Taxonomy of the genus Elasmopus (Crustacea, Amphipoda) in Japan and South Korea, with description of a new species

307 Taxonomy of the genus Elasmopus (Crustacea, Amphipoda) in Japan and South Korea, with description of a new species Hiro Yoshimura1, Chi-Woo Lee2, Ko Tomikawa1 1 Graduate School of Humanities and Social Sciences, Hiroshima University, 1-1-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8524, Japan 2 Nakdonggang National Institute of Biological Resources, Sangju 37242, Republic of Korea Corresponding author: Hiro Yoshimura (wrasseber[email protected]) Copyright: © Hiro Yoshimura et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract To advance the limited understanding of the East Asian Elasmopus fauna, field surveys were conducted in Japan and South Korea. In the present study, a new species, E. lumbiniger sp. nov., is described. The new species is distinguished from its congeners by a strongly projected posteroventral corner of the epimeral plate 3, long slender setae on the anterodistal corner of the male gnathopod 2 carpus, a mid-palmar ridge on the male gnathopod 2 propodus, and a smooth posterior margin on the basis of pereopods 5–7. In addition, E. koreanus is recorded in Japan for the first time, and the known distribution range of E. mukuinu has been significantly extended. The nucleotide sequences of the mitochondrial cytochrome c oxidase subunit I (COI) of these species were determined, and the genetic distances among Elasmopus species were provided. A key to the species of Elasmopus found in East Asia is also provided. Key words: COI, East Asia, genetic distance, Kimura two-parameter distance, morphology, p-distance Introduction The genus Elasmopus Costa, 1853 is distributed in the shallow waters of tropical and temperate regions worldwide (Lowry and Hughes 2009; Vader and Krapp-Schickel 2012). To date, 124 Elasmopus species have been described (Vader and Krapp-Schickel 2012; Alves et al. 2016; Horton et al. 2025; Yoshimura and Tomikawa 2025). Of these, 13 species of Elasmopus are currently known from East Asia (Table 1). In Japan, E. japonicus and E. projectus have been recorded from temperate coastal areas, and E. mukuinu and E. nkjaf from subtropical zones (Stephensen 1932; Nakamura et al. 2019; Sir and White 2022; Yoshimura and Tomikawa 2025). Two species, E. koreanus and E. rapax, have been recorded from South Korean waters (Kim and Kim 1991; Shin et al. 2005; Kim et al. 2023; Shin et al. 2025). However, many regions in Japan and South Korea remain taxonomically unstudied with regard to Elasmopus. The presence of several unrecorded or undescribed species of Elasmopus has been suggested (Hirayama 1995; Ariyama 2022). To clarify the Elasmopus fauna in Japan and South Korea, we conducted field surveys at multiple sites. We found E. koreanus and E. mukuinu in temperate Academic editor: Luiz F. Andrade Received: 23 June 2025 Accepted: 9 November 2025 Published: 2 December 2025 ZooBank: https://zoobank. org/9DCB403A-2E55-4D80-B00E9E0977AB9C00 Citation: Yoshimura H, Lee C-W, Tomikawa K (2025) Taxonomy of the genus Elasmopus (Crustacea, Amphipoda) in Japan and South Korea, with description of a new species. ZooKeys 1261: 307–335. https://doi.org/10.3897/ zookeys.1261.162923 ZooKeys 1261: 307–335 (2025) DOI: 10.3897/zookeys.1261.162923 308 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan regions of Japan, where they had not been previously recorded, as well as an undescribed species, E. lumbiniger sp. nov. Additionally, we sequenced the mitochondrial cytochrome c oxidase subunit I (COI) gene of the collected samples and calculated the genetic distances among the Elasmopus species. Materials and methods Field sampling The specimens were collected from the intertidal and shallow subtidal zones during low tide, depth range 0–1 m. The sampling sites are shown in Fig. 1. While still alive, some specimens were photographed using a digital camera (EOS M6; Canon, Tokyo, Japan) with a lens (LAOWA 65 mm F2.8 Macro Photo Lens; Venus Optics, Heifei, China). After photographs were taken, some specimens were fixed and preserved in 99% ethanol while others were frozen, fixed, and preserved in propylene glycol. Morphological examinations All appendages were dissected in 50% ethanol and mounted in gum-chloral medium on glass slides under a stereomicroscope (SZ61; Olympus, Tokyo, Japan). The specimens were examined using a light microscope (Eclipse Ni-U; Nikon, Tokyo, Table 1. Occurrence records of Elasmopus species in East Asia. Species Occurrence in East Asia Notes References E. alkhiranensis Myers & Momtazi, 2015 South China Sea, China Ren (2012) recorded this species as E. pectenicrus (Bate, 1862); however, it was referred to E. alkhiranensis by Myers and Momtazi (2015). Ren (2012); Myers and Momtazi (2015) E. hawaiensis Schellenberg, 1938 South China Sea, China - Ren (2012) E. hooheno Barnard, 1970 South China Sea, China - Ren (2012) E. japonicus Stephensen, 1932 Philippine Sea, Japan -Stephensen (1932) E. koreanus Kim & Kim, 1991 Sea of Japan (East Sea), South Korea, and Japan; Pacific Ocean, Japan -Kim and Kim (1991); Kim et al. (2023); Shin et al. (2025); the present study E. lumbiniger sp. nov. Sea of Japan, Japan (East Sea); Philippine Sea, Japan; Pacific Ocean, Japan - present study E. mukuinu Sir & White, 2022 Philippine Sea, Japan -Sir and White (2022); present study E. nanshaensis Ren, 1998 South China Sea, China - Ren (2012) E. nkjaf Nakamura, Nakano, Ota & Tomikawa, 2019 Philippine Sea, Japan -Nakamura et al. (2019) E. projectus Yoshimura & Tomikawa, 2025 Sea of Japan (East Sea), Japan; Pacific Ocean, Japan - Yoshimura and Tomikawa (2025) E. rapax Costa, 1853 South China Sea, China; Sea of Japan (East Sea), South Korea -Shin et al. (2005); Ren (2012); Shin et al. (2025) E. spinicarpus Berents, 1983 South China Sea, China - Ren (2012) E. spinidactylus Chevreux, 1907 South China Sea, China - Ren (2012) E. spinimanus Walker, 1904 South China Sea, China - Ren (2012) 335 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Ortiz M, Lalana R (1994) Two new species of the genus