Systematics

Order Amphipoda

Family Pardaliscidae Boeck, 1871

Genus Princaxelia Dahl, 1959

Princaxelia malohi

Duffy & Wainwright sp. nov.

246DFB8B-7AFE-5602-8CBB-560DDAF9A584

https://zoobank.org/E268BA46-CB89-458F-92EF-234240557140

Figs 2–8

Material examined.

Holotype. • Mature ♂, body length 35.1 mm; Tonga Trench (23°29.238'S, 174°59.890'W); 8,200 m; 18 July 2024; Baited lander. NMNZ accession number: CR.028062, GenBank® accession number CO1: PZ585448; 16S: PZ611777; 28S: PZ611786.

Other material.

• 2 ♂, collection data same as holotype. • 1 ♂, 20.0 mm; Tonga Trench (23°29.238'S, 174°59.890'W); 8,200 m; 18 July 2024; baited lander. • Juvenile, 16.6 mm; Tonga Trench (23°28.806'S, 174°59.906'W), 8,350 m; 18 July 2024; baited lander. GenBank® accession numbers 16S: PZ611774–PZ611776; 28S: PZ611783–PZ611785.

Diagnosis.

Primary flagellum article 1 of male antennae 1 elongate. Accessory flagellum of male antennae 1 article 1 enlarged and flattened, with two small articles at distal end. Maxilla 1 inner plate with two plumose setae; palp article 2 expanded with 11 or 12 apical robust setae. Maxilla 2 outer plate with more than three plumose setae. Maxilliped inner plate with two apical setae. Dactylus of gnathopods 1 and 2 with a single strong projection on posterior margin near the base. Dorsal margin of coxa 5 straight. Ventral margin of coxa 7 shallowly concave. Telson lobes uniformly tapering distally with one apical spine per lobe.

Description based on holotype, male.

Head. (Fig. 4A–C) Small, similar to length of pereonite 1; rostrum very short, slightly pointed; lateral cephalic lobe prominent; corner rounded, angled distally upward; eyes absent. Antennae 1 peduncle articles 1–3 with length ratio 1.0: 0.64: 0.35; peduncular article one broadened, with cluster of setae on ventral margin, some weakly plumose; posterior of peduncular articles 2 and 3 with short setae clusters. Primary flagellum article one thin and elongate with length 4× width; dorsal submargin with large clusters of thin sensory setae (aesthetascs), forming a callynophore. Accessory flagellum 3-articulate; article 1 elongate and flattened with setae on posterodorsal margin; articles 2 and 3 very short, about five percent of accessory flagellum. Primary flagella broken at article 19. Antennae 2 length 0.4× body length. Posterior margin of peduncular articles 1 and 2 with short setae clusters; peduncular article 3 slightly longer than article 4. Flagellum with 36 articles.

Mouthparts. (Fig. 4D–O) Upper lip medial face asetose, with asymmetrical setose ventral margin. Mandible lacinia mobilis present on left and right side, highly asymmetric, left lacinia mobilis broad and multi-dentate, right lacinia mobilis thin with broad projection at proximal end; setal row with 21 robust setae Mandible incisors with strong posterodistal projection, slightly asymmetric; molar absent. Mandible palp 3-articulate, article 1 asetose, short; article 2 arched medially with 12 setae on medial margin; article 3 0.85× length of article 2, with 17 setae across all margins. Lower lip setose with distinct outer lobes present; mandibular process absent or severely reduced. Maxilla 1 inner plate small with two apical plumose setae; outer plate arched medially with nine robust apical setae, one large claw-like projection at the distal end. Maxilla 1 palp 2-articulate; article 1 subrectangular, setae on posterior; article 2 expanded distally with 13 apical and eight marginal robust setae, apical submargin with setae. Maxilla 2 inner plate with 15 plumose setae along apical to medial margin, outer plate slightly longer and thinner than inner plate with four plumose setae along the apical margin and two along lateral margin. Maxilliped with inner and outer plates and palp; inner plate small and subtriangular with two apical plumose setae; outer plate rounded, reaching base of palp article 2, with 29 robust setae along medial to apical margin. Maxilliped palp long, 4-articulate; article 1 short, lateral margin with sparse setae, medial margin with dense setae; article 2 longest article, similar pattern of setae as article 1; article 3 long, slightly shorter than article 2 with similar setae pattern; article 4 slender with four strong claw-like projections on medial margin, tapered distally.

