A new species of Princaxelia from Shinkai Seep Field, Mariana Trench (Crustacea, Amphipoda, Pardaliscidae) Author Tomikawa, Ko Graduate School of Humanities and Social Sciences, Hiroshima University, Higashi-Hiroshima 739 - 8524, Japan tomikawa@hiroshima-u.ac.jp Author Watanabe, Hiromi Kayama X-STAR, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2 - 15 Natsushima-cho, Yokosuka, Kanagawa 237 - 0061, Japan Author Tanaka, Katsuhiko Department of Marine Biology, School of Marine Science and Technology, Tokai University, 3 - 20 - 1, Orido, Shimizu, Shizuoka, Shizuoka 424 - 8610, Japan Author Ohara, Yasuhiko Hydrographic and Oceanographic Department of Japan, 3 - 1 - 1 Kasumigaseki, Chiyoda-ku, Tokyo 100 - 8932, Japan & Research Institute for Marine Geodynamics (IMG), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2 - 15 Natsushima-cho, Yokosuka, Kanagawa 237 - 0061, Japan & Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464 - 8602, Japan text ZooKeys 2021 2021-02-04 1015 115 127 http://dx.doi.org/10.3897/zookeys.1015.59683 journal article http://dx.doi.org/10.3897/zookeys.1015.59683 1313-2970-1015-115 9CD39543892243088FCBF91306EC3806 6A7A97D24F8B582BA5549DDC3ADAD8DE Princaxelia marianaensis Tomikawa & Watanabe sp. nov. Figures 2 , 3 , 4 , 5 Material examined. Holotype : female (BL 23.9 mm), AMNH _IZC 00361360, the surface of the chimney which was named as "Chim 4" in CH 3 site in the Shinkai Seep Field ( Okumura et al. 2016 ), Mariana Trench ( 11°39.36'N , 143°2.88'W ), 5,689-5,683 m, collected by H. K. Watanabe, 17 July 2014. Diagnosis. Posterodistal corner of epimeral plate 3 quadrate. Primary flagellum article 1 of female antenna 1 not elongate; accessory flagellum article 1 longer than each of the articles 2-6. Maxilla 1 inner plate with 1 terminal plumose seta; palp article 2 expanded, with 8 or 9 apical robust setae. Dactylus of gnathopods 1 and 2 with three strong projections on posterior margin proximal to base. Dorsal margin of coxa 5 highest at proximal end. Venral margin of coxa 7 weakly concave. Telson lobe uniformly tapering distally. Description (female). Head (Fig. 2 ) as long as pereonites 1 and 2 combined; rostrum short, pointed; lateral cephalic corner rounded; eyes absent. Pleon (Fig. 2 ) with dorsal surfaces of pleonites 1-3 smooth; epimeral plates 1-3 (Fig. 3A-C ) with setae on ventral submargin and posterior margin; posterodistal corner of epimeral plates 2 and 3 quadrate. Dorsal margin of urosomites 1 and 2 (Fig. 2 ) with distally oriented projection. Figure 2. Princaxelia marianaensis Tomikawa & Watanabe, sp. nov., holotype female (BL 23.9 mm). Habitus, lateral view. Figure 3. Princaxelia marianaensis Tomikawa & Watanabe, sp. nov., holotype female (BL 23.9 mm) A epimeral plate 1, lateral view B epimeral plate 2, lateral view C epimeral plate 3, lateral view D antenna 1, lateral view, some distal articles of primary flagellum omitted E antenna 2, lateral view, flagellum omitted F upper lip, anterior view G left mandible, medial view H left mandible, medial view I right mandible, medial view J lower lip, anterior view K maxilla 1, dorsal view L palp of maxilla 1, dorsal view M maxilla 2, dorsal view N maxilliped, dorsal view. Antenna 1 (Fig. 3D ) length 0.3 times BL (distal part broken off); peduncular articles 1-3 with length ratio 1.0: 0.7: 0.3; peduncular article 1 broadened, with anterolateral cluster of setae, some weakly plumose; posterior margin of peduncular articles 2 and 3 with clusters of short setae; primary flagellum article 1 length 1.2 times width, 3.0 times as long as article 2; accessory flagellum 6-articulated, article 1 0.9 times as long