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

Ko Tomikawa; Hiromi Kayama Watanabe; Katsuhiko Tanaka; Yasuhiko Ohara

doi:10.3897/zookeys.1015.59683

Abstract

A new pardaliscid amphipod, Princaxelia marianaensis sp. 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.

Keywords

COI, deep sea, first record, hadal zone, Princaxelia marianaensis, systematics

Introduction

The benthic amphipod genus Princaxelia Dahl, 1959 occurs in deep waters of the Pacific Ocean (Lörz 2010). To date, four species have been described: P. abyssalis Dahl, 1959 from 6,435–9,530 m in the Aleutian, Kurile-Kamchatka, Izu-Ogasawara, Yap, Japan, Philippine, Bougainville, and Kermadec Trenches (Kamenskaya 1981, 1997); P. jamiesoni Lörz, 2010 from 7,055–9,583 m in the Kurile-Kamchatka, Japan, and Izu-Ogasawara Trenches (Lörz 2010; Jażdżewska and Mamos 2019); P. magna Kamenskaya, 1977 from 7,190–7,250 m in the Yap Trench; and P. stephenseni Dahl, 1959, the type species of the genus, from 1,505 m off the coast of Iceland. Princaxelia abyssalis and P. jamiesoni are reported to prey on other amphipods, suggesting that this genus is carnivorous (Jamieson et al. 2012).

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. 2013, 2016; Chen 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.

Material and methods

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.

Figure 1.

Sampling location and habitat of Princaxelia marianaensis Tomikawa & Watanabe, sp. nov. A map indicating sampling location (circle) (map data from GEBCO Compilation Group [2020]) B sampling site at 5,686 m depth (Okumura et al. 2016).

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).

Systematics

Family Pardaliscidae Boeck, 1871

Genus Princaxelia Dahl, 1959

Princaxelia marianaensis Tomikawa & Watanabe, sp. nov.

http://zoobank.org/B127A8B4-7BDA-4027-A7DA-8C04F61EA6BA

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 Lörz, 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.

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Morphological comparison of Princaxelia species.

	P. marianaensis Tomikawa & Watanabe, sp. nov.	P. abyssalis Dahl, 1959	P. jamiesoni Lörz, 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)	Lörz (2010); this study	Kamenskaya (1977)	Dahl (1959); Lörz (2010)

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, 1997; Lörz 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.

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.

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).

1	Palp article 2 of maxilla 1 expanded	2
–	Palp article 2 of maxilla 1 not expanded	P. stephenseni Dahl, 1959
2	Inner plate of maxilla 1 with 1 terminal plumose seta	3
–	Inner plate of maxilla 1 with several plumose setae	P. magna Kamenskaya, 1977
3	Primary flagellum article 1 of female antenna 1 not elongate; dorsal margin of coxa 5 highest at proximal end; telson lobe uniformly tapering distally	4
–	Primary flagellum article 1 of female antenna 1 elongate; dorsal margin of coxa 5 straight; telson lobe tapering from distal 1/3	P. jamiesoni Lörz, 2010
4	Posterodistal corner of epimeral plate 3 rounded; accessory flagellum article 1 of female antenna 1 equal to length of remaining articles; ventral margin of coxa 7 straight	P. abyssalis Dahl, 1959
–	Posterodistal corner of epimeral plate 3 quadrate; accessory flagellum article 1 of female antenna 1 longer than each of remaining articles; ventral margin of coxa 7 weakly concave	P. marianaensis Tomikawa & Watanabe, sp. nov.

Acknowledgements

We thank the captain and crew of the R/V Yokosuka and the operation team of DSV Shinkai 6500 for their cooperation in the YK14-13 cruise led by Y.O. The Shinkai Seep Field is a part of the Mariana Trench Marine National Monument of the United States of America, and we greatly appreciate the U.S. Fish and Wildlife Service for approving our study in the monument under the special use permit #12541-14001. The sample number under this permit used in the present study is #12541-14001-6K-1402-B01. This work was partly supported by the Japan Society for the Promotion of Science KAKENHI grants JP17K15174 and JP17H00820 to the first author.

References

Boeck A (1871) Crustacea amphipoda borealia et arctica. Forhandlinger i Videnskabs-Selskabet i Christiania 1870: 83–279. https://doi.org/10.5962/bhl.part.9800

Chapman JW (2007) Gammaridea. In: Carlton JT (Ed.) The Light and Smith manual: Intertidal Invertebrates from Central California to Oregon (4^th edn. ). University of California Press, Berkeley, 545–618.

