A New Species of the Genus Rhachotropis (Crustacea: Amphipoda: Eusiridae) from Japan

. A new eusirid amphipod, Rhachotropis lowryi sp. nov., is described from off Amamioshima Island, Japan, at 402 m depth. This new species differs from its congeners by the features of urosomite 1, pereopods 5–7 bases, uropod 3, and telson. A key to species of Rhachotropis from Japanese and adjacent waters is provided. Additionally, a nucleotide sequence of mitochondrial cytochrome c oxidase subunit I from the holotype of R. lowryi was determined for the future study.

Field surveys of the deep-sea amphipod fauna around Amamioshima Island, Amami Islands, Japan, have produced several previously undescribed species.One of these was described recently as Rhachotropis reiwa Okazaki, Ohtsuka & Tomikawa, 2020.In this study, we describe another new species of Rhachotropis.

Materials and methods
Collection.The present specimens were collected using a beam trawl (mouth opening 50 cm × 170 cm; mesh 15.5 mm) deployed from the TRV Toyoshio-Maru (Hiroshima University).The specimens were preserved in 99% ethanol on-board ship.For DNA extraction, muscle tissue was removed from the dorsal side of the pleon of the holotype.

Morphological examination.
Appendages were dissected in 70% ethanol and mounted in gum-chloral medium on glass slides under a stereomicroscope (Olympus SZX7).The specimen was examined using a light microscope (Nikon Eclipse Ni) and illustrated with the aid of a camera lucida.Bodies were dehydrated through a graded ethanol series, and dried using hexamethyldisilazane (HMDS) (Nation, 1983).They were then sputter-coated with gold and observed using scanning electron microscopy (SEM, JSM-6510LV).The body length from the tip of the rostrum to the base of the telson was measured along the dorsal curvature to the nearest 0.1 mm.The specimens are deposited in the Tsukuba Collection Center of the National Museum of Nature and Science, Tokyo (NSMT).
DNA sequencing.The extraction of genomic DNA from pleon muscle followed Tomikawa et al. (2014).The cytochrome c oxidase subunit I (COI) gene [LCO1490 and HCO2198 (Folmer et al., 1994)] primer set was used for PCR and cycle sequencing (CS) reactions.PCR reactions and DNA sequencing were performed following Tomikawa et al. (2016).DNA sequences have been deposited with the International Nucleotide Sequence Database Collaboration (INSDC) through the DNA Data Bank of Japan (DDBJ).
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  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 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 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.