as articles 2-6 combined; primary flagellum with at least 47 articles. Antenna 2 (Fig. 3E ) length 0.4 times BL; anterior margin of peduncular article 2 with setae; peduncular articles 4 and 5 with clusters of short setae on anterior margin, article 4 1.1 times longer than article 5; flagellum with 42 articles. Upper lip (Fig. 3F ) asetose, with asymmetrically incised ventral margin. Mandibles (Fig. 3G-I ) slightly asymmetric, incisor margins broad, anteroventral corner with strong tooth; left lacinia mobilis (Fig. 3H ) broad, about 0.7 times as long as incisor, multi-dentate; right incisor (Fig. 3I ) with three teeth on proximal to anterodorsal corner; right lacinia weak, with two teeth; accessory setal row of left and right mandibles each with about 20 robust setae; molar absent; mandibular palp 3-articulated with length ratio 1.0: 1.7: 1.5; article 1 asetose; article 2 posteriorly reflected, articles 2 and 3 with 18 and 22 setae, respectively. Lower lip (Fig. 3J ) with broad outer and distinct inner lobes. Maxilla 1 (Fig. 3K, L ) with inner and outer plates and palp; inner plate small with apical plumose seta; outer plate subrectangular, with 9 robust apical setae and strong projection; palp 2-articulate; article 1 with marginal setae; article 2 expanded distally with nine and eight robust setae on apical margin of left and right maxilla 1, respectively, and with apical submargin and medial margin lined with setae. Maxilla 2 (Fig. 3M ) with inner plate bearing row of 13 plumose setae along apical to medial margin; outer plate slightly longer than inner plate, with three apical plumose setae. Maxilliped (Fig. 3N ) with inner and outer plates and palp; inner plate small, subtriangular, not reaching base of palp, with plumose apical seta and short subapical seta; outer plate oval, reaching base of article 2 of palp, with setae along apical to medial margin; palp 4-articulate, long: article 2 longest with inner marginal rows of setae, article 3 with clusters of setae on dorsal and ventral faces and medial marginal setae, and article 4 slender, with robust setae on medial margin. Gnathopod 1 (Fig. 4A, B ) coxa subrectangular, length 1.8 times width, ventral margin straight, posterior submargin and medial face with setae; basis arched, with anterior and posterior margins with numerous setae in a row; posterior margin of merus with sparse setae; carpus oval, length 2.5 times width, posterior margin and medial face setose; propodus slender, length 0.6 times that of carpus, posterior margin weakly convex with setae; dactylus slender, slightly curved, posterior margin with three strong projections proximal to base. Gnathopod 2 (Fig. 4C, D ) coxa tapering anteriorly, length 1.8 times width, posterior submargin with setae; basis slender and straight, anterior and posterior margins densely setose; carpus widely produced posteriorly with numerous long setae, length 2.3 times width; propodus and dactylus similar to gnathopod 1. Figure 4. Princaxelia marianaensis Tomikawa & Watanabe, sp. nov., holotype female (BL 23.9 mm) A gnathopod 1, lateral view B dactylus of gnathopod 1, lateral view C gnathopod 2, lateral view D dactylus of gnathopod 2, lateral view E pereopod 3, lateral view F dactylus of pereopod 3, lateral view G pereopod 4, lateral view H pereopod 5, lateral view I dactylus of pereopod 5 J pereopod 6, lateral view K pereopod 7, lateral view. Pereopod 3 (Fig. 4E, F ) coxa weakly rounded ventrally, with submarginal setae; basis long, posterior margin strongly setose; merus, carpus, propodus, and dactylus in length ratio 1.0: 1.4: 1.4: 0.5; posterior margin of propodus lined with short setae. Pereopod 