Elasmopus (Amphipoda: Gammaridea), from the Cuban marine waters. Travaux du Museum d’histoire naturelle. Grigore Antipa 34: 293–302. Ren X (1998) Studies on family Melitidae (Crustacea: Amphipoda) from Nansha Islands, South China Sea. Studies on Marine Fauna and Flora and Biogeography of the Nansha Islands and Neighboring Waters 3: 193–218. Ren X (2012) 43 Fauna Sinica. Invertebrata. Vol. 43 Crustacea: Amphipoda: Gammaridea (II). Chinese Academy of Sciences (Ed.) Science Press, Beijing, China, Beijing, China, 651 pp. Schellenberg A (1938) Litorale Amphipoden des tropischen Pazifiks. Kungliga Svenska Vetenskapsakademiens Handlingar (Ser. 3) 16(6): 1–105. Shin M-H, Kim I-H, Lee KS (2005) Three Species of Gammaridean Amphipods (Crustacea) Associated with Cultured Abalones (Gastropoda) in Korea. Korean Journal of Systematic Zoology 21: 157–169. Shin S-Y, Choi J-H, Kim J, Kim K-W, Han S-H, Kim Y-H (2025) The marine amphipods (Arthropoda: Crustacea: Malacostraca) fauna from Ulleungdo Island in Korea. Journal of Species Research 1: 48–73. https://doi.org/10.12651/JSR.2025.14.1.048 Sir S, White KN (2022) Maerid amphipods (Crustacea: Amphipoda) from Okinawa, Japan with description of a new species. Zootaxa 5093(5): 569–583. https://doi. org/10.11646/zootaxa.5093.5.6 Stephensen K (1932) Some New Amphipods from Japan. Annotationes Zoologicae Japonenses 13: 487–501. Stout VR (1913) Studies in Laguna Amphipoda. Zoologische Jahrbücher. Abteilung für Systematik, Geographie und Biologie der Tiere 34: 633–659. https://doi.org/10.5962/ bhl.part.19898 Tamura K, Stecher G, Kumar S (2021) MEGA11: Molecular Evolutionary Genetics Analysis Version 11. Molecular Biology and Evolution 38(7): 3022–3027. https://doi. org/10.1093/molbev/msab120 Thomas JD, Barnard JL (1988) Elasmopus balkomanus, a new species from the Florida Keys (Crustacea, Amphipoda). Proceedings of the Biological Society of Washington 10(4): 838–842. Tomikawa K, Kobayashi N, Kyono M, Ishimaru SI, Grygier MJ (2014) Description of a new species of Sternomoera (Crustacea: Amphipoda: Pontogeneiidae) from Japan, with an analysis of the phylogenetic relationships among the Japanese species based on the 28S rRNA gene. Zoological Science 31(7): 475–490. https://doi.org/10.2108/zs140026 Vader W, Krapp-Schickel T (2012) On some maerid and melitid material (Crustacea: Amphipoda) collected by the Hourglass Cruises (Florida). Part 2: Genera Dulichiella and Elasmopus, with a key to world Elasmopus. Journal of Natural History 46(19–20): 1179–1218. https://doi.org/10.1080/00222933.2011.652984 Walker AO (1904) Report on the Amphipoda collected by Professor Herdman, at Ceylon, in 1902. Report to the Government of Ceylon on the Pearl Oyster Fisheries of the Gulf of Manaar, with supplementary reports upon the marine biology of Ceylon. Part II. The Royal Society, London, 229–300. Yoshimura H, Tomikawa K (2025) A new species of the genus Elasmopus (Crustacea: Amphipoda: Maeridae), from Honshu, Japan. Zootaxa 5653(2): 286–296. https://doi. org/10.11646/zootaxa.5653.2.8 310 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan MEGA 11 (Tamura et al. 2021). The DNA sequence has been deposited with the International Nucleotide Sequence Database (INSD) through the DNA Data Bank of Japan (DDBJ). The INSD accession numbers are LC872568–LC872582 (Table 2). Genetic distances Genetic distances within and among E. koreanus, E. lumbiniger sp. nov., E. mukuinu, E. nkjaf, E. projectus, and E. rapax were calculated. Nucleotide sequences of E. nkjaf, E. projectus, and E. rapax were obtained from NCBI database (https://www.ncbi.nlm.nih.gov/) (Table 2). Sequence alignment was performed using the MUSCLE algorithm implemented in MEGA 11. Genetic divergences were estimated based on uncorrected pairwise distance (p-distance) and Kimura’s two-parameter model (K2P) (Kimura 1980). Results Systematics Family Maeridae Krapp-Schickel, 2008 Genus Elasmopus Costa, 1853 Table 2. Specimen details and GenBank accession numbers used in the present study. Newly obtained sequences are indicated by an asterisk. Species Sample number Locality Coordinates (decimal degrees) GenBank accession number E. koreanus NSMT-Cr 33060 Shirahama, Chiba, Japan 34.9031°N, 139.8862°E *LC872568 NSMT-Cr 33062 Uradome, Tottori, Japan 35.5915°N, 134.3220°E *LC872569 NNIBRIV137288 Songjeonggudeokpo-gil, Busan, South Korea 35.1682°N, 129.1975°E *LC872570 E. lumbiniger sp. nov. NSMT-Cr 33064 Kannonzaki Coast, Kanagawa, Japan 35.2592°N, 139.7436°E *LC872571 NSMT-Cr 33072 Arasaki Coast, Kanagawa, Japan 35.1947°N, 139.5996°E *LC872572 NSMT-Cr 33073 Uchiura Beach, Chiba, Japan 35.1262°N, 140.1893°E *LC872573 NSMT-Cr 33074 Shirahama, Chiba, Japan 34.9031°N, 139.8862°E *LC872574 NSMT-Cr 33076 Takurazaki, Wakayama, Japan 34.2671°N, 135.0605°E *LC872575 NSMT-Cr 33078 Uradome, Tottori, Japan 35.5921°N, 134.3198°E *LC872576 E. mukuinu NSMT-Cr 33079 Okinawa Island, Okinawa, Japan 26.3324°N, 127.9222°E *LC872577 NSMT-Cr 33080 Ono Beach, Shizuoka, Japan 34.6329°N, 138.8975°E *LC872578 NSMT-Cr 33082 Shikinejima Island, Tokyo, Japan 34.3307°N, 139.2083°E *LC872579 NSMT-Cr 33084 Hachijôjima Island, Tokyo, Japan 33.1007°N, 139.7696°E *LC872580 E. nkjaf KUZ Z1864 Miyako Island, Okinawa, Japan 24.7997°N, 125.3342°E LC215812 KUZ Z1862 Miyako Island, Okinawa, Japan 24.7997°N, 125.3342°E LC215813 E. projectus NSMT-Cr 32973 Kannonzaki Coast, Kanagawa, Japan 35.2592°N, 139.7436°E LC851065 NSMT-Cr 32974 Kannonzaki Coast, Kanagawa, Japan 35.2592°N, 139.7436°E *LC872581 NSMT-Cr 32980 Uradome, Tottori, Japan 35.5921°N, 134.3198°E *LC872582 E. rapax SFAM20-013 Somewhere along the west coast of Portugal -KF369125 SFC18-003 Viana do Castelo, Minho, Portugal 41.70°N, 8.85°W KX224027 SFAM12-002 Viana do Castelo, Minho, Portugal 41.70°N, 8.85°W KX224028 SFAM20-001 Viana do Castelo, Minho, Portugal 41.70°N, 8.85°W KX224029 SFAM20-002 Viana do Castelo, Minho, Portugal 41.70°N, 8.85°W KX224030 SFAM20-003 Viana do Castelo, Minho, Portugal 41.70°N, 8.85°W KX224031 311 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Elasmopus koreanus Kim & Kim, 1991 Figs 2–5 [New Japanese name: