Elasmopus koreanus Kim & Kim, 1991

Elasmopus koreanus Kim & Kim, 1991 View in CoL

Figs 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , 5 [New Japanese name: Kourai-iso-yokoebi View Figure 5 ]

Elasmopus koreanus Kim & Kim, 1991: 329, figs 6, 7. View in CoL

Material examined.

• Male, 10.1 mm ( NSMT -Cr 33059 ), Kannonzaki Coast , Yokosuka, Kanagawa, Japan ( 35.2590°N, 139.7429°E) (Fig. 1 A View Figure 1 ), shallow subtidal zone, rocky shore, among holdfast of the brown alga Sargassum fusiforme , 1 March 2022, H. Yoshimura coll. GoogleMaps ; • male, 8.3 mm ( NSMT -Cr 33060 ), Shirahama , Minamiboso, Chiba, Japan ( 34.9031°N, 139.8862°E) (Fig. 1 D View Figure 1 ), intertidal zone, rocky shore, 5 June 2023, H. Ogawa coll. GoogleMaps ; • male, 8.3 mm ( NSMT -Cr 33061 ), Oura Beach , Shikinejima, Tokyo, Japan ( 34.3307°N, 139.2083°E) (Fig. 1 F View Figure 1 ), shallow subtidal zone, rocky shore, among the red algae, 3 August 2024, H. Yoshimura coll. GoogleMaps ; • male, 6.4 mm ( NSMT -Cr 33062 : Fig. 2 A View Figure 2 ), ovigerous female, 6.2 mm ( NSMT -Cr 33063 : Fig. 2 B View Figure 2 ), Uradome Beach , Iwami, Tottori, Japan ( 35.5915°N, 134.3220°E) (Fig. 1 H View Figure 1 ), 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. GoogleMaps ; • male, 5.0 mm ( NNIBRIV 137288 ), Songjeonggudeokpo-gil , Haeundae-gu, Busan, South Korea ( 35.1682°N, 129.1975°E) (Fig. 1 K View Figure 1 ), tide pool, rocky shore, among the brown alga Sargassum thunbergii , 26 September 2024, H. Yoshimura, C. W. Lee and K. Tomikawa coll. GoogleMaps

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. 2 A View Figure 2 ): 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. 3 A View Figure 3 ): 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. 3 B View Figure 3 ) 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. 3 C View Figure 3 ): 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. 3 D, E View Figure 3 ) 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. 4 A View Figure 4 ): 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 4 B View Figure 4 , 5 B View Figure 5 ): 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. 3 F View Figure 3 ): 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.

Description of female

( NSMT -Cr 33063 ), sexually dimorphic characters. Gnathopod 1 (Fig. 4 C View Figure 4 ): propodus anterior margin with one single and three clusters of setae, medial surface with five clusters of setae. Gnathopod 2 (Fig. 4 D View Figure 4 ): 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.

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. 5 A – C View Figure 5 ): 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 View Figure 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 ( K 2 P), respectively (Tables 3 View Table 3 , 4 View Table 4 ). These values were not particularly large compared to the intraspecific genetic distances of the other Elasmopus species examined in this study (Tables 3 View Table 3 , 4 View Table 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.