with 1 short, simple seta medially and 1 slender, simple spine and short, strong spine laterally.

Left incisor process with 5 teeth with cuticular wrinkles, ventral-most tooth largest, remaining teeth decreasing in size to dorsal side. Lacinia mobilis of left mandible broad, bearing 4 prominent teeth and 1 small tooth. Setal row of 4 curved spines and 2 slender, simple setae. Molar process finger-like, with 7 long, slender, simple setae.

Right incisor process with 5 broad teeth of approximately same size, with cuticular wrinkles. Setal row of 5 curved spines, each spine serrated with 7 strong, short teeth; between curved spines some long and slender simple setae. Molar process finger-like, distal with 12 long, slender, simple setae.

Maxilla 1 ( Fig. 3e, f View FIGURE 3 ): Outer lobe of left maxilla 1 terminally with 11 spine-like setae: 6 stout and rough serrated setae, 4 long setae serrated and equipped with a dense comb of fine setae and 1 simple, fine, ventral seta. Outer lobe of right maxilla 1 terminally with 11 spine-like setae of comparable size: 7 stout and rough serrated setae, 2 setae serrated and equipped with dense comb of fine setae and ventrally 2 simple fine setae. Inner lobe of right maxilla 1 with 3 long and slender setae.

Maxilla 2 ( Fig. 4a, b View FIGURE 4 ): middle and outer lobe subequal in length. Tips of middle and outer lobes with 3 long, single, side, plumose seta and 1 simple, media seta. Outer lobe also with 2 combs of 4 fine setae laterodistally. Inner lobe shorter and broader than middle and outer lobes. Inner lobe with 11 simple setae: 8 short on tip and 3 long on median margin. Proximal to these with 3 long and 7 small additional setae.

Maxilliped ( Fig. 3a–d View FIGURE 3 ): epipodite small and triangular, carrying numerous fine setae on its surface and lateral margins, reaching only half of length of lobe. Lateral margin of lobe irregularly rounded, tip truncated, with simple setae. Distal part of lateral margin of lobe with row of fine setae. Medial margin of lobe straight. Lobes connected by 5 (left lobe 3, right lobe 2) retinacula ( Fig. 3b, d View FIGURE 3 ). Retinacula with 5 prominent teeth and cuticular wrinkles. Palpus long and slender, about twice as long as lobe. Palpus tapering distally, composed of 5 articles. All articles with numerous fine setae on surface and lateral margins. Length ratio of articles: 0.3:0.7:1:1.5:1.2. All articles with thin, long setae as follows: 1 with single seta, 2 with 2, 3 with 4, 4 with 3 and 5 with 7 long setae.

Pereopods ( Fig. 4c–f View FIGURE 4 ): all pereopods insert on elongated lateral projection ( Fig. 1a, e View FIGURE 1 )

Pereopod 1 ( Fig. 4d View FIGURE 4 ): robust, length 1.3 cephalothorax-pereon length, subchelate with prominent, strong, unequally bifid setae of altering length on basis, ischium, merus and carpus. Basis with 1 unequally bifid seta on ventral margin. Ischium length 0.5 basis length, with 3 unequally bifid setae and 2 simple setae on ventral and 2 unequally bifid setae on dorsal margin. Merus short, with 6 unequally bifid setae, 3 on ventral and 3 on dorso-frontal margin. Carpus elongated, longer than basis, with 11 strong unequally bifid on ventral margin opposite to propodus and dactylus and 5 additionally unequally bifid setae on proximal medial surface.

Propodus with row of 5 fine setae on ventral and 2 simple setae on dorsal margin. Dactylus with 9 long simple setae ( Fig. 4f View FIGURE 4 ).

Pereopods 2–7 slender, sub-similar walking legs. Coxa of pereopods 2 to 7 recognizable but fused with pereonites and only slightly movable. All coxae with 3–5 strong, simple spines.

Pereopod 2 ( Fig. 4c View FIGURE 4 ): length 1.4 cephalothorax-pereon length. Basis with 3 slender simple setae on ventral margin (see remarks). Ischium as long as broad, with 5 unequally bifid setae, 1 on ventral and 4 on dorsal margin. Merus with 2 setae on ventral and 4 setae on dorsal margin. Carpus long, slender, with row of long unequally bifid setae and long simple setae on ventral margin opposite to propodus. 4 unequally bifid setae dorsally and additionally 1 broom seta distally. Propodus narrower, but as long as carpus. Propodus with more than 18 long, robust, unequally bifid setae. Terminally with 7 slender, simple setae. Dactylus with 7 long simple setae ( Fig. 4e View FIGURE 4 ).

Pleopods ( Fig. 5a–e View FIGURE 5 ): male pleopods 1 and 2 forming posterio-ventral part of pleotelson, completely covering branchial cavity.

Pleopod 1 ( Fig. 5a View FIGURE 5 ): male pleopod 1, length 3.7 width, lateral margins nearly parallel, tapering in the distal fifth. Ventral surface with row of 4 strong spines each. Disto-lateral corners with 8 small, hair-like setae.

Pleopod 2 ( Fig. 5e View FIGURE 5 ): male pleopod 2, length 3.0 width,with 2 rows of 7 cuticular spines on external surface. Lateral margin with dense row of long, slender setae. Stylet (endopodite) short, robust, stylet length 0.33 protopod length. Exopodite forms a small lobe disto-medial of endopodite.

Pleopod 2 ( Fig. 1g View FIGURE 1 ): female pleopod 2 tapering distally, truncate proximally, completely covering branchial cavity. External surface with irregularly distributed cuticular spines.

Pleopod 3 ( Fig. 5b View FIGURE 5 ): exopodite elongated and hemispherical, 2-articulated, lateral and disto-medial margins with rows of numerous long, hair-like setae, distal article with 1 simple seta apically. Endopod length 1.4 width, distally rounded, with 3 strong, plumose, terminal setae and hair-like, medial and apical setae.

Pleopod 4 ( Fig. 5c View FIGURE 5 ): transparent, small, length 2.5 width, without setae, exopodite slender.

Pleopod 5 ( Fig. 5d View FIGURE 5 ): transparent, small, length 2.8 width ratio, without setae, exopodite reduced.

Uropods ( Fig. 1c, f View FIGURE 1 ): inserting beneath broad keel on ventral margin of branchial cavity close to tip of pleotelson, uniramous, cylindrical. Distal part swollen, rounded, with 5 hair-like setae.

Remarks. The copulatory male described bares only the antenna 1 and the first pereopods. The rest of the long appendages were broken off at their basis. For description of the lost antenna and the pereopod 2, a comparably large paratype was used. Except for the individuals described here, the long and extremely fragile antenna 1, antenna 2 and pereopods 3 to 7 were broken off on all specimens of T. namibiensis sp. nov., or the specimens were significantly smaller. Male and female of T. namibiensis show no sexual dimorphism, besides the female may be expanded if oostegites are developed.

The inner lobe of the left maxillula and the distal part of the merus of pereopod 2 were damaged during dissection. The cuticular wrinkles on the surface of pleopod 3 may be artefacts.