Rhachotropis reiwa Okazaki & Ohtsuka & Tomikawa 2020, sp. nov.

Rhachotropis reiwa sp. nov.

[New Japanese name: Reiwa-ryūgū- yokoebi]

( Figs 1–5)

Material examined. Holotype: NSMT-Cr 26313, male (6.5 mm), off Amamioshima Island, northwestern Pacific Ocean, sledge net, 28.233716°N, 129.659316°E– 28.235583°N, 129.664716°E, 291–294 m depth, collected by K. Tomikawa on 18 May 2018. Paratype: NSMT-Cr 26314, male (6.3 mm), off Amamioshima Island, northwestern Pacific Ocean, beam trawl, 27.9725°N, 129.4010°E– 27.9776°N, 129.4086°E, 400–402 m depth, collected by K. Tomikawa on 26 May 2008.

Diagnosis. Head with long rostrum and large eyes. pereonite 7 with small middorsal tooth. Pleonites 1 and 2 with middorsal and dorsolateral teeth. Basis of pereopod 5 strongly produced posteriorly. Basis of pereopod 6 triangular. Telson cleft for 38%.

Description of male (holotype, NSMT-Cr 26313). Head (Figs 1, 2A): smooth, rostrum longer than half of head; eyes large, ovate; lateral cephalic lobe produced, rounded; antennal sinus quadrate. Pereonites 1–6: dorsally smooth. Pereonite 7 (Fig. 2B) with small middorsal tooth. Pleonites 1–2 (Fig. 2B) each with middorsal and dorsolateral teeth. Pleonite 3 (Fig. 2B) weakly carinate, lacking tooth. Epimeral plates 1–3 (Fig. 2 C–E): posterodistal corners rounded; posterior margin of epimeral plate 3 weakly serrate. Dorsal margin of urosomites smooth.

Antenna 1 (Fig. 2F): length ratio of peduncular articles 1–3 1.0: 0.7: 0.2; peduncular article 1 with long setae on posterior margin; peduncular article 2 with calceoli (Fig. 2G) on anterior margin, posterior margin lined with setae; primary flagellum with 23+ articles, bearing calceoli, article 1 elongate, swollen; accessory flagellum unrecognized. Antenna 2 (Fig. 2H): peduncular articles 4 with calceoli on anterior margin, posterior margin with long setae; peduncular article 5 long, length 1.5 times as long as article 4, with calceoli on anterior margin, posterior margin with a few short setae; flagellum with 20+ articles, article 4 with calceolus.

Upper lip (Fig. 2I): ventral margin weakly convex, with minute setae. Mandible (Fig. 2 J–L): incisor with toothed distally, left lacinia mobilis wide, 4-dentate (Fig. 2L), right one (Fig. 2K) slender, 2-demtate, left and right accessory setal rows (Fig. 2K, L) each with 4 blade setae, molar process large, triturative, edges lined with short blades; palp 3-articulate, length ratio of article 1–3 1.0: 3.1: 3.8, article 1 bare, article 2 with 14 setae, posterior margin of article 3 lined with setae. Lower lip (Fig. 2M): outer lobes broad, setulose; inner lobes distinct, fused medially. Maxilla 1 (Fig. 3A): inner plate narrow with 2 plumose setae; outer plate rectangular with 9 serrate robust setae; palp 2-articulate; article 1 subrectangular, lacking setae; article 2 with 7 apical and subapical slender setae. Maxilla 2 (Fig. 3B): inner plate broad; outer plate almost as long as inner plate, bearing long setae on apical margin. Maxilliped (Fig. 3C): inner plate subrectangular, short, not reaching half of palp article 1, with 7 robust setae subapically; outer plate slightly exceeding distal part of palp article 1, medial margin almost straight, lined with setae; palp 4-articulate, article 2 broad.

Gnathopod 1 (Fig. 3 D–F): coxa strongly produced anteriorly, with short seta; basis weakly arched, anterior and posterior margins with short setae, anterodistal corner with 2 long setae; carpus lobate posteriorly with setae; pro- podus oval, width 0.5 times as long as length, anterior submargin with short setae, palmar margin convex roundly with 2 rows of plumose setae (Fig. 3E), proximal part of palmar margin with 12 robust setae (Fig. 3F); dactylus long, slender, reaching end of palm. Gnathopod 2 (Fig. 3G, H): basis weakly arched, anterior margin with a few short setae, posterior margin with short setae, anterodistal corner with setae; carpus lobate posteriorly with setae; propodus oval, width 0.5 times as long as length, anterior margin and submargin with short setae, palmar margin convex roundly with 2 rows of setae, proximal part of palmar margin with 7 robust setae (Fig. 3H); dactylus long, slender, reaching end of palm.

