Stygiopontius spinifer, Lee & Kim & Kim, 2020

Lee, Jimin, Kim, Dongsung & Kim, Il-Hoi, 2020, Copepoda (Siphonostomatoida: Dirivultidae) from Hydrothermal Vent Fields on the Central Indian Ridge, Indian Ocean, Zootaxa 4759 (3), pp. 301-337 : 317-320

Stygiopontius spinifer n. sp.

(Figs 11, 12)

http://zoobank.org/ BBD02F2A-B2ED-4EEF-A084-EACA8134C4BE

Material examined. Fifty-five females from sediments at GTV 1809 (11°24.883´S, 65°25.425´E, depth 2022 m), the Onnuri vent field on the Central Indian Ridge, 23 June 2018. Holotype (female, MABIK CR00244729) and paratypes (30 females, MABIK CR00244730) have been deposited in the Marine Biodiversity Institute of Korea ( MABIK), Seocheon. Other specimens are retained in the collection of the junior author.

Additional material examined. Ten females (one female dissected) from washings of invertebrates at GTV 1702 (19°33.387´S, 65°50.893´E, depth 2507 m), the Solitaire vent field on the Central Indian Ridge, 01 August 2017; four females from washings of invertebrates at GTV1807 (19°33.395´S, 65°50.889´E, depth 2634 m), the Solitaire vent field on the Central Indian Ridge, 20 June 2018 .

Female. Body (Fig. 11A) dorsoventrally flattened, 1.51 mm long. Prosome 865 × 742 μm, oviform in dorsal view; posterolateral corners pointed in cephalothorax and second pedigerous somite, but rounded in third and fourth pedigerous somites. Cephalothorax 523 μm long. Urosome (Fig. 11B) 5-segmented. Fifth pedigerous somite trap- ezoidal, 199 μm wide, with blunt lateral apices. Genital double-somite 184 × 180 μm; anterior third broader than posterior two thirds, with claw-like, posteriorly directed lateral process on both sides near genital aperture; narrower posterior part gradually narrowing posteriorly. Three free abdominal somites 85 × 119, 61 × 109, and 61 × 107 μm, respectively. Anal somite with five or six spinules along both sides of posteroventral border (Fig. 11C). Caudal rami (Fig. 11C) parallel; each ramus 97 × 45 μm measured in ventral view, 2.16 times as long as wide, with six setae (setae II–VII); two larger mid-terminal setae pinnate along distal two thirds; inner terminal seta unilaterally pinnate along inner margin; other three setae naked.

Rostrum absent.Antennule (Fig. 11D) 430 μm long and 12-segmented; third segment longest; armature formula FIG. 11. Stygiopontius spinifer n. sp., female. A, habitus, dorsal; B, urosome, dorsal, C, caudal rami, ventral; D, antennule; E, antenna; F, mandible; G, maxillule; H, maxilla; I, maxilliped. Scale bars: A = 0.2 mm; B = 0.1 mm; C–I = 0.05 mm.

FIG. 12. Stygiopontius spinifer n. sp., female. A, leg 1; B, leg 2; C, leg 3; D, leg 4; E, leg 5; F, left genital aperture. Scale bars: 0.05 mm.

1, 2, 12, 8, 2, 4, 2, 2, 2, 2, 2 + aesthetasc, and 13; aesthetasc on penultimate segment more than twice as long as terminal segment; setae generally short, all of them naked. Antenna (Fig. 11E) with short, unarmed syncoxa. Basis smooth. Exopod small, 13 × 9 μm, with three setae. Endopod 2-segmented; proximal segment 62 × 29 μm, with fine spinules along distal half of outer margin; distal segment 43 × 21 μm, with two spines, two setae, and few setules.

