Argathona lineata, Bruce & Wong & Merrin, 2022

Bruce, Niel L., Wong, Helen P. - S. & Merrin, Kelly L., 2022, The genus Argathona Stebbing, 1905 (Crustacea: Isopoda: Corallanidae) in Singapore marine waters, with description of two new species, Raffles Bulletin of Zoology 70, pp. 492-510 : 494-500

Argathona lineata , new species

( Figs. 1–5 )

Argathona sp. 1 .— Bruce & Wong, 2015: 160.

Argathona cf. setosa .— Bruce & Wong, 2015: 160.

Material examined (all Singapore). Holotype: Female (ovig. 9.9 mm), Southern Fairway near St. John’s Island, 01°12.561–669′N, 103°51.322–460′E, 27 May 2013, rectangular dredge; depth 46.1–72.0 m, stn. DR91, SS-1971, coll. Lim Swee Cheng & party ( ZRC.2020.0137) . Paratypes: 4 females (ovig. 10.1 [uropod only], 9.7 [full dissection], 8.9; non-ovig. 10.2 [dissected P1, mxpd, mx1, mx2] mm), same data as holotype ( ZRC.2020.0138). Female (ovig. 11.9 mm), off Eastern Bunkering A, 01°18.861–595′N, 104°05.128–197′E, 28 May 2013, beam trawl, depth 26.7–33.7 m, stn. TB99, SS-1966, coll. Lim Swee Cheng & party ( ZRC.2020.0139). Female (non-ovig. 11.5 mm), Kusu Island , 01°13.274′N, 103°51.659′E, 03 Jun 2013, depth 19.1 m, SCUBA, stn. SD166, SS-4218, coll. Tan Heok Hui & party ( ZRC.2020.0140). 2 females (non-ovig. 9.9. 8.2, mm; large specimen damaged), off Eastern Bunkering A, 01°18.861–595N, 104°05.128–197′E, 28 May 2013, beam trawl, depth 26.7–33.7 m, stn. TB99, SS-1968, coll. S.C. Lim & party ( ZRC.2020.0141). Female (non-ovig. 11.3 mm), off Eastern Bunkering A, 01°18.861–595′N, 104°05.128–197′E, 28 May 2013, beam trawl, depth 26.7– 33.7 m, stn. TB99, SS-5397, coll. Lim Swee Cheng & party ( ZRC.2020.0143). Female (ovig. 8.1 mm), imm. (5.1mm), west of Pulau Pawai , 01°11.104–387′N, 103°42.061–185′E, 15 Oct 2014, rectangular dredge, depth 23.6–23.7 m, stn. DR506, SEA-9225, coll. Chim Chee Kong & party ( ZRC.2022.0033). Female (non-ovig. 9.1 mm), Eastern Bunkering A, 01°18.861–595′N, 104°05.128–197′E, 28 May 2013, beam trawl, depth 33.7– 26.7 m, stn. TB99, SS-1967, coll. Lim Swee Cheng & party ( ZRC.2022.0034). Female (ovig. 7.5 mm), imm. (6.0mm), near Eastern Bunkering A, 01°18.425–502′N, 104°04.607–844′E, 28 May 2013, beam trawl, depth 22.7– 22.4 m, stn. TB97, SS-1970, coll. Lim Swee Cheng & party ( ZRC.2022.0035) .

