Holotype. MBM 286555, ♀ (6.2 mm), dissected, Okinawa Trough, 27°33'N, 126°58'E, RY0067, ROV-3, depth 1243 m, 16 April 2014. Paratype: MBM 286555, ♂ (5.6 mm), same collection data as holotype.

MBM 286566, 4♀♂, Okinawa Trough, 27°33'N, 126°58'E, RY0069, ROV-3, depth 1243 m, 16 Apr. 2014.

Body smooth; epimerons 1–3 smooth, posteroventral margin rounded; urosomite 1 bearing dorsal notch. Head: head deeper than long; lateral cephalic lobe large, subtriangular, subacute apically. Eyes: no trace visible in ethanol-preserved materials. Antenna 1 with peduncular article 1 longest, length ~1.5× width, without dorsodistal protrusion, article 3 subequal to article 2; flagellum 9-articulate, with callynophore, without calceoli; accessory flagellum long, 5-articulate, article 1 longest, not forming cap. Antenna 2 longer than antenna 1; peduncle without brush setae, article 4 longer than article 5; flagellum 7-articulate, shorter than peduncle, without calceoli.

Mouthparts. Upper and lower lip typical for the genus. Mandible incisors symmetrical, smooth, subtriangular; left lacinia mobilis a long, slender peg; accessory setal row without distal setal tuft, both left and right with 3 short, slender, simple setae; molar well developed, columnar; palp 3-articulate, attached lower than molar, article 1 shortest, article 3 blade-like, shorter than article 2, fringed with long simple setae. Maxilla 1 with inner plate narrow with 2 pappose apical setae; outer plate with 11 setal-teeth in 7/4 crown arrangement; palp large, 2-articulate, with 9 terminal short robust setae and 1 long, plumose seta. Maxilla 2 inner plate narrower and slightly shorter than outer plate. Maxilliped with inner plate rectangular, distal margin with teeth and 1 or 2 robust setae; outer plate does not extend to distal margin of palp article 3, with 2 apical, robust setae; palp 4-articulate, dactylus nearly as long as article 3, unguis present.

Figure 1. 

Orchomenella compressa sp. nov., MBM 286555 A holotype, ♀ (6.2 mm) B paratype, ♂ (5.6 mm).

Coxae 1–4 longer than broad, overlapping. Gills present on coxae 2–7; pereopod 5 and 6 bearing accessory gill. Oostegites slender, present on coxae 2–5.

Pereopods. Gnathopod 1 subchelate; coxa anterior margin slightly concave, posterior margin nearly straight; basis slender, anterior margin fringed with thin, simple setae; ischium shorter than merus; merus shorter than carpus, with distal margin bearing 4 long setae; carpus slightly shorter than propodus; propodus subrectangular, posterior margin straight, posterodistal corner with a group of simple setae, without robust setae; dactylus simple, as long as palm. Gnathopod 2 minutely chelate, with distal 3 articles distinctly compressed compared to other articles; coxa large, subequal in size to coxa 3; ischium 1.8× longer than merus; carpus compressed, 1.6× longer than propodus, posterior margin strongly convex, anterior margin nearly straight; propodus subrectangular; dactylus reaching corner of palm, tufts of setae covering most of distal part of propodus. Pereopod 3 coxa large; merus with posterior margin bearing simple, long setae; propodus with posterior margin bearing small, robust setae; dactylus slender. Pereopod 4 coxa large, with large posteroventral lobe, anterior margin convex; ischium to carpus with posterior margin bearing simple, long setae; propodus nearly as long as merus, posterior margin with small robust setae; dactylus slender. Pereopod 5 coxa slightly deeper than wide, posterior lobe slightly larger than anterior one; basis broadly expanded with posterior and anterior margin bearing small robust setae; merus longer than carpus, slightly expanded posteriorly; propodus with anterior margin bearing 4 small, robust setae; dactylus slender. Pereopod 6 coxa posterior lobate; basis expanded, posterior and anterior margin bearing small, robust setae; merus subequal in length to carpus, anterior margin with robust setae, posterior margin bearing 1 distal and 1 subdistal seta; propodus with small, robust setae along anterior and posterior margins; dactylus slender. Pereopod 7 with coxa small; basis wider than that of pereopod 6, posterior margin broadly rounded; distal 5 articles similar to that of pereopod 6.

