Bathycrinus longipinnus Mironov 2019, n. sp.

Bathycrinus longipinnus n. sp.

Figs 4 A–J, 5A–H, 6A–K

Synonymy: Bathycrinus sp. A — Belyaev 1966: 119; Bathycrinus sp. E — Belyaev 1966: 119.

Etymology: The species name is derived from the Latin longus (long) and pinna (feather), in reference to long pinnule.

Holotype. IO RAS, No. XV-63-7, RV Vityaz, St. 3340, RR-ring with arms, RRD 4.16 mm.

Type locality. RV Vityaz, cruise 20, St. 3340, TS, 0 1.06.1955, Aleutian Trench, 53°53.2′ N, 166°55.6′ E, 6410– 6757 m.

Material examined. RV Vityaz, cruise 20, St. 3340, TS, 0 1.06.1955, Aleutian Trench, 53°53.2′ N, 166°55.6′ E, 6410–6757 m, holotype, 6 RR-rings with arms (paratypes), IO RAS Cat. no. Ech00751–Ech00755, RRD from 1.65 to 4.67 mm, fragments of arms and stalks; RV Vityaz, cruise 23, St. 3593, TG, 22.05 1957, Japan Trench, 40°54.9′ N, 144°53.0′ E, 6380 m, aboral cup with arms and RR-ring with proximal arms and stalk (paratypes), IO RAS Cat. no. Ech00756; RRD 1.87 and 2,48 mm. RV Akademik Mstislav Keldysh, cruise 22, St. 2294, TS, 25.07.1990, Aleutian Trench, 54°52.8′ N, 165°42.7’E – 54°52.2′ N, 165°36.4′ E, 6856–6865 m, RR-ring with arms, RRD 2.40 mm), IO RAS Cat. no. Ech00757; RV Sonne, KuramBio II expedition, St. 43, AGT, 30.08.2016, Kuril-Kamchatka Trench, 45°38.51′ N, 152°56.77′ E – 45°39.36′ N, 152°58.38′ E, 7241–7245 m, 4 RR-ring with arms, RRD 2.93–3.69 mm, SMF. Total: 13 RR-rings with arms, 1 aboral cup with arms and stalk, fragments of arms and stalks; RRD from 1.65 to 4.67 mm.

Description of the holotype (Figs 4 A–E; 5A). RR-ring slightly funnel-shaped (Figs 4A, 5A); RRH/RRD 0.48. Surface of RR smooth. Tegmen rising to top of Br2–middle of Br3. Sides of tegmen without plates. Height of primibrachitaxis (IBr1+2ax) 4.35 mm. Ratio IBr1H/RRH 1.21, IBr1H/Br2H 1.26, IBr1H/Br1W 1.07, Br2H/Br2W 0.69. IBrs 1–2 with smooth surface and low longitudinal median convexity; their sides flattened into wide lateral flanges continuing on to secundibrachs up to Br5. Five calcite plaques (outgrowths) on interradial sutures between RR and IBr1; each plaque lies on upper corners of two adjacent radial plates and lower corners of two adjacent IBrs 1 (Fig. 4A). The plaque is not one-piece plate; it dissociates into numerous small pieces in 5% sodium hypochlorite. All arms incomplete. Best preserved arm about 48 mm in length and comprises 54 Brs, 11 Ps on side of arm. Arm fragments suggest that complete arm consisted of about 80 Brs. Profile of arm (view from side) smooth. Proximal IIBrs low, IIBr1H/Br1W about 0.83. Longitudinal median convexity low, rounded, with smooth surface. Five rounded calcite plaques along the proximal margin of IIBr1-ring. Each tubercle lies on lower corners of IIBrs 1 (Fig. 4A).

All arms incomplete; secundibrachials 5 to 54. Proximal free arm pattern 1+2 4+5 7+8 9+10 11 (9 cases) and 1+2 4+? (1 cases). Successive brachial pairs in middle and distal arm. P1 on Br10 (6 cases), Br8 (3 cases) and lacking (1 case). P1 of 10 pinnulars, 5.37 mm in length; P2 of 11 pinnulars, 6.1 mm in length; PА2 of 12 pinnulars, 6.8 mm in length; more distal Ps are incomplete. Longest pinnule preserved on the arm fragment, likely belonging to holotype, is 10 th pinnule from top of arm (or about P8); it is 10.8 mm in length, comprises 15 pinnulars, and about 3–5 pinnulars were lost (Figs 4 C–-E). 6 th pinnule from top of arm (or about P12) is complete, 8.7 mm in length, 14 pinnulars. Pinnulars are V-shaped in cross section, their edges from slightly to strongly sinuous (Fig. 5G). Pinnulars 2 and 3 hemifused. Covering plates large and thin (Fig. 6D); ones from proximal Brs have local thickenings (Fig. 6C). Genital expansions without additional plates. Ambulacral tube-foot plates long and x-shaped (Fig. 6B).

