Haploniscus hades Knauber & Riehl 2025, sp. nov.

Haploniscus hades Knauber & Riehl sp. nov. Figs 1, 14, 15, 16, 17, 26 Holotype. KBII Hap 137, adult male (stage VI), 3.7 mm, SMF 56419. Paratypes. KBII Hap 104, adult female (stage IV; genome), SMF 56388; KBII Hap 213, adult female (stage IV), 2.4 mm, SMF 56495. Type locality. St. SO 250–042, RV “ Sonne ”, KuramBio II expedition, EBS, 7123 m, 45°39.62'N, 152°56.39'E, Northwest Pacific, hadal Kuril-Kamchatka Trench near Bussol Strait. Further records. St. SO 250–020: KBII Hap 116 (adult female) SMF 56399; St. SO 250–028: KBII Hap 208 (adult male) SMF 56490; St. SO 250–030: KBII Hap 209 (manca) SMF 56491, KBII Hap 210 (adult female) SMF 56492; St. SO 250–040: KBII Hap 220 (ovigerous female) SMF 56502, KBII Hap 221 (ovigerous female) SMF 56503, KBII Hap 231 (manca) SMF 56513, KBII Hap 232 (manca) SMF 56514; St. SO 250–042: KBII Hap 105 (manca) SMF 56389, KBII Hap 106 (manca) SMF 56390, KBII Hap 165 (adult male) SMF 56447, KBII Hap 198 (manca) SMF 56480, KBII Hap 230 (manca) SMF 56512; St. SO 250–086: KBII Hap 226 (adult male) SMF 56508; St. SO 250–097: KBII Hap 201 (manca) SMF 56483, KBII Hap 204 (adult male) SMF 56486, KBII Hap 205 (adult female) SMF 56487, KBII Hap 206 (ovigerous female) SMF 56488, KBII Hap 214 (manca) SMF 56496, KBII Hap 215 (adult female) SMF 56497, KBII Hap 216 (adult male) SMF 56498, KBII Hap 217 (adult female) SMF 56499, KBII Hap 227 (manca) SMF 56509, KBII Hap 233 (manca) SMF 56515, KBII Hap 234 (manca) SMF 56516; St. SO 250–098: KBII Hap 203 (manca) SMF 56485. Distribution. Northwest Pacific, Kuril-Kamchatka Trench, depth 5493–8191 m. Visualized in Fig. 27. Etymology. The specific epithet “ hades ” is a noun in apposition derived from “ Hades ” (Ancient Greek: Ἅιδης), the god and ruler of the underworld in Greek mythology. This name refers to this species distributional range within the hadal depths of the Kuril-Kamchatka Trench and the novel feature of the pleotelson posterior margin tergal plates projecting above the uropods in males. These render the uropods “ invisible ” from dorsal view, reminiscent of Hades’ cap of invisibility. Synonymy. Haploniscus KKT hadal (see Knauber et al. 2022). Diagnosis. Haploniscus hades sp. nov. differs from other species of the belyaevi - complex in the following characters: rostrum curved upwards, basally with dorsal bulge in males; Plt with posterolateral tergal ridge on posterolateral process and posterior margin tergal plates projecting above uropods (solely present in sister species H. kerberos sp. nov. otherwise); AII article 3 dorsal projection hook-shaped; Plp I distally with hook-shaped projection. Molecular diagnosis. Differing in the 16 S gene from other species of the belyaevi - complex in the nucleotide A (position 219 of the alignment) as well as the nucleotides G (103), A (211), T (217), C (232), G (256), T (328), A (349), C (364), C (370), C (442), A (469), C (499), and C (571) of the COI gene. Description. Male. Body (Figs 1 A, 14 B) length 2.3 width; oval; anterior body length (Ceph – Prn 4) 0.84 posterior body length (Prn 5 – Plt); lateral margin continuous. Cephalothorax (Figs 1 A, 14 B, D) length 0.28 width, 0.07 body length, width 0.56 body width; frontal margin width 0.53 Ceph width; rostrum curved upwards, basally with dorsal bulge. Pereonite 1 (Figs 1 A, 14 B) posterior tergite margin through Prn 5 anterior tergite margin delicately serrated, setose; Prn 2–4 anterolateral angles slightly projecting; Prn 2–4 posterolateral angles slightly projecting; Prn 4 lateral margin length 0.89 Prn 5 lateral margin length; Prn 5 anterolateral angle not projecting, rectangular. Pleotelson (Figs 1 A, 14 B, 26 F) length 0.75 width, 0.27 body length, trapezoidal, posterior margin concave; tergite surface tuberculate, less distinct than in remaining body; with posterolateral tergal ridge on posterolateral process and posterior margin tergal plates projecting above uropods; posterolateral processes minute, 0.04 Plt length, straight, oriented posteriorly. Antenna I (Fig. 15 B) length 0.17 body length; flagellum with 6 articles. Antenna II (Fig. 15 A) length 0.67 body length; article 3 dorsal projection hook-shaped, projection length 0.39 article 3 length; article 5 projection length 0.26 article 5 length; flagellum with 16 articles. Mandible (Fig. 16 B, C) incisor with 6 cusps, left Md lacinia mobilis with 5 cusps. Maxillipeds (Fig. 16 A) with 3 coupling hooks each. Pereopod I (Fig. 15 C) length 0.41 body length. PIII length 0.53 body length. PIV length 0.54 body length. PV length 0.67 body length. PVI (Fig. 15 E) length 0.70 body length. PVII length 0.69 body length; P lengths gradually increasing from PI to PVI, PVII shorter than PVI. Pleopod I (Figs 17 A, 26 F) medial lobes convexly rounded, tapering to an obtuse point; distally with hook-shaped projection; separated at the apex by a narrow gap. Pleopod II (Fig. 17 B, C) protopod elongated, suboval; endopod stylet 2.0 protopod length. Female. Differs from male in the following characters: Body (Fig. 14 A) length 2.4 width; anterior body length (Ceph – Prn 4) 1.0 posterior body length (Prn 5 – Plt). Cephalothorax (Fig. 14 A, C) length 0.52 width, 0.14 body length, width 0.63 body width; frontal margin width 0.55 Ceph width. Pereonite 4 (Fig. 14 A) lateral margin length 0.93 Prn 5 lateral margin length. Pleotelson (Figs 14 A, 26 E) length 0.75 width, 0.25 body length, posterior margin straight; posterolateral processes 0.08 Plt length. Antenna I (Fig. 14 A) length 0.15 body length; flagellum with 4 articles. Antenna II (Fig. 14 A) length 0.70 body length; flagellum with 11 articles. Operculum (Fig. 26 E) length 1.1 width, 0.74 Plt length; distal margin with numerous, evenly distributed long setae; lateral margins with up to 3 short setae each.