Kourai-iso-yokoebi] Elasmopus koreanus Kim & Kim, 1991: 329, figs 6, 7. Material examined. • Male, 10.1 mm (NSMT-Cr 33059), Kannonzaki Coast, Yokosuka, Kanagawa, Japan (35.2590°N, 139.7429°E) (Fig. 1A), shallow subtidal zone, rocky shore, among holdfast of the brown alga Sargassum fusiforme, 1 March 2022, H. Yoshimura coll.; • male, 8.3 mm (NSMT-Cr 33060), Shirahama, Minamiboso, Chiba, Japan (34.9031°N, 139.8862°E) (Fig. 1D), intertidal zone, rocky shore, 5 June 2023, H. Ogawa coll.; • male, 8.3 mm (NSMT-Cr 33061), Oura Beach, Shikinejima, Tokyo, Japan (34.3307°N, 139.2083°E) (Fig. 1F), shallow subtidal zone, rocky shore, among the red algae, 3 August 2024, H. Yoshimura coll.; • male, 6.4 mm (NSMT-Cr 33062: Fig. 2A), ovigerous female, 6.2 mm (NSMT-Cr 33063: Fig. 2B), Uradome Beach, Iwami, Tottori, Japan (35.5915°N, 134.3220°E) (Fig. 1H), shallow subtidal zone, rocky shore, among holdfast of the red alga Hypnea sp., 16 June 2024, H. Ogawa, Y. Mukaida, M. Ooga, and H. Yoshimura coll.; • male, 5.0 mm (NNIBRIV137288), Songjeonggudeokpo-gil, Haeundae-gu, Busan, South Korea (35.1682°N, 129.1975°E) (Fig. 1K), tide pool, rocky shore, among the brown alga Sargassum thunbergii, 26 September 2024, H. Yoshimura, C.W. Lee and K. Tomikawa coll. Type locality. Sadong, Ulleungdo Island, South Korea (37.472°N, 130.888°E; assumed from Kim and Kim 1991: fig. 1). Diagnosis. Epimeral plate 3 posteroventral corner without distinct projection. Antenna 1 peduncles bearing slender setae on lateral and medial margins, with bi-articulate accessory flagellum. Antenna 2 flagellum articles distinctly broader than long. Mandibular palp article 3 short. Male gnathopod 2 propodus posterior margin with a few clusters of slender setae, palmar margin with subtriangular process near hinge of dactylus, mid-palmar margin with subrectangular hump. Length of setae on coxae 2 and 3 shorter than width of each coxa. Pereopods 5–7 basis posterior margin smooth, without long setae. Telson cleft, border than long; lobe trapezoid, with shallow incision at apex, each lobe bearing one plumose and two or three robust setae apically. Description of male (NSMT-Cr 33062). Body (Fig. 2A): smooth, not carinate, with a few short setae dorsally on pereon and abdomen. Head: eyes oval; lateral cephalic lobe broad, anteroventral margin with notch; dorsal surface with a few short setae. Epimeral plate 3 (Fig. 3A): ventral margin with seven robust setae, posterior margin with five short setae, posteroventral corner without distinct projection. Antenna 1: length 0.6 × body length; peduncular articles 1–3 with slender setae on lateral and medial margins; accessory flagellum (Fig. 3B) bi-articulate, reaching 2/3 of primary flagellar article 1, accessory flagellar article 2 tiny; primary flagellum 21-articulate, with slender setae. Antenna 2 (Fig. 3C): length 0.4 × antenna 1; peduncular articles 4 and 5 almost same length, with slender setae on lateral and medial margins; flagellum 8-articulate, articles 1–5 shorter than wide, with slender setae. Mandible (Fig. 3D, E) with left and right incisors with two and four teeth, respectively; both left and right lacinia mobilis with five teeth; accessory setal row consisting of four setae on each of left and right mandibles; molar process well developed, triturative; palp well developed, tri-articulate; palp article 1 without setae, article 2 with six setae, article 3 falcate, length 2.5 × width. 312 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Figure 2. Elasmopus koreanus. A. Male, 6.4 mm (NSMT-Cr 33062), lateral view; B. Female, 6.2 mm (NSMT-Cr 33063), lateral view. Scale bar: 1.0 mm. Figure 3. Elasmopus koreanus, male, 6.4 mm (NSMT-Cr 33062). A. Left epimeral plate 3, lateral view; B. Right accessory flagellum, medial view; C. Left antenna 2, dorsal view; D. Incisor, lacinia mobilis, accessory setal row and molar process of left mandible, medial view; E. Right mandible, medial view; F. Telson, dorsal view. Scale bars: 0.3 mm (A, C); 0.1 mm (B, D–F). 313 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Gnathopod 1 (Fig. 4A): subchelate; coxa anteroventral corner weakly produced, ventral margin with long and short setae; basis with long setae on posterior margin and medial surface; carpus subequal in length to propodus, with slender seta on anterodistal corner, with dense setae on posterior margin and medial surface; propodus with three and six clusters of setae on anterior margin and medial surface, respectively, posterior margin with row of slender and robust setae, palmar margin almost transverse, minutely serrate, with rows of robust setae on medial and lateral palmar submargins. Gnathopod 2 (Figs 4B, 5B): subchelate; coxa with long and short setae on ventral margin, length of the longest seta 0.6 × width of coxa; basis with long setae on posterior margin; carpus with dense setae on posterior margin; propodus with four setae on anterolateral submargin, posterior margin with nine clusters of slender setae, palmar margin with subrectangular hump at midpoint and robust seta at proximal end where tip of dactylus contacts, lateral palmar margin with subtriangular hump near hinge of dactylus. Telson (Fig. 3F): broader than long, cleft for 60% of length; lobe trapezoid, with shallow incision at apex, each lobe bearing one plumose and three robust setae apically, lateral margin of each lobe with none or one plumose setae. Figure 4. Elasmopus koreanus, A, B. Male, 6.4 mm (NSMT-Cr 33062); C, D. Female, 6.2 mm (NSMT-Cr 33063). A. Right gnathopod 1, medial view; B. Left gnathopod 2, lateral view; C. Right gnathopod 1, medial view; D. Right gnathopod 2, medial view. Scale bars: 0.3 mm. 314 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Description of female (NSMT-Cr 33063), sexually dimorphic characters. Gnathopod 1 (Fig. 4C): propodus anterior margin with one single and three clusters of setae, medial surface with five clusters of setae. Gnathopod 2 (Fig. 4D): propodus with nine and eight clusters of setae on anteromedial margin and medial surface, respectively, palmar margin almost straight, with minutely Figure 5. Elasmopus