Bathycrinus kirilli n. sp.

Figs 2 A–E, 3A–G

Synonymy: Bathycrinus sp. F — Belyaev 1966: 119. Etymology: The species is named for Kirill Mironov, the son of the author.

Holotype. IO RAS, No. XV-63-6, RV Vityaz, St. 3494, 0 8.10.1955, aboral cup with proximal arms and stalk, RRD = 2.69 mm.

Type locality. RV Vityaz, cruise 22, St. 3494, TS, 0 8.10.1955, Izu-Bonin Trench, also known as Izu-Ogasawara Trench, 29°09.3′ N, 142°53.1′ E, 9715–9735 m .

Material examined. RV Vityaz, St. 3494, holotype; six aboral cups with proximal arms, proxistele and upper mesistele (paratypes 1–6), RR-ring with proximal arms (paratype 7), BB-ring with proxistele and upper mesistele (paratype 8); stalk fragments. RRD from 1.85 to 2.94 mm. IO RAS Cat. no. Ech00746–Ech00750.

Description of the holotype (Figs 2 A–C). CH 1.77 mm, RRD 2.69 mm, BBd 1.22 mm. Aboral cup low, CH / RRD 0.66. RR-ring conical, forming an angle with BB-ring; profile of aboral cup funnel-shaped (Fig. 2A). Both BB-ring and RR-ring strongly pentalobate (quinquelobate) in cross section. RRH/RRD 0.42; BBH/RRH 0.56. BB hemi-fused (hemi-ankylosed); BBH/BBD 0.42. Surface of BB and RR grooved (Fig. 3A). Tegmen rising to top of Br2. Height of primibrachitaxis (IBr1+2ax) 2.92 mm. Ratio IBr1H/RRH 1.53, IBr1H/Br2H 1.28, IBr1H/Br1W 1.21, IBr2H/Br2W 0.86. All free arms incomplete; number of secundibrachials 1 (4 cases), 5 (2 cases), 7 (3 cases) and 10 (1 case). Both primibrachials and secundibrachials with high, translucent longitudinal crest (Fig. 2A). Sides of Brs 1–6 flattened into lateral flanges continuing on to middle arm as low lateral crest. Profile of arm (view from side) serrate (Fig. 2C). Proximal free arm pattern 1+2 4+5 7 (3 cases) and 1+2 4+5 7+8 10 (1 case). Position of P1 is distal, because pinnule absent on Brs 6–10. Cover plates on proximal Brs thick (not flattened) with rough surface, appearing as two series of rough protuberances. Remaining proximal stalk attached to aboral cup of 34 columnals, 15 mm in length (Figs 2 A–B). Stalk diameter decreasing from proximalmost (1.21 mm) to 34 th columnal (0.75 mm); maximum H/D 2.2 in columnal 34. Transition from proxistele to mesistele is very short: 24 columnals are discoidal with H/D about 0.1, and columnal 27 is higher than wide. Articulate facets heptalobate in proxistele and round in upper mesistele.

Other specimens. CH / RRD varies from 0.54 to 0.66; RRH/RRD 0.33–0.49; BBH/RRH 0.45–0.61; BBH/ BBD 0.34–0.42. Sutures between BB absent or indistinct. Tegmen rising to top of Br 2 — base of Br 3; Sides of tegmen without plates. IBr1H / RRH 1.38 – 1.89; IBr1H/Br2H 1.05–1.31; IBr1H/Br1W 0.85–1.36; IBr2H/Br2W 0.76–1.00. IBrs 1–2 without knobby processes (Figs 3 B–C). Primibrachials and secundibrachials always with high longitudinal crest (Figs 3 D–F). The best preserved free arms in paratype 1, RRD 2.71 mm; proximal free arm pattern 1+2 4 (2 cases) and 1+2 4+5 7 (8 cases). Pinnule absent on Brs 6–7. Numerous fragments of stalks up to 50 mm in length. Length of complete stalk likely exceeded 100 mm. Maximum diameter of stalk near radix up to 1.9 mm. Number of short proximal columnals varies from 15 to 39. Maximum H /D 3.2 in upper mesistele. Columnals of proximal mesistele cylindrical or barrel-shaped. More distal columnals with slight middle constriction (Fig. 2D). Proxistele with longitudinal ridges, from 6 to 9 in number. Accordingly, facet shape of the proximal columnals varies from six-lobate to nine-lobate. Five longitudinal ridges of interradial position are more developed than the ridges of radial position (Figs 2A, 3H). In addition, surface of columnal from lower proxistele and upper mesistele with numerous small longitudinal ridges (Figs 3H, L). Facets ovoid with longitudinal fulcral ridge in proximal mesistele (Fig. 3I), circular or slightly elliptical (D/d = 1.01) with nearly transverse fulcral ridge in distal mesistele (Fig. 3J) and elliptical with longitudinal fulcral ridge in dististele (Fig. 3K). D/d in dististele up to 1.2. Relief of fulcral ridge irregular. Radix attached to pebble, tubes of polychaets or stalk of another specimen of B. kirilli . When stalk attached to large pebble, radicles are short and branched in plane of pebble surface. When stalk attached to small pebble or polychaete tube, radicles long and branched within sediment (Fig. 2E) .

Remarks. B. kirilli n. sp. is very similar to B. volubilis, but differs from it by the presence of the high longitudinal keel on Brs, grooved surface of RR and serrate profile of arms. In contrast to B. kirilli n. sp., B. volubilis has smooth surface of RR and Brs (Fig. 1 A–B), and smooth profile of arms. Knobby processes are absent in both species (Table 2).

Distribution. Izu-Bonin Trench, 9715–9735 m. So far B. kirilli n. sp. is the deepest dwelling crinoid species that have even been discovered. The stalked 10-armed crinoids seen in videos and on photos in the Izu-Bonin Trench, at 8994–9102 m depth (Oji et al. 2009) are likely B. kirilli .