Stygiopontius senokuchiae, Uyeno & Shimanaga, 2018

Uyeno, Daisuke & Shimanaga, Motohiro, 2018, A new dirivultid copepod (Siphonostomatoida) from hydrothermal vent fields of the Izu-Bonin Arc in the North Pacific Ocean, Zootaxa 4415 (2), pp. 381-389 : 383-387

Stygiopontius senokuchiae n. sp.

( Figs 2 –4 )

Type material. Holotype: adult female (NSMT-Cr 26842), ex populations of Paralvinella spp . (Polychaeta: Alvinellidae ), Myojin-sho Caldera (31°53.042N, 139°58.220E), off northern Ogasawara Islands, North Pacific Ocean, depth 795 m, 24 April 2013. Allotype: adult male (NSMT-Cr 26843), collection data same as that of holotype. Paratypes: six adult females and five adult males (NSMT-Cr 26844), collection data same as that of holotype.

Description of holotype adult female. Body ( Fig 2 A ) 792 long, cyclopiform composed of cephalothorax, second to fifth pedigerous somites, genital double somite, and 3-segmented abdomen. Cephalothorax wider than long, 391 × 455, bearing pair of posterolateral pointed processes. Second to fourth pedigerous somites gradually narrower. Prosome 556 long. Genital double somite ( Fig 2 A, C ) slightly longer than wide, 125 × 115, bearing pair of conical, blunt process on lateral to genital openings and pair of pointed posterolateral processes. Abdomen ( Fig 2 A, C ) progressively narrower posteriorly, composed of three free somites, 50 × 88, 47 × 78, and 47 × 71, respectively. Caudal rami ( Fig 2 C, E ) 1.31 times longer than wide, 36 × 27, with six setae.

Rostrum ( Fig 2 B ) bearing without distinct apex. Antennule ( Fig 2 F ) 12-segmented; armature formula 1, 2, 12, 8, 2, 4, 2, 2, 2, 2, 2 + 1 aesthetasc, and 14; all setae naked, except single plumose seta on fifth segment. Antenna ( Fig 2 G ) biramous, composed of coxa, basis, endopod, and exopod; coxa and basis unarmed; endopod 2- segmented, bearing single subterminal and three terminal setae; exopod unsegmented, bearing two setae. Oral cone ( Fig 2 B ) ovoid. Mandible ( Fig 2 H ) thin, slender, bearing row of spinules on subterminal part of inner margin and 10 distal teeth. Maxillule ( Fig 2 I ) bilobed: large inner lobe (endite) and small outer lobe (palp) bearing four setae, respectively. Maxilla ( Fig 2 J ) composed of syncoxa and claw; syncoxa robust bearing long seta near articulation with claw; claw elongate bearing spatulated and spinulated tip with spinules. Maxilliped ( Fig 3 A ) subchelate, comprising syncoxa, basis, and 3-segmented endopod; syncoxa and basis bearing single seta, respectively; proximal endopodal segment bearing two tiny setae; middle segment of endopod bearing single seta; distal endopodal segment bearing single seta and claw with row of fine spinules on inner margin.

Legs 1 to 4 ( Fig 3 B–E ) biramous; both rami bearing 3-segmented rami, except endopod of leg 4. Leg armature formula as follows:

Intercoxal sclerites of legs 1 to 4 ( Fig 3 B–E ) unornamented. All spines spatulate, bearing serrated margins. All setae plumose. Leg 1 with basis bearing conical inner distal process. Leg 2 with row of setules on inner side of basis. Legs 3 and 4 with rounded protrusion between rami. Rami of legs 1 to 4 bearing pointed processes on outer and distal margins. Leg 5 ( Fig 3 F ) indistinctly 2-segmented; proximal segment bearing 1 seta; distal segment bearing three distal setae; all of these 4 setae plumose. Leg 6 ( Fig 2 D ) represented by single small seta at genital opening.

Description of allotype adult male. Body ( Fig 4A ) cyclopiform, 471 long, composed of cephalothorax, second to fifth pedigerous somites, genital somite, and 4-segmented abdomen. Cephalothorax wider than long, 187 × 257, bearing pair of posterolateral pointed processes. Prosome 320 long. Genital somite ( Fig 4A, B ) wider than long, 45 × 76. Abdomen ( Fig 4A ) progressively narrower posteriorly, composed of four free somites, 34 × 59, 40 × 55, 35 × 47, and 27 × 43, respectively. Caudal rami ( Fig. 4A ) 1.41 times longer than wide, 24 × 17, with six setae.

Antennule ( Fig 4C ) 11-segmented; armature formula 1, 2, 12, 6, 6, 4, 2, 2, 4, 4 + 1 aesthetasc, and 12; all setae naked. Antenna, mandible, maxillule, and maxilla as in female. Proximal endopodal segment of maxilliped ( Fig 4D ) bearing two setae.

Armature formula of legs 1 to 4 as in female. Armature formula of leg 2 as in female but single outer, two distal, and single inner plumose setae of third endopodal segment replaced into thick, spine-like setae. Leg 5 ( Fig 4B ) composed of protopod bearing 1 outer seta and exopod bearing three outer and two rod-shaped distal setae. Leg 6 ( Fig 4B ) represented by genital flap bearing single serrated spine and single seta on outer margin; posterior part of genital flap bearing row of spinules.