koreanus, ontogenetic morphological change in male gnathopod 2, merus to dactylus of right gnathopod 2, medial view. A. Male, 10.1 mm (NSMT-Cr 33059); B. Male, 6.4 mm (NSMT-Cr 33062); C. Male, 5.0 mm (NNIBRIV137288). Scale bars: 0.3 mm. 315 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan serrate on the distal one third of the palmar margin. Oostegites on gnathopod 2 and pereopods 3–5, slender with long setae. Variations. Mandible: left and right incisors with two or three and three or four teeth, respectively; lacinia mobilis with four or five teeth. Gnathopod 2: length of setae on coxa 0.4–0.9 × width of coxa. Pereopod 3: length of setae on coxa 0.6–0.9 × width of coxa. Uropod 3: the length-to-width ratio of inner ramus 1.5–2.5. Telson: each lobe bearing two or three robust setae apically. As body length increases, male gnathopod 2 shows morphological changes (Fig. 5A–C): in adult males (features of immature males in parentheses), propodus elongate (semioval), hump on palmar margin strongly produced (weakly produced), mid-palmar margin with long ridge (without ridge), posterior margin without robust seta (with a robust seta at distal end of posterior margin). Coloration in life. Eyes black; antennae 1 and 2 peduncular articles brown, flagellum articles have two black patterns on a white background; body laterally brown, dorsally black, with small white dot patterns on head, pereon, and abdomen. Males and females have similar body coloration. Distribution. South Korea: Busan (present study); Ulleungdo Island in Gyeongsangbuk-do (Kim and Kim 1991; Shin et al. 2025); Chujado Islands in Jeju-do (Kim et al. 2023). Japan: Chiba, Kanagawa, and Tottori; Shikinejima Island in Tokyo (present study) (Fig. 1). Remarks. Our specimens obtained from South Korea and Japan are well identical to the original description of Elasmopus koreanus by Kim and Kim (1991). A comparison of E. koreanus from Japan and South Korea showed no remarkable morphological differences other than their ontogenetic changes. The intraspecific genetic distances of COI between Japanese and Korean specimens of this species were at most 0.024 (p-distance) and 0.025 (K2P), respectively (Tables 3, 4). These values were not particularly large compared to the intraspecific genetic distances of the other Elasmopus species examined in this study (Tables 3, 4). Therefore, we concluded that E. koreanus from Japan and South Korea should be regarded as conspecific, since they show no significant morphological or genetic differences that would support their separation as distinct species. Elasmopus koreanus is most similar to E. mutatus Barnard, 1962, with the following five features: 1) epimeral plate 3 posteroventral corner without distinct projection; 2) male gnathopod 2 propodus mid-palmar margin with subrectangular hump; 3) male gnathopod 2 propodus medial surface without dense setae; 4) pereopods 5–7 basis smooth, without long setae; and 5) telson broader than long. Kim and Kim (1991) distinguished E. koreanus from E. mutatus based on four morphological characters: 1) antennae of E. koreanus bear fewer setae than those of E. mutatus; 2) male gnathopod 2 palmar margin ridge where tip of dactylus contacts lacks a robust seta in E. koreanus, whereas it bears a robust seta in E. mutatus; 3) posterior margin of male gnathopod 2 propodus covered with fewer setae in E. koreanus than in E. mutatus; and 4) uropod 3 inner ramus broader in E. koreanus than in E. mutatus. However, all examined male specimens in the present study possessed a robust seta on the palm of the gnathopod 2 at the point where the tip of the dactylus contacts. This suggests that either Kim and Kim (1991) overlooked this robust seta, or that the presence or absence of this robust seta represents intraspecific variation in E. koreanus. Therefore, the presence or absence of this robust seta is not a diagnostic character distinguishing E. koreanus from E. mutatus. Additionally, regarding the 322 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Figure 11. Elasmopus lumbiniger sp. nov. A–D. Holotype male, 9.8 mm (NSMT-Cr 33064); E–I. Paratype female, 8.7 mm (NSMT-Cr 33068). A. Left uropod 1, lateral view; B. Left uropod 2, lateral view; C. Right uropod 3, medial view; D. Telson, dorsal view; E. Right gnathopod 1, medial view; F. Palmar margin of right gnathopod 1 propodus, medial view; G. Left gnathopod2, lateral view, coxal gill and oostegite omitted; H. Palmar margin of left gnathopod 2 propodus, lateral view; I. Left pereopod 3, lateral view. Scale bars: 0.3 mm (A–C); 0.1 mm (D, F, H); 0.5 mm (E, G, I). 323 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan peduncular article 1 with three robust setae on posteroproximal margin, robust seta on posterodistal corner; accessory flagellum tri-articulate, accessory flagellar article 3 tiny; primary flagellum 34-articulate, with slender setae. Antenna 2 (Fig. 8B): length 0.5 × antenna 1; peduncular article 2 with six short spines on anterodistal corner, gland cone of peduncular article 2 exceeding distal margin of peduncular article 3; peduncular article 4 slightly longer than 5, with slender setae on lateral and medial margins; flagellum 13-articulate, with slender setae. Mouthparts. As holotype upper lip missing, that of paratype (male, 9.9 mm, NSMT-Cr 33066) illustrated. Upper lip (Fig. 8C) with rounded anterior margin, bearing fine setae. Mandible (Fig. 8D–F) with left and right incisors two and four teeth, respectively; left lacinia mobilis five teeth and right lacinia mobilis four teeth; accessory setal row three setae on the left and four setae on the right; molar process well developed, triturative; palp (Fig. 8F) well developed, tri-articulate; palp article 1 without setae, article 2 with ten setae, article 3 falcate, length 3.0 × width. Lower lip (Fig. 8G) with outer lobes laterally expanded, bearing apical setae; inner lobes ovate, apically covered with setae. Maxilla 1 (Fig. 8H) with inner plate bearing two plumose setae apically; outer plate with seven robust dentate setae; palp bi-articulate, article 1 marginally