Seborgia cavernicola sp. nov. from a submarine cave on Okinawa Island, Ryukyu Islands, southwestern Japan (Crustacea, Amphipoda, Seborgiidae) Author Shimomura, Michitaka 0000-0002-2524-7160 Seto Marine Biological Laboratory, Kyoto University, 459 Shirahama-cho, Nishimuro-gun, Wakayama 649 - 2211, Japan. shimomura. michitaka 5 w @ kyoto-u. ac. jp; https: // orcid. org / 0000 - 0002 - 2524 - 7160 shimomura.michitaka5w@kyoto-u.ac.jp Author Fujita, Yoshihisa Okinawa Prefectural University of Arts, 1 - 4 Shuri-Tonokura, Naha, Okinawa 903 - 8602, Japan. text Zootaxa 2021 2021-02-11 4927 1 133 142 journal article 8128 10.11646/zootaxa.4927.1.9 9c0d1bc5-ef52-4ac2-868a-404f96e14820 1175-5326 4534081 6C5A8B9F-F813-4240-B584-F94A87260456 Seborgia cavernicola sp. nov. [New Japanese name: Dokutsu-mijin-yokoebi] Figs 1–5 Material examined. Holotype , 1.16 mm (KMNH IvR 500966), sex unknown (lacking oostegites or penile papillae), submarine cave “ Hedo Dome (Ginama) cave ”, located at the northern end of Okinawa Island, collected by Y. Fujita , 10 August 2016 . Paratype , 0.89 mm (KMNH IvR 500967), sex unknown, collection data as for holotype. Description of the holotype . Body ( Fig. 1A ) integument without microspinules. Head ( Fig. 1 A–C) with triangular rostrum, reaching distally proximal half of first article of antennular peduncle; head lobe well developed, evenly rounded, extending slightly beyond tip of rostrum; antennary sinus broad, shallowly excavated; posterodorsal margin of pleonites lacking serrations. Eyes ( Fig. 1A, B ) faintly pigmented without ommatidia. Epimeral plates 1 and 2 ( Fig. 1A ) with posterodistal angle weakly produced into pointed process, lacking seta on distal margin; plate 1 longer than plate 2; plate 2 as large as plate 3. Antennule ( Figs 1A , 2A ) broken off excluding peduncle articles 2 and 3: article 1 with two dorsal and one distolateral penicillate setae; article 2 slightly shorter and narrower than article 1, distally with three penicillate and two simple setae. Antenna ( Figs 1A , 2B ): gland cone short, directed anteriorly; peduncle articles 4 and 5 length ratio 14:15; flagellum 4-articulated, shorter than peduncle article 5, each article with short aesthetasc distally. Labrum ( Fig. 2C ) with slightly produced epistome; anterior margin sparsely ornamented with long and stout setules; both lobes of bilobed distal margin microspinulate medially. Paragnaths ( Fig. 3D ): inner lobes shorter than outer, distally with short setules; outer lobes distally with short setules and what seems to be pair of stout setae with truncate tip. Left mandible ( Fig. 2D, E ): incisor and lacinia mobilis coarsely strongly dentate, 4- and 5-dentate, respectively; palp articles length ratio 27: 61: 40; article 2 distally with 4 setulate setae; article 3 with three rows of A-setae, 3 D-setae and 3 E-setae. Right mandible ( Fig. 2F ): molar process with one long pappose seta and densely-set setules anteriorly. Maxillule ( Fig. 3A ) with smooth conical coxal endite; basal endite with 7 stout setae distally and 5 fine simple setae mediodistally. Endopod articles length ratio 11:17; proximal article with two fine simple setae distally; distal article with three short stiff and 5 fine simple setae distally. Maxilla ( Fig. 3B ) bilobed; outer lobe with two setulate setae apically, and with three long setae and 6 short simple setae laterally; inner lobe with one long setulate seta apically, two long and 4 short simple setae laterally and with 5 short simple setae medially. Maxilliped ( Fig. 3C ) palp relative length of carpus: propodus: dactylus: unguis as 46: 42: 17: 7. Carpus with 9 long and three short simple setae mediodistally; propodus with 5 long and some short setae mediodistally and 16 long simple setae laterally; dactylus with many short setae distally. Pereopodal coxal plate 1 ( Figs 1A , 3E ) 1.5 times as long as wide, with three short