Body. (Figs 2, 3) Pereon with subrectangular pereonites, dorsal and ventral margin smooth; pleon with dorsal surface smooth; dorsal margin of urosomites 1 and 2 with single thorn-like projection each, tapered distally, urosomite 3 dorsal margin smooth. Coxal gills present on gnathopod 2, pereopods 3–6; gills 2–4 elongate, gills 5 and 6 short and ovular.

Figure 2.: Princaxelia malohi sp. nov., holotype ♂ (body length 35.1 mm). Habitus, lateral view.

Figure 3.: Princaxelia malohi sp. nov. A. Holotype ♂, 35.1 mm; B. Adult ♂, 39.5 mm; C. Adult ♂, 20.0 mm; D. Juvenile, 16.6 mm.

Figure 4.: Princaxelia malohi sp. nov., holotype ♂. A. Antenna 1, left side, lateral, some primary flagella articles omitted; B. Head, lateral; C. Antenna 2, lateral, flagella omitted; D. Left mandible, dorsal; E. Left mandible lacinia mobilis, dorsal; F. Upper lip; G. Lower lip, dorsal, left lobe omitted; H. Maxilliped, dorsal; I. Maxilliped palp article 4; dorsal; J. Right mandible, dorsal, palp omitted; K. Maxilliped inner plate, dorsal; L. Maxilla 1 palp, lateral; M. Maxilla 1 inner plate, lateral; N. Maxilla 1, dorsal; O. Maxilla 2, dorsal.

Gnathopods. (Figs 5A–F, 6) Gnathopod 1 coxa subrectangular and slightly bilobate, ventral margin straight, posterior submargin with setae; basis slightly arched distally, with sparse setae on medial face, length 2.8× width; ischium small and subquadrate, single clump of setae near base of merus on posterior margin; merus broad, concave along proximal margin, ventral margin with evenly separated clumps of setae; carpus deep and rounded, length 2× width, dorsal margin smooth with setae sparse, lateral face asetose, posterior ventral submargin and medial face with dense clumps of setae; propodus narrower than carpus, posterior margin with dense setae; dactylus thin, dorsal and ventral margin with setae, posterior margin with one strong projection near base. Gnathopod 2 coxa subrectangular and slightly bilobate, ventral submargin with sparse setae; basis straight and elongate, length 3.8× width, sparse setae on posterior margin; ischium similar to gnathopod 1 with denser posterior setae clump; merus more elongate than gnathopod 1, numerous setae clumps on ventral submargin; carpus deep, subrectangular, length 2.2× width, dorsal margin smooth with sparse setae, lateral face asetose, ventral submargin and medial face densely setose; propodus similar to gnathopod 1, with the exception of having barbed robust setae (can be seen on the topmost setae of Fig. 6C); dactylus with majority of posterior margin asetose, otherwise similar to gnathopod 1.

Figure 5.: Princaxelia malohi sp. nov., holotype ♂. A. Gnathopod 1, lateral; B. Gnathopod 1, medial, basis and ischium omitted; C. Gnathopod 1 dactylus, lateral; D. Gnathopod 2 with coxal gill, lateral; E. Gnathopod 2, medial, basis and ischium omitted; F. Gnathopod 2 dactylus, lateral; G. Pereopod 3 with coxal gill, lateral; H. Pereopod 3 dactylus; lateral; I. Pereopod 4 with coxal gill, lateral; J. Pereopod 4 dactylus, lateral.

Figure 6.: SEM micrographs of Princaxelia malohi sp. nov., adult ♂, 39.5 mm. A. Gnathopod 1, lateral; B–E. Princaxelia malohi sp. nov., adult ♂, 33.5 mm; B. Gnathopod 2, lateral; C. Gnathopod 2 dactylus projection, lateral; D. Detailing of surface texture on ventral margin of gnathopod 2 dactylus; E. Backscattered composite image of gnathopod 2 propodus, lateral.

