Elasmopus projectus, Yoshimura & Tomikawa, 2025

Yoshimura, Hiro & Tomikawa, Ko, 2025, A new species of the genus Elasmopus (Crustacea: Amphipoda: Maeridae), from Honshu, Japan, Zootaxa 5653 (2), pp. 286-296 : 287-295

publication ID	https://doi.org/10.11646/zootaxa.5653.2.8
publication LSID	lsid:zoobank.org:pub:03A959AB-44E6-4FC6-9F81-7A6FBF8C06C9
DOI	https://doi.org/10.5281/zenodo.15822069
persistent identifier	https://treatment.plazi.org/id/F0346A15-FFD3-FFFD-FF0D-EBD9FCA17FBF
treatment provided by	Plazi
scientific name	Elasmopus projectus
status	sp. nov.

Treatment

Elasmopus projectus sp. nov.

urn:lsid:zoobank.org:act:5DE95ECD-2BEA-49DE-98A7-C75CC2AA3C36

[New Japanese name: Osero-iso-yokoebi]

( Figs. 2–8 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 )

Material examined. Holotype: male, 9.3 mm (NSMT-Cr 32973), Kannonzaki Coast , Yokosuka, Kanagawa, Japan (35°15’33.1”N, 139°44’37.0”E) ( Fig. 1A View FIGURE 1 ), shallow subtidal zone, among holdfast of the brown alga Eisenia bicyclis , 4 February 2023, collected by H. Yoshimura. GoogleMaps Paratypes: ovigerous female, 6.9 mm (NSMT-Cr 32974), 4 females, 7.3, 7.1, 8.1, 9.9 mm (NSMT-Cr 32975, 32981–32983), 5 males, 6.7, 9.6, 11.2, 7.1, 8.2 mm (NSMT-Cr 32976–32978, 32984, 32985), same data as for the holotype; ovigerous female, 6.4 mm (NSMT-Cr 32979), Arasaki Coast , Yokosuka, Kanagawa, Japan (35°11’38.99”N, 139°35’56.55”E) ( Fig. 1B View FIGURE 1 ), shallow subtidal zone, among the red alga Corallina sp. , 9 December 2022, collected by H. Yoshimura; female, 9.2 mm (NSMT-Cr 32980), Uradome Beach, Iwami, Tottori, Japan (35°35’31.7”N, 134°19’11.1”E) ( Fig. 1C View FIGURE 1 ), shallow subtidal zone, among sporophyll of the brown alga Undaria pinnatifida lying on the sea bottom, 16 June 2024, collected H. Ogawa, Y. Mukaida, M. Ooga, and H. Yoshimura GoogleMaps .

Etymology. The specific name, projectus , refers to the well-developed process on the mid-palmar margin of the male gnathopod 2. The Japanese name “Osero” is derived from the black and white bands along the dorsal body in life ( Fig. 2 View FIGURE 2 ), which resemble the pieces used in the board game Othello.

Diagnosis. Epimeral plate 3 with weakly pointed posterodistal corner.Antenna 1 peduncles bearing slender setae on lateral and medial margins, with 3-articulate accessory flagellum. Mandibular palp article 3 short. Male gnathopod 2 propodus with dense slender setae, length of the longest seta reaching 0.9 × width of propodus, mid-palmer margin with well-developed process. Female gnathopod 2 propodus mid-palmar margin with robust seta. Pereopods 5–7 basis with smooth posterior margin, without long setae. Telson border than long.

Description of male (holotype, NSMT-Cr 32973). Body ( Fig. 2A View FIGURE 2 ): smooth, not carinate, with a few short setae dorsally on pereonites and pleonites. Head ( Fig. 3A View FIGURE 3 ): eyes oval; lateral cephalic lobe broad, anteroventral margin with notch. Epimeral plates 1–3 ( Fig. 3B–D View FIGURE 3 ): each ventral margin with 1–2 cluster of robust setae, 1 cluster and single robust setae and 2 clusters and 2–3 robust setae, respectively; each posterior margin with 4, 4 and 3 short setae, respectively; posteroventral corner of epimeral plates 1–3 weakly produced.

