Rhachotropis lowryi Okazaki & Tomikawa 2023, sp. nov.
Creators: name: Okazaki, Masakuni
name: Tomikawa, Ko
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Amphipoda
Eusiridae
Rhachotropis
Rhachotropis lowryi

description: Rhachotropis lowryi sp. nov.
urn:lsid:zoobank.org:act: F841B29D-1A1B-4D50-B7D1-EFBA213708A2
[New Japanese name: Amami-ryūgū-yokoebi]
Figs 1–5
Holotype: NSMT-Cr 30790, ovigerous female (6.4 mm, G1746), off Amamioshima Island, Kagoshima, Japan, 27.9675°N, 129.4005°E, 402 m depth, coll. K. Tomikawa, 26 May 2008. Paratypes: NSMT-Cr 30791, female (5.6 mm); NSMT-Cr 30792 female (5.5 mm); NSMT-Cr 30793, female (4.8 mm); NSMT-Cr 30794, 2 females (4.6 mm, 5.9 mm); data as for holotype.
Diagnosis. Head with developed rostrum; eyes large. Pereonites 1–7 dorsally smooth, weakly rugose. Pleonites 1–3 each with middorsal and dorsolateral teeth, those of pleonite 3 minute. Epimeral plate 3 with strongly serrate posterior margin. Pereopods 5–7 with posteriorly produced basis. Telson cleft for 44%.
Description of holotype. Head (Fig. 1A, C, D) dorsally smooth; rostrum longer than half of head, pointed; eyes large, ovate; lateral cephalic lobe weakly produced, anterodistal corner squarish; antennal sinus absent. Pereonites 1–7 (Fig. 1A) dorsally smooth, weakly rugose. Pleonites 1–3 (Fig. 1A, B, E) each with middorsal and dorsolateral teeth, those of pleonite 3 minute. Epimeral plates 1–3 (Figs 1B, 2A–C) with posterodistal corner rounded, posterior margin of epimeral plate 3 strongly serrate. Dorsal margin of urosomites smooth (Fig. 1A, E).
Antenna 1 (Fig. 2D) with peduncular articles 1–3 1.0: 0.7: 0.2 in length ratio; peduncular article 1 with long setae on posterior margin; peduncular article 2 with calceoli (Fig. 1F) on anterior margin; primary flagellum with 9 articles, bearing calceoli; accessory flagellum not seen. Antenna 2 (Fig. 2E): peduncular articles 4 with calceoli on anterior margin, posterior margin with long setae; peduncular article 5 slightly shorter than article 4, with calceoli on anterior margin, posterior margin with a few short setae; flagellum 12-articulate with calceoli.
Upper lip (Fig. 2F) with ventral margin weakly convex, with setae. Mandible (Fig. 2G, H) incisor with distal tooth, left lacinia mobilis (Fig. 2G) wide, 6-dentate, right (Fig. 2H) narrow with minute teeth; accessory setal row with blade setae, molar process weakly triturative, edges lined with short blades; palp 3-articulate, length ratio of articles 1–3 1.0:2.3:2.3, article 1 with minute setae, article 2 with 15 setae, anterior margin of article 3 lined with setae. Lower lip (Fig. 2I) with broad outer lobes, setulose; inner lobes distinct, fused medially. Maxilla 1 (Fig. 2J) with narrow inner plate, bearing 2 plumose setae apically; outer plate subrectangular with 9 serrate robust setae; palp 2-articulate, article 1 subrectangular with 2 setae; article 2 with 7 apical and 2 inner marginal robust setae, and subapical slender setae. Maxilla 2 (Fig. 2K) with broad inner plate; outer plate slightly longer than inner plate, bearing long setae on apical margin. Maxilliped (Fig. 2M) with ovate inner plate, short, not reaching half-length of palp article 1; outer plate exceeding distal part of palp article 1, medial margin almost straight, lined with setae; palp 4-articulate.
Gnathopod 1 (Fig. 3A), coxa strongly produced anteriorly, with short setae; basis weakly curved, anterior and posterior margins with short setae, anterodistal corner with long setae; carpus lobate posteriorly with setae; propodus oval, width 0.5 times length, anterior submargin with short setae, palmar margin convex, setose; dactylus long, slender, reaching end of palm. Gnathopod 2 (Fig. 3B), coxa subrectangular, bearing short setae; basis curved, anterior and posterior margins with short setae, anterodistal and posterodistal corners with long setae; carpus lobate posteriorly with setae; propodus oval, width 0.5 times length, anterior margin without setae, palmar margin convex, setose; dactylus long, slender, reaching end of palm.
