Stygiopontius horridus, 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 : 320-326

Stygiopontius horridus n. sp.

(Figs 13–16)

http://zoobank.org/ C29CE293-4E24-4182-A215-D555443CA734

Material examined. Twenty three females, eight males, and one copepodid I in amplexus with a male adult, from washings of invertebrates at GTV1702 (19°33.387´S, 65°50.893´E, depth 2507 m), the Solitaire vent field on the Central Indian Ridge in the Indian Ocean, 01 August 2017. Holotype (female, MABIK CR00244731 ) and paratypes (20 females and seven males, MABIK CR00244732 ) have been deposited in the Marine Biodiversity Institute of Korea ( MABIK), Seocheon. Dissected paratypes (two females and one male) are retained in the collection of the junior author.

Additional material examined. Seven females and two males from washings of invertebrates, at GTV 1807 (19°33.395´S, 65°50.889´E, depth 2634 m), the Solitaire vent field, 20 June 2018.

Female. Body (Fig. 13A) moderately broad and 1.24 mm long. Prosome 710 × 545 μm. Cephalothorax 445 μm long, with angular posterolateral corners. Three metasomites with rounded posterolateral corners. Urosome (Fig. 13B) 5-segmented. Fifth pedigerous somite 149 μm wide; lateral apices not pointed. Genital double-somite 190 × 163 μm, distinctly longer than wide, with slightly expanded anterior two-fifths; genital aperture located dorsolaterally at 30% region of double-somite length. Three free abdominal somites 73 × 104, 49 × 99, and 46 × 98 μm, respectively, smooth without spinules or setules on all surfaces. Caudal rami (Fig. 13C) stout and slightly convergent; each ramus 72 × 42 μm, 1.71 times as long as wide, with tapering posteroventral margin and six naked setae; innermost distal seta as long as outermost distal seta. Egg sac (Fig. 13D) containing two or three eggs; each egg about 195 μm in diameter.

Rostrum absent.Antennule (Fig. 13E) relatively short, 273 μm long, and 10-segmented; third segment short and incompletely articulated from second segment; first segment the longest; armature formula 15, 8, 2, 4, 2, 2, 2, 2, 2 + aesthetasc, and 13; setae naked and mostly short. Antenna (Fig. 13F) massive. Articulation between coxa and basis incomplete. Exopod small, 10 × 6 μm, with three setae. First endopodal segment unarmed but with large tubercle on inner side. Second endopodal segment (Fig. 13G) 31 × 21 μm, with two blunt spiniform setae (one on inner margin and the other on distal margin) and two robust spines tipped with bundle of spinules.

Oral cone short, stout. Mandible (Fig. 13H) with more than ten teeth distally and hyaline lamella subdistally. Maxillule (Fig. 13I) with both lobes bearing nearly parallel lateral margins; inner lobe with three setae; outer lobe subequal in length to inner lobe, with three setae; setae of outer lobe distinctly longer than those of inner lobe. Maxilla (Fig. 13J) with broad, unarmed syncoxa; basis hook-like, with fine spinules near middle; seta arising between segments naked and much shorter than basis. Maxilliped (Fig. 13K) 4-segmented; seta on syncoxa and basis small; endopod 2-segmented, but proximal segment subdivided by rudimentary articulation; proximal segment with two small setae; distal segment about 28 μm long, distally with one large seta; terminal claw 93 μm long, smooth, with denticle subdistally.

Legs 1–3 (Fig. 14A–C) with 3-segmented rami. Leg 4 (Fig. 14D) with 3-segmented exopod and 2-segmented endopod. All of these legs lacking inner coxal seta. Basis of leg 1 with mammillary process (indicated by arrowhead in Fig. 14A) at inner distal corner and thin, needle-shaped seta near base of endopod. Leg 4 with first exopodal segment bearing almost naked inner seta; third exopodal segment armed with three spines and four setae; first endopodal segment small, 26 × 23 μm; second endopodal segment 87 × 37 μm, much broader than first segment, with slightly undulating outer margin and terminal spine of 63 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; 0-2; 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. 14E) 1-segmented, clearly articulated from somite, 77 × 54 μm, 1.43 times as long as wide, with four naked setae, innermost one of them thin. Leg 6 not seen in genital aperture (Fig. 13B).

Male. Body (Fig. 15A) markedly smaller than that of female, 776 μm long. Prosome 465 × 375 μm. Cephalo- thorax 306 μm long, frontally truncate. Urosome (Fig. 15B) 6-segmented. Fifth pedigerous somite 95 μm wide, with angular lateral apices. Genital somite 79×138 μm, much wider than long, with rounded corners. Four abdominal somites 50 × 98, 45 × 86, 28 × 72, and 33 × 72 μm, respectively, with convex lateral margins. Anal somite with several minute spinules on ventral surface near base of caudal rami (Fig. 15C). Caudal ramus (Fig. 15C) 50 × 30 μm, 1.67 times as long as wide; ventrodistal apex bilobed.

FIG. 13. Stygiopontius horridus n. sp., female. A, habitus, dorsal; B, urosome, dorsal; C, anal somite and caudal rami, dorsal; D, egg sac; E, antennule; F, antenna; G, terminal segment of antenna; H, mandible; I, maxillule; J, maxilla; K, maxilliped. Scale bars: A, D = 0.2 mm; B = 0.1 mm; C, E, K = 0.05 mm; F–J = 0.02 mm.

