Aphotopontius limatulus Humes, 1987

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 : 310-314

Aphotopontius limatulus Humes, 1987

(Figs 6–8)

Material examined. One female and one male from washings of invertebrates at GTV 1702 (19°33.387´S, 65°50.893´E, depth 2507 m), the Solitaire vent field in the Indian Ocean, 01 August 2017; two females from sediments at GTV1701 (19°33.398´S, 65°50.899´E, depth 2540 m), the Solitaire vent field, 31 July 2017 ; seven females from washing of invertebrates, at GTV1807 (19°33.395´S, 65°50.889´E, depth 2634 m), the Solitaire vent field, 20 June 2018 ; one female from sediments at GTV 1809 (11°24.883´S, 65°25.425´E, depth 2022 m) from the Onnuri vent field, 23 June 2018 .

Female. Body (Fig. 6A) rather narrow, 1.17 mm long. Prosome oval, 740 μm long, with greatest width 555 μm. Cephalothorax 416 μm long, wider than long, with pointed posterolateral corners. Second pedigerous somite also with pointed posterolateral corners. Urosome (Fig. 6B) slender. Fifth pedigerous somite tapering posterolaterally, 157 μm wide, wider than genital double-somite, with angular lateral apices. Genital double-somite 109 × 144 μm, much wider than long; anterior two-thirds broader than posterior third; genital aperture located dorsolaterally, slightly posterior to midlength. Three free abdominal somites 39 × 87, 22 × 78, and 43 × 70 μm, respectively. Anal somite narrowing distally, with patch of spinules on posteroventral surfaces near bases of caudal rami. Caudal ramus (Fig. 6C) elongate, 177 × 20 μm, 8.85 times as long as wide, with three patches of spinules on ventral surface (two patches near middle and one on distal regions), and armed with six setae distally; two mid-terminal setae weakly pinnate, other four setae naked; inner distal seta very small, spinule-like.

Rostrum (Fig. 6D) distinct, tapering, with rounded posterior apex. Antennule (Fig. 6E) slender, elongate, 617 μm long, and 11-segmented; first segment longest; armature formula 15, 10, 2, 4, 2, 2, 2, 2, 2 + aesthetasc, 6, and 7; setae small and naked. Antenna (Fig. 6F) slender. Short syncoxa unarmed. Basis with setules on inner margin and row of fine spinules on outer margin near base of exopod. Exopod small, 12 × 5 μm, with two unequal setae distally. Endopod 2-segmented; first segment 51 × 19 μm, with setules along inner and outer margins; second segment 21 × 16 μm, with four setiform elements, inner subterminal one rudimentary.

Oral cone stout. Mandible (Fig. 6G) with about ten teeth distally and small hyaline lamella at distal third. Maxillule (Fig. 6H) with three setae on outer lobe and four setae + one small setule on inner lobe. Maxilla (Fig. 6I) with a pore at base of syncoxa; endopod attenuated, with setules at distal region, distal one of these setules longer than others; setae between syncoxa and basis large, longer than basis and ornamented with long setules. Maxilliped (Fig. 6J) 4-segmented; inner seta of syncoxa large, 90 μm long, finely spinulose along its proximal margin. Inner seta on basis naked, 70 μm long, and directed proximally; first endopodal segment with two setae and one thin setule; sec- ond endopodal segment 30 μm long, with one seta distally; terminal claw slender, 105 μm long, and finely spinulose along inner margin.

Legs 1–3 (Fig. 7A–C) with three segmented rami. Leg 4 (Fig. 7D) with 3-segmented exopod and 2-segmented endopod. Inner coxal seta present in legs 2 and 3, but lacking in legs 1 and 4. Inner spine on basis of leg 1 moderately slender and 47 μm long. Two distal setae on third endopodal segment of leg 2 spiniform. Leg 4 with slender rami; inner seta on first exopodal segment with fine spinules along its distal half; first endopodal segment 54 × 35 μm, its inner seta shorter than second endopodal segment; second endopodal segment 133 × 28 μm, setulose along proximal half of both margins and finely spinulose along distal half of outer margin; distal spine 98 μm long, with fine spinules on both margins. 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: 0-1 1-0 I-1; I-1; III, I, 4 0-1; 0-2; 1, 2, 3

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

Leg 5 (Fig. 7E) clearly 2-segmented; protopod 43 × 28 μm, with naked outer seta; exopod 30 × 20 μm, with three naked setae. Leg 6 represented by one small seta in genital aperture (Fig. 6B).

