Thysaneroserolis orbicula, Bruce, 2009

Bruce, Niel, 2009, New genera and species of the marine isopod family Serolidae (Crustacea, Sphaeromatidea) from the southwestern Pacific, ZooKeys 18 (18), pp. 17-76 : 65-70

Thysaneroserolis orbicula sp. n.

urn:lsid:zoobank.org:act:DDA89194-4F26-4B9F-9FBC-6A02B3D69AC9

Figs 25–27

Material. Holotype: ♀ (ovig., 7.0 mm), off Lord Howe Island , 23°51.30–31´S, 161°43.13–42.96´E, 5 Oct 2005, BBISCOL stn DW2482, 400– 430 m, manganese ( MNHN Is.6024, 2 microslides).

Description. Body 0.9 as long as wide (1.2 as long as overall length inclusive of antenna and uropods), widest at coxae 2, dorsal surfaces polished in appearance and irregularly nodular. Head anterolateral lobes straight, anterior submarginal ‘ridge’ entire; dorsally with pair of low sub-median tubercles anterior to eyes, posterior margin

without median tubercle. Eyes minute (less than 5% greatest width of head), reniform, ommatidia not distinct (possibly absent). Pereonites all entire, articulating; Pereonite 1 anterolateral margin continuously convex; dorsal surfaces without tubercles. Coxae of pereonites 2–4 articulated, with dorsal sutures, distal margins weakly convex; coxae 4 extending to anterior margin of pleotelson; coxae 5 extending posteriorly along 0.5 of pleotelson length; coxae 6 extending to posterior margin of pleotelson, and along 0.8 of pleotelson length. Pleonites extending posteriorly along 1.1 pleotelson lateral margin; pleonite 1 sternal plates with single median lobe, sternal plate 1 without median ridge. Pleotelson 0.8 times as long as anterior width; dorsal surface with median longitudinal carina (not reaching posterior margin), without paired sublateral carinae; lateral margins straight; posterior margin sub-truncate (with median point), without distinct median excision.

Antennule peduncle article 2 1.1 times as long as wide; articles 3 and 4 1.1 times as long as article 2; article 3 3.8 times as long as wide; flagellum 0.6 as long as peduncle articles 3 and 4, with 2 or 3 articles, extending to posterior margin of eye. Antenna peduncle article 4 2.7 times as long as wide, 3.8 times as long as article 3; article 5 0.9 times as long as article 4, 3.9 times as long as wide; antennal flagellum 0.3 as long as peduncle article 5, with 1 or 2 articles, extending to posterior of pereonite 1.

Epistome with obtuse median point. Mandible incisor even or minutely irregular, left mandible lacinia mobilis 0.8 as wide as incisor, right mandible lacinia mobilis distally multicuspid, mandibular spine simple or with 1 or 2 teeth; palp article 2 with 7 distolateral setae, article 3 with 13 biserrate setae. Maxilla mesial lobe with 8 long, finely serrate setae; middle lobe with 2 long simple setae; lateral lobe with 2 distal simple setae. Maxilliped palp article 2 proximomesial margin with 2 setae, distomesial margin with 6 setae, lateral margin distally with 3 setae; article 3 lateral margin with 0 setae, distal margin with 8 setae; endite distal margin RS serrate.

Pereopod 1 carpus RS simple; propodus 2.5 times as long as wide, inferior margin with ̴48 RS; narrow RS simple, deeply bifid; dactylus with unguis distally bifid. Pereopod 2 basis 3.8 times as long as greatest width; 0.5 times as long as basis, ischium 2.1 times as long as wide; merus 0.6 as long as ischium, 1.4 times as long as greatest width, inferior margin with 1 cluster of setae (of 1), superior distal angle with 0 setae; carpus 0.7 as long as ischium, 1.7 times as long as wide, inferior margin with 1 cluster of setae (of 1); propodus 0.9 as long as ischium, 3.0 times as long as wide, inferomesial margin with 2 RS; dactylus 0.2 as long as propodus, unguis acute, with prominent secondary unguis. Pereopod 6 basis 3.3 times as long as greatest width; ischium 0.9 as long as basis, 3.5 times as long as wide, inferior margin with 3 clusters of setae (of 1, short), superior distal angle with 0 RS; merus 0.5 as long as ischium, 2.5 times as long as wide, inferior margin with 1 cluster of setae (of 1), superior distal angle with 1 seta (minute); carpus 0.6 as long as ischium, 3 times as long as wide, inferior margin with 2 clusters of setae (1 and 1 RS), superior distal angle with 5 setae; propodus 0.6 as long as ischium, 3.1 times as long as wide, inferior margin with 2 clusters of setae (RS), distal margin with 3 setae (RS), inferior distal angle with 1 RS; dactylus 1.6 as long as proximal width. Pereopod 7 similar to, but 0.7 as long as pereopod 6. Setae on inferior margins of pereopods 4–7 simple. Inferior margins of pereopods 2–7 setulose fringe absent.

Pleopod 1 peduncle 3 times as long as wide, mesial margin with 3 coupling setae; exopod 2.3 as long as wide, with 11 PMS; endopod 2.7 times as long as wide, 0.7 as long as exopod, with 6 PMS. Pleopod 2 peduncle 1.5 as long as wide, mesial margin with 2 coupling setae; exopod 2.2 as long as wide, with 15 PMS; endopod 2.8 as long as greatest width. Pleopod 3 exopod with 24 PMS, endopod with 13 PMS. Pleopod 4 exopod with complete transverse suture, endopod without transverse suture. Pleopod 5 exopod with complete transverse suture, endopod without transverse suture.

Uropods (rami + peduncle) 0.7 as long as pleotelson, peduncle 0.7 as long as endopod. Endopod 1.8 as long as wide; distally narrowly rounded. Exopod 0.6 as long as endopod, 2.3 times as long as wide, distally acute.

Remarks. The two other species in the genus, both from the western Atlantic, are abundantly different. Thysanoserolis orbicula can be immediately identified and separated from its congeners (and all other serolids) by the almost circular body outline (body shorter [0.9] than wide, compared to 1.4 as long as wide in T. elliptica and 1.2 as long as wide in T. completa ), very flat body, conspicuously flattened and wide antennal peduncle articles that form part of the continuous body outline, the very short antennule flagellum, and geniculate antennal flagellum.

Distribution. New Caledonia; at depths of 400–430 metres.

Etymology. Th e epithet is derived from the Latin orbis meaning round.

Figures

Figure 25. Thysanoserolis orbicula sp. n. Holotype. A dorsal view B lateral view C head, frons and antennae, ventral view D head, anterior margin E pleon and pleotelson, ventral view F coxae 2–5, ventral G right mandible H left mandible I maxilliped Į maxilliped palp.

Figure 26. Thysanoserolis orbicula sp. n. Holotype. A–D pereopods 1, 2, 6 and 7 respectively a detail of pereopod 1 propodal palm setae E maxillule F maxilla G mandible palp H pereopod 1, dactylus unguis.

Figure 27. Thysanoserolis orbicula sp. n. Holotype. A–E pleopods 1–5 respectively.