Pereopod 3 (Fig. 4A, B): coxa subrectangular; basis long, straight, anterior and posterior margins with short setae; length ratio of merus, carpus and propodus and dactylus 1.0: 1.8: 1.9: 2.2; dactylus (Fig. 4B) slender, slightly arched, with short setae. Pereopod 4 (Fig. 4C): coxa with shallow posterior concavity; anterior and posterior margins of basis with short setae. Pereopod 5 (Fig. 4D): coxa bilobate, anterior and posterior lobes equal in size; basis strongly expanded posteriorly, posterior margin with small setae; anterior and posterior margins of merus with short setae. Pereopod 6 (Fig. 4E): coxa bilobate, posterior lobe with small seta on posterodistal corner; basis strongly expanded posteriorly, triangular, posterior margin with short setae; anterior and posterior margins of merus with short setae. Pereopod 7 (Fig. 4F): coxa quadrate with seta on posterodistal corner; basis broad, posterior margin with short setae; merus with short setae on anterior and posterior margins.

Coxal gills (Fig. 4A, C): large, broad, present on gnathopod 2, pereopods 3–7.

Pleopods 1–3 (Fig. 5A): peduncle broad, inner distal corner with paired retinacula.

Uropod 1 (Fig. 5B): medial and lateral margins of peduncle along with robust setae, respectively; inner ramus length 1.1 times as long as peduncle, with 7 and 10 robust setae on medial and lateral margins, respectively; outer ramus length 0.8 times as long as inner ramus, medial margin lacking setae, lateral margin with 6 robust setae. Uropod 2 (Fig. 5C): peduncle with robust setae on medial and lateral margins; inner ramus length 1.9 times as long as peduncle, with 19 and 18 robust setae on medial and lateral margins, respectively; outer ramus length 0.8 times as long as inner ramus, medial and lateral margins with 11 and 10 robust setae, respectively. Uropod 3 (Fig. 5D): peduncle with robust setae on medial and lateral margins; inner ramus length 2.1 times as long as peduncle, medial margin with robust setae and plumose setae, lateral margin with 11 robust setae; outer ramus length 0.9 times as long as inner ramus, medial and lateral margins with 10 and 6 robust setae, respectively. Telson (Fig. 5E): elongate, length 2.5 times as long as width, cleft for 38%, with lateral plumose setae on basal part.

Nucleotide sequence. One COI sequence of holotype was determined. Accession number is LC487680 (658 bp). Among available data in a database (INSDC), the sequence of R. reiwa sp. nov. and that of R. chathamensis Lörz, 2010 have the highest similarity with 15.5% in p- distance.

Distribution. Japan, off Amamioshima Island, northwestern Pacific Ocean, 291–402 m depth.

Etymology. The specific name is from the new Japanese era Reiwa that began 1 May 2019.

Remarks. Rhachotropis reiwa sp. nov. differs from its congeners by the combination of following features: 1) eyes present; 2) pereonite 7 with middorsal tooth; 3) pleonite 3 and urosomite 1 without middorsal and dorsolateral teeth; 4) the basis of pereopod 5 strongly produced posteriorly, and 5) the basis of pereopod 6 triangular. The new species shares posteriorly produced bases of pereopods 5 and 6 with R. aculeate (Lepechin, 1780), R. oweni Lörz, 2015, and R. palporum Stebbing, 1908. However, the new species differs from these three species by the following features: from R. aculeate, 1) head smooth (versus raised dorsally), 2) pereonite 6 and pleonite 3 without middorsal and dorsolateral teeth (versus present), 3) urosomite 1 weakly carinate, lacking tooth (versus bearing bidentate middorsal ridge and posterolateral marginal tooth), 4) coxa of gnathopod 1 straightly extended (versus arched downward), 5) basis of pereopod 5 broadly produced posteriorly (versus acutely produced), and 6) basis of pereopod 6 triangular (versus not triangular); from R. oweni, 1) head and pereonites 1–7 without dorsal humps (versus dorsal humps are present), 2) eyes large (versus small), 3) pereonites 7 with middorsal tooth (versus lacking), 4) pleonite 1 with middorsal tooth (versus lacking), 5) pleonite 3 without dorsal tooth (versus middorsal and dorsolateral teeth are present), 6) basis of pereopod 7 without posterior large expansion (versus present), and 7) telson cleft for 38% (versus 10%); and from R. palporum, 1) pleonite 3 without middorsal and dorsolateral teeth (versus present), 2) mandibular palp article 3 developed, longer than article 2 (versus reduced, shorter than article 2), 3) basis of pereopod 5 broadly produced posteriorly (versus acutely produced), 4) basis of pereopod 6 triangular (versus not triangular), 5) coxa without posterior tooth (versus present), and 6) basis of pereopod 7 unproduced posteriorly (versus strongly produced). Rhachotropis reiwa sp. nov. is distinguished from R. chathamensis by 1) pereonite 7 with a small middorsal tooth (versus lacking tooth), 2) pleonite 3 without dorsal tooth (versus bearing tooth), 3) gnathopod 1 with coxa anteriorly rounded (versus anteriorly pointed), 4) pereopods 5 and 6 with strongly expanded bases posteriorly (versus not expanded), and 5) telson cleft (versus entire).