bare, article 2 with numerous apical and subapical setae, outer margin bare. Maxilla 2 (Fig. 8I) with inner plate bearing long apical setae, with setae on outer and inner margins; outer plate bearing long apical setae, with setae on outer margin. Maxilliped (Fig. 8J) with inner plate bearing plumose setae apically; outer plate obovate, with spatula plumose setae on apical and inner margins, reaching more than half of palp article 2; palp article 1 with two setae on inner margin, with seta on outer margin; palp article 2 slender, lined with setae on inner margin, outer margin bare; nail of palp article 4 distinct. Gnathopod 1 (Fig. 9A–D): subchelate; coxa anteroventral corner weakly produced, anterior margin weakly concave, ventral margin with long and short setae; basis with long setae on posterior margin and medial surface; carpus subequal in length to propodus, with slender setae on anterodistal corner, with dense pinnate sided setulated setae on medial surface and posterior margin; propodus with six clusters of setae on anterior margin, with seven clusters of setae on medial surface; palmar margin (Fig. 9C, D) almost transverse, minutely serrate, with Figure 12. Elasmopus lumbiniger sp. nov., paratype male, 13.8 mm (NSMT-Cr 33073). Abdomen, lateral view. Scale bar: 0.5 mm. 324 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan rows of robust setae on medial and lateral palmar submargins, palmar corner with two long robust setae; dactylus with slender seta on anteroproximal margin, with six small setae on lateral margin, with spatulate seta and slender seta laterally at the base of nail. Gnathopod 2 (Fig. 9E–G): subchelate; coxa with long and short setae on ventral margin, of which two long setae retrorse; basis with long setae on posterior margin; merus distoventral corner subquadrate bearing long setae; carpus with slender setae on anterodistal corner, length of these setae 0.9 × width of carpus, with dense setae on posterior margin; propodus with five clusters, two pairs and two single setae on anterolateral submargin, length of these setae 0.4 × width of propodus, anteromedial submargin with eight clusters of setae; lateral palmar margin weakly concave with trapezoidal hump (Fig. 9G) on near insertion of dactylus, with one clusters, three pairs and one single setae on lateral surface, with six and seven robust setae on lateral and medial margins, respectively; mid-palmar margin with ridge; posterior margin with row of plumose setae, the proximal seta longer than the distal seta, length of the longest seta 0.9 × width of propodus; dactylus strongly curved, subacute, length 0.6 × length of propodus, with seta on anteroproximal margin. Pereopod 3 (Fig. 10A): coxa longer than broad, ventral margin with numerous short and a few long setae, of which long seta retrorse; posterior margins of basis with long slender and robust setae, merus with slender setae, carpus and propodus with robust and slender setae; dactylus with plumose seta on anteroproximal margin, with one robust and two slender setae at the base of nail. Pereopod 4 (Fig. 10B): coxa expanded with posterior concavity, ventral margin arched bearing numerous short and a few long setae, of which long seta retrorse; posterior margins of basis with long slender and robust setae, merus with slender setae, carpus and propodus with robust and slender setae; dactylus with plumose seta on anteroproximal margin, with one robust and two slender setae at the base of nail. Pereopod 5 (Fig. 10C): coxa bilobed, ventral margin of posterior lobe with two robust setae, posterior margin of posterior lobe with five short setae; basis broad, subrectangular, posterior margin smooth, with numerous short setae; anterior and posterior margins of carpus with robust setae; propodus with robust setae on anterior margin, with one cluster setae on posteromedial submargin; dactylus with plumose seta on posteroproximal margin, with one robust and two slender setae at the base of nail. Pereopod 6 (Fig. 10D): coxa bilobed, anterior margin of anterior lobe with slender setae, ventral margin of posterior lobe with three robust setae, posterior margin of posterior lobe with five short setae; basis weakly expanded proximally, tapering distally, posterior margin smooth with numerous short setae; anterior margin of carpus with robust and slender setae, posterior margin with robust setae; anterior margin of propodus with robust setae, with two clusters setae on posteromedial submargin; dactylus with plumose seta on posteroproximal margin, with one robust seta and two slender setae at the base of nail. Pereopod 7 (Fig. 10E): coxa rounded, with slender setae on anterior and short setae on posterior margins; basis expanded, subovate, posterior margin rounded, smooth, with numerous short setae; anterior and posterior margins of carpus with robust and slender setae; anterior margin of propodus with robust and slender setae, with three clusters setae on posteromedial submargin; dactylus with plumose seta on posteroproximal margin, with one robust and two slender setae at the base of nail. Coxal gills on gnathopod 2 and pereopods 3–6. 325 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Pleopods (Fig. 10F–K): peduncles with long setae, inner distal corner of peduncle with two retinacula (Fig. 10G, I, K), peduncle of pleopod 3 broader than pleopod 1 and 2. Uropods. Uropod 1 (Fig. 11A): peduncle length 1.2 × length of outer ramus, with six dorsolateral, seven dorsomedial and one basofacial robust setae; outer ramus slightly shorter than inner ramus, medial and lateral margins each with four robust setae, distal part with four robust setae; inner ramus medial and lateral margins each with four robust setae, distal part with five robust setae. Uropod 2 (Fig. 11B): peduncle length 0.9 × length of outer ramus; outer ramus slightly shorter than inner ramus, with three and five robust setae on medial and lateral margins, respectively, distal part with four robust setae; inner ramus five and four robust setae on medial and lateral margins, respectively, distal part with five robust setae. Uropod 3 (Fig. 11C): peduncle length 0.8 × length of outer ramus, with pair of robust setae on outer margin; rami distally truncated; outer ramus length 1.5 × length of inner ramus, with three and one clusters of robust setae on lateral and distal margins, respectively, with robust seta and one pair of robust setae on medial margin and surface, respectively; inner ramus with cluster of robust setae distally and two robust setae on medial margin. Telson (Fig. 11D): broader than long, cleft for 75% of length; lobe trapezoid, with shallow incision at apex, each lobe bearing one plumose five robust setae apically, lateral margin of each lobe with one or two plumose setae. Description of female (paratype, NSMT-Cr 33068), sexually dimorphic characters. Gnathopod 1 (Fig. 11E, F): propodus anterior margin with 1 single and 4 clusters of setae, medial surface with 6 clusters of setae. Gnathopod 2 (Fig. 11G, H): slender setae on carpus anterodistal corner reaching width of carpus; propodus with two clusters, three pairs, and two2 single setae on anterolateral submargin, length of these setae 0.7 × width of propodus, anteromedial submargin with 13 clusters; palmar margin (Fig. 11H) almost straight, with minutely serrate on the distal 1/5 of the palmar margin. Pereopod 3 (Fig. 11I): basis with slender setae on anterior margin, the longest seta 0.8 × width of basis. Oostegites on gnathopod 2 and pereopods 3–5, slender with long setae. Variations. Mandible: left and right incisors with two or three and three or four teeth, respectively; lacinia mobilis with four or five teeth. Gnathopod 2: length of setae on carpus anterodistal corner 0.8–1.1 and 0.9–1.1 × width of carpus in male and female, length of setae on propodus anterolateral submargin 0.3–0.7 and 0.6–0.7 × width of propodus in male and female, respectively. Telson: each lobe bearing 4–6 robust setae apically. Large male > 10 mm bearing slender setae on ventral margins of epimeral plates 2 and 3, and urosomite 1 (Fig. 12). Coloration in life. Male: eyes black; antenna 1 peduncular articles brown, flagellum with two patterns, brown proximally and black distally, on a white background; body color varies among individuals ranging from reddish brown to brown or black, with small white dot patterns on head, pereon, abdomen, and coxae, with one dark black band along dorsal midline of pleonite 3 and urosomite 1. Female: body black, without distinct black band on dorsal midline of abdomen. Distribution. Japan: Chiba, Kanagawa, Wakayama, and Tottori (present study) (Fig. 1). Ecological note. This new species was found among brown and red algae or under stones in rocky shore. E. lumbiniger sp. nov. and E. projectus were 326 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan collected from the same individual algae holdfasts at two sites: Kannonzaki Coast (Fig. 1A) and Uradome Beach (Fig. 1H). Etymology. The specific name lumbiniger, from the Latin lumbus (= loin, lower back) and niger (= black), refers to the dark coloration along the dorsal midline of male pleonite 3 and urosomite 1 in life (Fig. 6A). The Japanese name is derived from this band, which resembles a vein running along the back of a shrimp, known as “Sewata”. Remarks. Elasmopus lumbiniger sp. nov. is morphologically similar to E. antennatus (Stout, 1913), E. balkomanus Thomas & Barnard, 1988, and E. lemaitrei Ortiz & Lalana, 1994, in having the following features: 1) epimeral plate 3 posteroventral corner produced into well-developed acute spine; 2) male gnathopod 2 propodus palm with dense slender setae; 3) male gnathopod 2 mid-palmar margin with ridge; 4) male gnathopod 2 propodus with hump on near insertion of dactylus; 5) pereopods 5–7 basis posterior margin smooth, without long setae; and 6) telson broader than long. However, E. lumbiniger sp. nov. can be distinguished from the above three species by length of the longest seta on the anterodistal corner of male gnathopod 2 carpus 0.8–1.1 × width of carpus (less than 0.2 in these three species). From E. antennatus, this new species is distinguishable by the following two characters in addition (features of E. antennatus in parentheses): 1) hump on propodus of male gnathopod 2 trapezoidal, produced (semioval, weekly produced); and 2) the length of setae on anterior margin of pereopods 5–7 merus exceeding half the width of each merus (less than half of the width of each merus). Elasmopus lumbiniger sp. nov. can be further distinguished from E. balkomanus and E. lemaitrei by the following two additional features (features of E. balkomanus and E. lemaitrei in parentheses): 1) ridge on mid-palmar margin of male gnathopod 2 propodus not serrate (serrate); and 2) the length of setae on posterior margin of pereopod 7 carpus almost equal to that on anterior margin (more than twice that on anterior margin). In this study, we calculated the genetic distances of COI among five Elasmopus species recorded in East Asia and Elasmopus lumbiniger sp. nov. E. lumbiniger sp. nov. differs from the other five Elasmopus species by large genetic distances (0.158–0.205 in p-distance; 0.181–0.244 in K2P), which were larger than the intraspecific distances among the Elasmopus species examined in this study (Tables 3, 4). In addition, E. lumbiniger sp. nov. is morphologically distinguished from its congeners. Therefore, it can be concluded that E. lumbiniger sp. nov. is a distinct new species. Elasmopus mukuinu Sir & White, 2022 Figs 13–15 [New Japanese name: Mukuinu-iso-yokoebi] Elasmopus mukuinu Sir & White, 2022: 574, figs 1C, D, 4–9. Material examined. • Male, 9.3 mm (NSMT-Cr 33079: Fig. 13A), Kaichu Doro, Uruma, Okinawa, Japan (26.3324°N, 127.9222°E) (Fig. 1J), intertidal zone, coral beach, among the green alga Ulva sp., 14 March 2025, H. Yoshimura coll.; • ovigerous female, 7.5 mm (NSMT-Cr 33080), Ono Beach, Shimoda, Shizuoka, Japan (34.6329°N, 138.8975°E) (Fig. 1E), shallow subtidal zone, rocky shore, under 327 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan the stone, 24 February 2024, H. Yoshimura coll.; • male, 8.7, 7.4 mm (NSMT-Cr 33081: Fig. 13B, NSMT-Cr 33082), ovigerous