Antenna 1 ( Fig. 3E View FIGURE 3 ): length 0.6 × body length; peduncular articles 1 to 3 in length ratio of 1.1: 1.0: 0.5, with slender setae on lateral and medial margin; peduncular article 1 with 2 robust setae on posteroproximal margin, robust seta on posterodistal corner; accessory flagellum ( Fig. 3F View FIGURE 3 ) 3-articulate, reaching to distal margin of primary flagellar article 2; primary flagellum 21-articulate, with slender setae. Antenna 2 ( Fig. 3G View FIGURE 3 ): length 0.6 × antenna 1; peduncular article 2 with 2 short spines on anterodistal corner, gland cone of peduncular article 2 exceeding distal margin of peduncular article 3; peduncular articles 4 and 5 almost same length, with slender setae on lateral and medial margins; flagellum 10-articulate, with slender setae.

Mouthparts. Upper lip ( Fig. 3H View FIGURE 3 ) with rounded anterior margin, bearing fine setae. Mandible ( Fig. 3I, J View FIGURE 3 ) with left and right incisors 2- and 4-dentate, respectively; left and right lacinia mobilis 5- and 4-dentate, respectively; accessory setal row consisting of 4 setae on each of left and right mandibles; molar process well developed, triturative; palp well developed, 3-articulate; palp article 1 without setae, article 2 with 13 setae, article 3 falcate, length 2.5 × width. Lower lip ( Fig. 3K View FIGURE 3 ) with outer lobes laterally expanded, bearing apical setae; inner lobes ovate, apically covered with setae. Maxilla 1 ( Fig. 3L, M View FIGURE 3 ) with inner plate bearing 2 plumose setae apically; outer plate with 7 robust dentate setae ( Fig. 3M View FIGURE 3 ); palp 2-articulate, article 1 marginally bare, article 2 with numerous apical and subapical setae, outer margin without seta. Maxilla 2 ( Fig. 3N View FIGURE 3 ) 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. 4A View FIGURE 4 ) with inner plate bearing plumose setae apically, lacking nodular setae; outer plate obovate, reaching more than half of palp article 2; palp article 1 with seta on inner margin, with 2–3 setae 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. 4B, C View FIGURE 4 ): subchelate; coxa anteroventral corner weakly produced, anterior margin weakly concave, ventral margin with long and short setae, of which long seta retrorse; basis with long setae on posterior margin and medial surface; carpus subequal in length to propodus, with slender setae on anterodistal corner and medial surface, with density pinnate sided setulated setae on posterior margin; propodus with 5 clusters of setae on anterior margin, with 7 clusters of setae on medial surface, palmar margin almost transverse, minutely serrate, with rows of robust setae on medial and lateral palmar submargins, palmar corner with long robust seta; dactylus with seta on anteroproximal margin, with 5 small setae on lateral margin, with spathulate seta and slender seta laterally at the base of nail. Gnathopod 2 ( Fig. 4D–H View FIGURE 4 , 8B View FIGURE 8 ): subchelate; coxa with long and short setae on ventral margin, of which 2 long setae retrorse; basis with long setae on posterior margin; merus distoventral corner subquadrate bearing long setae; carpus with short setae on anterodistal corner, length of these setae reaching 0.2 × width of carpus, with density pinnate sided setulated setae ( Fig. 4H View FIGURE 4 ) medially on posterior margin; propodus with 1 clusters, 3 pairs and single setae on anterolateral submargin, length of these setae reaching 0.2 × width of propodus, anteromedial submargin with 6 clusters of setae, palmar margin weakly concave with subtriangular hump ( Fig. 4F View FIGURE 4 ) on near insertion of dactylus, with 2 clusters, 2 pairs and single setae, with 10 and 7 robust setae on lateral and medial margins, respectively; mid-palmar margin with well-developed process, 3 rows of weakly plumose setae ( Fig. 4G View FIGURE 4 ), the proximal seta longer than the distal seta, length of the longest seta reaching 0.9 × width of propodus; dactylus strongly curved, subacute, length 0.6 × length of propodus, with seta on anteroproximal margin.