Pereopod 3 (Fig. 3C), coxa subrectangular; basis long, straight, anterior and posterior margins with short setae and a few long setae; length ratio of merus, carpus, propodus, and dactylus 1.0:1.6:1.7:1.3; dactylus slender, slightly curved, with short setae. Pereopod 4 (Fig. 3D), coxa with shallow posterior concavity; posterior margins of basis with long setae; merus and carpus with long setae on posterior margins; length ratio of merus, carpus, propodus, and dactylus 1.0:1.5:1.4:1.4. Pereopod 5 (Fig. 3E), coxa bilobate, anterior and posterior lobes equal in size; basis produced posteriorly; anterior margin of merus with long setae; length ratio of merus, carpus, propodus, and dactylus 1.0:1.0:1.9:1.0. Pereopod 6 (Fig. 4A) with coxa bilobate, posterior lobe larger than anterior one; basis strongly expanded posteriorly, posterodistal margin with long setae; anterior margin of merus with long setae; length ratio of merus, carpus, and propodus and dactylus 1.0:1.0:1.9:0.9. Pereopod 7 (Fig. 4B), coxa rounded; basis broad, strongly expanded posteriorly with short setae.
Coxal gills (Figs 3C–E, 4A) large, broad, present on gnathopod 2 to pereopod 7.
Pleopods 1–3 (Fig. 4C–E), peduncle broad, inner distal corner with paired retinacula (Fig. 4F).
Uropod 1 (Fig. 5A) peduncle long, length 3.8 times width, with robust setae on medial and lateral margins; inner ramus 1.1 times length peduncle, with 3 robust setae on medial margin; outer ramus 0.9 times length of inner ramus, lateral margin with 5 robust setae. Uropod 2 (Fig. 5B), peduncle length 2.5 times width, with robust setae on medial and lateral distal corners; inner ramus 1.9 times length of peduncle, with 7 and 3 robust setae on medial and lateral margins, respectively; outer ramus 0.8 times length of inner ramus, with 7 robust setae on lateral margin. Uropod 3 (Fig. 5C), peduncle short, length 1.6 times width, with slender setae on medial margin; inner ramus, length 2.4 times peduncle, medial and lateral margins with 4 and 2 robust setae, respectively; outer ramus almost as long as inner ramus, lateral margin with 5 robust setae. Telson (Fig. 5D) length 1.9 times width, cleft for 44%, with lateral plumose setae on basal part.
11 eggs.
Nucleotide sequence. One 658 bp COI sequence of holotype was determined (GenBank accession number LC727553). Among available data in the INSDC database, the sequence of R. lowryi sp. nov. and that of R. reiwa Okazaki, Ohtsuka & Tomikawa, 2020 have the highest similarity (23.8% uncorrected p -distance).
Distribution. Known only from the type locality.
Etymology. Named after Dr James K. Lowry.
Remarks. Rhachotropis lowryi sp. nov. differs from its congeners by the combination of the following features: 1) urosomite 1 without dorsal process; 2) pereopod 5 basis with produced posterior margin; 3) uropod 3 with peduncle shorter than 0.5 times the outer ramus; 4) uropod 3 with inner and outer rami of the same length; and 5) telson cleft for 44% of its length. The new species is similar to R. reiwa Okazaki, Ohtsuka & Tomikawa, 2020, described from off Amamioshima Island, in having pereopods 5 and 6 with a posteriorly produced basis and relatively deeply incised telson (more than 38% of telson length). However, R. lowryi sp. nov. is distinguished from R. reiwa by the following features (features of R. reiwa in parentheses): (1) pereonite 7 dorsally smooth (bearing middorsal tooth); (2) epimeral plate 3 with strongly serrate posterior margin (weakly serrate); (3) pereopod 7 with basis strongly produced posteriorly (rounded but not produced); and (4) uropod 3 with inner and outer ramus of the same length (outer ramus slightly shorter than inner). Although this new species and R. reiwa occur in the same geographic area, the two species are highly differentiated genetically (23.8% uncorrected p -distance), indicating that they are clearly distinct species. Rhachotropis lowryi sp. nov. shares the features of a prominent posterior margin of the basis of pereopod 5 with the following five species: R. aculeata (Lepechin, 1780), R. gubilata J. L. Barnard, 1964, R. oweni Lörz, 2015, R. palporum Stebbing, 1908, and R. reiwa. A comparison of features with these five species is shown in Table 1.
ACKNOWLEDGEMENTS. We express our sincere thanks to Professor Susumu Ohtsuka, the captain and crew of the TRV Toyoshio-Maru of Hiroshima University for their cooperation at sea. Thanks are also due to Drs Anne-Nina Lörz and Michael H. Thurston for their critical reading and valuable comments on our manuscript. This study was partly supported by JSPS KAKENHI Grant Numbers 22H01011 and 22K06373.
description: Published as part of Okazaki, Masakuni & Tomikawa, Ko, 2023, A New Species of the Genus Rhachotropis (Crustacea: Amphipoda: Eusiridae) from Japan, pp. 507-514 in Records of the Australian Museum (Rec. Aust. Mus.) (Rec. Aust. Mus.) 75 (4) on pages 508-512, DOI: 10.3853/j.2201-4349.75.2023.1888, http://zenodo.org/record/10413640