Carapace almost reaching medial margin of pleonite 4. Rostrum long and narrow, 2.4 times as long as wide. Surface of eyes smooth. Article 4 of antennule with row of four simple setae and four robust distal spines. Article 3 of antenna with eight spines and nine spine-like setae along proximal half, two thin setae and six spine-like setae on external lateral face, six spines and four simple setae on terminal margin. Article 1 of endopod of second maxilla longer than article 2. Exopod of second maxilla longer than article 1 of endopod. Article 2 of mandibular palp with two thin setae. Pleonites 6 and 7 with distally rounded denticles along posterior border. Exopod of pleopod 1 with a single row of approximately 21 stout serrated spines along lateral margin. Anal plates with no distinct lateral ‘shoulder’. Furcal rami longer than combined length of pleonite 7 and telson.

Twenty-eight specimens were collected using the JAMSTEC ROV “Hyper-Dolphin” of JAMSTEC during dive 886 in the Wakamiko Caldera during the R/V “Natsushima” NT08-17 Leg-1 cruise. “Hyper-Dolphin” dive 886: the Tagiri Site on the Wakamiko Caldera: 30°40.068'N, 130°45.690'E; 200 m; 7 Aug 2008. 14 ♂♂ (TL: 3.7–6.0 mm), 13 ♀♀ (TL: 2.0–6.1 mm).

Holotype : (NSMT-Cr 26758), adult ♀ of 10.7 mm TL. Allotype: (NSMT-Cr 26759), adult ♂ of 7.0 mm TL. Paratypes: 3 adult ♂♂ (TL: 4.1–6.0 mm) (NSMT-Cr 26760, NSMT-Cr 26761, NSMT-Cr 26762) 3 adult ♀♀ (TL: 3.9–6.0 mm) (NSMT-Cr 26763, NSMT-Cr 26764, NSMT-Cr 26765).

Female holotype. Carapace (Fig. 2A) oval, ca. 1.5 times as long as wide, almost reaching pleonite 4.

Rostrum (Fig. 2B) long and narrow, 2.4 times as long as width, with round apex.

Compound eye (Fig. 2C): ommatidial part covering two-thirds of eye-stalk. Supraocular plate reaching to ommatidial part.

Antennule (Fig. 2D): peduncle composed of four articles. Article 2 longer than article 3, with single long plumose seta on anterior margin, 5 long and three short plumose setae arising subterminally and cluster of simple setae on anterior margin, respectively. Article 3 shorter than article 2, widest distally, with terminal cluster of simple setae and long simple seta arising on anterior margin and five long plumose setae and two thin plumose setae on posterio-distal margin. Article 4 much shorter than article 3, with row of four simple setae and four robust spines distally. Antennular scale oval, twice as long as width. Flagellum slightly longer than peduncle, composed of 12 articles.

Antenna (Fig. 2E): peduncle composed of 3 articles. Article 2 2.3 times as long as wide, with stout spine at dorso-distal portion. Article 3 longer than article 2, with different pattern of spines or setae along medial anterior margin as follows:

(1) proximal row of ca. six simple setae and plumose seta on inner surface;

(2) eight spines and nine spine-like setae along proximal half, the distalmost being the longest, respectively;

(3) two thin setae and six spine-like setae on external lateral face;

(4) seven thin plumose setae, three plumose setae and seven simple setae, each associated proximal spines;

(5) six robust spines increasing in length distally and four simple setae at apex, 21 long plumose setae arising from posterior distal margin, and robust plumose seta arising sub-terminally. Flagellum longer than peduncle, composed of 15 articles.

Mandible (Fig. 2F) well developed. Mandibular palp composed of three articles. Article 2 equal in length as article 3, article 2 with two thin setae at mid-length on lateral margin and sub-terminal on superior margin, respectively. Article 3 cylindrical, with marginal setae-row covering four-fifths length of article. Molar process shorter than palp article 1, distal margin with row of teeth forming grinding surface. Incisor process broad basally with acute terminal process and minute tooth along lateral margin.

First maxilla (Fig. 2G): proximal endite (Fig. 2H) with rounded medial margin, bearing short robust plumose setae. Distal endite (Fig. 2I) carrying two rows of stout, spatulate setae and two long plumose seta. Palp long, ca. 4.5 times longer than combined length of both endites, bearing 20 setae.

Second maxilla (Fig. 2J): protopod sub-divided into four endites bearing plumose setae. Endite 1 rectangular and endite 3 rounded approximately equally sized to endite 1; endite 2 oval, smaller than endite 1 and 3; endite 4 smaller than other endites. Endopod composed of two articles, article 1 ca. twice as long as article 2, lateral margin with plumose setae, article 2 with three terminal plumose setae. Exopod reaching beyond apex of endopod article 1, bearing 23 plumose setae on lateral margin.

