Haploniscus nyx Knauber & Riehl 2025, sp. nov.

Haploniscus nyx Knauber & Riehl sp. nov. Figs 22, 23, 24, 25, 26 Holotype. SKB Hap 34, adult male (stage VI), 3.4 mm, MIMB 50320. Paratypes. SKB Hap 21, adult female (stage IV), 3.0 mm, MIMB 50321. Type locality. St. LV 71–10 – 07, RV “ Akademik M. A. Lavrentyev ”, SokhoBio expedition, EBS, 4469 m, 46°07.8'N, 152°10.3'E, Northwest Pacific, abyssal branch of the Kuril-Kamchatka Trench into the Bussol Strait. Further records. St. LV 71–10 – 07: SKB Hap 35 (adult male), SMF 56552. Distribution. Only known from type locality. Northwest Pacific, abyssal region adjacent to the Kuril-Kamchatka Trench to the northwest, depth 4769 m. Visualized in Fig. 27. Etymology. The epithet “ nyx,” a noun in apposition, is derived from “ Nyx ” (Ancient Greek Νύξ), the goddess of the night in Greek mythology. Synonymy. Haploniscus SO - WTA (see Knauber et al. 2022). Diagnosis. Haploniscus nyx sp. nov. differs from other species of the belyaevi - complex in the following character: pleotelson posterior margin concave in males; Plp I medial lobes convexly rounded, tapering to an obtuse point, distally without a hook-shaped protrusion. Molecular diagnosis. differing in the 16 S gene from other species of the belyaevi - complex in the nucleotides G (position 25 of the alignment) and C (312) as well as the nucleotides C (124), G (148), C (193), T (206), C (304), G (334), C (346), G (481), T (496), C (508), C (520), G (553), G (556), and C (628) of the COI gene. Description. Male. Body (Fig. 22 B) length 2.2 width; subrectangular; anterior body length (Ceph – Prn 4) 0.99 posterior body length (Prn 5 – Plt); lateral margin continuous. Cephalothorax (Fig. 22 B, D) length 0.30 width, 0.07 body length, width 0.55 body width; frontal margin width 0.46 Ceph width; rostrum curved upwards. Pereonite 1 (Fig. 22 B) posterior tergite margin through Prn 5 anterior tergite margin delicately serrated, setose; Prn 2–5 anterolateral angles slightly projecting; Prn 1–4 posterolateral angles slightly projecting; Prn 4 lateral margin length 1 Prn 5 lateral margin length. Pleotelson (Figs 22 B, 26 L) length 0.72 width, 0.23 body length, trapezoidal, posterior margin concave; tergite surface smooth; with posterolateral tergal ridge between uropod insertion and posterolateral process; posterolateral processes minute, 0.13 Plt length, straight, oriented posteriorly. Antenna I (Fig. 23 B) length 0.16 body length; flagellum with 5 articles. Antenna II (Fig. 23 A) length 0.61 body length; article 3 dorsal projection triangular, projection length 0.36 article 3 length; article 5 projection length 0.23 article 5 length; flagellum with 15 articles. Mandible (Fig. 24 B, C) incisor with 5 cusps, left Md lacinia mobilis with 4 cusps. Maxillipeds (Fig. 24 A) with 3 coupling hooks each. Pereopod I (Fig. 23 C) length 0.37 body length. PII (Fig. 23 D) length 0.45 body length. PIII (Fig. 23 E) length 0.52 body length. PV length 0.65 body length. PVI length 0.70 body length; P lengths gradually increasing from PI to PVI, PVII shorter than PVI. Pleopod I (Figs 25 A, 26 L) medial lobes convexly rounded, tapering to an obtuse point; adjoining at the apex. Pleopod II (Fig. 25 B, C) protopod semi-circular, with distal lobe extending beyond protopod distal margin; endopod stylet 2.1 protopod length. Female. Differs from male in the following characters: Body (Fig. 22 A) length 2.1 width; oval; anterior body length (Ceph – Prn 4) 0.98 posterior body length (Prn 5 – Plt). Cephalothorax (Fig. 22 A, C) length 0.46 width, 0.12 body length, width 0.55 body width; frontal margin width 0.46 Ceph width. Pereonite 4 (Fig. 22 A) lateral margin length 1.05 Prn 5 lateral margin length. Pleotelson (Figs 22 A, 26 K) length 0.83 width, 0.27 body length, posterior margin rounded, convex; posterolateral processes short, 0.19 Plt length. Antenna I (Fig. 22 A) length 0.16 body length; flagellum with 4 articles. Antenna II (Fig. 22 A) length 0.50 body length; flagellum with 15 articles. Operculum (Fig. 26 K) length 0.98 width, 0.66 Plt length; distal margin with numerous, evenly distributed long setae; lateral margins with fewer, evenly distributed short setae.