4 (Fig. 4G ) similar to pereopod 3, with coxa tapering anteriorly. Pereopod 5 (Fig. 4H, I ) coxa subtriangular, dorsal margin highest at proximal end, anterior and ventral submargins with setae; basis length 2.9 times width, with clusters of setae on anterior margin proximal to base, posterodistal corner weakly produced; merus, carpus, propodus, and dactylus in length ratio 1.0: 0.8: 1.2: 0.3; carpus and propodus with robust setae on anterior and posterior margins. Pereopod 6 (Fig. 4J ) coxa weakly concave; basis length 2.5 times width, posterodistal corner quadrate; merus, carpus, propodus, and dactylus in length ratio 1.0: 1.0: 1.2: 0.3. Pereopod 7 (Fig. 4K ) coxa weakly concave; basis length 1.9 times width, weakly expanded anteriorly, posterodistal corner quadrate. Coxal gills (Fig. 2 ) on gnathopod 2, pereopods 3-6; coxal gills 2-4 elongate, coxal gill 2 longest, its length exceeding the distal part of basis of gnathopod 2, coxal gill 6 shortest. Pleopods 1-3 (Fig. 5A-C ) each with paired retinacula (Fig. 5B ) on inner distal margin of peduncle, and bifid (clothespin) setae (Fig. 5C ) on inner basal margin of inner ramus; rami articles wide and flattened. Figure 5. Princaxelia marianaensis Tomikawa & Watanabe, sp. nov., holotype female (BL 23.9 mm) A pleopod 1, anterior view, some setae on rami omitted B retinacula on peduncle of pleopod 1, anterior view C bifid (clothespin) plumose seta on inner basal margin of inner ramus of pleopod 1, anterior view D uropod 1, dorsal view, distal part of outer ramus broken E uropod 2, dorsal view F telson, dorsal view. Princaxelia jamiesoni Loerz , 2010, holotype female (BL 56.2 mm) G palp of maxilla 1, dorsal view H dactylus of gnathopod 1, lateral view I dactylus of gnathopod 2, medial view J telson, dorsal view. Uropod 1 (Fig. 5D ) peduncle longer than rami, with 14 basofacial setae, distomedial peduncular projection very strong; inner ramus length 0.8 times that of peduncle, outer ramus distally damaged, rami with setal row along medial and lateral margins. Uropod 2 (Fig. 5E ) peduncle slightly longer than rami, with four basofacial setae, distomedial peduncular spine shorter than that of uropod 1; inner ramus length 1.2 times that of outer ramus, rami with setal row along medial and lateral margins. Uropod 3 missing (damaged). Telson (Fig. 5F ) length 2.3 times width, with cleft extending 80% its length; lobes tapering distally with facial setae; apex of each lobe shallowly incised with small robust seta. Etymology. The specific name is an adjective derived from the type locality, the Mariana Trench. DNA sequence. A single nucleotide sequence of COI was obtained from the holotype (AMNH_IZC 00361360; 658 bp). Remarks. The morphologies of P. marianaensis sp. nov. and congeners are summarized in Table 1 . Princaxelia marianaensis sp. nov. is most similar to P. abyssalis Dahl, 1959 in having a short first flagellar article of the female antenna 1, a weakly setose maxilla 1, coxa 5 with its dorsal margin highest at the proximal end and its distal margin rounded, and a uniformly tapering telson. However, P. marianaensis sp. nov. differs from the description of P. abyssalis in having the posterodistal corner of epimeral plate 3 quadrate in P. marianaensis sp. nov. but rounded in P. abyssalis ; the accessory flagellum article 1 of the female antenna 1 longer than each of the articles 2-6 in P. marianaensis sp. nov. but equal to the length of the remaining segments in P. abyssalis ; and the ventral margin of the coxa 7 weakly concave in P. marianaensis sp. nov. but straight in P. abyssalis . Table 1. Morphological comparison of Princaxelia species.