Pereopods. (Figs 5G–J, 7) Pereopod 3 coxa ovular, slightly bilobate along dorsal margin, ventral submargin rounded with sparse setae; basis elongate, anterior and posterior margin setose, large clump of setae on posterodistal angle; ischium anterior margin concave, dense clump of setae on posterodistal margin; merus, carpus, propodus, and dactylus length ratio 1.0: 1.22: 1.19: 0.52; merus elongate, distal margin tapered, dorsal and ventral margins straight, posterior margin setose; carpus elongate, rounded; dorsal margin asetose, posterior margin with dense setae. Propodus elongate, thin, anterior margin asetose, posterior margin with short setae; dactylus slender with single robust seta near distal end. Pereopod 4 coxa ovular, not bilobate, slightly tapered proximally, asetose; basis, ischium, merus, carpus, and dactylus shape and setae arrangement similar to pereopod 3. Pereopod 5 coxa subrectangular, tapered near proximal end, dorsal margin slightly convex, ventral margin straight, sparsely setose, length 2.6× width; basis broad and rectangular, length 2.9× width, posterior margin with many thin setae, anterior margin sparsely setose, posterodistal corner rounded; ischium short and rectangular, anterodistal corner with clump of setae, anterior margin slightly concave; merus, carpus, propodus, and dactylus length ratio 1.0: 0.89: 1.47: 0.43; merus thin, anterior and posterior margin setose; medial face setose; carpus slightly shorter than merus, with similar setae arrangement; propodus elongate, 60% longer than carpus, setae along anterior submargin and posterior margin; dactylus slender, one robust seta near the distal end. Pereopod 6 1.28× longer than pereopod 5; coxa subrectangular, length 2.8× width, slightly tapered at distal end, ventral margin slightly concave, distal end sparsely setose; entire pereopod long and slender, 1.28× longer than pereopod 5; basis broad, length 2.4× width, posterodistal corner quadrate; ischium similar to pereopod 5; merus, carpus, propodus, and dactylus similar to pereopod 5, length ratio 1.0: 1.16: 1.46: 0.31. Pereopod 7 1.04× longer than pereopod 6; coxa slender, bilobate, rounded at distal end ventral margin slightly concave, posterodistal margin with sparse setae, medial margin with penile papillae; penial papillae large, rounded and faced medially; basis slightly broader than pereopod 6, length 2.11× width; ischium, merus, carpus, propodus, and dactylus similar to pereopod 6.

Figure 7.: Princaxelia malohi sp. nov., holotype ♂. A. Pereopod 5 with coxal gill, lateral; B. Pereopod 6, lateral; C. Pereopod 7, lateral; D. Pereopod 5 dactylus, lateral; E. Pereopod 6 dactylus, lateral; F. Pereopod 7 dactylus; lateral.

Pleon and urosome. (Fig. 8) Pleopods with paired coupling setae (retinacula) on medial distal margin of peduncle; coupling setae with five hooks on inner margin; rami of similar length; inner ramus with numerous bifid (clothespin) setae on inner basal margin; ramus articles wide, with dense plumose setae along both side margins, article at distal end with additional plumose setae on ventral margin. Epimeron setose on ventral submargin; epimeral plate 1 ventral margin curved, with strong groove above the ventral margin, posterodistal corner rounded; epimeral plate 2 subrectangular with strong groove above the ventral margin, posterodistal corner subquadrate; epimeral plate 3 subrectangular, posterodistal corner quadrate. Uropod 1 peduncle longer than rami, seven basofacial setae, distomedial projection weak; inner ramus 0.8× length of peduncle, medial margin setose, sparse setae on lateral margin; outer ramus damaged near the distal end, medial margin setose, lateral margin with few setae. Uropod 2 peduncle about same length as rami, three basofacial setae, distomedial peduncular projection similar to uropod 1; rami with setae along medial margins, sparse setae on lateral margin; inner ramus 1.1× length of outer ramus. Uropod 3 peduncle short and subrectangular, length 2.2× width, medial and lateral margins setose; rami paddle-like, wide, and ovular; inner ramus with setae on all margins; outer ramus with very short setae along medial margin, a single large bump on distal end. Telson length 2.5× width, cleft extending 78% of length, lobes weakly tapered distally with many setae, apex of each lobe concave, with one robust seta each.

Figure 8.: Princaxelia malohi sp. nov., holotype ♂. A. Pleopod 2, posterior; B. Coupling spines (retinacula) on pleopod; C. Bifurcate (clothespin) setae on outer lobe of pleopod; D. Uropod 1, dorsal; outer ramus broken distally; E. Uropod 2, dorsal; F. Uropod 3, dorsal, most rami setae broken or missing; G. Telson, dorsal; H. Epimeron 1, lateral; I. Epimeron 2, lateral; J. Epimeron 3, lateral.

Variations.

The maxilla 2 inner plate setae number varies with specimen size. One specimen (BL 33.5 mm, ♂) has four apical setae on the right maxilla 2 inner plate and three on the left. The robust setae of the outer plate of the maxilla 1 vary from seven to nine. Article 3 of the maxilliped with numbers of projections ranging from three to five.

Supplementary material.

