Rhachotropis reiwa Okazaki, Ohtsuka, and Tomikawa 2020
- Dataset
- A new species of the genus Rhachotropis from off Amamioshima Island northwestern Pacific (Crustacea: Amphipoda: Eusiridae)
- Rank
- SPECIES
- Published in
- Okazaki, Masakuni, Ohtsuka, Susumu, Tomikawa, Ko (2020): A new species of the genus Rhachotropis from off Amamioshima Island northwestern Pacific (Crustacea: Amphipoda: Eusiridae). Zootaxa 4750 (2): 182-190, DOI: 10.11646/zootaxa.4750.2.2
Classification
- kingdom
- Animalia
- phylum
- Arthropoda
- class
- Malacostraca
- order
- Amphipoda
- family
- Eusiridae
- genus
- Rhachotropis
- species
- Rhachotropis reiwa
description
(Figs 1 – 5)
description
Description of male (holotype, NSMT-Cr 26313). Head (Figs 1, 2 A): 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. 2 B) with small middorsal tooth. Pleonites 1 – 2 (Fig. 2 B) each with middorsal and dorsolateral teeth. Pleonite 3 (Fig. 2 B) 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. 2 F): 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. 2 G) 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. 2 H): 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. 2 I): ventral margin weakly convex, with minute setae. Mandible (Fig. 2 J – L): incisor with toothed distally, left lacinia mobilis wide, 4 - dentate (Fig. 2 L), right one (Fig. 2 K) slender, 2 - demtate, left and right accessory setal rows (Fig. 2 K, 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. 2 M): outer lobes broad, setulose; inner lobes distinct, fused medially. Maxilla 1 (Fig. 3 A): 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. 3 B): inner plate broad; outer plate almost as long as inner plate, bearing long setae on apical margin. Maxilliped (Fig. 3 C): 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. 3 E), proximal part of palmar margin with 12 robust setae (Fig. 3 F); dactylus long, slender, reaching end of palm. Gnathopod 2 (Fig. 3 G, 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. 3 H); dactylus long, slender, reaching end of palm. Pereopod 3 (Fig. 4 A, 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. 4 B) slender, slightly arched, with short setae. Pereopod 4 (Fig. 4 C): coxa with shallow posterior concavity; anterior and posterior margins of basis with short setae. Pereopod 5 (Fig. 4 D): 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. 4 E): 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. 4 F): 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. 4 A, C): large, broad, present on gnathopod 2, pereopods 3 – 7. Pleopods 1 – 3 (Fig. 5 A): peduncle broad, inner distal corner with paired retinacula. Uropod 1 (Fig. 5 B): 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. 5 C): 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. 5 D): 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. 5 E): 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 LC 487680 (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.
diagnosis
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 %.
discussion
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).
distribution
Distribution. Japan, off Amamioshima Island, northwestern Pacific Ocean, 291 – 402 m depth.
etymology
Etymology. The specific name is from the new Japanese era Reiwa that began 1 May 2019.
materials_examined
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.