Pereopod 3 ( Fig. 5A View FIGURE 5 ): 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 1 robust and 2 slender setae at the base of nail. Pereopod 4 ( Fig. 5B View FIGURE 5 ): 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 1 robust and 2 slender setae at the base of nail. Pereopod 5 ( Fig. 5C View FIGURE 5 ): coxa bilobed, ventral margin of posterior lobe with 3 robust setae, posterior margin of posterior lobe with 2 short setae; basis broad, subrectangular, smooth, with numerous short setae; anterior margin of merus with robust and slender setae, posterior margin with robust setae; anterodistal corner of carpus with robust and slender setae; propodus with robust setae on anterior margin, with slender setae on posterodistal corner shorter than length of dactylus; dactylus with plumose seta on anteroproximal margin, with 1 robust and 2 slender setae at the base of nail. Pereopod 6 ( Fig. 5D View FIGURE 5 ): coxa bilobed, anterior margin of anterior lobe with slender setae, ventral margin of posterior lobe with 6 robust setae, posterior margin of posterior lobe with 2 short setae; basis weakly expanded proximally, tapering distally, posterior margin smooth with numerous short setae; anterior margin of merus with robust and slender setae, posterior margin with robust setae; anterior margin of carpus with robust setae; anterior margin of propodus with robust setae, slender setae on posterodistal corner shorter than length of dactylus; dactylus with plumose seta on anteroproximal margin, with 1 robust seta and 2 slender setae at the base of nail. Pereopod 7 ( Fig. 5E View FIGURE 5 ): coxa rounded, with 3 slender setae on ventral and 5 short setae on posterior margins; basis expanded, subovate, posterior margin rounded, smooth, with numerous short setae; anterior margins of merus and carpus with robust, and slender setae, length of the longest seta on anterior margin almost equal to width of merus and carpus, posterior margins of merus and carpus with robust setae; anterior margin of propodus with robust setae, slender setae on posterodistal corner shorter than length of dactylus; dactylus with plumose seta on anteroproximal margin, with 1 robust and 2 slender setae at the base of nail.

Coxal gills ( Figs 4E View FIGURE 4 , 5A–5D View FIGURE 5 ) on gnathopod 2 and pereopods 3–5 broad, gill on pereopod 6 small.

Pleopods ( Fig. 6A–F View FIGURE 6 ): peduncles with long setae, inner distal corner of peduncle with 2 retinacula ( Fig. 6B, 6D, 6F View FIGURE 6 ), peduncle of pleopod 3 broader than pleopod 1 and 2.

Uropods. Uropod 1 ( Fig. 6G View FIGURE 6 ): peduncle with basofacial robust seta, length 1.1 × length of outer ramus, medial and lateral margins each with 5 robust setae, distal part with 2 robust setae; inner ramus with 3 and 5 robust setae on medial and lateral margins, respectively, distal part with 5 robust setae; outer ramus slightly shorter than inner ramus. Uropod 2 ( Fig. 6H View FIGURE 6 ): peduncle length 1.0 × length of outer ramus; outer ramus slightly shorter than inner ramus, with 5 marginal and 4 distal robust setae; medial and lateral margins of inner ramus each with 4 robust setae, distal part with 5 robust setae. Uropod 3 ( Fig. 6I View FIGURE 6 ): peduncle length 0.9 × length of outer ramus, with pair of robust setae on outer margin; rami distally truncated; outer ramus length 1.6 × length of inner ramus, with 2, 1 and 1 clusters of robust setae on outer, distal and inner margins, respectively, 1 slender seta apically; inner ramus with robust seta on inner margin and cluster of robust setae distally.