female, 5.1 mm (NSMT-Cr 33083: Fig. 13C), Oura Beach, Shikinejima, Tokyo, Japan (34.3307°N, 139.2083°E) (Fig. 1F), shallow subtidal zone, rocky shore, under the stone, 3 August 2024, H. Yoshimura coll.; • male, 14.6, 7.9, 6.5 mm (NSMT-Cr 33084–33086), Figure 13. Elasmopus mukuinu. A. Male, 9.3 mm (NSMT-Cr 33079), lateral view; B. Male, 8.7 mm (NSMT-Cr 33081), lateral view; C. Female, 5.1 mm (NSMT-Cr 33083), lateral view. Scale bars: 1.0 mm. 328 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Yakengahama Beach, Hachijojima, Tokyo, Japan (33.1007°N, 139.7696°E) (Fig. 1G), rocky shore, 20 December 2022, K. Tomikawa coll. Type locality. Kaichu Doro, Okinawa Island, Japan (26.3321°N, 127.9146°E). Diagnosis. Epimeral plate 3 posterior margin weakly serrated. Antenna 1 peduncles bearing slender setae on lateral and medial margins, with bior tri-articulate accessory flagellum. Mandibular palp article 3 short. Gnathopod 2 carpus anterodistal corner with slender setae, length of the longest seta reaching 0.3–0.6 and 0.4–0.6 × width of carpus in male and female. Male gnathopod 2 propodus posterior margin and inner surface with dense slender setae, length of the longest seta 0.9 × width of propodus, palmar margin with semicircular hump near hinge of dactylus, mid-palmer margin with ridge. Pereopods 5–7 basis posterior margin smooth, without long setae. Telson cleft, border than long; lobe trapezoid, with shallow incision at apex, inner apical corner rounded and produced, each lobe bearing one plumose and one or two robust setae apically. Description of male (NSMT-Cr 33081). Body (Fig. 13B): smooth, not carinate, with a few short setae dorsally on pereon and abdomen. Head: eyes oval; lateral cephalic lobe broad, anteroventral margin with notch; dorsal surface with a few short setae. Epimeral plate 3 (Fig. 14A): ventral margin with two pairs and four single robust setae, posterior margin weakly serrated, with four short setae. Antenna 1: length 0.6 × body length; peduncular articles 1–3 with slender setae on lateral and medial margins; accessory flagellum (Fig. 14B) tri-articulate, reaching to distal margin of primary flagellar article 1, accessory flagellar article 3 tiny; primary flagellum 28-articulate, with slender setae. Figure 14. Elasmopus mukuinu. A–F. Male, 8.7 mm (NSMT-Cr 33081); G. Female, 5.1 mm (NSMT-Cr 33083). A. Right epimeral plate 3, lateral view; B. Right accessory flagellum, medial view; C, D. Incisor, lacinia mobilis, accessory setal row and molar process of left and right mandible, medial view; E. Right mandibular palp articles 2, 3, medial view; F. Telson, dorsal view; G. Right accessory flagellum, medial view. Scale bars: 0.3 mm (A); 0.1 mm (B–G). 329 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Mandible (Fig. 14C–E) with left and right incisors three and four teeth, respectively; left and right lacinia mobilis three and four teeth, respectively; accessory setal row consisting of four setae on each of left and right mandibles; molar process well developed, triturative; palp well developed, tri-articulate; article 2 with seven setae, article 3 falcate, length 3.2 × width. Gnathopod 1 (Fig. 15A): subchelate; coxa anteroventral corner weakly produced; basis with long setae on posterior margin and medial surface; carpus subequal in length to propodus, with slender seta on anterodistal corner, with dense setae on posterior margin and medial surface; propodus with four and five clusters of setae on anterior margin and medial surface, respectively, posterior margin with row of slender and robust setae, palmar margin almost transverse, minutely serrate, with rows of robust setae on medial and lateral palmar submargins. Gnathopod 2 (Fig. 15B): subchelate; basis with long setae on posterior margin; carpus with slender setae on anterodistal corner, length of these setae reaching 0.6 × width of carpus, with dense setae on posterior margin; propodus with one cluster, one pair and three single setae on anterolateral submargin, length of these setae reaching 0.2 × width of propodus, with nine clusters of setae on anteromedial submargin, lateral palmar margin with hump on near insertion of dactylus, mid-palmar margin with ridge, posterior margin with row of plumose setae, the proximal seta longer than the distal seta, length of the longest seta reaching 0.9 × width of propodus. Telson (Fig. 13F): broader than long, cleft for 65% of length; lobe trapezoid, with shallow incision at apex, inner apical corner rounded and produced, each Figure 15. Elasmopus mukuinu. A, B. Male, 8.7 mm (NSMT-Cr 33081); C, D. Female, 5.1 mm (NSMT-Cr 33083). A. Right gnathopod 1, medial view; B. Right gnathopod 2, medial view; C. Right gnathopod 1, medial view; D. Right gnathopod 2, medial view. Scale bars: 0.3 mm. 330 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan lobe bearing one plumose and two robust setae apically, lateral margin of each lobe with one plumose seta. Description of female (NSMT-Cr 33083), sexually dimorphic characters. Antenna 1: accessory flagellum (Fig. 14G) bi-articulate. Gnathopod 1 (Fig. 15C): propodus anterior margin with three clusters of setae, medial surface with five clusters of setae. Gnathopod 2 (Fig. 15D): slender setae on carpus anterodistal corner 0.6 × width of carpus; propodus with four slender setae on anterolateral submargin, the longest setae 0.4 × width of propodus, with eight and ten clusters of setae on anteromedial margin and medial surface, respectively; palmar margin almost straight, with minutely serrate on the distal 2/5 of the palmar margin. Variations. Mandible: both incisors with 2–4 teeth; right lacinia mobilis with four or five teeth. Gnathopod 2: length of setae on carpus anterodistal corner 0.3–0.6 and 0.4–0.6 × width of carpus in male and female, length of setae on propodus anterolateral submargin 0.2–0.3 and 0.3–0.5 × width of propodus in male and female, respectively. Telson: each lobe bearing one or two robust setae apically. Coloration in life. Eyes black; antennae 1 and 2 peduncular articles brown, flagellum articles have two black patterns on a white