Leipsuropus seisuiae sp. nov.

(Figs. 2–6)

Zoobank: urn:lsid:zoobank.org:act: 0F13E56C-CDA3-4514-95CC-09F621D1DBA8

Type material. Holotype: male, 3.4 mm, ICHUM6353; TR/ V “Seisui-maru”, Stn. 1, Kumano Sea, Northwestern Pacific (34°06.92’N 136°42.73’E to 34°04.98’N 136°42.25’E), 340– 338 m depth, biological dredge, 7 November 2017, Y. Matsumoto and K. Kakui coll.

Diagnosis (male). Pereonites 2–5 each with five narrow spiniform projections on tergal plate. Pereonites 6 and 7 segmented. Telson with dorsal tubercle. Gnathopod-2 palm with small, proximal, denticulate projection.

Description of holotype. Body(Figs.2, 3A–C, 6D) 3.4 mm long, depressed. Head with mid-dorsal spiniform projection, pair of dorsolateral spiniform projections, and pair of ventral projections at insertion of antenna 2. Pereonite 1 with 2 mid-dorsal spiniform projections and pair of lateral spiniform projections.Pereonites2–7 segmented, each with mid-dorsal spiniform projection, pair of dorsolateral spiniform projections, and pair of lateral spiniform projections. Pleonites 1–2 each with mid-dorsal spiniform projection and pair of dorsolateral projections.Pleonite3 without projection. Urosomites 1–3without projection;urosomite1 with posterodorsal simple seta. Telson (Figs. 3B, 6D) linguiform, with dorsal tubercle bearing dorsal simple seta.

Antenna 1 (Fig. 4A) with peduncular article 1 having ventral setulose setae and dorsal bifurcate setae; other parts lost. Antenna 2 (Fig. 4B) with peduncular articles 1–2 (fused) having distolateral projection and antennal gland cone directed ventrodistally; peduncular article 3 with ventral setulose setae and dorsal bifurcate setae; other parts lost.

Upper lip (Fig. 4C; partly broken) ventrally setulated, with ventral hollow. Epistome (Fig. 4D) produced anteriorly. Mandibles (Fig. 4E, E 1, F, F1) with incisor bearing 5 cusps; lacinia mobilis with 3 cusps; subdistal region with 3 (left) or 2 (right) accessory blades and 3 (left) or 1 (right) plumose seta; molar with plate-like structure bearing pectinate margin. Palp with naked article 1; article 2 with medial setulose setae; article 3 distally expanded, with setulose setae in distal half. Lower lip (Fig. 4G) with developed inner lobes and mandibular processes; distal region of inner and outer lobes covered with thin setae. Maxilla 1 (Fig. 4H) with vestigial inner plate; outer plate with 8 distal robust setae (6 bifurcate, 2 simple). Palp with naked article 1; article 2 with 7 distal robust setae (1 broken, 3 serrate, 3 simple), ventral simple seta, and 3 dorsal setulose setae.Maxilla 2 (Fig. 4I) with inner and outer plates having distal setulose setae and covered with thin setae. Maxillipeds (Fig. 4J, J 1) with inner plate bearing 6 distal setulose setae and 3 distal short robust setae; outer plate with 4 distal setulose setae and 5 medial robust setae and several medial simple setae. Palp with4 articles; article 1 naked; article 2with distal and medial setulose setae; articles 3–4 with setulose setae on almost all surfaces.

Gnathopod 1 (Figs. 3A, 5A, A 1) subchelate. Coxa with slight lateral projection. Basis with 1 anterior and 1 posterodistal simple setae. Ischium with posterodistal setulose setae. Merus with posterior setulose setae. Carpus with 1 medial and several posterior setulose setae. Propodus with setulose setae on almost all margins; palmar margin slightly concave, with proximomedial 4 robust setulose setae. Dactylus slightly curved posteriorly, with 2 posterosubdistal denticles.