FIG. 14. Stygiopontius horridus n. sp., female. A, leg 1; B, leg 2; C, leg 3; D, leg 4; E, leg 5. Scale bars: 0.05 mm. FIG. 15. Stygiopontius horridus n. sp., male. A, habitus, dorsal; B, urosome, ventral; C, caudal rami, ventral; D, antennule; E, coxa, basis and endopod of leg 1; F, leg 5. Scale bars: A = 0.1 mm; B, E = 0.05 mm; C, D, F = 0.02 mm.

Rostrum absent. Antennule (Fig. 15D) stout, strongly recurved, and 13-segmented; armature formula 1, 2, 12, 1, 4, spine+1, 1, 4, 2, 2, 2, 1+aesthetasc, and 12; fifth segment with about three vestiges of articulations on posterior side; sixth segment with outgrowth bearing two spiniform processes, its terminal spine with small warts on all surfaces and tipped with short seta; eighth segment with two blunt processes on anterior margin, each tipped with seta; eleventh and twelfth segments respectively with two and three distally-directed processes on anterior margin. Antenna as in female.

FIG. 16. Stygiopontius horridus n. sp., copepodid I. A, habitus, dorsal; B, urosome, ventral C, antennule; D, antenna; E, mandible; F, maxillule; G, maxilla; H, maxilliped; I, leg 1; J, leg 2. Scale bars: A = 0.1 mm; B, I, J = 0.05 mm; C, D, G, H = 0.02 mm; E, F = 0.01 mm.

Oral cone, mandible, maxillule, maxilla, and maxilliped as female.

Leg 1 with first endopodal segment covered with numerous spinules on outer surface (Fig. 15E). Legs 2–4 as in female.

Leg 5 (Fig. 15F) 2-segmented, but protopod short and not articulated from somite, with long outer seta; exopod 27 × 25 μm, with three setae on outer margin (middle one longer than other two) and two spiniform, blunt setae on distal margin; latter two distal setae sclerotized in proximal two-thirds and lamellate in distal third. Leg 6 represented by two unequal setae on genital operculum (Fig. 15B).

Copepodid I. Body (Fig. 16A) 5-segmented, 422 μm long. Prosome consisting of cephalothorax and second pedigerous somite. Cephalothorax 213 × 193 μm, gradually narrowing posteriorly. Urosome (Fig. 16B) 3-seg- mented; first urosomite being third pedigerous somite. Second urosomite 30 × 57 μm, broadening posteriorly, with angular posterolateral corners. Anal somite 50 × 53 μm, with convex lateral margins. Caudal ramus 32 × 20 μm, with six setae; longest inner distal seta bipinnate; second longest outer distal seta pinnate along inner margin and finely spinulose along outer margin.

Rostrum absent. Antennule (Fig. 16C) 3-segmented; second segment short; armature formula 3, 1, and 11 + 2 aesthetascs. Antenna (Fig. 16D) stout. Syncoxa and basis smooth. Exopod with two distal setae. Endopod 2-segmented; first segment unarmed; second segment with two setae and two broad, spiniform elements, one of latters with spinules at distal region.

Oral cone short. Mandible (Fig. 16E) denticulate distally, with hyaline lamella at distal three-fourths. Maxillule (Fig. 16F) bilobed; outer and inner lobes with three and two setae, respectively. Maxilla (Fig. 16G) basically as in adult. Maxilliped (Fig. 16H) 4-segmented; syncoxa and basis unarmed; first and second endopodal segments each with one seta; terminal claw with spinules at distal region.

Leg 1 (Fig. 16I) and leg 2 (Fig. 16J) biramous, both rami 1-segmented and lacking inner coxal seta. Basis of leg 1 with spinules along inner distal margin. Armature formula of these two legs as follows:

Coxa Basis Exopod Endopod

Leg 1: 0-0 1-0 IV, I, 3 1, 2, 4

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

Leg 3 (Fig. 16B) bilobed; outer lobe (exopod) with two setae; inner lobe unarmed. Legs 3–6 absent.

Etymology. The specific name horridus , from Latin horrid (prickly), alludes to the prickly tip of the distal spines of the antenna.

Remarks. Stygiopontius horridus n. sp. possesses the characteristic antenna and maxillule, typifying the new species. The antenna has a large tubercle on the first endopodal segment and two spinule-tipped distal spines on the second endopodal segment. The maxillule has only three (not four) setae on the inner lobe. Because these features are not shared by its congeners, the new species is easily distinguishable from other species in the genus.

Ivanenko (1998) recorded copepodid I of a dirivultid copepod found in plankton over a hydrothermal vent on the Mid-Atlantic Ridge. This copepodid I appears to be different from our specimen from the Indian Ocean mainly in body length (0.37 mm in Ivanenko’s specimens), antennular segmentation (4-segmented in Ivanenko’s specimens) and setation, and the morphological features of the antenna (three setae on the exopod and an elongate terminal spine on the second endopodal segment in Ivanenko’s specimens).

The discovery of a copepodid I juvenile in amplexus with a male adult in the vent community implies that copepodid I of this species stays on the bottom of the vent field and that mate guarding may take place as early as the female copepodid I stage.