Benthoxynus constrictus, 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 : 314-317

Benthoxynus constrictus n. sp.

(Figs 9, 10)

http://zoobank.org/ 13B0DBCD-6362-49E6-BE95-0509CBCB4863

Material examined. Two females 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, 01 August 2017. Holotype (female, MABIK CR00244728 ) has been deposited in the Marine Biodiversity Institute of Korea ( MABIK), Seocheon. Dissected paratype is retained in the collection of the junior author.

Female. Body (Fig. 9A) narrow, 1.78 mm long. Prosome oviform, 930 × 750 μm. Cephalothorax 632 μm long, with tapering posterolateral corners. Second to fourth pedigerous somites with rounded posterolateral corners. Urosome (Fig. 9B) slender. Fifth pedigerous somite laterally constricted in middle, with dorsal posterolateral extensions. Genital double-somite rhomboidal, 222 × 236 μm, widest at proximal third; genital aperture located dorsolaterally slightly posterior to widest region. Three free abdominal somites 139 × 113, 90 × 100, and 90 × 102 μm, respectively. Abdominal somite and caudal rami smooth, without setules or spinules on all surfaces. Caudal rami (Fig. 9C) slightly divergent; each ramus 209 × 43 μm, 4.86 times as long as wide, armed with six setae (setae II–VII); dorsal seta located subdistally and other five setae on distal margin; two larger mid-terminal setae weakly pinnate along distal half; inner distal seta characteristically small, obscure.

Rostrum absent. Antennule (Fig. 9D) 710 μm long and 12-segmented; third segment longest, with five trans- verse sclerotization bands on one surface (not shown in Fig. 9D); armature formula 1, 2, 12, 10, 2, 4, 2, 2, 2, 2, 2 + aesthetasc, and 13; aesthetasc on penultimate segment slender, slightly longer than terminal segment. Antenna (Fig. 9E) with short, unarmed syncoxa. Basis smooth. Exopod small, 19 × 9 μm, with three setae distally. Endopod 2-segmented; proximal segment 98 × 43 μm, unarmed; distal segment 72 × 35 μm, with four setae (one small inner, two subdistal, and one large distal) and several setules near base of outer subdistal seta.

Oral cone stout as usual in the family. Mandible (Fig. 9F) as flattened stylet, with more than ten teeth distally and hyaline lamella along distal fourth of inner margin. Maxillule (Fig. 9G) bilobed; outer lobe with four setae (three distal and one subdistal); inner lobe with strongly protruded inner margin and four distal setae; both outer and inner lobes smooth without setules or spinules. Maxilla (Fig. 9H) 2-segmented; syncoxa unarmed, with pore at basal region; basis elongate, with fine spinules and setules at distal region, one of setules large; one large seta present, arising between syncoxa and basis. Maxilliped (Fig. 9I) 5-segmented; syncoxa with one inner distal seta of 42 μm long; basis with inner seta of 71 μm long; endopod 3-segmented, with two, one, and one setae, respectively; two setae on first endopodal segment minute, obscure; third endopodal segment 52 μm; terminal claw 174 μm long, weakly arched, with spinules along distal half of inner margin.

Legs 1–4 (Figs. 10A–D) lacking inner coxal seta; outer seta on basis thin and naked; setae of these legs, especially those of endopod, swollen in proximal third and weakly pinnate in distal half. Second exopodal segment of leg 1 small, with outer spine and inner seta; all of other elements on leg 1 setiform. Inner distal seta on basis of leg 1 minute, needle-like. First endopodal segment of leg 3 unarmed, lacking inner seta. First and second endopodal FIG. 9. Benthoxynus constrictus n. sp., female. A, habitus, dorsal; B, urosome, dorsal C, caudal rami, dorsal; D, antennule; E, antenna; F, mandible; G, maxillule; H, maxilla; I, maxilliped. Scale bars: A = 0.5 mm; B, D, E = 0.1 mm; C, F–I = 0.05 mm.

FIG. 10. Benthoxynus constrictus 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–D = 0.1 mm; E = 0.02 mm; F = 0.05 mm.

segments of leg 4 smooth, 86 × 41 and 133 × 35 μm, respectively; terminal seta 145 μm long. Armature formula of legs 1–4 as follows:

Coxa Basis Exopod Endopod

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

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

Leg 5 (Fig. 10E) 1-segmented, clearly articulated from somite, 45 × 23 μm, about twice as long as wide, with three naked setae (one dorsal and two distal). Leg 6 absent (Fig. 10F).

Male. Unknown.

Etymology. The specific name constrictus refers to the lateral constriction of the fifth pedigerous somite.

Remarks. Benthoxynus spiculifer Humes, 1984 and B. tumidiseta Humes, 1989 , the two known members of the genus, were recorded from hydrothermal vent fields in the East Pacific. These two congeners of B. constrictus n. sp. have the following features which are useful for differentiating them from the new species:

(1) Leg 5 is lobate, unarticulated from the fifth pedigerous somite (vs. free in B. constrictus n. sp.).

(2) The antennule is 18-segmented in B. spiculifer and 11-segmented in B. tumidiseta (vs. 12-segmented in B. constrictus n. sp.)

(3) The caudal ramus is longer than that of n. sp., 240 μm in B. spiculifer and 313 μm in B. tumidiseta ( Humes, 1984, 1989) (vs. 209 μm in B. constrictus n. sp.), although their bodies are smaller than that of B. constrictus n. sp. (recorded as 1.68 and 1.67 mm long, respectively, in their original descriptions).

(4) The fifth pedigerous somite is not constricted laterally (vs. strongly constricted in B. constrictus n. sp.).

Genus Stygiopontius Humes, 1987