Elasmopus mukuinu Sir & White, 2022

Elasmopus mukuinu Sir & White, 2022 View in CoL

Figs 13 View Figure 13 , 14 View Figure 14 , 15 [New Japanese name: Mukuinu-iso-yokoebi View Figure 15 ]

Elasmopus mukuinu Sir & White, 2022: 574, figs 1 C, D, 4–9. View in CoL View Cited Treatment

Material examined.

• Male, 9.3 mm ( NSMT -Cr 33079 : Fig. 13 A View Figure 13 ), Kaichu Doro , Uruma, Okinawa, Japan ( 26.3324°N, 127.9222°E) (Fig. 1 J View Figure 1 ), intertidal zone, coral beach, among the green alga Ulva sp. , 14 March 2025, H. Yoshimura coll. GoogleMaps ; • ovigerous female, 7.5 mm ( NSMT -Cr 33080 ), Ono Beach , Shimoda, Shizuoka, Japan ( 34.6329°N, 138.8975°E) (Fig. 1 E View Figure 1 ), shallow subtidal zone, rocky shore, under the stone, 24 February 2024, H. Yoshimura coll. GoogleMaps ; • male, 8.7, 7.4 mm ( NSMT -Cr 33081 : Fig. 13 B View Figure 13 , NSMT -Cr 33082 ), ovigerous female, 5.1 mm ( NSMT -Cr 33083 : Fig. 13 C View Figure 13 ), Oura Beach , Shikinejima, Tokyo, Japan ( 34.3307°N, 139.2083°E) (Fig. 1 F View Figure 1 ), shallow subtidal zone, rocky shore, under the stone, 3 August 2024, H. Yoshimura coll. GoogleMaps ; • male, 14.6, 7.9, 6.5 mm ( NSMT -Cr 33084 –33086), Yakengahama Beach , Hachijojima, Tokyo, Japan ( 33.1007°N, 139.7696°E) (Fig. 1 G View Figure 1 ), rocky shore, 20 December 2022, K. Tomikawa coll. GoogleMaps

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 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.

Description of male

( NSMT -Cr 33081 ). Body (Fig. 13 B View Figure 13 ): 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. 14 A View Figure 14 ): 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. 14 B View Figure 14 ) 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. 14 C – E View Figure 14 ) 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. 15 A View Figure 15 ): 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. 15 B View Figure 15 ): 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. 13 F View Figure 13 ): 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.

Description of female

( NSMT -Cr 33083 ), sexually dimorphic characters. Antenna 1: accessory flagellum (Fig. 14 G View Figure 14 ) bi-articulate. Gnathopod 1 (Fig. 15 C View Figure 15 ): propodus anterior margin with three clusters of setae, medial surface with five clusters of setae. Gnathopod 2 (Fig. 15 D View Figure 15 ): 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 View Figure 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. 1 J View Figure 1 ) and females from Shizuoka (Fig. 1 E View Figure 1 ) and Shikinejima Island (Fig. 1 F View Figure 1 ), the accessory flagellum consisted of two articles, consistent with the original description (like Fig. 14 G View Figure 14 ). In contrast, males from Shikinejima Island and Hachijojima Island (Fig. 1 G View Figure 1 ) possessed a tri-articulate accessory flagellum (like Fig. 14 B View Figure 14 ). 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 ( K 2 P), respectively, representing the largest intraspecific genetic distance among the Elasmopus species examined in this study (Tables 3 View Table 3 , 4 View Table 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.