Additional specimens. manca (6.7 mm), off Pulau Senang, 01°09.942–949′N, 103°43.458–471′E, 05 Jun 2013, beam trawl, depth 24.3–24.5 m, stn. TB185, SS-5409, coll. Lim Swee Cheng & party ( ZRC.2020.0144). Female (preovig. 9.0 mm), off Changi Naval Base 01°18.702–838’N, 104°03.234–479’E, 08 Apr 2014, rectangular dredge, depth 12.9–14.7 m, stn. DR363, SEA-6135, coll. Teresa Tay & party ( ZRC.2020.0145). Female (non-ovig. 9.9 mm), off Changi Naval Base 01°18.702–838′N, 104°03.234–479′E, 08 Apr 2014, rectangular dredge, depth 12.9–14.7 m, stn. DR363, SEA-6137, coll. Teresa Tay & party ( ZRC.2020.0146). Female (non-ovig. 7.2 mm), off Changi Naval Base, 01°18.522–874′N, 104°02.751–672′E, 08 Apr 2014, rectangular dredge, depth 10.9–17.5 m; stn. DR362, SEA-6700, coll. Teresa Tay & party ( ZRC.2020.0147). Female (non-ovig. 7.9 mm), off Changi Naval Base 01°18.702– 838′N, 104°03.234–479′E, 08 Apr 2014, rectangular dredge, depth 12.9–14.7 m, stn. DR363, SEA-6833, coll. Teresa Tay & party ( ZRC.2020.0148). 2 imm. (6.6, 6.8 mm), 1 manca (5.2 mm), between Pulau Ubin and Pulau Tekong, 06 Mar 2012, rectangular dredge, depth 16 m, stn. CMBS-D06 , coll. CMBS party ( ZRC.2020.0149). 2 imm. (6.8, 7.0 mm), 1 manca (5.8 mm) between Pulau Ubin and Pulau Tekong, 06 Mar 2012, rectangular dredge, depth 16 m, stn. CMBS-D06 , coll. CMBS party ( ZRC.2020.0150) .

Description of female. Body 3 times as long as greatest width, widest at pereonite 5, dorsal surfaces finely setose; lateral margins subparallel, coxae 4–7 and pleonites 3 and 4 laterally setose. Rostral point present, projecting anteriorly, minute. Eyes separated by about 46% width of head; eye colour black. Pereonite 1 and coxae 2–3 each with posteroventral angle rounded; coxae 5–7 with entire oblique carina; posterior margins of pereonites 7 with fine nodules laterally. Pleon with pleonite 1 largely concealed by pereonite 7; pleonites 3–5 posterior margin with continuous small nodules along posterior margin; posterolateral angles of pleonite 2 not visible, not posteriorly produced; pleonite 3 with posterolateral margins extending to posterior margin of pleonite 4, narrowly rounded; posterolateral margin of pleonite 4 sub-truncate with ventral point, not extending beyond posterior margin of pleonite 5; pleonite 5 with posterolateral angles overlapped by lateral margins of pleonite 4. Pleotelson 0.7 times as long as anterior width, dorsal surface with two sub-median fields of stiff hyaline setae; lateral margins weakly convex, margins smooth, posterior margin sub-truncate, without median point, with 6 robust setae.

Antennula peduncle articles 1 and 2 entirely fused; articles 3 and 4 0.6 times as long as combined lengths of articles 1 and 2; article 3 1.2 times as long as wide; flagellum with 11 articles, extending to anterior of pereonite 1. Antenna peduncle article 4 2.4 times as long as wide, 2.2 times as long as article 3, inferior margin with 2 short simple setae; article 5 1.1 times as long as article 4, 3.5 times as long as wide, inferior margin with 1 pappose setae; anterodistal angle with cluster of 2 short simple setae; anterodistal angle with cluster of 2 pappose setae; flagellum with 37 articles, extending to posterior of pereonite 4.

Frontal lamina pentagonal, ventral surface entirely flat, 2.8 times as long as posterior width, lateral margins concave, anterior margin acute, without small median point.

Mandible palp article 2 with 16 distolateral setae; article 3 with 20 smooth RS, terminal seta serrate. Maxillula mesial lobe distally truncate with 1 serrate acute RS; lateral lobe with 1 large acute and 3 falcate RS. Maxilla lateral lobe distally truncate, with 4 short simple setae at distomesial angle. Maxilliped palp article 2 mesial margin with 2 slender setae, lateral margin distally with 1 slender setae; article 3 mesial margin with 5 slender setae, lateral margin with 2 slender setae; article 4 mesial margin with 10 slender setae, lateral margin with 3 slender setae; article 5 distal margin 9 setae, lateral margin with 3 setae.

