Stygiopontius quadripaxillifer, 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 : 332-336

Stygiopontius quadripaxillifer n. sp.

(Figs 20–22)

http://zoobank.org/ 2CF78CF5-28FB-453F-B062-F9C501E55D3F

Material examined. One female (holotype, MABIK CR00244735) and one male (paratype, MABIK CR00244736) from sediments at GTV 1701 (19°33.398´S, 65°50.899´E, depth 2540 m), the Solitaire vent field in the Indian Ocean, 31 July 2017. All specimens have been dissected and mounted on the slide, and deposited in the Marine Biodiversity Institute of Korea ( MABIK), Seocheon.

Female. Body (Fig. 20A) moderately narrow and dorsoventrally flattened. Length 1.08 mm. Prosome 612 × 447 μm. Cephalothorax 340 μm long, with pointed posterolateral corners. Second to fourth pedigerous somites with rounded posterolateral corners. Urosome (Fig. 20B) 5-segmented. Fifth pedigerous somite 165 μm wide, lat- erally tapering, with pointed lateral apices. Genital double-somite 142 × 155 μm; anterior third slightly expanded with pointed claw-like process on both sides at level of genital apertures (Fig. 21F); posterior two-thirds 131 μm wide, with slightly convex lateral margins and angular posterolateral corners; genital aperture located dorsolaterally. Three free abdominal somites 80 × 116, 54 × 97, and 43 × 84 μm, respectively. First free abdominal somite with convex lateral margins and angular posterolateral corners. Anal somite shorter than preceding somite, with several spinules on posteroventral surface (Fig. 20C). Caudal rami (Fig. 20C) directed straightly backwards, 82 × 35 μm, 2.34 times as long as wide, with six naked setae, and several spinules on ventrodistal surface; posteroventral margin with short tube bearing pore at tip; innermost distal seta characteristically inflated proximally.

Rostrum absent. Antennule (Fig. 20D) 304 μm long and 13-segmented; articulation between last two segments faint; third segment longest; armature formula 1, 2, 12, 9, 2, 4, 2, 2, 2, 2, 2 + aesthetasc, 6, and 7; first segment with several setules on anterior margin; aesthetasc on penultimate segment constricted in middle. Antenna (Fig. 20E) with unarmed, short syncoxa; basis with several spinules on outer and inner margins. Exopod small, 10 × 6 μm, with three setae. First endopodal segment unarmed, with few minute spinules on inner margin. Second endopodal segment 28 × 16 μm, with two spines (83 and 49 μm, respectively), two setae, and three groups of setules.

Oral cone stout. Mandible (Fig. 20F) and maxilla (Fig. 20H) similar to those of S. geminus n. sp. Maxillule (Fig. 20G) bilobed; outer lobe small, half as long as inner lobe with three large and one minute setae; inner lobe with four large and one minute setae and with few spinules subdistally and setules on inner margin. Maxilliped (Fig. 20I) 5- segmented; inner distal seta on syncoxa 40 μm long; basis with few spinules on outer margin, seta on inner margin 34 μm long; three endopodal segments with three, one, and one setae, respectively; three setae on first endopodal segment small; terminal claw 97 μm long, slightly arched, spinulose along inner margin, about four times as long as third endopodal segment (24 μm long).

Legs 1–4 (Fig. 21A–D) lacking coxal seta. Leg 1 basis with patch of spinules near inner distal corner. Inner distal region of leg 2 basis protruded, with six or seven spinules on its anterior surface (Fig. 21B). Leg 4 endopodal segments 29 × 20, and 57 × 26 μm, respectively; terminal spine on second endopodal segment setiform and 68 μm long. Armature formula of legs 1–4 as follows:

Coxa Basis Exopod Endopod

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

Leg 5 (Fig. 21E) 2-segmented; segmentation represented by transverse unsclerotized band; proximal segment (protopod) about 24 × 19 μm, with large outer seta of 92 μm long; distal segment (exopod) 34 × 13 μm, about 2.6 times as long as wide, with three setae, largest proximal one of latters 84 μm long. Leg 6 (Fig. 21F) represented by one naked seta in genital aperture.

