Syscenus karu, Bruce, Niel L., 2005

Bruce, Niel L., 2005, Two new species of the mesopelagic isopod genus Syscenus Harger, 1880 (Crustacea: Isopoda: Aegidae) from the southwestern Pacific, Zootaxa 1070, pp. 31-42 : 38-41

Syscenus karu sp. nov. ( Figs 5 , 6 )

Material examined: Holotype: Ψ (22.5 mm), Vanuatu, 19°19.62’S, 169°26.43’E, 23 Sep 1994, MUSORSTOM 8, stn. CP984, 480– 455 m, coll. B. Richer de Forges ( MNHN Is.5847).

Description: Body 3.4 times as long as greatest width, lateral margins subparallel. Rostrum simple, anteriorly narrow. Eyes not medially united (lateral in position), separated by about 70% width of head; each eye made up of ~6 transverse rows of ommatidia, each row with ~6 ommatidia; eye colour pale brown. Coxae 2–3 each with posteroventral angle rounded; 5–7 with entire oblique carina. Pleon with pleonite 1 visible in dorsal view. Pleotelson 1 times as long as anterior width, anterior dorsal surface with 2 sub­median depressions, posterior margin sub­truncate (with shallow median indentation), without RS. Antennule peduncle article 3 similar to other of the genus; flagellum with 8 articles, extending to anterior posterior margin of head. Antenna peduncle article 3 similar to others of the genus; flagellum with 32 articles, extending to pereonite 6.

Frontal lamina as wide as long, anteriorly acute. Mandible palp article 3 with 2 setae. Maxillule with 5 RS. Maxilla mesial lobe with 2 hooked RS (small); lateral lobe with 1 hooked RS (small). Maxilliped palp article 1 distomesial angle without RS; article 2 with 2 hooked RS; article 3 with absent.

Pereopod 1 basis 2.2 times as long as greatest width; ischium 0.4 times as long as basis, inferior margin without RS, superior distal margin without setae; merus inferior margin without RS, superior distal angle with 1 seta (acute RS); inferior margin without RS; propodus 2 times as long as proximal width, propodal palm simple, without blade or process, without setae, inferior margin without RS; dactylus 0.9 times as long as propodus. Pereopods 2 and 3 similar to pereopod 1. Pereopod 6 similar to pereopod 7. Pereopod 7 basis 1.8 times as long as greatest width, inferior margins without palmate setae; ischium 0.8 as long as basis, inferior margin without RS, superior distal angle with 4 RS, inferior distal angle with 1 robust seta; merus 0.5 times as long as ischium, 1.4 times as long as wide, inferior margin without RS, superior distal angle with 3 RS, inferior distal angle with 2 RS; carpus 0.6 times as long as ischium, 2 times as long as wide, inferior margin without RS, superior distal angle with 11 RS, inferior distal angle with 1 robust seta; propodus 0.6 as long as ischium, 2.8 times as long as wide, inferior margin without RS, superior distal angle with 2 slender setae, inferior distal angle with 1 RS. Pereopods distal margins of ischium to carpus without setae; with strong carina on basis.

Pleopod 1 exopod 1.3 times as long as wide, lateral margin straight, mesial margin strongly convex, with PMS on central one­third; endopod 2 times as long as wide, lateral margin straight; peduncle mesial margin with 9 coupling hooks. Pleopods 2–5 peduncle distolateral margin each without acute RS.

Uropod peduncle ventrolateral margin without RS, posterior lobe about one­third as long as endopod. Uropod rami with endopod and exopod co­planar, rami extending to pleotelson apex. Endopod lateral margin convex, lateral margin without RS, mesial margin weakly convex, without RS. Exopod not extending to end of endopod, 2.8 times as long as greatest width; lateral margin convex, without RS; mesial margin straight, without RS; distal margin rounded.

Size: Holotype 22.5 mm.

Remarks: Syscenus karu sp. nov. can be distinguished from all but one species of Syscenus by the presence of conspicuous, faceted eyes. Syscenus peruanus Menzies & George, 1972 is described as having ‘eyes degenerate, lacking pigment but bearing ocelli’. The eyes of S. karu are conspicuous, pale brown in colour, and the ommatidia are distinct. S. karu is further distinguished by the shape of anterior margin of the head which has the median portion strongly produced, the basis of pereopods 4–7 each bearing a strong mesial blade, and the relatively stout pereopods (proportional measurements are provided in the descriptions; in S. karu pereopod 7 is 6.1 times as long as the width of the basis, for S. moana and S. springthorpei that proportion is 13.7 and 12.2 respectively). This species is unusual within the genus in having pleopod 5 peduncle provided with coupling hooks, and also in pleopods 1–3 exopods having marginal setae only on the distomesial margin. and the endopods of pleopods 1–5 all lacking marginal setae.

Prey: Not known.

Distribution: Known only from the type locality off Vanuatu.

Etymology: The epithet is a Polynesian word for eye; noun in apposition.

Figures

FIGURE 5. Syscenus karu sp. nov. A, dorsal view; B, lateral view; C, head, dorsal view; D, frons; E, maxillule; F, maxillule apex; G, maxilla; H, maxilla apex; I, maxilliped; J, maxilliped palp article 2.

FIGURE 6. Syscenus karu sp. nov. A – C, pereopods 1, 2, 7 respectively; D – F, pleopods 1 – 3 respectively; G, uropod.