Figure 2. 

Orchomenella compressa sp. nov., MBM 286555, holotype, ♀ (6.2 mm): UL, upper lip; LL, lower lip; Md L, left mandible; M×2, maxilla 2; Mxp, maxilliped; U1 L, left uropod 1; U2 L, left uropod 2; U3 L, left uropod 3; T, telson.

Uropods and telson. Uropod 1 peduncle longer than rami, with 4 dorsomedial and 1 apicomedial robust setae; outer ramus slightly longer than inner ramus; inner ramus with 2 dorsal robust setae. Uropod 2 peduncle subequal in length to rami, with 1 apicolateral, 1 dorsolateral, and 1 apicomedial robust seta; outer ramus slightly longer than inner ramus, with 3 dorsal, robust setae; inner ramus with 2 dorsal, robust setae. Uropod 3 peduncle slightly shorter than outer ramus, with 6 robust setae; outer ramus 2-articulate, article 1 with 2 lateral and 2 distal robust setae, article 2 about 0.5× length of article 1; inner ramus not reaching to base of article 2 of outer ramus, with 2 lateral robust setae. Telson cleft, about 68%, bearing 3 dorsal and 1 apical robust setae on each side.

Figure 3. 

Orchomenella compressa sp. nov., MBM 286555, paratype, ♂ (5.6 mm): Md R, right mandible; M×1 R, right maxilla 1 with SEM picture of outer plate; A1, antenna 1; A2, antenna 2; G1 R, right gnathopod; G2 R, right gnathopod 2.

Figure 4. 

Orchomenella compressa sp. nov., MBM 286555, holotype, ♀ (6.2 mm): P3 L, left pereopod 3; P4 L, left pereopod 4; P5 L, left pereopod 5; P6 L, left pereopod 6; P7 L, left pereopod 7; H, head; A1, antenna 1; A2, antenna 2; E1–3, epimeral plates 1–3.

From the Latin compressa (= compressed), referring to the compressed distal 3 articles of the gnathopod 2.

Presently known only from Okinawa Trough, at a depth of 1243 m.

This is the first time that a species of Orchomenella has been reported from hydrothermal vents. The new species can be distinguished from other Orchomenella species in lacking of eyes and having the distal three articles of gnathopod 2 compressed. The new species is morphologically most similar to O. tabasco (Barnard, 1967), which was collected from the Cedros Trench at 1720–1728 m. However, O. compressa sp. nov. differs from O. tabasco by following characters: carpus of the gnathopod 1 shorter than propodus in the new species, rather than subequal in length in O. tabasco, and telson cleft more than 50% in the new species, rather than cleft only 40% in O. tabasco (Barnard 1967). The molecular analysis shows O. compressa sp. nov. clustering with O. pinguis (Boeck, 1861) and O. minuta (Krøyer, 1846) (Fig. 5). Morphologically, O. compressa sp. nov. can be distinguished from O. pinguis by the unserrated posterior margin of epimera 2 and 3 (Boeck 1861; Gurjanova 1962; Hirayama 1986), and from O. minuta by having three pairs of dorsal spines on the telson (Gurjanova 1962).

Figure 5. 

Phylogenetic relationships among Orchomenella compressa sp. nov. and nine species of Orchomenella registered in GenBank, analyzed by the maximum-likelihood (ML) method with two Orchomenyx species as outgroup taxa. Bootstrap values of ML (>50) are indicated above branches of clades.

The ML tree inferred from partial COI sequences from 10 species of Orchomenella, including the new species, is shown in Fig. 5. Orchomenella compressa sp. nov. is clustered with O. pinguis and O. minuta with high bootstrap support (100%). Interspecific genetic divergence (K2P) among these 10 species is summarized in Table 2. The pairwise distance was 0.09%–0.21%. The new species is closest to O. pinguis and O. minuta genetically (0.096 and 0.092, respectively), although morphological characters do not support this close relationship. It is a pity that the COI sequence of O. tabasco, which is morphologically most similar to O. compressa sp. nov., is still unavailable. Of the species analyzed, O. rotundifrons (Barnard, 1932), is most genetically divergent from the new species (0.215) (Havermans et al. 2010; Hupalo et al. 2022).