setae ventrally, anterior margin of plate strongly convex, posterior margin nearly straight. Coxal plate 2 ( Figs 1A , 3G ) 2.3 times as long as wide, with two short setae ventrally and one short seta posteriorly; anterior margin of plate slightly convex, posterior margin nearly straight. Coxal plate 3 ( Figs 1A , 3B ) 2.5 times as long as wide, with two short setae ventrally; anterior margin of plate slightly convex, posterior margin slightly concave. Coxal plate 4 ( Figs 1A , 4D ) 1.7 times longer than wide, widest just below square posterior excavation; anterior margin of plate slightly convex, posterior margin nearly straight; excavation occupying proximal 15% of posterior margin. Coxal plate 5 ( Figs 1A , 4F ) with two short setae ventrally; anterior margin of plate concave, posterior margin nearly straight. Coxal plate 6 ( Figs 1A , 4F ) with one short seta ventrally; anterior margin of plate convex, posterior margin slightly convex. Coxal plate 7 ( Figs 1A , 5C ) with one short seta on posterior corner. Gnathopod 1 ( Fig. 3E, F ) subchelate: basis longest article, with one short simple seta posterodistally; ischium trapezoidal, with one short simple seta posterodistally; merus trapezoidal, as long as ischium, with three long simple setae posterodistally; carpus subtriangulate, with 5 long simple setae posterodistally; propodus 1.1 times as long as wide; palm angle evenly rounded, located at 45% of maximum length of article, with two stout robust bifid setae. Palm margin nearly straight, covered with short triangular denticles and with two short setae located submarginally. Dactylus with terminal spine on medial margin. Unguis long and slender, 29% length of dactylus. Gnathopod 2 ( Fig. 3G, H ) subchelate: basis longest article, with two long simple setae posteriorly; ischium trapezoidal, with one long simple seta posterodistally; merus quadrate, as long as ischium, with one long simple seta posterodistally; carpus subtriangular, with 4 long simple setae posterodistally; propodus subrectangular, as long as wide, with one long simple seta medially; palm margin slightly convex, covered with short triangular denticles and with three short setae submarginally; palm angle armed with one stout robust bifid, two short robust and one slender setae; posterior margin with many fine setae. Dactylus with short unguis; unguis 15% length of dactylus. Pereopods 3 and 4 ( Fig. 4A, C ) slender, similar, with unguis not incorporated into dactylus. Pereopod 5 ( Fig. 4E ): basis moderately broad, 1.9 times as long as wide: both margins of basis convex. Pereopods 6 and 7 ( Fig. 5A, B ): bases broad, 1.6 and 1.3 times as long as wide, respectively; posterior margins strongly convex. Pleopods 1–3 ( Fig. 5 D–G) similar in shape: protopods smooth, lacking setulation, each with 2 retinacles (= coupling hooks); both rami 3-articulate, with plumose setae. Uropods ( Figs 1A, D , 5H ) progressively shorter towards posterior. Uropod 1 ( Figs 1A , 5H ) protopod slightly longer than rami, with one short robust bifid seta posterodistally; endopod and exopod subequal in length, each with one short robust bifid setae posteriorly; setae on both rami located around subterminal notch. Uropod 2 ( Fig. 1A, D ) similar to uropod 1 except for its length: protopod shorter than endopod, with one short robust bifid seta posterodistally; endopod 1.5 times as long as exopod, with two robust bifid setae posterodistally; exopod with one robust bifid seta posterodistally. Uropod 3 ( Fig. 1A, D ) without setae: protopod 1.2 times as long as endopod; endopod longer than exopod; both rami without notches subapically. Telson ( Fig. 1D ) 1.5 times as long as wide, with evenly rounded distal margin, without setae. Description of the paratype . Antennule ( Fig. 2G, H ) peduncle articles length ratio 32: 21: 21. Peduncle-tomain flagellum length ratio 37: 45. Main flagellum 4-articulate: article 2 