Male. Body (Fig. 8A) more slender than that of female. Length 1.01 mm. Prosome 602 μm long and greatest width 442 μm. First three prosomal somites with acutely pointed posterolateral corners. Urosome 6-segmented. FIG. 6. Aphotopontius limatulus Humes , female. A, habitus, dorsal; B, urosome, dorsal; C, right caudal ramus, ventral; D, rostrum; E, antennule; F, antenna; G, mandible; H, maxillule; I, maxilla; J, maxilliped. Scale bars: A = 0.2 mm; B = 0.1 mm; C–F = 0.05 mm; G–J = 0.02 mm.

FIG. 7. Aphotopontius limatulus Humes , female. A, leg 1; B, leg 2; C, leg 3; D, leg 4; E, leg 5. Scale bars: 0.05 mm. FIG. 8. Aphotopontius limatulus Humes , male. A, habitus, dorsal; B, anal somite and caudal rami, ventral; C, antennule; D, left legs 5 and 6, ventral. Scale bars: A = 0.2 mm; B–D = 0.05 mm.

Fifth pedigerous somite 102 μm wide. Genital somite 76 × 113 μm, much wider than long, with spinules on ventral surface of genital operculum (Fig. 8D). Four abdominal somites 46 × 73, 34 × 68, 24 × 64, and 40 × 60 μm, respec- tively. Caudal ramus (Fig. 8B) 146 × 20 μm, 7.30 times as long as wide. Anal somite and caudal rami with spinules on ventral surface, as in female.

Rostrum as in female. Antennule (Fig. 8C) 11-segmented; armature formula 1, 14, 8, 3, 2, 4, 2, 2, 2, 2 + aesthetasc, and 11; fourth segment with outgrowth tipped by two spiniform elements and with one minute seta; proximal seta on penultimate segment minute, obscure; antepenultimate segment with additional spiniform process. Antenna as in female.

Mandible, maxillule, maxilla, maxilliped, and legs 1–4 as in female.

Leg 5 (Fig. 8D) 2-segmented; protopod short unarticulated from somite, with one large outer seta; exopod 27 × 21 μm, with three long outer setae and two inner knife-like setae. Leg 6 (Fig. 8D) represented by two long setae on genital operculum.

Remarks. Aphotopontius limatulus Humes, 1987 has previously been reported from the East Pacific Rise and Galapagos Rift in the Pacific Ocean ( Humes 1998). Our material from the Central Indian Ridge does not differ significantly from the type specimens. The only notable differences were in the caudal ramus and antennule. The length-to-width ratio of the caudal ramus of the type specimens was described 9.94 in the female and 8.6 in the male, while it is 8.85 and 7.30 in our female and male specimens, respectively. The two ventral patches of spinules on the mid-region of the caudal ramus are not mentioned in the original description. The antennule of the type specimens is described as 10-segmented in the female and 12-segmented in the male, whereas it is 11-segmented in both sexes of our specimens. We consider these differences to be within species variation or artificial and insufficient to separate them as different species.

Four species in the Dirivultidae have been recorded as transoceanic, from both the Atlantic and Pacific oceans: Aphotopontius forcipatus Humes, 1987 , Stygiopontius mirus Humes, 1996 , S. pectinatus Humes, 1987 , and S. rimivagus Humes, 1997 ( Humes 1990, 1996, 1998; Gollner et al. 2010). Here, A. limatulus is reported as an additional transoceanic species from the Pacific and Indian Oceans.

Genus Benthoxynus Humes, 1984