Schizoproctus frigidus Kim & Boxshall 2021, sp. nov.

Schizoproctus frigidus sp. nov.

(Figs. 73, 74)

Type material. Holotype ♀ (MNHN-IU-2018-1980, dissected and mounted on a slide) from Cçrella antarctẚca Sluiter, 1905; Antarctic Ocean, Eltanin cruise 12, Stn 1078 (61°26´S, 41°55.4´W), depth 604 m, 12 April 1964.

Etymology. The specific name is derived from the Latin frẚg (cold), referring to its discovery in Antarctic waters.

Description of female. Body (Fig. 73A) robust, 2.88 mm long. Anterior part of body unsegmented and inflated; cephalosome defined from metasome by faint dorsal constriction. Posterior part of body (Fig. 73B) consisting of genital somite and 4-segmented abdomen. Genital somite much wider than long (205×487 μm) incompletely ar- ticulated from first abdominal somite. First and second abdominal somites 354×436 and 230×385 μm, respectively; third and fourth abdominal somites indistinctly articulated from each other, 128×297 and 210×270 μm, respectively. Caudal ramus (Fig. 73C) about 107×135 μm, armed with 4 claws and 2 setae; lengths of claws 58, 76, 50, and 25 μm from inner to outer.

Rostrum (Fig. 73D) broad, steeply tapering towards nipple-shaped, semicircular apex. Antennule (Fig. 73E) 252 μm long and indistinctly 5-segmented; first segment broad, forming right angle with distal 4 segments; 2 distal articulations incomplete; armature formula 11, 6, 3, 2, and 8+aesthetasc. Left antenna (Fig. 73F) consisting of unarmed coxa, basis and unsegmented endopod; endopod 163× 65 μm, armed with 8 short setae and with rudimentary suture line subdistally defining proximal 3-setae part and distal 5-setae part; longest distal seta 96 μm. Right an- tenna segmented and armed as left antenna; endopod (Fig. 73G) 167×65 μm, subdivided by subdistal sclerotization band.

Labrum (Fig. 73H) much broader than long, with large, soft posteromedian protuberance. Mandible (Fig. 73I) as usual for genus, with 3 teeth on coxal gnathobase and 9 setae on palp. Maxillule (Fig. 73J) with 8 setae on arthrite, distal seta rudimentary; palp unsegmented with vestigial seta representing epipodite, 2 setae on medial margin, 3 setae on outer margin, 1 tubercle distally, and 3 setae on endopod; endopod fused with basis. Maxilla (Fig. 74A) 2-segmented with 2 setae on first segment and 7 setae on second. Maxilliped (Fig. 74B) robust, 4-segmented, but second endopodal segment completely fused with terminal claw; syncoxa broadening distally; basis much wider than long, bearing 2 setae; first endopodal segment unarmed; second endopodal segment + terminal claw complex unarmed, with 1 minute papilla at proximal third.

Legs 1-4 biramous with 1-segmented rami; coxa unarmed; basis with small outer seta. Legs 1 (Fig. 74C) and 2 with same armature. Legs 3 (Fig. 74D) and 4 also with same armature. Outer spines on exopods gradually becoming longer from proximal to distal; longest distal spine shorter than width of exopodal segment. Endopods distinctly tapering. Two anterior (ventral) setae on endopod of legs 3 and 4 naked. Armature formula for legs 1-4 as follows:

[table omitted]

Leg 5 (Fig. 74E) circular, fleshy, lamellate, 100×92 μm, with 1 rudimentary seta distally. Leg 6 (Fig. 73K) repre- sented by 2 small spinules and 1 spiniform process on genital operculum.

Male. Unknown.

Remarks. The most distinctive diagnostic feature of S. frẚgẚdus sp. nov. is the presence of 7 spines on the exopods of legs 1-4, because in other species of the genus the maximum number of spines on the exopod of any leg is 6, although several species bear an additional rudimentary seta on the exopod. The armature sequence on the endopods of legs 1-4 (8-8-7-7) is also an outstanding diagnostic feature shared only with SK bẚsetatus sp. nov. (Table 3), which differs from S. frẚgẚdus sp. nov. in the presence of 2 setae on the lamellate exopod of leg 5. The fusion of the second endopodal segment and the terminal claw of the maxilliped of S. frẚgẚdus sp. nov. is an unusual feature within the genus, which is only known in S. ẚnflatus, as figured by Sars (1921) and S. fusiformis sp. nov.