Xenuraega bythionekta Shimomura & Bruce 2019, sp. nov.

Xenuraega bythionekta sp. nov.

Figs 1–4

Material examined. Holotype, manca (2.0 mm), off northwest of Okinoerabujima Island, Kagoshima Pref., southern Japan 27°22.90´N, 128°23.70´E– 27°24.74´N, 128°21.98´E, 26 May 2000, depth 1178–1179 m, stn. 11, ORI net, coll. TRV Toyoshio-Maru (KMNH IvR 500922).

Description. Body 3.5 times as long as greatest width, dorsal surfaces sparsely punctate, widest at pereonite 4, lateral margins weakly ovate. Rostrum simple, anteriorly subtruncate. Eyes not medially united, separated by about 83% width of head; each eye made up of ~4 transverse rows of ommatidia, each row with ~8 ommatidia; eye colour pale orange. Coxae all wholly (2 and 3) or anteriorly (4–6) fused to pereonite; coxae 2–3 each with posteroventral angle rounded; coxae 4–6 without oblique carina. Pleon with pleonite 4 posterolateral margins not extending to posterior margin of pleonite 5; pleonite 5 with posterolateral angles quadrate. Pleotelson 2.1 times as long as anterior width, anterior dorsal surface without 2 sub-median depressions, dorsal surface smooth; lateral margins sinuate, posterior two-thirds strongly narrowed and weakly concave; posterior margin narrowly rounded, without RS, with 26 long plumose setae; longest setae 4 times longer than pleotelson length.

Antennula peduncle article 3 1.1 times as long as combined lengths of articles 1 and 2, 2.5 times as long as wide; flagellum with 4 articles, extending to posterior of pereonite 1. Antenna peduncle articles 1 and 2 fused; article 4 2.1 times as long as article 3, 2.6 times as long as wide, inferior margin without setae; article 5 1.3 times as long as article 4, 3.8 times as long as wide, inferior margin without setae, anterodistal angle with cluster of 3 short simple setae; antenna flagellum with 16 articles, extending to posterior of pleotelson.

Mandible molar process absent; palp article 2 without marginal setae; palp article 3 with 1 terminal elongate seta. Maxillula with 5 terminal RS. Maxilla mesial lobe absent; lateral lobe with 3 hooked RS. Maxilliped article 2 with 2 hooked RS; article 3 without hooked RS, fused to article 2.

Pereopod 1 basis 1.8 times as long as greatest width; inferior margin without RS, superior distal margin without setae; merus inferior margin with 1 small RS, set as single row, superior distal angle with 1 RS; carpus 0.9 times as long as merus, inferior margin without RS; propodus 2.1 times as long as proximal width, propodal palm simple, without blade or process, without setae, inferior margin without RS; dactylus 1.1 times as long as propodus. Pereopods 2 and 3 similar to pereopod 1. Pereopod 4 basis 1.4 times as long as greatest width, inferior margins without palmate setae; ischium 0.9 as long as basis, inferior margin with 1 RS, superior distal angle with 2 RS, inferior distal angle with 4 RS; merus 0.9 times as long as ischium, 3.2 times as long as wide, inferior margin without RS, superior distal angle with 3 RS, inferior distal angle with 3 RS; carpus 1.7 times as long as ischium, 8.2 times as long as wide, inferior margin without RS, superior distal angle with 1 RS, inferior distal angle with 5 RS; propodus 2.7 as long as ischium, 11.9 times as long as wide, inferior margin with 0 RS, superior distal angle without slender setae, inferior distal angle with 2 RS. Pereopod 5 similar to pereopod 4, but carpus and propodus less elongate. Pereopod 6 similar to 5 but carpus and propodus less elongate. Pereopods distal margins of ischium to carpus without slender setae; without strong carina on basis.

Pleopod 1 exopod 3.3 times as long as wide, lateral margin weakly convex, mesial margin straight, with PMS on distal margin only, with ~20 PMS; endopod 3.3 times as long as wide, 1.0 as long and 0.7 as wide as exopod, with ~9 PMS, lateral margin weakly sinuate, with PMS on distal margin only, mesial margin with PMS on distal half; peduncle mesial margin with 4 coupling hooks. Pleopod 2 exopod with ~27 PMS, endopod with ~9 PMS. Pleopod 3 exopod with ~24 PMS, endopod without PMS. Pleopod 4 exopod with ~25 PMS, endopod without PMS. Pleopod 5 exopod with ~22 PMS. Pleopods 2–5 peduncle distolateral margin each without acute RS; pleopods 3–5 endopods smaller (0.7–0.8 length; 0.7–0.8 width) than respective exopod.

Uropod peduncle ventrolateral margin without RS, posterior lobe about absent. Uropod rami with endopod and exopod co-planar, rami extending beyond pleotelson. Exopod 1.1 times as long as pleotelson, endopod 1.2 as long as pleotelson; 1.1 as long as exopod, marginal setae in single tier; lateral margin distally straight, with 3 slender setae, mesial margin straight, with 3 lateral and 3 terminal plumose setae. Endopod extending to just beyond end of endopod, 7.0 times as long as greatest width; lateral margin straight, without RS (3 simple setae); mesial margin straight, with 2 terminal plumose setae; distal margin truncate. Uropod rami both terminally truncate.

Color in life. Body and appendages transparent white. Eyes pale orange.

Remarks. This species can be recognized and identified by both uropodal rami being elongate (seven times as long as proximal width) with elongate plumose setae (Fig. 1A) together with the posteriorly narrowed rounded pleotelson also provided with long plumose setae. Xenuraega ptilocera Tattersall, 1909 differs in having the uropodal exopod reduced to a stub and the pleotelson broadly rounded and lacking setae.

Etymology. The epithet derived from the Greek words bythios (of the deep) with nektos (swimming), alluding to the presumed bentho-pelagic habitat of the species.