longest; articles 2–4 each with short aesthetasc distally. Accessory flagellum bearing two simple and one penicillate setae apically. Remarks . Seborgia cavernicola sp. nov. can be readily identified and separated from other species of the genus by the following combination of characters: body without microspinules; eyes fainted, without ommatidia; epimeral plates with posterodistal angle produced into pointed process; main flagellum of antennule 4-articulate; article 2 of main flagellum of antennule longest; propodus of gnathopod 2 as long as wide; gland cone of antenna directed anteriorly; maxilla bilobed; uropod 3 biramous; and telson without setae and setules. Seborgia cavernicola sp. nov. is close to S. sanctensis Jaume et al. , 2009 from an anchialine pool on Vanuatu ( Jaume et al. 2009 ), with which it shares a bilobed maxilla and biramous uropod 3. Seborgia cavernicola sp. nov. is distinguished from S. sanctensis by the following features (those of S. sanctensis in parentheses): body integument without microspinules (with densely-set microspinules); eyes fainted, without ommatidia (fully developed); article 2 of main flagellum of antennule longest (article 5 longest); gland cone of antenna directed anteriorly (directed posteriorly); excavation of coxal plate 4 occupying proximal 15% of posterior margin (26%); propodus of gnathopod 2 as long as wide (about 1.3 times as long as wide); and telson without setae (2 pairs of penicillate and 5 trifid setae dorsally). Habitat and biological implication. The submarine cave popularly known as “Hedo Dome” is located on the north coast of Okinawa Island , Ryukyu Island , southwestern Japan . The cave’s entrance lies at 15 m below sea level , and is 14 m wide and 3 m high. The interior of the cave is about 53 m long, and is continued by a stepwise tunnel, with some frontage road. The innermost part of the cave ends in an air pocket, and the water was very clear with the salinity about 23 ‰ and the temperature 24.5 °C. Seborgia cavernicola sp. nov. was collected only at the innermost part of the cave . FIGURE 1. Seborgia cavernicola sp. nov. holotype: A, habitus, lateral; B, head, lateral; C, head and peduncle articles 1 and 2 of antennule, dorsal; D, pleonites 2 and 3, telson and uropods 2 and 3, dorsal. Scales = 100 µm. FIGURE 2. Seborgia cavernicola sp. nov. A–F, holotype, G, H, paratype (KMNH IvR 500967): A, right antennule, lateral; B, right antenna, lateral; C, labium, dorsal; D, incisor of left mandible, medial; E, palp of left mandible, ventral; F, right mandible, ventral; G, head, antennule and antenna, lateral; H, left antennule, lateral. Scales = 100 µm. FIGURE 3. Seborgia cavernicola sp. nov. holotype: A, left maxillule, dorsal; B, right maxilla, ventral; C, left maxilliped, dorsal; D, lower lip, dorsal; E, left pereopod 1, medial; F, distal part of left pereopod 1, medial; G, left pereopod 2, medial; H, distal part of left pereopod 2. Scales = 100 µm. FIGURE 4. Seborgia cavernicola sp. nov. holotype: A, left pereopod 3, lateral; B, left coxal plate 3, lateral; C, left pereopod 4, lateral; D, left coxal plate 4 medial; E, left pereopod 5, lateral; F, left coxal plates 5 and 6, lateral. Scale = 100 µm. FIGURE 5. Seborgia cavernicola sp. nov. holotype: A, left pereopod 6, medial; B, left pereopod 7, lateral; C, left coxal plate 7 and pleonite 1, lateral; D, left pleopod 1, medial; E, seta on inner ramus of left pleopod 1, medial; F, left pleopod 2, medial; G, left pleopod 3; H, left uropod 1, medial. Scale = 100 µm. A thermosbaenacean, Halosbaena okinawaensis Shimomura & Fujita, 2017 (Halosbaenidae) was recently recorded from this cave ( Shimomura & Fujita 2017 ). The other cave fauna inhabiting various portions of this cave, including sponges, molluscs, and decapod crustaceans, will be reported in a forthcoming paper. Etymology . The specific epithet refers to the habitat of this new species.