Rhachotropis lowryi, Okazaki & Tomikawa, 2023

Okazaki, Masakuni & Tomikawa, Ko, 2023, A New Species of the Genus Rhachotropis (Crustacea: Amphipoda: Eusiridae) from Japan, Records of the Australian Museum (Rec. Aust. Mus.) 75 (4), pp. 507-514 : 508-512

publication ID	https://doi.org/ 10.3853/j.2201-4349.75.2023.1888
publication LSID	lsid:zoobank.org:pub:750C3002-9644-4904-863F-EDAF30891928
DOI	https://doi.org/10.5281/zenodo.10955796
persistent identifier	https://treatment.plazi.org/id/F841B29D-1A1B-4D50-B7D1-EFBA213708A2
taxon LSID	lsid:zoobank.org:act:F841B29D-1A1B-4D50-B7D1-EFBA213708A2
treatment provided by	Felipe
scientific name	Rhachotropis lowryi
status	sp. nov.

Treatment

Rhachotropis lowryi View in CoL sp. nov.

urn:lsid:zoobank.org:act:F841B29D-1A1B-4D50-B7D1-EFBA213708A2

[New Japanese name: Amami-ryūgū-yokoebi]

Figs 1–5 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 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 GoogleMaps . 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 GoogleMaps .

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 View Figure 1 ) 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 View Figure 1 ) dorsally smooth, weakly rugose. Pleonites 1–3 ( Fig. 1A, B, E View Figure 1 ) each with middorsal and dorsolateral teeth, those of pleonite 3 minute. Epimeral plates 1–3 ( Figs 1B View Figure 1 , 2A–C View Figure 2 ) with posterodistal corner rounded, posterior margin of epimeral plate 3 strongly serrate. Dorsal margin of urosomites smooth ( Fig. 1A, E View Figure 1 ).

Antenna 1 ( Fig. 2D View Figure 2 ) 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 View Figure 1 ) on anterior margin; primary flagellum with 9 articles, bearing calceoli; accessory flagellum not seen. Antenna 2 ( Fig. 2E View Figure 2 ): 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 View Figure 2 ) with ventral margin weakly convex, with setae. Mandible ( Fig. 2G, H View Figure 2 ) incisor with distal tooth, left lacinia mobilis ( Fig. 2G View Figure 2 ) wide, 6-dentate, right ( Fig. 2H View Figure 2 ) 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 View Figure 2 ) with broad outer lobes, setulose; inner lobes distinct, fused medially. Maxilla 1 ( Fig. 2J View Figure 2 ) 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 View Figure 2 ) with broad inner plate; outer plate slightly longer than inner plate, bearing long setae on apical margin. Maxilliped ( Fig. 2M View Figure 2 ) 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 View Figure 3 ), 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 View Figure 3 ), 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 View Figure 3 ), 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 View Figure 3 ), 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 View Figure 3 ), 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 View Figure 4 ) 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 View Figure 4 ), coxa rounded; basis broad, strongly expanded posteriorly with short setae.

Coxal gills ( Figs 3C–E View Figure 3 , 4A View Figure 4 ) large, broad, present on gnathopod 2 to pereopod 7.

Pleopods 1–3 ( Fig. 4C–E View Figure 4 ), peduncle broad, inner distal corner with paired retinacula ( Fig. 4F View Figure 4 ).

Uropod 1 ( Fig. 5A View Figure 5 ) 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 View Figure 5 ), 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 View Figure 5 ), 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 View Figure 5 ) 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.