Lebbeus parvirostris Komai & Chen, 2024, sp. nov.

Lebbeus parvirostris sp. nov. [New Japanese name: Kyorasan-ibara-mo-ebi] (Figs. 1–5) Lebbeus cf. shinkaiae.—Chen et al. 2023: fig. 3M. Material examined. Holotype. CBM-ZC 17848, female (cl 7.4 mm), preserved in 99% ethanol, Kyorasan site, Amami Rift hydrothermal vent field (28°36.3611’N, 128°44.0891’E; 628 m deep), by suction sampler mounted on HOV Shinkai 6500, R/V Yokosuka cruise YK23-16S, 19 September 2023. Paratype. CBM-ZC 17849, 1 female (cl 4.6 mm), same preservation and data as holotype. Description. Holotype female. Body (Fig. 1) robustly built. Rostrum (Fig. 2A, B) short, 0.26 times as long as carapace, laterally compressed, reaching beyond midlength of first article of antennular peduncle or distal corneal margin of forwardly directed ocular peduncle; dorsal margin sloping down anteriorly, serrated with 5 small equidistant teeth, including 2 postorbital, posteriormost tooth located at 0.1 carapace length; ventral margin slightly concave, armed with 1 small subdistal tooth, blade not developed. Carapace (Figs. 1, 2A, B) glabrous, with low postrostral ridge not reaching midlength; supraorbital spine moderately small, forwardly directed; conspicuous U-shaped notch below base of supraorbital spine; suborbital lobe well developed, triangular with subacute tip, falling slightly short of moderately small antennal spine; anterolateral margin below antennal spine sinuous with deep concavity just below antennal spine and rounded, unarmed pterygostomial margin; lateral face smooth, without ridges. First to third pleomeres (Fig. 1) with pleura rounded marginally; fourth pleomere pleuron damaged on both sides (Figs. 1, 2C); fifth pleomere pleuron (Fig. 2C) with moderately large posteroventral tooth. Second pleomere tergum (Fig. 1) with shallow transverse groove. Third pleomere tergum (Fig. 1) with moderately produced posterior margin. Sixth pleomere (Fig. 1) 0.5 times as long as carapace, 1.4 times as long as fifth pleomere, 2.1 times as long as proximal height, with small posteroventral tooth; posterolateral process terminating in acute tooth. Telson (Fig. 2D, E) 1.2 times as long as sixth pleomere, 2.6 times as long as wide, with 3 (right) or 4 (left) dorsolateral spiniform setae; lateral margins subparallel in anterior 0.4, tapering thereafter; posterior margin convex, without median tooth, with 2 pairs of unequal spiniform setae and 1 mesial pair of setulose setae. Eye (Fig. 2A, B) subpyriform; cornea moderately large, wider than stalk, width no more than 0.2 times of carapace length; no ocellar spot. Antennular peduncle (Fig. 2A, B) reaching distal 0.2 of antennal scaphocerite. First article distinctly longer than distal 2 articles combined, armed with 1 (left) or 2 (right) spines on dorsodistal margin; stylocerite acuminate, reaching distolateral margin of basal article. Second article with 1 small spine at dorsodistal lateral angle. Third article about half-length of second article, with small spine on dorsodistal margin. Outer flagellum with aesthetascbearing portion moderately stout, about half length of carapace; inner flagellum slender, subequal in length to outer flagellum. Antennal peduncle (Fig. 2A, B, F) with stout basicerite, armed with small ventrolateral distal spine; dorsolateral distal angle produced in blunt lobe. Carpocerite reaching beyond midlength of scaphocerite. Scaphocerite 0.6 times as long as carapace, 2.6 times as long as wide; lateral margin nearly straight, terminating in slender tooth reaching as far as rounded distal margin of lamella. Mouthparts not dissected. Strap-like epipods present on third maxilliped and first to third pereopods, corresponding to setobranchs on first to fourth pereopods. Third maxilliped (Fig. 3A, B) without exopod; endopod overreaching distal end of antennal scaphocerite by about half-length of ultimate article. Antepenultimate article subequal in length to distal 2 articles combined, depressed in proximal part; distolateral margin with slender spiniform seta; lower lateral distal angle also with minute spiniform seta. Ultimate article 2.7 times as long as penultimate article (= carpus), abruptly tapering in distal 0.2, distal part circumscribed by 6 darkly pigmented, robust spiniform setae. First pereopod (Figs. 3C–E, 4A; only right one preserved) relatively long and stout, overreaching distal end of scaphocerite by half-length of fingers. Basis short, stout. Merus 4.0 times as long as wide, with short row of minute spiniform setae on lower margin proximally and minute spiniform tubercle proximally on dorsolateral surface. Carpus widened distally, with grooming apparatus on flexor surface distally (grooming apparatus consisting of deep concavity partially surrounded by row of stiff setae). Chela 1.6 times as long as carpus, 3.8 times as long as wide; palm 1.1 times as long as carpus, 2.6 times as long as wide, with short row of stiff setae, consisting of grooming apparatus, on mesial face proximally; fingers each with tufts