Argathona trichota, 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 : 500-505

Argathona trichota , new species

( Figs. 6–10 )

Argathona sp. 2 .— Bruce & Wong, 2015: 160.

Material examined

Holotype. Female (ovig. 10.4 mm), off Eastern Boarding Ground A, 01°12.974–958′N, 103°52.960–832′E, 30 May 2013, beam trawl, depth 113–128 m, stn. TB127, SS-4153, coll. Lim Swee Cheng & party ( ZRC.2020.0151).

Paratypes. 2 females (ovig. 8.8 [dissected], 8.2 mm), Pulau Semakau , 01°12.389′N, 103°45.240′E, 23 May 2013 SCUBA coral brushing, depth 5 m, stn. SB41, SS-5382, coll. Heok Hui Tan & party ( ZRC.2020.0152). Female (non-ovig. 8.8 mm), St. John’s Island, 01°13.116′N, 103°51.079′E, 29 May 2013, gill net, depth 2 m, stn. SW104, SS-0858, M. R bin Duriat & D. Uyeno ( ZRC.2020.0153). Female (non-ovig. 9.2 mm), south of Pulau Sebarok, 01°10.876′N, 103°47.660′E, 03 Sep 2013, rectangular dredge; depth 32.5 m, stn. DR197, SEA-0102, coll. Helen Wong & party ( ZRC.2020.0154) .

Description of female. Body 2.7 times as long as greatest width, widest at pereonite 5, dorsal surfaces all densely setose, lateral margins subparallel. Rostral point present, projecting anteriorly, minute. Eyes separated by about 54% width of head, each eye made up of ~6 transverse rows of ommatidia, each row with ~11 ommatidia, eye colour dark brown. Pereonite 1 and coxae 3 each with posteroventral angle right-angled, coxae 2 rounded; coxae 5–7 with entire oblique carina; posterior margins of pereonites 5 and 6 smooth, that of pereonite 7 very finely nodular. Pleon with pleonite 1 largely concealed by pereonite 7; pleonites 3 and 5 posterior margins smooth, pleonite 4 with fine nodules; posterolateral angles of pleonite 2 concealed, not posteriorly produced; pleonite 3 with posterolateral margins not extending to posterior margin of pleonite 5, acute; posterolateral margin of pleonite 4 subtruncate with ventral point, not extending beyond posterior margin of pleonite 5; pleonite 5 with posterolateral angles overlapped by lateral margins of pleonite 4. Pleotelson 0.6 times as long as anterior width, dorsal surface without longitudinal carina; lateral margins weakly sinuate, smooth; posterior margin sub-truncate, without median point, with 6 robust setae.

Antennule peduncle articles 1 and 2 entirely fused; articles 3 and 4 0.7 times as long as combined lengths of articles 1 and 2, article 3 2 times as long as wide; flagellum with 10 articles, extending to anterior of pereonite 1. Antenna peduncle article 4 3.0 times as long as wide, 2.8 times as long as article 3, inferior margin with 1 pappose seta, and 1 short simple seta; article 5 1.2 times as long as article 4, 3.9 times as long as wide, inferior margin without pappose setae; anterodistal angle with cluster of 2 pappose setae; anterodistal angle without cluster of short simple setae; anterodistal angle with cluster of 2 long simple setae; flagellum with 32 articles, extending to middle of pereonite 4.

Frontal lamina 0.7 times as long as posterior width, lateral margins straight, diverging slightly towards anterior, anterior margin acute, without small median point.

Mandible palp article 2 with 18 simple and biserrate distolateral setae; article 3 with 20 robust biserrate setae. Maxillula mesial lobe with distal margin truncate and concave, distomesial angle with two recurved RS; lateral lobe with 1 large, 1 small falcate and 1 small straight terminal robust setae. Maxilla distally rounded, with 2 small simple setae and 2 stiff setae. Maxilliped palp article 2 mesial margin with 3 slender setae, lateral margin distally with 1 slender setae; article 3 mesial margin with 3 slender setae, lateral margin with 2 slender setae; article 4 mesial margin with 8 slender setae, lateral margin with 3 slender setae; article 5 distal margin 14 setae, lateral margin with 3 setae.