Thoracopod 1 (Fig. 3A): endopod composed of large article and three small distal articles, with numerous plumose setae along outer margin, terminal setae not reaching the terminal margin of exopod. Exopod oval, not reaching beyond the terminal article of endopod, with single long setae on terminal margin and 21 thin setae along inner margin. Epipod large, proximal lobe beyond the basis, distal lobe reaching beyond the middle of exopod.

Thoracopod 2–6 (Fig. 3B–F): Except for the exopod, shape of each limbs same. Eleven thin setae of inner margin of exopod in thoracopod 2, five in thoracopod 3, seven in thoracopod 4, six in thoracopod 5, seven in thoracopod 6. Shape of exopod gradually changes from oval to triangular from thoracopods 2–6.

Thoracopod 7 (Fig. 3G) endopod composed of one large article and two small distal articles, with numerous plumose setae along outer margin, terminal setae beyond the terminal margin of exopod. Exopod rounded and distal part expanded like a triangle, reaching beyond the terminal article of endopod, with eight thin setae along inner margin. Epipod large and triangular, proximal lobe beyond the basis, distal lobe reaching beyond the middle of exopod, with three thin setae along distal margin.

Thoracopod 8 (Fig. 3H): smaller than other thoracopods. Endopod composed of a large article and three small distal articles, with numerous plumose setae along outer margin, terminal setae not reaching the terminal margin of exopod. Exopod oval, reaching beyond the terminal article of endopod, with five setae along inner margin. Epipod small, proximal lobe beyond the basis, distal lobe not reaching beyond the middle of exopod.

Pleon (Fig. 2A, 5A): composed of seven segments. Posterior margin of pleonite 1 smooth. Pleonite 2 with narrowly triangular denticles on half-length of posterior margin with wide flat margin. Pleonite 3 with round minute denticles. Pleonite 4 with round minute denticles, posterolateral margin expanded and forming narrow acute process. Pleonites 5, 6, and 7 with minute rounded denticles.

Pleopod 1 (Fig. 4A): protopod twice as long as width, with three short simple setae arising proximally, simple seta medially, two spine-like setae near base of endopod, spine-like seta near base of exopod. Endopod composed of two segments, longer than exopod, distal segment with acute process at apex, bearing long robust simple spine, lateral and medial margin each with plumose setae, nine short setae on proximal medial margin, appendix interna of proximal segment with three short recurved hooks. Exopod with row of 21 stout serrated spines along lateral margin, five stout simple spines on distolateral margin, distal one longest, plumose setae along distal inner margin.

Pleopod 2 (Fig. 4B): protopod 2.2 times as long as wide, six simple setae and seven simple setae on proximal and distal part of inner side, respectively; simple seta near base of exopod, with blade-like process between exopod and endopod. Endopod composed of two segments, longer than exopod, distal segment with acute process at apex, bearing long robust simple spine, lateral and medial margins each with plumose setae, ten short setae on proximal medial margin, appendix interna of proximal segment with three short recurved hooks. Exopod with row of six pairs of robust spines along lateral margin, three stout simple spines on distal margin, plumose setae along distal inner margin.

Pleopod 3 (Fig. 4C): protopod 2.2 times as long as wide, five simple setae on proximal and distal part of inner side respectively, simple seta near base of exopod, with blade-like process between exopod and endopod. Endopod composed of two segments, longer than exopod, distal segment with acute process at apex, bearing long robust simple spine, lateral and medial margin each with plumose setae, eight short setae on proximal medial margin, appendix interna of proximal segment with three short recurved hooks. Exopod with row of seven pairs of robust spines along lateral margin, three stout simple spines on distal margin, plumose setae along distal inner margin.

Pleopod 4 (Fig. 4D): protopod twice as long as width, bearing five simple setae along lateral proximal border, eleven short simple setae along ventral proximal border and four simple setae along posterior border. Posterolateral corner of peduncle produced as acute point, with blade-like process between exopod and endopod, posterior margin lacking serrations, posterolateral corner with acute projection. Endopod composed of two segments, longer than exopod, distal segment of endopod with acute process at apex, bearing long robust simple spine, lateral and medial margin each with plumose setae, ten short setae on proximal medial margin, appendix interna of proximal segment with three short recurved hooks. Exopod with row of seven pairs of robust spines along lateral margin, three stout simple spines on distal margin, plumose setae along distal inner margin.

Pleopod 5 (Fig. 4E, F): acute process between bases of rami. Protopod composed of two segments. Distal segment 4.5 times as long as wide, bearing five simple spines and ten short setae, lateral margin with ca. 33 simple setae.

Pleopod 6 (Fig. 4G, H): acute process between bases of rami, rami bearing five simple spines and six simple setae.

