Stygiopontius geminus, 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 : 326-331

Stygiopontius geminus n. sp.

(Figs 17–19)

http://zoobank.org/ 76F9CD5C-BB8F-4818-B658-65D745145164

Material examined. Sixteen females and five males 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. Holotype (female, MABIK CR00244733 ) and paratypes (11 females and three males, MABIK CR00244733 ) 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.

Female. Body (Fig. 17A) dorsoventrally flattened and moderately broad. Length 1.18 mm. Prosome 772 × 533 μm. Cephalothorax 413 μm long, with angular posterolateral corners in dorsal view. Three metasomal somites with blunt or rounded posterolateral corners. Urosome (Fig. 17B) 5-segmented. Fifth pedigerous somite 164 μm wide, with angular lateral apices and straight posterodorsal margin. Genital double-somite 162 × 158 μm, slightly longer than wide, with roundly expanded anterior third and pointed denticle at posterolateral corners; genital aperture located dorsolaterally at 37% region of double-somite length. Three free abdominal somites, 66 × 107, 41 × 91, and 42 × 95 μm, respectively. Caudal rami (Fig. 17C) directed backwards, 67 × 40 μm measured in ventral view, 1.68 times as long as wide, with bilobed posteroventral margin bearing spinules, transverse row of several spinules on ventral surface near posteroventral margin, and armed with six setae; two mid-terminal setae pinnate along their distal two-thirds; innermost distal seta pinnate along inner margin.

Rostrum absent. Antennule (Fig. 17D) 373 μm long and 12-segmented; armature formula 1, 14, 10, 2, 4, 2, 2, 2, 2, 2 + aesthetasc, 6, and 7 + aesthetasc; third segment with trace of articulation subdistally; aesthetasc on antepenultimate segment constricted near middle; aesthetasc on terminal segment very small. Antenna (Fig. 17E) with short, unarmed syncoxa. Basis with setules on inner margin and several spinules on outer margin near base of exopod. Exopod small, 12 × 6 μm, with three setae. Endopod 2-segmented; first segment unarmed but with row of spinules on outer side; second segment 29 × 16 μm, with two spines (108 and 58 μm long, respectively), two setae, and three groups of minute setules.

Oral cone stout and short as in other species in the genus. Mandible (Fig. 17F) with about ten teeth distally and three transparent lamellae on inner margin, distal two of latters finely pectinate and proximal one short and smooth. Maxillule (Fig. 17G) bilobed; shorter outer lobe with three large and one small setae; inner lobe with four large and one minute setae distally and setules on inner margin. Maxilla (Fig. 17H) consisting of syncoxa, basis, and intermediate seta; syncoxa with short, flexible tube at base; basis with spatulate tip bearing spinules and setules and with transverse row of spinules in middle; intermediate seta inserted on fleshy expansion arising between syncoxa and basis, articulated at base, nearly as long as basis, and pinnate distally. Maxilliped (Fig. 17I) 5-segmented; inner setae on syncoxa and basis 49 and 28 μm long, respectively; endopod with two, one, and one setae on first to third segments, respectively; third endopodal segment 22 μm long; terminal claw elongate, 125 μm long, more than five times as long as third endopodal segment, with fine spinules along inner margin.

Legs 1–3 (Fig. 18A–C) with 3-segmented rami. Leg 4 (Fig. 18D) with 3-segmented exopod and 2-segmented endopod. Inner coxal seta present in legs 1 and 2 but absent in legs 3 and 4. Inner distal seta on basis of leg 1 needlelike. Second endopodal segment of legs 1–3 with bicuspid outer distal corner. Basis of leg 2 with five or six spinules on anterior surface near inner distal corner. First segment of leg 4 endopod 29 × 20 μm; second segment 56 × 28 μm, with setules on outer margin, its terminal spine setiform and 75 μm long. Armature formula of legs 1–4 as follows:

Coxa Basis Exopod Endopod

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

Leg 2: 0-1 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. 18E) obscurely 2-segmented; articulation represented by transverse unsclerotized area. Proximal segment (protopod) 37 × 32 μm, its outer seta 116 μm long. Distal segment (exopod) narrowing distally, 27 × 22 μm, with three setae, longest one 96 μm long. Leg 6 (Fig. 17F) represented by 1 pinnate seta in genital aperture.