-	P. marianaensis Tomikawa & Watanabe, sp. nov.	P. abyssalis Dahl, 1959	P. jamiesoni Loerz , 2010	P. magna Kamenskaya, 1977	P. stephenseni Dahl, 1959
Maximum body size	female 23.9 mm	male 21 mm, female 32 mm	male 57 mm, female 61 mm	male 52 mm	male 10 mm, female 11 mm
Epimeral plate 3 posterodistal corner	quadrate	rounded	quadrate	quadrate	weakly rounded
Dorsal projections on urosomites 1 and 2	pointing toward distal end	unknown	pointing toward distal end	pointing upright	pointing toward distal end
Upper lip	strongly asymmetrical	unknown	slightly asymmetrical	strongly asymmetrical	nearly asymmetrical
Maxilla 1 palp article 2	expanded	expanded	expanded	expanded	not expanded
Maxilla 1 palp article 2	9 apical robust setae	less than 14 apical robust setae	25 apical robust setae	approx. 10 apical robust setae	7 apical robust setae
Maxilla 1 inner plate	1 plumose seta	1 plumose seta	1 plumose seta	6 plumose setae	1 plumose seta
Female antenna 1 primary flagellum article 1	not elongated	not elongated	elongated	unknown	elongated
Female antenna 1 accessory flagellum article 1	longer than each of the rest	equal to length of remaining articles	longer than each of the rest	unknown	unknown
Gnathopods 1 and 2 dactyli	3 strong projections near the base	unknown	8-9 strong projections near the base	4 strong projections near the base	unknown (absent?)
Coxa 5 dorsal margin	highest at proximal end	highest at proximal end	straight	convex	straight / convex
Coxa 5 distal margin	rounded	rounded	rounded	slightly pointed	straight
Coxa 7 ventral margin	shallowly concave	straight	slightly concave	slightly concave	straight
Telson lobe	uniformly tapering distally	uniformly tapering distally	tapering from distal 1/3	weakly tapering distally	unknown
References	This study	Dahl (1959)	Loerz (2010) ; this study	Kamenskaya (1977)	Dahl (1959) ; Loerz (2010)

Princaxelia jamiesoni Loerz , 2010 was described from 7,703 m and 9,316 m in the Japan and Izu-Ogasawara trenches, respectively ( Loerz 2010 ), and subsequently from 7,055-9,583 m in the Kurile-Kamchatka Trench ( Jazdzewska and Mamos 2019 ). Examination of the holotype of P. jamiesoni reveals new features not originally described which facilitate differentiation of this species from P. marianaensis sp. nov.: the palp article 2 of the maxilla 1 bears eight or nine robust apical setae in P. marianaensis sp. nov. but 25 robust apical setae in P. jamiesoni (Fig. 5G ); the dactylus of gnathopods 1 and 2 has three strong projections proximal to its base in P. marianaensis sp. nov., but eight or nine strong projections proximal to the base of the dactylus in P. jamiesoni (Fig. 5H, I ); and the telson lobe uniformly tapers distally in P. marianaensis sp. nov. but tapers from the distal 1/3 in P. jamiesoni (Fig. 5J ). While two projections on the dactylus of the left gnathopod 2 were originally described for P. jamiesoni , we report nine projections on the right gnathopod 2 of the holotype; we believe that Loerz (2010) described the damaged left gnathopod 2. The morphology of Princaxelia is consistent with an animal that swims in that its body is streamlined, flat, and has well-developed pleopods ( Loerz 2010 ). Analyses of the locomotion of Princaxelia species demonstrate they have a high swimming ability - a trait useful for preying on other amphipods in hadal trenches ( Jamieson et al. 2012 ). Amphipods lack a planktonic larval stage and generally have low dispersal ability ( Chapman 2007 ). Judging from known habitat depths of Princaxelia , with the exception of the bathypelagic P. stephenseni , the distributions of species might be expected to be restricted to individual trenches. However, P. abyssalis , and especially P. jamiesoni , are reported from multiple trenches (Fig. 6 ) ( Kamenskaya 1981 , 1997 ; Loerz 2010 ; Jazdzewska and Mamos 2019 ). Deep-sea amphipod species previously regarded as widely distributed have since been found to contain cryptic species (e.g., Narahara-Nakano et al. 2018 ). Loerz (2010) also considered that P. abyssalis , as reported from multiple trenches by Kamenskaya (1981) , may contain other or undescribed species. It is possible that P. abyssalis and P. jamiesoni represent species complexes, but a greater understanding of species diversity of this hadal-dwelling genus will require additional genetic and morphological analyses. Figure 6. Geographical distributions of the species of Princaxelia (map data from GEBCO Compilation Group [2020] ). The exact location of the distribution of P. abyssalis in the Aleutian Trench is uncertain.