• 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.

Sadong, Ulleungdo Island, South Korea (37.472°N, 130.888°E; assumed from Kim and Kim 1991: fig. 1).

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.

(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.

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.

(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 serrate on the distal one third of the palmar margin. Oostegites on gnathopod 2 and pereopods 3–5, slender with long setae.

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).

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.

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).

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 broadness of the uropod 3 inner ramus, the length-to-width ratio in E. koreanus ranged from 1.5 to 2.5. In E. mutatus, this ratio was 2.1, which fell within the intraspecific variation range of E. koreanus. Thus, E. koreanus and E. mutatus cannot be distinguished based on the broadness of the uropod 3 inner ramus. On the other hand, we propose newly found taxonomic features to distinguish the two species: 1) antennae of E. koreanus bear fewer setae than those of E. mutatus; 2) posterior margin of male gnathopod 2 propodus covered with fewer setae in E. koreanus than in E. mutatus; and 3) the longest seta on coxae 2 and 3 shorter than the width of each coxa in E. koreanus, whereas longer in E. mutatus.

Holotype : • male, 9.8 mm (NSMT-Cr 33064), Kannonzaki Coast, Yokosuka, Kanagawa, Japan (35.2592°N, 139.7436°E) (Fig. 1A), shallow subtidal zone, rocky shore, among holdfast of the brown alga Eisenia bicyclis, 4 February 2023, H. Yoshimura coll. Paratypes: • male, 10.4, 9.9, 8.5 mm (NSMT-Cr 33065–33067), ovigerous female, 8.7 mm (NSMT-Cr 33068), same data as for the holotype; • male, 9.6, 7.1 mm (NSMT-Cr 33069, 33070), female, 5.9 mm (NSMT-Cr 33071), Kannonzaki Coast, Yokosuka, Kanagawa, Japan (35.2592°N, 139.7432°E) (Fig. 1A), shallow subtidal zone, rocky shore, among holdfast of the brown alga Sargassum sp., 22 April 2023, H. Yoshimura coll.; • male, 7.0 mm (NSMT-Cr 33072), Arasaki Coast, Yokosuka, Kanagawa, Japan (35.1947°N, 139.5996°E) (Fig. 1B), shallow subtidal zone, rocky shore, among the red alga Corallina sp., 2 September 2023, H. Yoshimura coll.; • male, 13.8 mm (NSMT-Cr 33073: Fig. 6A), Uchiura Beach, Kamogawa, Chiba, Japan (35.1262°N, 140.1893°E) (Fig. 1C), shallow subtidal zone, rocky shore, among the red alga Corallina sp., 18 June 2022, H. Yoshimura coll.; • male, 10.0 mm (NSMT-Cr 33074), Shirahama, Minamiboso, Chiba, Japan (34.9031°N, 139.8862°E) (Fig. 1D), intertidial zone, rocky shore, 5 June 2023, H. Ogawa coll.; • female, 12.6, 10.3 mm (NSMT-Cr 33075: Fig. 6B, NSMT-Cr 33076), Takurazaki, Wakayama, Wakayama, Japan (34.2671°N, 135.0605°E) (Fig. 1I), shallow subtidal zone, rocky shore, under the stone, 10 May 2024, H. Yoshimura coll.; • male, 10.0 mm (NSMT-Cr 33077), female, 7.9 mm (NSMT-Cr 33078), Uradome Beach, Iwami, Tottori, Japan (35.5921°N, 134.3198°E) (Fig. 1H), shallow subtidal zone, rocky shore, among sporophyll of the brown alga Undaria pinnatifida lying on the sea bottom, 16 June 2024, H. Ogawa, Y. Mukaida, M. Ooga, and H. Yoshimura coll.

Epimeral plate 3 posteroventral corner produced into well-developed acute spine. Antenna 1 peduncles bearing slender setae on lateral and medial margins, with tri-articulate accessory flagellum. Mandibular palp article 3 short. Gnathopod 2 carpus anterodistal corner with slender setae, length of the longest seta reaching 0.8–1.1 and 0.9–1.1 × 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 reaching 0.9 × width of propodus, palmar margin with trapezoidal 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, each lobe bearing one plumose and 4–6 robust setae apically.

(holotype, NSMT-Cr 33064). Body (Fig. 7A): 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 plates 1–3 (Fig. 7B–D): each ventral margin with one cluster and one single robust seta, two clusters and one single robust seta, and two clusters and five robust setae, respectively; each posterior margin with three or four, four and four short setae, respectively; posteroventral corner of epimeral plates 1 and 2 weakly produced, epimeral plate 3 produced into well-developed acute spine.

Antenna 1 (Fig. 8A): length 0.55 × body length; peduncular articles 1–3 in length ratio of 1.1: 1.0: 0.6, with slender setae on lateral and medial margin; 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 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.

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.

(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.

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).

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.

Japan: Chiba, Kanagawa, Wakayama, and Tottori (present study) (Fig. 1).

This new species was found among brown and red algae or under stones in rocky shore. E. lumbiniger sp. nov. and E. projectus were collected from the same individual algae holdfasts at two sites: Kannonzaki Coast (Fig. 1A) and Uradome Beach (Fig. 1H).

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”.

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.

• 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 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), Yakengahama Beach, Hachijojima, Tokyo, Japan (33.1007°N, 139.7696°E) (Fig. 1G), rocky shore, 20 December 2022, K. Tomikawa coll.

Kaichu Doro, Okinawa Island, Japan (26.3321°N, 127.9146°E).

Epimeral plate 3 posterior margin weakly serrated. Antenna 1 peduncles bearing slender setae on lateral and medial margins, with bi- or 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.

(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.

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 lobe bearing one plumose and two robust setae apically, lateral margin of each lobe with one plumose seta.

(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.

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.

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.

Japan: Shizuoka, Izu Islands (Shikinejima Island and Hachijojima Island), Tokyo (present study); Okinawa Island, Okinawa (Sir and White 2022; present study) (Fig. 1).

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.