Telson ( Fig. 6J View FIGURE 6 ): broader than long, cleft for 72% of length, each lobe bearing 5 robust and 1 plumose setae apically, lateral margin of each lobe with 1–2 plumose setae.

Description of female (paratype, NSMT-Cr 32974), sexually dimorphic characters. Gnathopod 1 ( Fig. 7A, B View FIGURE 7 ): propodus anterior margin with 3 clusters of setae, medial surface with 4 clusters of setae. Gnathopod 2 ( Fig. 7C, D View FIGURE 7 ): propodus with 5 pairs and single setae on anterolateral submargin, anteromedial submargin with 6 clusters, 1 pairs and single setae; mid-palmar margin with robust seta; palmar margin almost straight, with minutely serrate on the distal quarter of the palmar margin. Oostegites ( Fig. 7E View FIGURE 7 ) on gnathopod 2 and pereopods 3–5, slender with long setae.

Coloration in life. Eyes black; antennae 1 and 2 peduncular articles brown, flagellum articles have 2 black patterns on a white background; body brown; small white dot patterns on coxae, pereonites, and pleonites; white bands aligned along the boundaries between body segments on the dorsal side.

Variations. Mandible: lacinia mobilis 4–5 dentate. Gnathopod 1: propodus anterior margin with 5–6 clusters of setae in males, 3–5 clusters in females; medial surface with 6–8 clusters of setae in males, 5–8 clusters in females. Gnathopod 2: length of setae on carpus anterodistal corner reaching 0.1–0.2 and 0.3–0.4 × width of carpus in male and female, respectively; length of setae on propodus anterolateral submargin reaching 0.2–0.3 and 0.2–0.4 × width of propodus in male and female, propodus anteromedial submargin with 6–8 and 7–9 clusters of setae in male and female, respectively; subtriangular hump on palmar margin in male with 6–10 robust setae on lateral margin and 5–7 robust setae on medial margin. Telson: each lobe bearing 3–5 robust setae apically. As body length increases, male gnathopod 2 shows morphological change ( Fig. 8A–C View FIGURE 8 ): in immature males, propodus thin and semioval, mid-palmer margin with no process, dactylus weakly curved ( Fig. 8A View FIGURE 8 ); in adult males, propodus thick and elongate, mid-palmer margin with well-developed process, dactylus strongly curved ( Fig. 8B, C View FIGURE 8 ).

Ecological note. This new species was found on a holdfast of brown and red algae inhabiting a subtidal rocky shore. Specimens of this new species were obtained in groups of 2–12 individuals from an alga.

Distribution. Known from Kanagawa and Tottori Prefectures, Japan.

Nucleotide sequence. The nucleotide sequence of the COI gene of holotype (NSMT-Cr 32973) was determined and deposited in the INSD with the GenBank accession number LC851065 View Materials .

Remarks. Elasmopus projectus sp. nov. is morphologically similar to E. arrawarra Hughes & Lowry, 2006 , E. leveque Hughes & Lowry, 2011 , and E. otus Hughes & Lowry, 2011 , with the following features: 1) mandibular palp article 3 short; 2) male gnathopod 2 propodus palm with long slender, dense setae; 3) male gnathopod 2 with mid-palmar margin bearing a well-developed process; 4) epimeral plates 3 posteroventral corner weakly produced; and 5) telson broader than long. However, E. projectus sp. nov. can be distinguished from these species by the following two features (features of the three species in parentheses): 1) length of the longest seta on the posterior margin of male gnathopod 2 reaching 0.9 × width of propodus (less than 0.6), and 2) pereopods 6–7 basis with smooth posterior margin (castelloserrate).

In comparison with other Japanese Elasmopus species, E. projectus sp. nov. is most similar to E. japonicus and E. mukuinu in having a propodus of male gnathopod 2 with densely slender setae. However, E. projectus sp. nov. can be distinguished from E. japonicus and E. mukuinu by a developed process on the mid-palmar margin of the propodus of the male gnathopod 2. This new species differs from other Japanese Elasmopus species based on distinct morphological characteristics shown in the key below.