background; body translucent white, laterally brown, with small white dot patterns on head, pereon, and abdomen, with orange pattern on epimeral plates. Male and female have similar body coloration. Distribution. Japan: Shizuoka, Izu Islands (Shikinejima Island and Hachijojima Island), Tokyo (present study); Okinawa Island, Okinawa (Sir and White 2022; present study) (Fig. 1). Remarks. The examined specimens agreed well with the original description of E. mukuinu by Sir and White (2022), but there was regional morphological variation in the number of accessory flagellum articles of antenna 1. In the male from Okinawa Island (Fig. 1J) and females from Shizuoka (Fig. 1E) and Shikinejima Island (Fig. 1F), the accessory flagellum consisted of two articles, consistent with the original description (like Fig. 14G). In contrast, males from Shikinejima Island and Hachijojima Island (Fig. 1G) possessed a tri-articulate accessory flagellum (like Fig. 14B). Therefore, the number of accessory flagellum articles in males differed between localities: E. mukuinu from Okinawa Island had two articles, whereas those from the Izu Islands (Shikinejima and Hachijojima Islands) had three. The genetic distance of COI between E. mukuinu from Okinawa Island and those from Shizuoka and the Izu Islands was 0.050 (p-distance) and 0.052 (K2P), respectively, representing the largest intraspecific genetic distance among the Elasmopus species examined in this study (Tables 3, 4). However, we believed that these morphological and genetic differences were not sufficient to support separation into distinct species, and we concluded that these specimens should be regarded as conspecific. Further examination of E. mukuinu specimens from other regions will help clarify the taxonomic status of local populations of this species. Genetic distances among Elasmopus species The intraand interspecific distances in Elasmopus were calculated using p-distance (Table 3) and K2P distance (Table 4). The maximum intraspecific distances were 0.050 (p-distance) and 0.052 (K2P), and the minimum interspecific distances were 0.158 (p-distance) and 0.181 (K2P). 331 ZooKeys 1261: 307–335 (2025), DOI: 10.3897/zookeys.1261.162923 Hiro Yoshimura et al.: Elasmopus diversity and a new species from Japan Key to the species of Elasmopus from East Asia The following dichotomous key excludes E. spinimanus, whose occurrence in East Asia is doubtful. 1 Urosomite 1 with triangular process dorsally ............................ E. japonicus – Urosomite 1 dorsally smooth .......................................................................2 2 Male gnathopod 2 propodus medial surface and palmar margin with few setae ...............................................................................................................3 – Male gnathopod 2 propodus medial surface and palmar margin with dense slender setae ......................................................................................6 3 Antenna 2 flagellum articles anteroposteriorly compressed; pereopods 5–7 bases without long setae on posterior margins ................ E. koreanus – Antenna 2 flagellum articles nearly cylindrical; pereopods 5–7 bases with long setae on posterior margins ..................................................................4 4 Telson shorter than wide ............................................................. E. hooheno – Telson longer than wide ................................................................................ 5 5 Foremost medial tooth on palm of male gnathopod 2 not exceeding palmar margin; posterodistal setae on propodi of pereopods 5–7 longer than length of dactyli ..................................................................................E. rapax – Foremost medial tooth on palm of male gnathopod 2 exceeding palmar margin; posterodistal setae on propodi of pereopods 5–7 shorter than length of dactyli .................................................................................. E. nkjaf 6 Posterior margin of pereopod 6 basis castelloserrate ...............................7 – Posterior margin of pereopod 6 basis smooth ...........................................8 7 Posterior margin of pereopod 7 basis castelloserrate; epimeral plate 3 posteroventral corner with distinct spine .............................E. nanshaensis – Posterior margin of pereopod 7 basis smooth; epimeral plate 3 posteroventral corner notched, without distinct spine ................... E. alkhiranensis 8 Inner margins of pereopods 3–7 dactyli serrate .................E. spinidactylus – Inner margins of pereopods 3–7 dactyli smooth ........................................ 9 9 Male gnathopod 2 carpus with a row of slender setae along anterior margin ...................................................................................... E. spinicarpus – Male gnathopod 2 carpus anterior margin with setae on anterodistal corner only, or on both anterodistal corner and mid-anterior margin ............10 10 Anterior setae on meri of pereopods 5–7 shorter than half width of meri . ......................................................................E. hawaiensis sensu Ren, 2012 – Anterior setae on meri of pereopods 5–7 longer than half width of meri ... ......................................................................................................................11 11 Male gnathopod 2 propodus mid-palmer margin with well-developed process; length of setae on gnathopod 2 carpus anterodistal corner less than 0.2 × width of carpus in male, less than 0.4 × in female; male pereopod 7 merus to propodus without slender setae on posterior margins ... E. projectus – Male gnathopod 2 propodus mid-palmer margin with ridge; length of setae on gnathopod 2 carpus anterodistal corner exceeding 0.3 × width of carpus in male, exceeding 0.4 × in female; male pereopod 7 merus to propodus with slender setae on posterior margins ..................................12 12 Epimeral plate 3 posteroventral corner without distinct projection; length of setae on gnathopod 2 carpus anterodistal corner 0.3–0.6 and 0.4–0.6 ×