In situ video footage was also captured from the baited landers dropped in the Tonga Trench (https://doi.org/10.5281/zenodo.20756281), which provides further insight on the locomotion and predation habits of Princaxelia.

Differential diagnosis.

Princaxelia malohi sp. nov. is the only member of Princaxelia to have two plumose setae on the inner plate of the maxilla 1 and consistently more than three plumose setae on the outer plate of the maxilla 2. It is also the only reported member of the genus to have a single basal projection on the dactylus of both gnathopods.

Etymology.

The specific epithet malohi is derived from the Tongan word for mighty or powerful. This refers to back to the status of this animal as a predator, where it uses its strength to subdue prey, as well as the adaptations it must have to survive in hadal depths. It is treated as an indeclinable noun in apposition.

Princaxelia kahat

Duffy & Wainwright sp. nov.

F10BB474-9B6B-589E-A118-2BDFD0945C5F

https://zoobank.org/1FEADE39-34F2-4EF4-BDF3-12288BD283F7

Figs 9, 10, 11, 12, 13, 14, 15

• Princaxelia magna: Jamieson and Weston 2023: table 2.

Material examined.

Holotype. • Mature ♂, body length 59.3 mm; Mariana Trench (12°17.316'N, 144°40.112'E); 8,143 m; 5 Dec. 2014; Baited lander. USNM accession number USNM 1775248. GenBank® accession number: 16S: PZ611773; 28S: PZ611782.

Other material.

• 4 ♂, 2 ♀; Mariana Trench (11°10.002'N, 142°30.972'E); 8,000 m; 26 Jan. 2017; Baited lander. GenBank® accession numbers: 16S: PZ611770–PZ611772; 28S: PZ611778–PZ611781.

Diagnosis.

Primary flagellum article 1 of male antennae 1 elongate and longer than half the body. Accessory flagellum of male antennae 1 article 1 enlarged and flattened, with three small articles at distal end. Upper lip slightly asymmetrical. Maxilla 1 inner plate with six plumose setae; palp article 2 expanded with 16 apical robust setae. Maxilla 2 outer plate with three plumose setae. Maxilliped inner plate with one apical plumose seta. Dactylus of gnathopods 1 with three and gnathopod 2 with five strong projections on the posterior margin near the base. Dorsal margin of coxa 5 slightly lower on distal end. Ventral margin of coxa 7 shallowly concave. Telson lobes broad and tapering distally at last 1/3.

Description primarily based on holotype, male.

Head. (Figs 9, 10) Slightly shorter than length of pereonites 1 and 2 combined; rostrum very short, slightly pointed; lateral cephalic lobe prominent; corner rounded; eyes absent.

Figure 9.: Princaxelia kahat sp. nov., holotype ♂ (59.3 mm). Habitus, lateral view.

Figure 10.: Princaxelia kahat sp. nov. A. Holotype ♂, 59.3 mm; B. Adult ♀, 61.8 mm; C. Adult ♀, 53.4 mm.

Antennae. (Fig. 11) Extremely elongate, over 70 percent body length. Peduncle articles 1–3 with length ratio 1.0: 0.5: 0.2; peduncular article one broadened, with several clusters of setae arranged in rows on ventral margin, some weakly plumose, sparse setae on dorsal margin; posterior of peduncular article 2 with short setae clusters. Primary flagellum article one thin and elongate with length 7× width, dorsal submargin with dense clusters of thin sensory setae (aesthetascs), forming a callynophore. Primary flagella with 108 articles. Accessory flagellum 4-articulate; article 1 elongate and scale-like with setae on posterodorsal margin; articles 2–4 tapered and setose. Antennae 2 dorsal margin of peduncular article 1 and ventral margin of article 2 with short setae clusters; peduncular article 3 elongate with sparse small setae. Article 4 damaged and flagella lost in collection.

Figure 11.: Princaxelia kahat sp. nov. A. Holotype ♂ right antennae 1, lateral; B. Holotype ♂ accessory flagellum article 2–4; C. Adult ♀ left antennae 1 lateral, distal articles of primary flagellum omitted; D. Adult ♀ antenna 1 accessory flagella; E. Holotype ♂ left antennae 2 lateral, article 5 and flagella missing.