Other specimens. RRD from 1.65 to 4.67 mm. RR-ring slightly funnel-shaped, sometimes conical. In specimens with RRD> 2 mm RRH/RRD varies from 0.35 to 0.57, IBr1H/ RRH 1.00–1.53, IBr1H/ Br 2H 1.04–1.30, IBr1H/ Br 1W 0.801.11, Br 2H/ Br 2W 0.57–0.76. Two specimens with RRD <2 mm differ in having relatively higher RR-rings (RRH / RRD 0.57 and 0.65), more elongated IBr2 (Br2H/Br2W 0.73 and 0.95). Each radial with two long processes on inner-upper edge (Fig. 5H). There are 8 fragments of stalks with BB ring and one small specimen with RR and BB-rings. BB fused or hemi-fused. BBH/BBD varies from 0.45 to 0.54 in large specimens (BBD> 1 mm), and from 0.60 to 0.70 in small specimens (BBD <1 mm). Tegmen rising to top of Br2 or Br3; anal sac rising to middle of Br3 or Br4. Ambulacral grooves not reaching oral opening, extend out to circumoral elevation of soft tissue. Large tube-feet and plates (Fig. 6A) located along aboral margin of circumoral elevation; two thick rounded ambulacral tubes in each interradius, and two flat rounded plates located aborally of tube feet. The paratype 3 was dissociated to examine knobby processes. IBrs2-circlet with 6 rudimentary knobby processes: two IBrs 2 with two processes (Fig. 5B), two IBrs 2 with one process, and one IBr2 without processes. Spines (on surface of knobby processes) not pointed and at tops of subparallel stereom columns (Fig. 5D). IBrs 1-circlet with 4 rudimentary knobby processes on upper part of ossicle (Fig. 5C). Proximal patterns and position of P1 examined in 60 arms (excluding 9 arms of the holotype). Most frequent proximal arm patterns 1+2 4+5 7+8 9+10 11 (28 cases or 47%) and 1+2 4+5 6+7 8+9 10 (21 cases or 35%). P1 on Brs 8–13; most frequent positions on Br 8 (24 cases or 40%) and Br 9 (19 cases or 32%; see also Table 1). Muscular and nonmuscular synarthrial facets of free arms with classical features (Figs 5E, F). The longest fragment of stalk (St. 3340) composes of 52 columnals, 116 mm in length (Figs 4G, H). Length of complete stalk more than 200 mm. Number of proximal short columnals varies from 17 to 32 in larger specimens (BBD> 1 mm), and from 10 to 15 in small specimens (BBD <1 mm). The thicker the stalk, the lower the maximum H/ D. So thin stalk (Fig. 4F) characterized by maximum H/D 3.51 (D of most elongate columnal 0.81 mm), and thicker stalk (Fig. 4G, H) displays maximum H/D 2.35 (D of most elongate columnal 1.73 mm). Facets slightly lobate in proxistele (Fig. 6E), elliptical (D/d up to 1.16) with longitudinal position of fulcral ridge in mesistele (Fig. 6I), slightly elliptical (D/d up to 1.03) with slightly transverse position of fulcral ridge in distal mesistele (Fig. 6J), and elliptical (D/d up to 1.6) with longitudinal position of fulcral ridge in dististele (Fig. 6K). Fulcral ridge of distal mesistele with sinuous borders (Fig. 6J). Facets of distalmost columnals with irregular relief on fulcral ridge (Fig. 6K). Differentiation of fulcral ridge and areola (ligament area) begins at distal proxistele, occurs along mesistele with maximally developed in dististele. Ligament fosse in distal proxistele and proximal mesistele with irregular bottom (Figs 6G, H) that results from deepening and grouping of several proximal ligament depressions.

Remarks. The knobby processes are totally absent in B. volubilis and B. kirilli, and rudimentary in B. longipinnus. Other species of the family Bathycrinidae have well developed knobby processes (Table 2). Presence of calcite plaques (outgrowths) on the suture between the RR-ring and IBrs as well as the plaques along the lower margin of IIBr1-ring in B. longipinnus are unique features within the family Bathycrinidae.

Irregular relief on the fulclar ridge of the distalmost columnals is known in B. longipinnus, B. volubilis, B. kirilli and B. equatorialis (Roux & Messing 2017). Other species of Bathycrinus have regular relief. B. volubilis and B. kirilli differ from B. longipinnus in having most frequent proximal pattern 1+2 4+5 7+8 10+ 11 13, most frequent positions of P1 on Brs 11–12, thick (not flattened) cover plates on the Brs 4–12, right margins on pinnulars, distinctly multilobate areola in proximal columnal facets, and less elliptical facets on distal columnals. They also differ in lacking the circumoral ring of plates, knobby processes and the calcite plates on suture between RR and IBrs. B. equatorialis differs from B. longipinnus in having relatively higher aboral cup, barrel-shaped BB-ring, developed knobby processes with the spines at top of convergent stereom meshes, most frequent proximal arm pattern 1+2 3+4 5+6 7, fulcral ridge with parallel borders in distal mesistele, axis of fulcral ridge always corresponding to greatest facet diameter, small encrusting disk attached to hard substrate. Roux & Messing (2017) noted that there are no tube-foot plates observed in B. equatorialis. However x-shaped tube-foot plates are visible on the photo of distal end of pinnule (Roux & Messing 2017, fig. 3I).

Distribution. Aleutian, Japan and Kuril-Kamchatka Trenches, 6380–7245 m.