Pereopod 1 basis 2.7 times as long as greatest width, superior distal angle without cluster of acute setae; inferior distal angle with cluster of 5 acute setae; ischium 0.5 times as long as basis, inferior margin with 3 setae, inferior distal margin with 1 molariform RS, superior distal margin with 1 RS; pereopod 1 merus inferior margin with 7 molariform RS, set as single row, inferior margin with 7 simple stiff acute setae, inferior distal margin with 0 RS, superior distal angle with 3 stiff setae; carpus inferior margin with 0 RS, and 1 simple setae; propodus 2.1 times as long as wide, inferior margin with 2 small RS, inferior margin with 0 simple setae, inferior distal margin with 1 large RS and 7 setae. superior distal with 2 simple setae; dactylus 0.8 times as long as propodus. Pereopod 2 ischium inferior margin with 6 short setae, inferodistal margin with 2 molariform RS, and 2 acute RS, distolateral margin with 4 simple setae superior distal angle with 0 long simple setae and 1 RS; merus inferior margin with 7 stout molariform RS set as two groups, inferior distal margin with 0 stout RS, superior distal margin with 5 stout acute RS, superior distal margin with 4 acute RS; carpus inferior distal margin with 2 small RS; propodus 2.3 as long as wide, inferior margin with 1 small RS; inferior distal margin with 1 stout RS and 5 simple setae; dactylus 0.8 times as long as propodus. Pereopod 3 similar to pereopod 2. Pereopod 6 similar to pereopod 7, but ischium with 8 RS on inferior margin. Pereopod 7 basis 2.5 times as long as greatest width, superior margin weakly convex, inferior margin with 1 palmate setae and row of short simple setae); ischium 0.5 times as long as basis, inferior margin with 7 RS (set as 3+1+3), superior distal angle with 6 simple and biserrate RS, inferior distal angle with 7 RS; merus 0.6 times as long as ischium, 1.4 times as long as wide, inferior margin with 3 RS, superior distal angle with 7 RS, inferior distal angle with 7 RS; carpus 0.7 times as long as ischium, 1.9 times as long as wide, inferior margin with 2 small RS, superior distal angle with 14 simple and biserrate RS, inferior distal angle with 6 RS; propodus 0.9 times as long as ischium, 3.5 times as long as wide, inferior margin with 3 single RS, superior distal angle with 2 slender setae. 2 pappose setae and 2 acute RS, inferior distal angle with 3 RS; dactylus 0.5 times as long as propodus.

Pleopod 1 exopod 1.7 times as long as wide; lateral margin straight, distally broadly rounded, mesial margin strongly convex, with PMS from distal one-third, with ~54 PMS; endopod 2 times as long as wide, distally broadly rounded, convex, with PMS on distal margin only, mesial margin with PMS on distal margin only, pleopod 1 endopod with ~24 PMS; peduncle 1.8 times as wide as long; mesial margin with 4 coupling setae. Pleopod 2 exopod with ~66 PMS, endopod with ~27 PMS. Pleopod 3 exopod with ~74 PMS, endopod damaged. Pleopod 4 exopod and endopod damaged. Pleopod 5 exopod damaged. Pleopods 2–5 peduncle distolateral margin without prominent acute RS, 3–5 endopods with distomesial serrate scales.

Uropod peduncle ventrolateral margin with 2 RS, lateral margin without medial short acute robust seta, posterior lobe about two-thirds as long as endopod; rami extending to pleotelson apex (or just a little beyond), marginal setae in single dense tier, apices broadly rounded; dorso-lateral surfaces with sparse acute hyaline setae. Endopod apically not bifid; lateral margin weakly convex, proximal lateral margin with 0 RS; distal lateral margin with 3 RS, mesial margin strongly convex, with 7 RS. Exopod not extending to end of endopod, 2.9 times as long as greatest width, apically not bifid; lateral margin weakly convex, with 8 RS; mesial margin weakly convex, with 3 RS.

Size. Ovigerous females 8.8–11.9 mm (mean = 9.8 mm); non-ovigerous females 7.2–11.5 mm (mean = 9.4 mm; mancas 5.2–6.7 (mean 5.9 mm); immature specimens (small, post-manca) 5.4–7.0 mm (mean = 6.5 mm).

Variation. The abundance of setae, particularly on the uropodal rami makes it difficult to count the number of robust setae. Of the specimens examined the range of marginal robust setae on the uropodal rami is (single extreme count in parentheses): uropodal endopod mesial margin 6–7 (0), uropodal endopod lateral margin 2–3 (1); uropodal exopod mesial margin 2–4, uropodal exopod lateral margin 7–8 (0).