Mouthparts. (Fig. 12) Upper lip medial face asetose, with asymmetrical ventral margin. Mandible lacinia mobilis present on left and right side, highly asymmetric, left lacinia mobilis broad and multi-dentate, 22 robust setae on setal row. Right lacinia mobilis thin with broad projection at proximal end; 21 robust setae on setal row. Mandible incisors with strong posterodistal projection, slightly asymmetric; molar absent. Mandible palp 3-articulate, article 1 asetose, short; article 2 arched medially with 28 setae on medial margin; article 3 with many setae arranged in rows. Lower lip with distinct outer lobes present; mandibular process absent or severely reduced. Maxilla 1 inner plate small with six apical plumose setae; outer plate arched medially with 14 robust apical setae with one large claw-like projection at the distal end. Maxilla 1 palp 2-articulate; article 1 subrectangular, setae on posterior; article 2 expanded distally with 16 apical and nine marginal robust setae, apical submargin with setae. Maxilla 2 inner plate with 16 plumose setae along apical to medial margin, outer plate thin with three plumose setae along the apical margin. Maxilliped with inner and outer plates and palp; inner plate small and subtriangular with one apical plumose setae, one additional robust setae on the ventral margin; outer plate rounded, reaching base of palp article 2, with robust setae along medial to apical margin. Maxilliped palp long, 4-articulate; article 1 short, lateral margin with sparse short setae, medial margin with dense setae; article 2 longest article, similar pattern of setae as article 1; article 3 long, slightly shorter than article 2 with similar setae pattern; article 4 slender with strong claw-like projections on medial margin, tapered distally.

Figure 12.: Princaxelia kahat sp. nov., holotype ♂. A. Lower lip, ventral; B. Upper lip; C. Right mandible, dorsal, palp omitted; D. Right mandible lacinia mobilis; E. Left mandible, dorsal; F. Left mandible lacinia mobilis; G. Maxilliped, dorsal; H. Maxilliped palp article 4; I. Maxilliped inner plate, dorsal; J. Maxilla 1 palp, medial; K. Maxilla 2, dorsal; L. Maxilla 1, ventral; M. Maxilla 1 outer plate, lateral.

Body. Pereon with subrectangular pereonites, dorsal and ventral margin smooth; pleon with dorsal surface smooth; coxal gills present on gnathopod 2, pereopods 3–6; gills 2–4 elongate, gills 5 and 6 short and ovular; dorsal margins of urosomites 1 and 2 with single thorn-like projection each, pointed upward in urosomite 1, tapered distally in urosomite 2, urosomite 3 dorsal margin smooth.

Gnathopods. (Fig. 13A–F) Gnathopod 1 coxa subrectangular and slightly bilobate, ventral margin straight; basis slightly arched distally, with sparse setae, length 1.78× width; ischium small and subquadrate, single clump of setae near base of merus on posterior margin; merus broad, concave along proximal margin, ventral margin with evenly separated clumps of setae; carpus broad and rounded, length 2.3× width, dorsal margin smooth with short sparse setae, lateral face asetose, posterior ventral submargin and medial face with dense clumps of setae; propodus narrower than carpus, posterior margin with dense setae, with one robust seta at ventral posterodistal corner; dactylus thin, ventral margin with setae, posterior margin with three strong projection near base. Gnathopod 2 coxa subrectangular and slightly bilobate; basis straight and elongate, length 1.7× width, setae on posterior margin; ischium similar to gnathopod 1; merus more elongate than gnathopod 1 with numerous dense setae clumps on ventral submargin; carpus deep, subrectangular, length 2.3× width, dorsal margin smooth with sparse setae, lateral face asetose, ventral submargin and medial face densely setose; propodus with numerous, broadly plumose setae, otherwise similar to gnathopod 1; dactylus with majority of posterior margin asetose with five strong projections near base.

Figure 13.: Princaxelia kahat sp. nov., holotype ♂. A. Gnathopod 1, lateral, coxa broken proximally; B. Gnathopod 1, medial, basis and ischium omitted; C. Gnathopod 1 dactylus, lateral; D. Gnathopod 2, lateral; E. Gnathopod 2, medial, basis and ischium omitted; F. Gnathopod 2 dactylus, lateral; G. Pereopod 3, lateral; H. Pereopod 3 dactylus, lateral; I. Pereopod 4, right lateral; J. Pereopod 4 dactylus, lateral; K. Adult ♀ 61.0 mm, dactylus of gnathopod 1, lateral.