Oral cone stout. Mandible (Fig. 11F) with about ten teeth distally, one blunt process near distal fourth of outer margin, and two hyaline lamellae (proximal and distal) on inner margin. Maxillule (Fig. 11G) bilobed; outer lobe with four setae, including three large, weakly pinnate and one small, naked ones; inner lobe with four setae distally and several setules on inner margin. Maxilla (Fig. 11H) as usual in the genus; seta between segments not extending over basis. Maxilliped (Fig. 11I) 5-segmented; syncoxa and basis each with one inner seta, 54 and 45 μm long, respectively, both spinulose in distal half; endopod with two, two, and one setae, respectively, on first to third segments; third segment 45 μm long; terminal claw 117 μm long, weakly arched, with spinules along distal half of inner margin.

Legs 1–4 (Fig. 12A–D) without inner seta on coxa. Second endopodal segment of legs 1–3 with bicuspid outer distal corner. Inner distal spine on basis of first leg 40 μm long and slender. Basis of leg 2 with five spinules on inner side. Leg 4 (Fig. 12D) with three spines and four setae on third exopodal segment; first endopodal segment 45 × 26 μm; second endopodal segment 76 × 32 μm, its distal spine 100 μm long. Armature formula of legs 1–4 as follows:

Coxa Basis Exopod Endopod

Leg 1: 0-0 1-I I-1; I-1; III, 2, 2 0-1; 0-2; 1, 2, 3

Leg 2: 0-0 1-0 I-1; I-1; III, I, 4 0-1; 0-2; 1, 2, 3

Leg 3: 0-0 1-0 I-1; I-1; III, I, 5 0-1; 0-2; 1, I, 3

Leg 4: 0-0 1-0 I-1; I-1; II, I, 4 0-0; 0, I, 1

Leg 5 (Fig. 12E) unsegmented but divided into proximal and distal parts by unsclerotized band on both surfaces; proximal part 46 × 32 μm, with large, feebly pinnate seta; distal part 23 × 23 μm, with three setae, larger outer seta twice as long as two smaller inner setae. Leg 6 (Fig. 12F) represented by one naked seta in genital aperture.

Male. Unknown.

Etymology. The specific name spinifer, Latin spin (=a spine) and fero (=to carry), alludes to the spiniform process on the lateral margins of the genital double-somite, as in several congeners.

Remarks. The genus Stygiopontius is characterized by the combination of the features, as follows: (1) the endopod of leg 1 is three-segmented in both sexes; (2) the first endopodal segment of leg 3 is armed with an inner seta; (3) the first endopodal segment of leg 4 lacks an inner seta; and (4) the second endopodal segment of leg 4 is armed with one distal spine and one inner seta. In the genus Stygiopontius , seven species are known to have, like S. spinifer n. sp., two (not three) outer spines on the third exopodal segment of leg 4 (armature formula II, I, 4), as follows: S. cinctiger Humes, 1987 , S. lomonosovi Ivanenko and Martinez Arbizu 2006 , S. mucroniferus Humes, 1987 , S. rimivagus Humes, 1997 , S. serratus Humes, 1996 , S. teres Humes, 1996 , and S. verruculatus Humes, 1987 . In six of these species, at least one of legs 1–4 has an inner seta on the coxa. In S. verruculatus , the remaining species, there is no inner seta on the coxa of any of legs 1–4, which is comparable with S. spinifer n. sp.

Stygiopontius verruculatus , known from the East Pacific Rise, was described based only on the male ( Humes 1987). Although a direct comparison between it and S. spinifer n. sp. may be difficult, some sexually non-dimorphic characters may be used to compare male S. verruculatus and female S. spinifer n. sp., as follows: (1) the epimeral regions of the fourth pedigerous somite are rounded in S. spinifer n. sp. but tapering and pointed in S. verruculatus ; (2) the innermost distal seta on the caudal ramus of S. spinifer n. sp. is unilaterally pinnate, whereas that of S. verruculatus is naked; and (3) the inner element on the basis of the maxilliped is a seta located at the proximal third in S. spinifer n. sp. but a ball-like process located near distal third in S. verruculatus .