Variability. The morphology of the female paratypes as in the holotype. The measurements of the body parts (n = 6) are as follows: body length 685–786 (727 ± 37); cephalothorax length 335–396 (365 ± 26); cephalothorax width 387–431 (402 ± 16); prosome length 484–538 (516 ± 23); genital double somite length 94–123 (114 ± 11); genital double somite width 107–126 (116 ± 7); first urosomite length 51–63 (55 ± 5); first urosomite width 78–88 (82 ± 4); second urosomite length 40–55 (45 ± 8); second urosomite width 73–80 (76 ± 2); anal somite length 28– 52 (38 ± 8); anal somite width 65–69 (67 ± 2); caudal ramus length 32–45 (39 ± 5); caudal ramus width 25–29 (26 ± 2). Caudal ramus 1.29–1.71 (1.48 ± 0.15) times longer than wide.

The morphology of the male paratypes as in the allotype. The measurements of the male body parts of other material (n = 5) are as follows: body length 446–483 (464 ± 15); cephalothorax length 208–258 (229 ± 21); cephalothorax width 221–272 (251 ± 19); prosome length 317–345 (331 ± 13); genital somite length 48–56 (50 ± 3); genital somite width 75–83 (78 ± 3); first urosomite length 30–42 (37 ± 4); first urosomite width 58–63 (59 ± 2); second urosomite length 26–42 (34 ± 6); second urosomite width 50–55 (52 ± 2); third urosomite length 19–33 (26 ± 6); third urosomite width 44–48 (47 ± 2); anal somite length 18–26 (22 ± 3); anal somite width 42–49 (44 ± 3); caudal ramus length 24–26 (25 ± 1); caudal ramus width 17–20 (18 ± 2). Caudal ramus 1.22–1.45 (1.38 ± 0.09) times longer than wide.

Remarks. So far 22 valid species are known in Stygiopontius . Seven species, S. cinctiger Humes, 1987 , S. lomonosovi Ivanenko & Martínez Arbizu, 2006 , S. mucroniferus Humes, 1987 , S. rimivagus Humes, 1997 , S. serratus Humes, 1996 , S. teres Humes, 1996 , and S. verruculatus Humes, 1987 , share three spines and four setae on the third exopodal segment of the leg 4 with S. senokuchiae n. sp. (vs. other 15 species have four spines and four setae) (see Humes 1987, 1989, 1990, 1991, 1996; Ivanenko et al. 2006; Ivanenko & Ferrari 2013). Among the seven species, S. cinctiger , S. lomonosovi , and S. teres shares coxal setae both on legs 1 and 2 (see Humes 1987, 1996; Ivanenko & Martínez Arbizu, 2006). Stygiopontius cinctiger differs from the new species by having an inner spine on the basis of the leg 1 (vs. single inner seta) and the genital double somite without lateral and posterolateral processes (vs. with both lateral and posterolateral processes) (see Humes 1987). Stygiopontius lomonosovi is separated from the new species by having an inner seta on the bases of legs 3 and 4 (vs. no inner seta) (see Ivanenko & Martínez Arbizu 2006). Stygiopontius teres is similar to S. senokuchiae n. sp. but is distinguished from the new species by the following characters: the genital double somite has no lateral processes (vs. bearing a pair of conical processes); the caudal rami bears a pointed process on the posterodorsal margin (vs. without processes); the basis of leg 1 without inner distal processes (vs. bearing one conical inner distal process) (see Humes 1996). Further, on S. teres , the segmentation of the female antennule differs from that of the new species (the first segment appears indistinctly 3 -segmented vs. distinctly 3 -segmented) (see Humes 1996, fig. 16b).

Etymology. The specific name, senokuchiae , was chosen after Ms Reina Senokuchi who formerly studied copepods on hydrothermal fields in Izu-Bonin Arc and found the specimens of the new species.

Newly established Japanese name. “Rengoku-kudakuchi-mijinko-ka” for the family, “Rengoku-kudakuchimijinko-zoku” for the genus, and “Rengoku-kudakuchi-mijinko” for the species. In Japanese, the Purgatory is translated to “Rengoku”, “kudakuchi” means a tube-like mouth of the copepod, and “mijinko” is a general term for tiny crustaceans.

Figures

FIGURE 2. Stygiopontius senokuchiae n. sp. adult female, holotype (NSMT-Cr 26842). A, habituS, dorSal; B, roStral area; C, uroSome, dorSal; D, genital aperture and leg 6, dorSal; E, right caudal ramuS, dorSal; F, left antennule, poSterior; G, right antenna, anterior; H, right mandible; I, right maXillule, poSterior; J, right maXilla, poSterior.Scale barS: A, 200 µm; B, 100 µm; C, 70 µm; D, I, J, 30 µm; E–G, 40 µm; H, 20 µm.

FIGURE3. Stygiopontius senokuchiae n. sp.adult female,holotype (NSMT-Cr 26842).A, right maXilliped, poSterior; B, right leg 1, anterior; C, left leg 2, anterior; D, left leg 3, anterior; E, left leg 4, anterior; F, right leg 5, dorSal.Scale barS:A, F, 30 µm; B–E, 40 µm.

FIGURE 4. Stygiopontius senokuchiae n. sp. adult male, allotype (NSMT-Cr 26843). A, habituS, dorSal; B, fifth pedigerouS Somite to firSt uroSomite, ventral; C, right antennule, poSterior; D, endopod of right maXilliped, poSterior. Scale barS: A, 100 µm; B, 40 µm; C, 30 µm; D, 20 µm.