Pereopods. (Figs 13G–J, 14) Pereopod 3 coxa ovular, slightly bilobate along dorsal margin, ventral submargin rounded with sparse setae; basis elongate, anterior and posterior margin setose, large clump of setae on posterodistal angle; ischium anterior margin concave, dense clump of setae on posterodistal margin; merus, carpus, propodus, and dactylus length ratio 1.00: 0.91: 0.37: 0.37; merus elongate, distal margin tapered, dorsal and ventral margins straight, posterior margin setose; carpus elongate, rounded; dorsal margin asetose, posterior margin with dense setae. Propodus elongate, thin, anterior margin asetose, posterior margin with short setae; dactylus slender with single robust seta near distal end. Pereopod 4 similar to pereopod 3. Pereopod 5 coxa subrectangular, tapered near proximal end, dorsal margin slightly convex, ventral margin straight, sparsely setose, length 2.4× width; basis broad and rectangular, length 3.4× width, posterior margin with many thin setae, anterior margin sparsely setose, posterodistal corner rounded; merus thin, anterior and posterior margin setose; medial face setose; carpus slightly shorter than merus, with similar setae arrangement. Pereopod 6 coxa subrectangular, slightly tapered at distal end, ventral margin slightly concave, distal end sparsely setose; entire pereopod long and slender; basis broad, length 2.3× width, posterodistal corner rounded. Pereopod 7 coxa slender and slightly tapered at distal end, ventral margin slightly concave, posterodistal margin with sparse setae, medial margin with large penile papillae faced medially.

Figure 14.: Princaxelia kahat sp. nov., holotype ♂. A. Pereopod 5, lateral, distal portion of propodus broken; B. Pereopod 6, lateral, distal portion of propodus broken; C. Pereopod 7, right lateral, coxa broken distally; D. Pereopod 7 coxae, left lateral.

Pleon and urosome. (Fig. 15) Epimeron setose on ventral submargin; epimeral plate 1 ventral margin curved with strong groove above the ventral margin, posterodistal corner pointed; epimeral plate 2 subrectangular, with strong groove above the ventral margin, posterodistal corner rounded; epimeral plate 3 subrectangular, posterodistal corner quadrate. Pleopods 1–3 with paired coupling setae (retinacula) on medial distal margin of peduncle; coupling setae with hooks on inner margin; rami of similar length; inner ramus with numerous bifid (clothespin) setae on inner basal margin; ramus articles wide. Uropod 1 peduncle longer than rami, 13 basofacial setae, distomedial projection strong; medial margin setose, sparse setae on lateral margin; outer ramus damaged, medial margin setose, lateral margin with few setae. Uropod 2 peduncle triangular, about same length as rami, eight basofacial setae, distomedial peduncular projection similar to uropod 1; rami with setae along medial margins, sparse setae on lateral margin; outer ramus damaged. Uropod 3 from holotype lost in collection. Female uropod 3 peduncle short and broad with sparse setae on the lateral margin and a row of plumose setae on the distomedial margin, rami ovular and flattened, lined with bases for long setae, most damaged or missing in collection, outer rami slightly tapered distally. Telson length 2.18× width, cleft extending 89% of length, lobes weakly tapered distally at last 1/3, apex of each lobe concave.

Figure 15.: Princaxelia kahat sp. nov. A–D. Holotype ♂; A. Pleopod 2, lateral; B. Bifurcate seta on pleopod; C. Uropod 1, right dorsal, outer ramus broken distally; D. Uropod 2, right dorsal, outer ramus broken distally; E–H: Adult ♀ 61.0 mm; E. Uropod 1, left dorsal; F. Uropod 2, left dorsal; G. Uropod 3, dorsal, most rami setae broken or missing; H. Telson, dorsal.

Sexual dimorphism.

Female specimens have a slender, setose 6 articulate accessory flagellum on antenna 1 (Fig. 11D). Female individuals also have a much stronger distomedial peduncular projection on uropod 1 than males (Fig. 15E), as well as shorter uropod rami. There is no significant difference in maximum sizes between male and female individuals.

Variable characters.

The number of basal projections on the dactylus of the gnathopods varied between specimens regardless of sex. The number of basal projections on gnathopod 1 varied from three to five, with four being the most common across examined specimens (see Fig. 13K for typical arrangement and placement). The number of basal projections on gnathopod 2 varied between four, five, and six, with five being the most common across examined specimens. The number of dactylus projections on gnathopod 2 is always greater or equal to that of gnathopod 1. The number of setae on the mandible setal row varies between specimens, with the left having 20–26 setae and right having 16–21 setae. The number of robust setae on the inner plate of maxilla 1 can vary between 7–10 setae.

Differential diagnosis.

Princaxelia kahat sp. nov. can be differentiated from most other Princaxelia species by the presence of six plumose setae on the inner plate of maxilla 1, with the sole exception of P. magna which also shows this feature. Princaxelia kahat sp. nov. is morphologically very similar to P. magna, with key differences seen in Princaxelia kahat sp. nov. being a longer antennae 1, telson lobes that are broader and tapered laterally, and an upper lip that is much blunter than that seen illustrated for P. magna.

