A new species of Princaxelia from Shinkai Seep Field, Mariana Trench (Crustacea, Amphipoda, Pardaliscidae)

Abstract A new pardaliscid amphipod, Princaxeliamarianaensissp. nov., is described from a single female captured at the Shinkai Seep Field, Mariana Trench, from a depth of 5,689–5,683 m. A key to species of Princaxelia is provided. This is the first species of Princaxelia to be described from the Mariana Trench, and the second report of this genus from this region.

The Shinkai Seep Field is a serpentinized, peridotite-hosted, cold-seep system which hosts an aggregation of chemosynthesis-based communities including Abyssogena clam, Provanna gastropod, and Phyllochaetopterus polychaete species. It is located northeast of the Challenger Deep, the deepest part of the Mariana Trench (Ohara et al. 2012;Okutani et al. 2013Okutani et al. , 2016Chen et al. 2018;Watanabe et al. in press). During one submersible dive on an expedition to this seep by R/V Yokosuka, a single specimen of a species referable to Princaxelia was collected. This is the first record of an identified Princaxelia species from the Mariana Trench. We here describe and illustrate this species as new.

Samples
The single Princaxelia specimen was collected from the Mariana Trench during dive 1402 of the deep-submergence vehicle (DSV) Skinkai 6500 aboard R/V Yokosuka (cruise YK14-13, PI: Yasuhiko Ohara) by H. K. Watanabe (Fig. 1). Aboard the ship, the specimen was fixed and preserved in 99.5% ethanol. The specimen was sorted by K. Tanaka in the laboratory.
The holotype of P. jamiesoni, which was collected from the Japan Trench, was borrowed from the Tsukuba Collection Center of the National Museum of Nature and Science, Tokyo (NSMT-Cr 21250, female BL 56.2 mm), for comparison.

Morphology
Appendages were dissected in 99% ethanol and mounted using gum chloral medium on glass slides with the aid of a stereomicroscope (Olympus SZX7). Appendages were examined by light microscopy (Nikon Eclipse Ni) and illustrated using a camera lucida. Body length (BL), from the tip of the rostrum to the base of the telson along the dorsal curvature, was measured to the nearest 0.1 mm. The only known specimen, the holotype, has been deposited in the collections of the American Museum of Natural History (AMNH).

PCR and DNA sequencing
Genomic DNA was extracted from pereopod muscle of the holotype following procedures detailed in Tomikawa et al. (2014). The primer set for the cytochrome c oxidase subunit I (COI) gene (LCO1490 and HCO2198; Folmer et al. 1994) was used for the polymerase chain reaction (PCR) and cycle sequencing reactions. PCR and sequencing followed the methods detailed by Tomikawa et al. (2017). The DNA sequence has been deposited with the International Nucleotide Sequence Database Collaboration (INSDC) through the DNA Data Bank of Japan (DDBJ).  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.
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.  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.
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 This study Dahl (1959) Lörz (2010; this study Kamenskaya (1977) Dahl (1959; Lörz (2010) 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. Princaxelia jamiesoni Lörz, 2010 was described from 7,703 m and 9,316 m in the Japan and Izu-Ogasawara trenches, respectively (Lörz 2010), and subsequently from 7,055-9,583 m in the Kurile-Kamchatka Trench (Jażdżewska 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 Lörz (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 (Lörz 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(Kamenskaya , 1997Lörz 2010;Jażdżewska 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). Lörz (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.
Key to species of Princaxelia modified from Lörz (2010) We added P. marianaensis sp. nov. to the key by Lörz (2010) and modified the key to include the characteristics of the telson, which was not considered by Lörz (2010).