Pereopod 1 basis 2.4 times as long as greatest width, superior distal angle with cluster of 2 acute setae; inferior distal angle with cluster of 7 acute setae; ischium 0.5 times as long as basis, inferior margin with 2 setae, inferior distal margin with 4 RS, superior distal margin with row of 9 long stiff setae; merus inferior margin with 9 molariform RS, set as two groups, inferior margin with row of 13 stiff acute setae, inferior distal margin without RS, superior distal angle with 5 setae and 1 RS; carpus inferior margin without RS, inferior margin with 2 simple setae; propodus 1.8 times as long as wide, inferior margin with 2 RS, inferior margin without simple setae, inferior distal margin with 1 large RS and 10 simple setae, superodistal angle with 2 pappose setae; dactylus 0.7 times as long as propodus. Pereopod 2 ischium inferior margin without stout, bluntly rounded RS, with 1 acute RS and 5 simple setae; distal margin with 1 RS; without molariform RS; ischium superior distal margin with 3 long simple setae; without RS; merus inferior margin with 6 molariform RS, set as two groups, inferior distal margin with 7 stout RS, superior distal margin with 2 stout RS and 2 setae; carpus inferior distal margin with 1 RS; propodus 2.5 times as long as wide; inferior margin with 2 RS; inferodistal margin with 1 long RS and 6 simple setae; dactylus 0.6 times as long as propodus. Pereopod 3 similar to pereopod 2. Pereopod 6 similar to pereopod 7. Pereopod 7 basis 2.7 times as long as greatest width, superior margin weakly convex, inferior margin without palmate setae; ischium 0.5 times as long as basis, inferior margin with 8 RS set as clusters of 3+3+2, superior distal angle with 4 simple and 1 biserrate RS, inferior distal angle with 8 RS and 2 setae; merus 0.7 times as long as ischium, 1.6 times as long as wide, inferior margin with 6 RS(set as 2+2, superior distal angle with 10 RS, inferior distal angle with 7 RS; carpus 0.6 times as long as ischium, 2.4 times as long as wide, inferior margin with 1 RS, superior distal angle with smooth and biserrate 12 RS, inferior distal angle with 5 RS; propodus 0.8 times as long as ischium, 3.9 times as long as wide, inferior margin with 4 single of RS, superior distal angle with 3 slender and 2 pappose setae, inferior distal angle with 2 simple setae; dactylus 0.4 times as long as propodus.

Pleopod 1 exopod 1.8 times as long as wide, lateral margin weakly convex, distally broadly rounded, mesial margin strongly convex, with PMS from distal one-third, with ~52 PMS; endopod 2.2 times as long as wide, distally subtruncate, lateral margin weakly concave, with PMS on distal margin only, mesial margin with PMS on distal margin only, endopod with ~19 PMS; peduncle 2.3 times as wide as long; mesial margin with 4 coupling setae. Pleopod 2 exopod with ~62 PMS, endopod with ~24 PMS. Pleopod 3 exopod with ~72 PMS, endopod with ~16 PMS. Pleopod 4 exopod with ~66 PMS, endopod with ~13 PMS. Pleopod 5 exopod with ~65 PMS. Pleopods 2–5 peduncle distolateral margin with prominent acute RS; pleopods 1–5 exopod lateral margins each with widely spaced serrations; pleopods 3–4 endopods with mesial serrate scale patch, 5 with mesial and distomesial scale patches.

Uropod peduncle and rami dorsolaterally densely covered with stiff, acute hyaline setae; peduncle ventrolateral margin RS not visible, lateral margin without medial short acute robust seta, posterior lobe about three-quarters as long as endopod; rami extending beyond pleotelson, marginal setae in single tier, apices broadly rounded. Endopod apically not bifid; lateral margin weakly convex, proximal lateral margin with 3 RS; mesial margin strongly convex, with 8 RS. Exopod not extending to end of endopod, 3.0 times as long as greatest width, apically not bifid; lateral margin weakly convex, with 4 RS, proximal RS slender not ‘peglike’; mesial margin weakly convex, with 2 RS.

Size. Ovigerous females 8.2–10.4 mm (mean = 9.1 mm); single non-ovigerous female 9.2 mm.

Remarks. Argathona trichota , new species, has a uniquely short frontal lamina that is shorter than the posterior width, effectively separating it from all other species in the genus. In addition, the ovigerous females have the dorsum covered by dense hyaline setae, which in combination with the relatively short antennal flagellum extending to mid-pereonite 4, sub-truncate pleotelson posterior margin and the rounded uropodal endopod, all serve to identify the species.

Argathona lineata , new species, is similar in general morphology, particularly the shape of the uropods and pleotelson but A. trichota differs in having a shorter frontal lamina, abundant hyaline setae all over the dorsum; fine pleonal tubercles only on the posterior margin of pereonite 7; a shorter antennal flagellum that extends to mid-pereonite 4 (to posterior of pereonite 4 in A. lineata ); more robust setae on the pereopods; and the mesial margin of the pleopodal exopods all have widely spaced serrations (smooth in A. lineata ).

Etymology. The epithet is derived from the Greek τριχωτός (trichotós), meaning ‘hairy’.

Figures

Fig. 6. Argathona trichota, new species, A–E, holotype, ZRC 2020.0151; F–I, female paratype, 8.8 mm, ZRC 2020.0152. A, dorsal view; B. lateral view; C, head, dorsal view; D, head and frons, ventral view; E, pleotelson, posterior margin; F, antennula; G, antenna peduncle; H, antenna, I, antenna flagellum, articles 14–15.

Fig. 7. Argathona trichota, new species, female paratype, 8.8 mm, ZRC 2020.0152. A, maxilliped, ovigerous female; B. maxilliped, palp article 5; C, maxillula; D, maxilla; E, left mandible incisor; F, right mandible incisor; G, mandible palp.

Fig. 8. Argathona trichota, new species, female paratype, 8.8 mm, ZRC 2020.0152. A, pereopod 1, dactylus; B, pereopod 1; C, pereopod 2; D, pereopod 7.

Fig. 9. Argathona trichota, new species, female paratype, 8.8 mm, ZRC 2020.0152. A–E, pleopods 1–5 respectively.

Fig. 10. Argathona trichota, new species, female paratype, 8.8 mm, ZRC 2020.0152. A, uropod; B, uropod, showing marginal robust setae only.