Chen C, Watanabe HK, Ohara Y (2018) A very deep Provanna (Gastropoda: Abyssochrysoidea) discovered from the Shinkai Seep Field, Southern Mariana Forearc. Journal of the Marine Biological Association of the United Kingdom 98(3): 439–447. https://doi.org/10.1017/S0025315416001648

Dahl E (1959) Amphipoda from depth exceeding 6000 meters. In: Galathea Report: Scientific Results of the Danish Deep-Sea Expedition Round the World 1950–1952 (Vol. 1). Danish Science Press, Copenhagen, 211–241.

Folmer O, Black M, Hoeh W, Lutz R, Vrijenhoek R (1994) DNA primers for amplification of mitochondrial cytochrome c oxidase subunit I from diverse metazoan invertebrates. Molecular Marine Biology and Biotechnology 3: 294–299.

GEBCO Compilation Group (2020) GEBCO 2020 Grid – a continuous terrain model of the global oceans and land. British Oceanographic Data Centre, National Oceanography Centre, NERC. https://doi.org/10.5285/a29c5465-b138-234d-e053-6c86abc040b9

Jamieson AJ, Lörz AN, Fujii T, Priede IG (2012) In situ observations of trophic behavior and locomotion of Princaxelia amphipods (Crustacea: Pardaliscidae) at hadal depths in four West Pacific Trenches. Journal of the Marine Biological Association of the United Kingdom 92: 143–150. https://doi.org/10.1017/S0025315411000452

Jażdżewska AM, Mamos T (2019) High species richness of Northwest Pacific deep-sea amphipods revealed through DNA barcoding. Progress in Oceanography 178: e102184. https://doi.org/10.1016/j.pocean.2019.102184

Kamenskaya OE (1977) Two new species of ultraabyssal amphipods from Yap Trench. Akademia Nauk SSSR 108: 105–114. [in Russian]

Kamenskaya OE (1981) The amphipods (Crustacea) from deep-sea trenches in the western part of the Pacific Ocean. Trudy Instituta Okeanologii 115: 94–107. [in Russian]

Kamenskaya OE (1997) Peculiarities of the vertical distribution of the amphipods in the region of the Kuril-Kamchatka Trench. Russian Academy of Sciences, P.P. Shirshov Institute of Oceanology, Moscow, 141–168. [in Russian]

Lörz AN (2010) Trench treasures: the genus Princaxelia (Pardaliscidae, Amphipoda). Zoologica Baetica 21: 65–84.

Narahara-Nakano Y, Nakano T, Tomikawa K (2018) Deep-sea amphipod genus Eurythenes from Japan, with a description of a new Eurythenes species from off Hokkaido (Crustacea: Amphipoda: Lysianassoidea). Marine Biodiversity 48: 603–620. https://doi.org/10.1007/s12526-017-0758-4

Ohara Y, Reagan MK, Fujikura K, Watanabe H, Michibayashi K, Ishii T, Stern RJ, Pujana I, Martinez F, Girard G, Ribeiro J, Brounce M, Komori N, Kino M (2012) A serpentinite-hosted ecosystem in the southern Mariana forearc. Proceedings of the National Academy of Sciences of the USA 109(8): 2831–2835. https://doi.org/10.1073/pnas.1112005109

Okumura T, Ohara Y, Stern RJ, Yamanaka T, Onishi Y, Watanabe H, Chen C, Bloomer SH, Pujana I, Sakai S, Ishii T, Takai K (2016) Brucite chimney formation and carbonate alteration at the Shinkai Seep Field, a serpentinite‐hosted vent system in the southern Mariana forearc. Geochemistry, Geophysics, Geosystems 17(9): 3775–3796. https://doi.org/10.1002/2016GC006449

Okutani T, Fujikura K, Watanabe H, Ohara Y (2013) Calyptogena (Abyssogena) mariana: discovery of a new vesicomyid clam from the Mariana Trench. Venus [Journal of the Malacological Society of Japan] 71: 39–47. https://doi.org/10.18941/venus.71.1-2_39

Tomikawa K, Kobayashi N, Kyono M, Ishimaru S-I, Grygier MJ (2014) Description of a new species of Sternomoera (Crustacea: Amphipoda: Pontogeneiidae) from Japan, with an analysis of the phylogenetic relationships among the Japanese species based on the 28S rRNA gene. Zoological Science 31: 475–490. https://doi.org/10.2108/zs140026

Tomikawa K, Nakano T, Hanzawa N (2017) Two new species of Jesogammarus from Japan (Crustacea, Amphipoda, Anisogammaridae), with comments on the validity of the subgenera Jesogammarus and Annanogammarus. Zoosystematics and Evolution 93: 189–210. https://doi.org/10.3897/zse.93.12125

Watanabe HK, Chen C, Nishi E, Ohara Y (in press) Cryptic diversity of the tube-dwelling polychaete Phyllochaetopterus in Shinkai Seep Field, Mariana Trench. Plankton and Benthos Research.