of short setae distally, fixed finger terminating in darkly pigmented, clearly demarcated unguis; dactylus 0.6 times as long as palm, gently curved, terminating in 2 darkly pigmented ungues. Second pereopods (Fig. 3F, G) slightly unequal with left longer than right, left overreaching distal end of scaphocerite by length of chela. Ischium 1.2 times as long as merus, with 3 closely set robust setae on lower margin proximally. Carpus divided into 7 segments in left, into 9 segments in right. Chela 0.25 times as long as carpus; dactylus 0.6 times as long as palm. Third pereopod (Figs. 3H, 4B) relatively slender, overreaching distal end of scaphocerite by 0.7 lengths of propodus. Ischium unarmed.Merus with row of 4 well-spaced spiniform setae on distal half of lateral surface adjacent to lower margin. Carpus 0.5 times as long as propodus, unarmed. Propodus with minute spiniform setae, arranged in 2 rows, on flexor margin. Dactylus 0.2 times as long as propodus, 3.5 times as long as wide, biunguiculate, with 6 accessory spiniform setae over entire length of flexor margin. Fourth pereopod (Figs. 3I, 4C) generally similar to third pereopod, overreaching distal end of scaphocerite by half-length of propodus. Merus with 3 spiniform setae. Dactylus 3.5 times as long as wide, with 6 accessory spiniform setae in addition to subterminal unguis. Fifth pereopod (Figs. 3J, 4D) generally similar to third and fourth pereopods, reaching distal end of scaphocerite by tip of dactylus. Merus with only 1 spiniform seta subdistally. Propodus with cluster of short stiff setae, consisting of grooming apparatus, in distal 0.2, followed by minute spiniform setae arranged in 2 rows. Dactylus 3.8 times as long as wide, with 6 accessory spiniform setae in addition to subterminal unguis. Pleopods without distinguishing features, not illustrated. Uropodal rami (Fig. 2D) slightly exceeding telson posterior margin. Protopod posterolateral angle produced into strong tooth. Exopod lateral margin terminating in small tooth, accompanied by spiniform seta longer than posterolateral tooth. Paratype female. Generally similar to holotype, although much smaller. Rostrum with 4 teeth on dorsal margin, including 2 postrostral teeth. Fourth pleuron armed with small posteroventral tooth. Telson with 3 pairs of dorsolateral spiniform setae; posterior margin with 2 pairs of spiniform setae and 2 mesial pairs of plumose setae. Second pereopod carpus divided into 7 segments on both sides. Meri of third to fifth pereopod armed with 3–4, 1, 1 spiniform setae, respectively; dactyli with 6, 6, 6–7 accessory spiniform setae, respectively. Colouration in life. Body and appendages orange-red overall; cornea brown (Fig. 5). Distribution. Presently known only from the Amami Rift at a depth of 628 m. Etymology. From the combination of the Latin “ parvus ” (= small, short) and “ rostris ” (= front), in reference to the short rostrum in the new species. Used as a noun in apposition. Remarks. Practically, Lebbeus has been subdivided into four informal species groups according to the development of epipods on the first to third pereopods (e.g., Rathbun 1904; Holthuis 1947; Butler 1980; Wicksten 1990; Hayashi 1992; Komai et al. 2004; Komai & Matsuzaki 2022), although the grouping does not always reflect the true phylogenetic relationship (e.g., Komai et al. 2019, 2021; Wang et al. 2023). Lebbeus parvirostris sp. nov. belongs to the group with epipods on the first to third pereopods, and is morphologically similar to species exhibiting the following characteristics: rostrum short, not reaching level of distal end of first article of antennular peduncle, dorsally armed with one or more teeth (in addition to one or two postrostral ones) and ventrally armed with one subterminal tooth; distinct notch present just below supraorbital tooth; first article of antennular peduncle can have more than one spine on dorsodistal margin; meri of third and fourth pereopods with spiniform setae. The species morphologically similar to L. parvirostris sp. nov. include L. microceros (Krøyer, 1841), L. mundus Jensen, 2006, L. saldanhae (Barnard, 1947), L. schrencki (Bražnikov, 1907), L. spongiaris Komai, 2001, and L. tosaensis Hanamura & Abe, 2003. Lebbeus parvirostris sp. nov. differs from all of these species in the absence of a pterygostomial tooth on the carapace in the female (Fig. 2A). In all of these species the pterygostomial tooth is present in the female, although it may be absent in the male depending on the species (Bražnikov 1907; Squires 1990; Fransen 1997; Komai 2001; Hanamura & Abe 2003; Komai & Takeda 2004; Jensen 2006). Further differences between the new species and these species are discussed below: Lebbeus microceros. The main differences are seen in the following specific characters: the rostrum overreaches the distal corneal margin in L. microceros (cf. Squires 1990: fig. 112b), rather than not reaching it in L. parvirostris sp. nov. (Fig. 2A, B); the antennular stylocerite reaches or overreaches th