Princaxelia marianaensis Tomikawa & Watanabe, 2021

Tomikawa, Ko, Watanabe, Hiromi Kayama, Tanaka, Katsuhiko & Ohara, Yasuhiko, 2021, A new species of Princaxelia from Shinkai Seep Field, Mariana Trench (Crustacea, Amphipoda, Pardaliscidae), ZooKeys 1015, pp. 115-127 : 115

publication ID	https://dx.doi.org/10.3897/zookeys.1015.59683
publication LSID	lsid:zoobank.org:pub:9CD39543-8922-4308-8FCB-F91306EC3806
persistent identifier	https://treatment.plazi.org/id/B127A8B4-7BDA-4027-A7DA-8C04F61EA6BA
taxon LSID	lsid:zoobank.org:act:B127A8B4-7BDA-4027-A7DA-8C04F61EA6BA
treatment provided by	ZooKeys by Pensoft
scientific name	Princaxelia marianaensis Tomikawa & Watanabe
status	sp. nov.

Treatment

Princaxelia marianaensis Tomikawa & Watanabe sp. nov. Figures 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , 5 View Figure 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 View Figure 2 ) as long as pereonites 1 and 2 combined; rostrum short, pointed; lateral cephalic corner rounded; eyes absent. Pleon (Fig. 2 View Figure 2 ) with dorsal surfaces of pleonites 1-3 smooth; epimeral plates 1-3 (Fig. 3A-C View Figure 3 ) 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 View Figure 2 ) with distally oriented projection.

Antenna 1 (Fig. 3D View Figure 3 ) 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 View Figure 3 ) 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 View Figure 3 ) asetose, with asymmetrically incised ventral margin. Mandibles (Fig. 3G-I View Figure 3 ) slightly asymmetric, incisor margins broad, anteroventral corner with strong tooth; left lacinia mobilis (Fig. 3H View Figure 3 ) broad, about 0.7 times as long as incisor, multi-dentate; right incisor (Fig. 3I View Figure 3 ) 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 View Figure 3 ) with broad outer and distinct inner lobes. Maxilla 1 (Fig. 3K, L View Figure 3 ) 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 View Figure 3 ) 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 View Figure 3 ) 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 View Figure 4 ) 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 View Figure 4 ) 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.

Pereopod 3 (Fig. 4E, F View Figure 4 ) 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 View Figure 4 ) similar to pereopod 3, with coxa tapering anteriorly. Pereopod 5 (Fig. 4H, I View Figure 4 ) 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 View Figure 4 ) 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 View Figure 4 ) coxa weakly concave; basis length 1.9 times width, weakly expanded anteriorly, posterodistal corner quadrate.

Coxal gills (Fig. 2 View Figure 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 View Figure 5 ) each with paired retinacula (Fig. 5B View Figure 5 ) on inner distal margin of peduncle, and bifid (clothespin) setae (Fig. 5C View Figure 5 ) on inner basal margin of inner ramus; rami articles wide and flattened.

Uropod 1 (Fig. 5D View Figure 5 ) 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 View Figure 5 ) 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 View Figure 5 ) 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 View 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 .

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 View Figure 5 ); 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 View Figure 5 ); and the telson lobe uniformly tapers distally in P. marianaensis sp. nov. but tapers from the distal 1/3 in P. jamiesoni (Fig. 5J View Figure 5 ). 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 View Figure 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.