Male. Body (Fig. 19A) 903 μm long. Prosome relatively broad, 510 × 449 μm. Cephalothorax 315 μm long, with pointed posterolateral corners. Urosome (Fig. 19B) 6-segmented, fifth pedigerous somite 108 μm wide. Genital somite wider than long, 93 × 136 μm, with rounded corners; genital operculum (Fig. 19H) with spinules on inner and outer sides. Four abdominal somites 80 × 97, 58 × 87, 33 × 78, and 29 × 76 μm, respectively; first abdominal somite with pointed denticle at posterolateral corners. Anal somite shortest among abdominal somites, with spinules near posteroventral margin (Fig. 19C). Caudal ramus (Fig. 19C) 52 × 33 μm, 1.58 times as long as wide.

FIG. 17. Stygiopontius geminus 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, F = 0.02 mm; D, E, G–I = 0.05 mm.

FIG. 18. Stygiopontius geminus n. sp., female. A, leg 1; B, leg 2; C, leg 3; D, leg 4; E, leg 5; F, right genital aperture. Scale bars: A–E = 0.05 mm; F = 0.02 mm.

FIG. 19. Stygiopontius geminus n. sp., male. A, habitus, dorsal; B, urosome, ventral; C, caudal rami, ventral; D, antennule; E, maxilliped; F, leg 2; G, leg 5; H, leg 6. Scale bars: A = 0.2 mm; B = 0.1 mm; C–E, G, H = 0.02 mm; F = 0.05 mm.

Rostrum absent. Antennule (Fig. 19D) strongly curved and 13-segmented; armature formula 1, 2, 12, 2, 4, spine, 2, 4, 2, 2, 2, 1 + aesthetasc, and 11; spine on sixth segment inserted on outgrowth of segment, strong, serrate on distal margin, and tipped with one small setule; eighth segment with round protrusion on anterior margin; tenth to twelfth segments with one, two, and three spiniform processes, respectively, on anterior margin. Antenna as in female.

Oral cone, mandible, maxillule, and maxilla as in female. Maxilliped (Fig. 19E) 5-segmented; inner seta on syncoxa and basis expanded, spinulose along inner (posterior) margin, and 34 and 22 μm long, respectively. Outer margin of basis smooth. Endopod with three, one, and one setae on first to third segments, respectively.

Leg 1 as in female. Leg 2 (Fig. 19F) with third endopodal segment bearing four strong spines and two setae; lengths of these spines 70, 25, 100, and 44 μm, from outer to inner. Legs 3 and 4 as in female.

Leg 5 (Fig. 19G) with protopod fused with somite, with long outer seta. Exopod small, 13×21 μm, with three setae on outer margin. Leg 6 (Fig. 19H) represented by two setae on genital operculum.

Etymology. The specific name geminus is derived from the Latin gemin (=twin), referring to the close similarity of the new species to Stygiopontius lomonosovi Ivanenko and Martinez Arbizu, 2006 in Ivanenko, Martinez Arbizu and Stecher (2006) .

Remarks. As diagnostic features of S. geminus n. sp., the third exopodal segment of leg 4 has three spines and four setae (armature formula II, I, 4) and the inner coxal seta present in legs 1 and 2. This feature is shared by three known species: S. cinctiger Humes, 1987 , S. teres Humes, 1996 , and S. lomonosovi Ivanenko and Martinez Arbizu, 2006 . Stygiopontius cinctiger has a broad, ventrally swollen genital double-somite in the female and is, therefore, easily distinguishable from the other three species. Stygiopontius teres differs also in the female from those of S. lomonosovi and S. geminus n. sp. in having an oviform prosome, tapering epimeral regions of the fourth pedigerous somite, unsegmented broad leg 5, and all bipinnate caudal setae. Females of the remaining two species, S. lomonosovi and S. geminus n. sp., are hardly distinguishable, except for a slight difference in the dimensions of the caudal ramus. The length-to-width ratio of the caudal ramus in the female S. lomonosovi is at most 1.45 based on the original illustration (Fig. 5E of Ivanenko et al. 2006), compared with 1.68 in S. geminus n. sp.

However, because males of the two species differ markedly, they cannot be treated as conspecific. For example, the inner distal corner of the maxilliped syncoxa of the male is strongly projected in S. lomonosovi but not projected in A. geminus n. sp., and a prominent outgrowth on the third antennular segment in the male described and illustrat- ed in S. lomonosovi (see Ivanenko et al. 2006) is absent in the male of S. geminus n. sp. These and other differences are summarized in Table 1.

*Length/width ratios of caudal rami of S. lomonosovi were measured based on the original illustrations.