Non-ovigerous specimens are almost entirely without setae on the dorsal surfaces, but are otherwise similar to ovigerous females. No males were present in the material examined.

Remarks. Argathona lineata , new species, can be recognised by the following combination of characters: a pentagonal frontal lamina 3.8 times longer that its posterior width, posterior margins of pleonites 3–5 with transverse row of nodules, rounded uropodal rami that only just extend past the pleotelson apex, a sub-truncate pleotelson posterior margin and the dorsal surface of the pleotelson with two dense sub-median fields of acute hyaline setae, with a few further hyaline setae on the dorsal surfaces of the uropodal rami. In fresh specimens there appear to be five or six longitudinal stripes of brown (or dark orange) chromatophores, but these fade with age, and that pattern is also present in some other species. Photos of this species can be seen at the website “The Biodiversity of Singapore ” at the following link: https://singapore.biodiversity.online/species/A-Arth-CrusIsopoda-000006

Argathona confine Hale, 1925 is similar, but lacks pleonal nodules, the uropodal endopod is distally more acute, the pleotelson apex is more narrowly and distinctly rounded, and the body surfaces are more dorsally setose than A. lineata new species

Argathona crenulata Bruce, 1982b is also similar but smaller (male mean length 6.6 mm; female mean length 7 mm), lacks dorsal setae, and also has posterior margins of pereonites 5 to 7 nodular. The shape of the pleotelson is less truncate and also has sub-lateral elongate ridges.

Argathona setosa Richardson, 1910 has an acute uropodal endopod with a falcate apex and strongly convergent pleotelson margins, and the body dorsal surfaces are more densely setose. The species is in need of redescription but is otherwise evidently very different to A. lineata new species

Argathona hirsuta Hobbins & Jones, 1993 is a deep-water species, has a differently shaped pleotelson, but notably has long robust setae on the pleotelson and uropodal margins; the pereonites and pleonites lack nodules and the frontal lamina is elongate-pentagonal with parallel lateral margins.

Argathona normani Stebbing, 1905 has dense setae over all of the dorsal body surfaces, has large widely separated nodules on posterior pereonites and pleonites ( A. lineata has small and more numerous nodules), and a relatively short uropodal exopod ( A. lineata has a longer uropodal exopod) and the uropodal exopod is narrowly rounded, with the uropodal endopod apex forming a distinct angle ( A. lineata has both uropodal rami with broadly rounded apices).

Etymology. The epithet is derived from the Old English word ‘line’ and the old French word ‘ligne’ both meaning rope, cord or string; and also from the Latin ‘linum’ (flax) and ‘linea’ (fibre) ( Brown, 1956); alluding to the dorsally lined chromatophore pattern of this species.

Figures

Fig. 1. Argathona lineata, new species, A–E, holotype, ZRC 2020.0137; F–I, female paratype, 9.7 mm, ZRC 2020.0138. A, dorsal view; B. lateral view; C, head, dorsal view; D, head and frons, ventral view; E, pleon, lateral view; F, pleotelson posterior margin; G, antennula; H, antennula, distolateral angle, article 6; I, antenna, articles 4 and 5.

Fig. 2. Argathona lineata, new species, female paratype, 9.7 mm, ZRC 2020.0138. A, maxilliped, ovigerous female; B. maxilliped palp; C, maxillula; D, maxilla; E, mandible incisor; F, antenna peduncle; G, antenna; H, antenna flagellum articles 16–18.

Fig. 3. Argathona lineata, new species, female paratype, 9.7 mm, ZRC 2020.0138. A, pereopod 1; B, pereopod 1, dactylus; C, pereopod 2; D, pereopod 6; E, pereopod 7.

Fig. 4. Argathona lineata, new species, female paratype, 9.7 mm, ZRC 2020.0138. A, pleopod 1; B, pleopod 1 peduncle, mesial margin; C, pleopod 1, marginal setae; D–G, pleopods 2–5 respectively

Fig. 5. Argathona lineata, new species, paratypes, A, female, 9.7 mm; B, female 10.1 mm; ZRC 2020.0138. A, uropod; B, uropod, showing marginal robust setae only.