Notoixys planiceps Kim & Boxshall 2020, gen. et sp. nov.

Notoixys planiceps gen. et sp. nov.

(Figs. 346, 347)

Typematerial. Holotype (intact ♀, MNHN-IU-2014- 21392), paratypes (3 intact ♀♀, MNHN-IU-2014-21393), and dissected paratypes (2 ♀♀, figured) from Aplidium cyaneum Monniot C. & Monniot F., 1983 (MNHN-IT-2008-295 = MNHNA 1/ APL.B/129), Cruise Eltanin 32, Stn 2127, Antarctic Peninsula (71°23’S, 171°36’E), depth 515-521 m, SOSC coll., 13 February 1968.

Etymology. The specific name of the new species is derived from the combination of the Latin plan (=flat) and ceps (=head), alluding to the flattened cephalosome.

Descriptionoffemale. Body (Fig. 346A) robust, 1.37 mmlong. Prosome 1.14 mmlong: dorsalcephalic shield dorsoventrally flattened (Fig. 346A), expanded ventrally, tapering anteriorly in dorsal view towards angularanterior apex, andwithpairedhorn-likeprocesses posterolaterally (Fig. 346B). Metasome swollen, unsegmented, 1.7 times longer than dorsoventral depth, with arched dorsal margin and straight ventral margin. Freeurosome (Fig. 346C) small, recurved ventrally, tapering posteriorly, distinctly 5-segmented: genital and 4 abdominalsomites 55×230, 64×175, 60×141, 44×116, and 56×96 μm, respectively. Anal somite and caudal rami covered with setules; anal somite with deep posteromedian incision. Caudalramus (Fig. 346D) about 2.9 times longer than wide (82×28 μm) and shorter than anal somite: armedwith 6 setae (outer lateral, dorsal, and 4 distal); outer distal and dorsal setae positioned at 48 and 60% of ramus length, respectively; all caudal setae small, less than half width of ramus.

Rostrum (Fig. 346E) large, elongate, densely covered with setules, proximal half with parallel margins, distal half tapering. Antennule (Fig. 346F) 7-segmented, but terminal segment bearing traces of 2 articulations; first and second segments much broader than other segments; setal armature not discernible due to similarity with setules. Antenna (Fig. 346G) 4-segmented; proximal 3 segments unarmed; terminal segment (second endopodal segment) 3.1 timeslongerthan wide (62×20 μm) and slightly longerthan first endopodal segment: armed with 9 small setae (arranged as 1, 1, 2, 2, and 3) plus small terminal claw, less than half length of segment.

Labrum (Fig. 346H) with broad, setulose, almost straight posteriormargin and large, semicircular, setulose posteromedian lobe. Mandible (Fig. 346I) with medial margin of coxal gnathobase bearing 3 teeth and short pectinate area; sizes and gaps between teeth variable (Fig. 346 J-L): basiswith 1 seta medially: exopodwith 4 equally large medial setae and 1 small outer seta (0.4 times as long as medial setae): endopod with 1 broad setaon first segment and 6 setae on second. Maxillule (Fig. 347A) with 6 setae on arthrite, 1 each on epipodite and basis, 4 onexopodand 3 on endopod. Maxilla (Fig. 347B) 5-segmented; syncoxawith 3, 1, 2, and 2 setaeon first to fourth endites, respectively; basis with 3 setae; first to third endopodal segments with 1, 1, and 3 setae, respectively; 3 setae on third endopodal segment very unequal. Maxilliped (Fig. 347C) as unsegmented small lobe bearing 5 setae (1 broadened).

Legs 1–3 (Fig. 347 D-F) with 3-segmented rami. Leg 4 (Fig. 347G) with 3-segmented exopod and 2-segmented endopod. Inner coxal seta present only in leg 1. Outer seta on basis of legs 1–4 large and pinnate. Inner distal spine on basis of leg 1 smooth, 15 μm long. Outer seta absent on second exopodal segment of leg 3 and first and second exopodal segments of leg 4. Inner seta absent on first exopodal segment of legs 3 and 4, on second endopodal segment of leg 3, and on first endopodal segment of leg 4. Setae on rami of legs 2–4 short and naked. Armature formula for legs 1–4 as follows:

Leg 5 (Fig. 347H) positioned on posteroventral margin of somite; consisting of outer seta and inner truncate exopodal lobe tipped with 1 seta.

Male. Unknown.

Remarks. Notoixys planiceps gen. et sp. nov. can be easily distinguished from its congeners by its characteristic, strongly depressed cephalosome, by the possession of a single setaon the basis of the maxilluleand by the presence of 2 setae on the third endopodal segment of the maxilla. The setation of the posterior swimming legs is also reduced relative to its congeners.

It is remarkable that Notoixys planiceps gen. et sp. nov. and N. ovata gen. et sp. nov. have a very similar form of the coxalgnathobase on the mandible despite the different body form of the adult females and the marked differences in the armature of the mouthparts and swimming legs.