Etymology.

The specific epithet kahat is derived from a Chamorro word meaning “to hunt stealthily, as if stalking or sneaking”, referring to the observed predatory behaviour of Princaxelia. This also reflects its habitat, as it hunts under the darkness of the hadal zone. The name is treated as an indeclinable noun in apposition.

Remarks.

The distinct morphological differences between Princaxelia species can be seen in Table 1, modified from Tomikawa et al. (2021). As seen in this table, the projection near the base of the dactylus is likely a diagnostic feature that should be noted for future Princaxelia captures. This was first suggested as a diagnostic feature of the genus in Tomikawa et al. (2021), and this description solidifies it as a distinct trait. This feature was not mentioned nor illustrated in the descriptions of P. abyssalis and P. stephenseni, and was also not noted in the review of the specimens by Lörz (2010). These are illustrated and mentioned briefly in the description of P. magna; however, the exact number is not specified and is obscured in the illustration of gnathopod 2 making this unclear. More specimens will need to be captured to confirm the presence or absence of this feature in those species.

Species | Princaxelia abyssalis | Princaxelia jamiesoni | Princaxelia magna | Princaxelia marianaensis | Princaxelia stephenseni | Princaxelia malohi sp. nov. | Princaxelia kahat sp. nov.
Literature | Dahl (1959) | Lörz (2010); Additional info from Tomikawa et al. (2021) | Kamenskaya (1977) | Tomikawa et al. (2021) | Dahl (1959); Additional info from Lörz (2010) | This study | This study
Location | Aleutian, Kuril – Kamchatka, Izu Bonin (Izu-Ogasawara), Yap, Japan, Philippine, Bougainville and Kermadec trenches | Kuril–Kamchatka, Japan and Izu-Ogasawara trenches | Yap Trench, dubious reports from Japan and Tonga trenches | Mariana Trench | North Atlantic: Southwest of Iceland | Tonga Trench | Mariana Trench
Depth | 6,620–8,300 m | 7,703–9,316 m | 7,190–7,250 m | 5,683 m | 1,505 m | 8,200–8,350 m | 8,000–8,964 m
Maximum known size | Male: 21 mm; Female: 32 mm | Male: 57 mm; Female: 61 mm | Male: 52 mm; Female: N/A | Male: N/A; Female: 23.9 mm | Male: 10 mm; Female: 11 mm | Male: 39.5 mm; Female: N/A | Male: 59.3 mm; Female: 61.8 mm
Epimeral plate 3 posterodistal corner | Rounded | Quadrate | Quadrate | Quadrate | Weakly Rounded | Quadrate | Quadrate
Dorsal projections on urosomites 1 and 2 | Unknown | Pointing toward distal end | Pointing upright | Pointing toward distal end | Pointing toward distal end | Pointing toward distal end | Urosomite 1 pointed upright; Urosomite 2 pointed distally
Upper lip | Unknown | Slightly asymmetrical | Strongly symmetrical | Strongly asymmetrical | Slightly asymmetrical | Strongly asymmetrical | Slightly asymmetrical
Maxilla 1 palp article 2 | Expanded; less than 14 apical robust setae | Expanded; 25 apical robust setae | Expanded; ~10 apical robust setae | Expanded; 9 apical robust setae | Not expanded; 7 apical robust setae | Expanded; 13 apical robust setae | Expanded; 14 apical robust setae
Maxilla 1 inner plate | 1 plumose seta | 1 plumose seta | 6 plumose setae | 1 plumose seta | 1 plumose seta | 2 plumose setae | 6 plumose setae
Maxilla 2 outer plate | 3 apical plumose setae | 3 apical plumose setae | 3 apical plumose setae | 3 apical plumose setae | 3 apical plumose setae | 6 apical plumose setae | 3 apical plumose setae
Gnathopods 1 and 2 dactyli basal projections | Unknown | 7–9 projections | 4 projections | 3 projections | Unknown | 1 projection | 3–5 (4 most common) projections on gnathopod 1; 4–6 (5 most common) projections on gnathopod 2
Coxa 5 dorsal margin | Highest at proximal end | Straight | Convex | Highest at proximal end | Straight/convex | Straight | Highest at proximal end
Coxa 5 distal margin | Rounded | Rounded | Slightly pointed | Rounded | Straight | Slightly pointed | Rounded
Coxa 7 ventral margin | Straight | Slightly concave | Slightly concave | Shallowly concave | Straight | Shallowly concave | Shallowly concave
Uropod distomedial corner projection | Unknown | Very strong | Unknown | Strong | Unknown | Weak | Very strong in females, average in males
Telson lobe | Thin; Uniformly tapering distally | Broad; Tapering from distal 1/3 | Thin; Tapering from distal 1/3 | Thin; Uniformly tapering distally | Unknown | Broad; Weakly tapering distally | Broad; Tapering from distal 1/3