Gnathopod 2 (Figs. 3C, 5B, B 1) subchelate. Coxa with lateral projection. Basis with 1 mid-posterior and 3 posterodistal simple setae and lateral anterodistal spiniform projection. Ischium with 2 posterodistal simple setae. Merus with 5 posterodistal simple setae and posterodistal spiniform projection. Carpus short, with simple setae on antero- and posterodistal corners. Propodus ovate, with simple setae on almost all margins; palmar margin with proximal small denticulate projection, proximal bifurcate robust seta, mid-plumose seta, and 11 robust setae. Dactylus curved posteriorly.

Pereopod 3 (Figs. 3A, C, 5C) 2.33 mm long, with length ratio of basis, ischium, merus, carpus, and propodus 1.00:0.24:0.64:1.01:1.37 (tip of dactylus broken). Coxa pointed laterally. Basis with anterior and posterior simple setae.Ischium with posterodistal simple seta. Merus with posterior simple setae, mid-anterior thick seta and anterodistal bifurcate thick seta. Carpus with anterior, posterior, and distal simple setae and mid-anterior, anterodistal, and posterodistal thick setae. Propodus with anterior simple setae and anterior, posterior, and distal thick setae. Dactylus slightly curved, with 2 anterior simple setae and mid-anterior plumose seta. Pereopod 4 (Figs. 3A, 5D) 2.32 mm long, with length ratio of articles from basis to propodus 1.00:0.25:0.73:1.16:1.50 (tip of dactylus broken). Similar to pereopod 3 except: basis with lateral anterodistal spiniform projection; dactylus without anterior simple setae. Pereopod 5 (Figs. 3A, 5E) 2.34 mm long, with length ratio of articles from basis to dactylus 1.00:0.27:0.61:1.30:1.65:1.08. Similar to pereopod 4. Pereopod 6 (Figs. 3A, 5F) 2.45 mm long, with length ratio of articles from basis to dactylus 1.00:0.27:0.68:1.23:1.63:1.05. Similar to pereopod 5. Pereopod 7 (Figs. 3A, 5G) 2.57 mm long, with length ratio of articles from basis to dactylus 1.00:0.28:0.68:1.22:1.65:1.13. Similar to pereopod 6.

Pleopods 1–3(Fig. 6A–C) 0.66 mm, 0.80 mm, 0.76 mm long, respectively. Peduncle with 4 mediodistal coupling hooks. Each segment of both rami bearing 1–3 plumose setae.

Uropod 1 (Fig. 6D) biramous. Peduncle with 1 distolateral and 2 mediodistal robust setae. Inner ramus with 3 lateral, 3 medial and 4 distal robust setae. Outer ramus with 2 lateral, 1 medial, and 2 distal robust setae. Uropod 3 (Fig. 6D, E) curled, with distal thin setae and 3 dorsal simple setae.

Etymology. The specific name is a noun in the genitive case and derived from TR/ V “Seisui-maru”, the vessel from which the type specimen was collected.

Type locality. Kumano Sea, Japan, Northwestern Pacific (34°06.92’N 136°42.73’E to 34°04.98’N 136°42.25’E), 340– 338 m depth.

Distribution. So far known only from the type locality.

Remarks. Leipsuropus seisuiae sp. nov. is the fifth named species in Leipsuropus. Pereonites 2–7 of male L. seisuiae sp. nov. each has five projections on the tergal plate: one dorsal, a pair of dorsolateral, and a pair of lateral projections. This condition was also found in male L. hongi, but L. seisuiae sp. nov. is distinguished from L. hongi by: i) dorsal projection on the pereonites is narrow and spiniform (broad, keel-like in L. hongi), ii) gnathopod-2 palmar margin has one small,proximal, denticulate projection (one wide and two large triangular projections in L. hongi), and iii) the telson has a dorsal tubercle (no tubercle in L. hongi) (Ariyama, 2012).Differences among congeners including L. seisuiae sp. nov. in a combination of five character states are presented in Tab. 1. A key to world Leipsuropus males is given below.