Male. Body (Fig. 22A) similar to that of female. Length 718 μm. Prosome 417 × 298 μm. Cephalothorax with four peg-like spinules on each anterolateral margin (Fig. 22B); these four spinules simple, about 9 μm long, not bifurcate at tip. Urosome 6-segmented. Fifth pedigerous somite narrow, 73 μm wide. Genital somite much wider FIG. 20. Stygiopontius quadripaxillifer 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, E–I = 0.02 mm; D = 0.05 mm.

FIG. 21. Stygiopontius quadripaxillifer n. sp., female. A, leg 1; B, leg 2; C, leg 3; D, leg 4; E, leg 5; F, left genital aperture. Scale bars: A–D = 0.05 mm; E, F = 0.02 mm.

FIG. 22. Stygiopontius quadripaxillifer n. sp., male. A, habitus, dorsal; B, anterior part of left lateral margin of cephalothorax, dorsal; C, caudal rami, ventral; D, antennule; E, first to eighth segments of antennule, medial; F, maxilliped; G, leg 2; H, right legs 5 and 6, ventral. Scale bars: A = 0.1 mm; B–H = 0.02 mm.

than long, 71 × 104 μm, with convex lateral margins; genital operculum with three groups of spinules (one lateral and two medial). Four abdominal somites 50 × 74, 47 × 69, 32 × 58, and 24 × 54 μm, respectively. Anal somite with more spinules on ventrodistal surface than in female (Fig. 22C). Caudal ramus (Fig. 22C) shorter than that of female, 43 × 23 μm, 1.87 times as long as wide.

Rostrum absent. Antennule (Fig. 22D, E) 12-segmented; armature formula 1, 2, 12, 6, spine, 2, 4, 2, 2, 2, 1+aesthetasc, and 11; fourth segment with three transverse articulation vestiges, with small process bearing bifurcate tip and a seta. Similar process present on seventh to ninth segments; spine of fifth segment denticulate in two rows and tipped with setule. Antenna as in female.

Oral cone, mandible, maxillule, maxilla as in female. Maxilliped (Fig. 22F) as in female, but distal seta on first endopodal segment larger than that of female.

Leg 1, 3 and 4 as in female. Leg 2 (Fig. 22G) with second exopodal segment bearing inflated, inwardly curved outer process on outer margin; third endopodal segment with four spines (armature formula I, II, I+2), 17, 34, 93, and 50 μm long, respectively, from outer to inner.

Leg 5 (Fig. 22H) consisting of naked outer seta on fifth pedigerous somite and free exopod. Exopod slightly longer than wide, with five naked setae (three on outer margin and two one distal margin). Leg 6 (Fig. 22H) consisting of outer seta and weakly spinulose distal spine on genital operculum.

Etymology. The specific name quadripaxillifer is derived from Latin quadr (=four), paxillus (=a peg) and fero (=to bear), alluding to the presence of the four peg-like spinules on each side of the cephalothorax in the male.

Remarks. Stygiopontius quadripaxillifer n. sp. has, as diagnostic features, a patch of spinules on the inner distal region of the basis of leg 1 in both sexes and peg-like spinules on the anterolateral margin of the cephalothorax in the male. One or both of these features are shared by four congeners: S. appositus Humes, 1989 , S. hispidulus Humes, 1987 , S. paxillifer Humes, 1989 , and S. sentifer Humes, 1987 , all of which known from the East Pacific Rise ( Humes 1987, 1989). Stygiopontius hispidulus and S. sentifer lack an inner seta on the coxa of legs 1 and 2, and are, therefore, not necessary to compare further with the new species. The remaining two species, S. appositus and S. paxillifer which were known based only on the male, are differentiated from the new species by their longer caudal ramus (more than 2.3 times as long as wide in both species, in contrast to 1.87 times as long as wide in the male of S. quadripaxillifer n. sp.), by their exopod of male leg 5 which is much wider than long (vs. longer than wide in S. quadripaxillifer n. sp.), and by their uninflated innermost distal caudal seta (vs. the same seta being inflated proximally in S. quadripaxillifer n. sp.).