Telson, anal plates, and furca (Fig. 5B, C): anal plates (Fig. 5C) with medial margin slightly convex, point acute, lateral margin with no distinct ‘shoulder’. Furcal rami (Fig. 5B) slightly longer than combined lengths of pleonite 7 and telson, 21 spines along outer edge, 14 robust setae and 16 plumose setae along inner margin, three long robust setae and thin spine on distolateral margin.

Color in life (Fig. 7): living specimens with dark red eyes and most of body transparent.

(adult male): antennule flagellum more swollen proximally than in female (Fig. 6B). Antenna flagellum composed of more than 50 articles (Fig. 6C). article 2 of endopod palp half-length of article 1 (Fig. 6D). Furcal rami sub-equal in length to combined length of pleonite 7 and telson (Fig. 6E).

Examination of five female specimens of various sizes shows several morphological variations in the number of robust distal spines of article 4 of antennule and lateral spines of exopod of pleopod 1. Article 4 of antennule with 3–5 spines, exopod of pleopod 1 with 21–24 spines on lateral margin, respectively.

The specific name tagiri originates from the Japanese word ‘tagiru’ meaning “boiling”, a reflection of the hydrothermal venting and bubbling of methane and carbon dioxide as found in the habitat of N. tagiri sp. nov.

Nebalia tagiri sp. nov. differs from the other 34 described species based on four unique characteristics: (1) smooth oval eye-stalk; (2) ommatidial part covering two-thirds of eye-stalk; (3) article 4 of antennule with 3–5 robust distal spines; (4) article 1 of second maxilla endopod longer than article 2. Nebalia tagiri sp. nov. can easily be distinguished from N. bipes (Fabricius, 1780), N. mortoni (Lee & Bamber, 2011), and N. koreana (Song et al., 2012) based on the following: (1) length of rostrum in N. tagiri, N. koreana, and N. mortoni ca. 2.4 times as long as width, N. bipes approximately twice times as long as width; (2) number of distal spines of article 4 of antennule is four in N. tagiri sp. nov. and N. mortoni, three in N. bipes and five in N. koreana; (3) length of antennular scale in N. tagiri, N. koreana and N. bipes ca. twice as long as width, N. mortoni ca. 2.7 times as long as width; (4) the different pattern of spines or setae of article 3 of antenna among related three species are summarized in Table. 1.

Nebalia tagiri sp. nov. showed different characteristics from N. koreana or N. mortoni in the following points: (1) number of spines or spine-like setae along proximal half in comparison with N. koreana and N. mortoni; (2) existence of spine-like setae on external lateral face in comparison with N. mortoni; (3) number of spines and setae on terminal margin in comparison with N. koreana and N. mortoni, N. bipes is lacking detailed description; (5) thin seta of article 2 of mandibular palp is two in N. tagiri sp. nov., N. koreana and N. mortoni and one in N. bipes; (6) number of lateral spines of exopod of pleopod 1 is 21–24 in N. tagiri sp. nov, 25 in N. mortoni, and more than 30 in N. bipes and N. koreana; (7) furcal rami are longer than the combined length of pleonite 7 and the telson in N. tagiri. sp. nov. and of the same length in N. bipes and N. mortoni, while in N. koreana they are shorter; (8) denticles of posterior margin of pleonites 6 and 7 are rounded in N. tagiri sp. nov., N. bipes, and N. koreana while in N. mortoni are square-shaped.

Most species of Nebalia have been reported from shallow water (< 10 m depth) in the world oceans with two exceptions, i.e., Nebalia abyssicola (Ledoyer, 1997) and N. schizophthlma (Haney et al., 2001) have been reported from ca. 100 m or deeper (Table 1). Nebalia schizophthalma was reported at a depth of 2886 m in the North Atlantic Ocean, which is the deepest record of this genus. Nebalia tagiri sp. nov. was collected from a hydrothermal vent chimney at a depth of 200 m in Wakamiko Caldera of Kagoshima Bay, Japan (Fig. 1). This depth of 200 m is relatively deep compared to similar species of this genus. The genus Nebalia has not been previously reported from any hydrothermal fields to date. The only species reported from a deep-sea hydrothermal field is Dahlella caldariensis (Hessler, 1984), observed on the mussel beds of hydrothermal vent areas at the Galapagos and the East Pacific Rise at depths deeper than 2000 m. On the other hand, N. tagiri sp. nov. was collected near the interior of chimney walls, which were close to the chimney vents spouting thermal water (ca. 200 °C).

This species was clearly observed on the inner surface of the chimney, suggesting the species is adapted to the hydrothermal environment. For a more accurate understanding of this species, further investigations are needed to determine their ecological and/or physiological aspects in relation to the hydrothermal fields.