An additional feature which may be diagnostic is the distomedial corner of the uropods, which is very weak in Princaxelia malohi sp. nov. Princaxelia kahat sp. nov. shows a much stronger projection in female individuals than males, but the projection in male individuals is stronger than that seen in male individuals of Princaxelia malohi sp. nov. This feature appears very strong in the other described species with illustrations, specifically in those illustrated from female individuals. This is likely a sexually dimorphic feature and should be noted for future Princaxelia descriptions.

Key to the genus Princaxelia

Updated dichotomous key to the genus Princaxelia, adapted from Tomikawa et al. (2021).

1 | Palp article 2 of maxilla 1 expanded | 2
– | Palp article 2 of maxilla 1 not expanded | P. stephenseni Dahl, 1959
2 | Maxilla 1 inner plate with two or less plumose setae | 3
– | Maxilla 1 inner plate with more than two plumose setae | 6
3 | Coxa 7 ventral margin concave | 4
– | Coxa 7 ventral margin straight | P. abyssalis Dahl, 1959
4 | Maxilla 1 palp article 2 with less than 20 apical robust setae | 5
– | Maxilla 1 palp article 2 with more than 20 apical robust setae | P. jamiesoni Lörz, 2010
5 | Maxilla 1 inner plate with single plumose setae, gnathopod dactyli with 3 basal projections | P. marianaensis Tomikawa & Watanabe in Tomikawa et al. 2021
– | Maxilla 1 inner plate with two plumose setae, gnathopod dactyli with single basal projection | Princaxelia malohi sp. nov.
6 | Antennae 1 shorter than half the length of body, strongly asymmetrical, sharp upper lip, thin telson lobes | P. magna Dahl, 1959
– | Antennae 1 longer than half the length of body, weakly asymmetrical, blunt upper lip, broad telson lobes | Princaxelia kahat sp. nov.

In situ behaviours

Princaxelia individuals were observed at 8,200 meters in the Tonga Trench capturing amphipod prey using the anterior appendages, holding prey between the gnathopods and maxillipeds while swimming, and then restraining prey against the sediment surface during consumption (Fig. 16A–C; https://doi.org/10.5281/zenodo.20756281). The typical free-swimming posture of the genus was also observed, showing a raised pereopod 5 (Fig. 16D). Large aggregations of Princaxelia were also observed on lander video footage in both the Tonga and Mariana (Fig. 16E) Trenches, providing better views on their behaviours.

Figure 16.: A–C. In situ observations of Princaxelia from 8,200 m in the Tonga Trench; A. Princaxelia individual swimming with prey item (amphipod; Hirondellea dubia); B. Individual positioning prey for feeding; C. Princaxelia individual in motion across seafloor, showing trailing pereopods six and seven; D. Free swimming posture captured from an individual in the Mariana Trench; E. Aggregation of seven individuals at 8,098 m around bait on a boulder field in the Mariana Trench.

Pairwise distances

Pairwise distances are presented in Suppl. material 2. In Princaxelia malohi sp. nov. no intraspecific variation was detected among the newly generated 16S rRNA sequences, with maximum uncorrected p-distances and Kimura 2-parameter (K2P) distance both equal to 0.00%. In contrast, Princaxelia kahat sp. nov. exhibited low intraspecific divergence, with maximum uncorrected p-distance and maximum K2P distance of 0.25%. Pairwise divergence between Princaxelia malohi sp. nov. and Princaxelia kahat sp. nov. was substantially greater, ranging from 15.76–16.42% for uncorrected p-distance and 17.71–18.54% K2P distance. For COI, a single newly generated Princaxelia malohi sp. nov. sequence was obtained, precluding estimation of within-species genetic distances. Pairwise comparisons with publicly available Princaxelia sequences showed uncorrected p-distance of 24.05% and K2P distance of 29.61% for Princaxelia sp. (PX720326), and 15.07% uncorrected p-distance from P. marianaensis (LC719249) of 15.97% and 16.90% for K2P.