Archinotodelphys unisetosus, Kim & Boxshall, 2020

Kim, Il-Hoi & Boxshall, Geoff A., 2020, A revision of the family Archinotodelphyidae Lang, 1949 (Copepoda: Cyclopoida Oithonida), with the recognition of 15 new species, Zootaxa 4801 (1), pp. 1-56 : 53-54

Archinotodelphys unisetosus sp. nov.

(Figs. 32, 33)

Type material. Holotype (♀, MNHN-IU-2014-21211, dissected and mounted on a slide). Type host. unidentified Phlébobranche.

Type locality. North Atlantic , ABYPLAINE cruise, N/0 Cryos, Stn DS 11, 42°59.7´N, 14°05.4´W, depth 5260 m, Monniot coll., 12-13 June 1981 .

Etymology. This name of the species alludes to the presence of a single seta only on the first endopodal segment of the mandible, a unique character state for the family.

Female. Body (Fig. 32A) small, length 815 μm: prosome 495 μm long; greatest width 298 μm across cephalo- some. Urosome (Fig. 32B) 5-segmented; fifth pedigerous somite 107 μm wide. Genital double-somite almost as long as wide (104×100 μm); paired genital apertures located dorsolaterally at 40% of double-somite length. Three free abdominal somites 45×72, 44×71, and 45×73 μm, respectively. Caudal rami separated from each other; each ramus (Fig. 32C) about 2.6 times longer than wide (67×26 μm), armed with 6 setae, all pinnate; outer lateral seta located at 38% of ramus length.

Rostrum (Fig. 32D) triangular, 51×42 μm. Antennule (Fig. 32E) 202 μm long and 12-segmented; armature formula 3, 5, 7, 2, 2, 6, 4+aesthetasc, 4, 2+aesthetasc, 2, 2 + aesthetasc, and 7+aesthetasc; all setae naked. Antenna (Fig. 32F) with unarmed coxa; basis with 2 subequal exopodal setae at outer distal corner; endopod 3-segmented with 1, 4, and 6+claw on first to third segments, respectively; terminal segment 2.3 times longer than wide.

Labrum (Fig. 32G) with large posteromedian lobe and setulose distal margin. Mandible (Fig. 32H) with 2 teeth and 2 setae on coxal gnathobase; basis with 1 seta on medial margin; exopod 4-segmented, each segment with 1 seta; endopod with 1 and 7 setae on first and second segments. Maxillule (Fig. 32I) with 9 setae on precoxal arthrite; coxa with 1 seta on endite and 2 setae on epipodite; basis with 6 setae on medial margin; exopod with 4 setae distally; endopod with 5 setae, 3 medial and 2 distal. Maxilla (Fig. 33A) 5-segmented; syncoxa with 4 (including 1 small), 1, 2, and 3 setae on first to fourth endites; basis with 1 smooth claw and 2 setae; endopod 3-segmented with 1, 2, and 3 setae on first to third segments, respectively. Maxilliped (Fig. 32J) 3-segmented with 4 (1, 1, and 2), 1, and 3 setae on first to third segments, respectively.

Legs 1–4 biramous with 3-segmented rami (Figs. 33B-D); armature formula for legs 1–4 as generic diagnosis. All setae on legs pinnate. Inner distal spine on basis of leg 1 (Fig. 33B) 33 μm long, longer than first endopodal segment, proximally pinnate but distally spinulose.

Leg 5 (Fig. 33E) protopodal segment wider than long, probably with 1 outer seta (detached but attachment scar discernible); exopodal segment about 1.9 times longer than wide (50×26 μm), with 3 large, pinnate setae and 1 smaller, naked seta. Leg 6 (Fig. 33G) represented by 1 spine and 1 seta on genital operculum.

Male. Unknown.

Remarks. This deep-water species exhibits extreme reduction of the mandibular endopod with the fewest setae of any known species: 1 on the first segment and 7 on the second. In other species the first segment is armed with between 4 and 6 setae, and the second segment between 8 and 10 setae. The antennule of the female is 12-segmented in A. unisetosus sp. nov., which is another unique character state. Most other species have more expressed segments (14 to 17) and only A. profundus has fewer, with only 10 (cf. Huys & Boxshall, 1991: Fig. 2.8.9A). The maxillule has only 5 setae on the endopod in A. unisetosus sp. nov. This compares with 8 to 11 in all other species except A. polynesiensis and A. phallusiae comb. nov., each of which apparently has 12 setae ( Hansen, 1923), although these counts require confirmation as 12 is more than is found in any other known member of the order Cyclopoida ( Huys & Boxshall, 1991) .

This is the deepest known record of any described species of Archinotodelphyidae but the family inhabits considerably deeper waters. Khodami et al. (2019) sequenced unidentified archinotodelphyids collected from depths of 8734 to 9013 m at the Kuril Trench in the North Pacific.