Thalassocrinus clausus Mironov & Sorokina, 1998 Figure 17, Tables 16–17. Synonymy: Thalassocrinus clausus Mironov & Sorokina, 1998: 36 –39, fig. 11, pls. 5 (5–6), 6 (1–2), 10 (6); Roux 2002: 173– 174, 184; Roux et al. 2002: 822; Roux & Lambert 2011: 48, 51. Material examined. USNM 1005456 from South of Gambier Islands, SE Pacific basin, 42°01’ à 41°57’S –130°02’ à 13°01’W, depth 4,831–4,851 m: one specimen with aboral cup and proximal arms, and three stalk fragments. Description. Theca very similar to holotype, except cup higher than wide, orals without terminal finger-like projection, and anal cone slightly higher than oral cone. Measurements are given in Tables 16 and 17. Proximal arm pattern 1+2 3 4+5 with first pinnule on Br5, one arm missing, three arms broken after Br5, the fifth separated (length 9.2 mm) with successive brachial pairs beyond Br6. A single row of H-shaped plates in proximal pinnules only, probably indicative of an immature specimen. Length of preserved stalk 22.6 mm. Proxistele rough and hexalobate in cross section proximally, distally becoming smoother and hexagonal in cross section beyond columnal 25. Columnal diameter decreasing from 1.3 mm below basal ring to 1.05 mm at a distance of 5.4 mm and up to 0.95 mm at 14.5 mm, and then increasing up to 1.0 mm at distal end of preserved stalk. Columnal height <0.3 mm proximally, increasing to 0.5 mm at a distance of 6.0 mm from aboral cup, 1.0 mm at 14.5 mm, and 1.25 mm distally. Maximum ratio height to diameter 1.25 in distalmost columnal. Columnal symplexies of 6 crenular units of 1 crenula (Fig.17). Axial canal poorly differentiated in proximal mesistele (Fig. 17 A) and highly differentiated distally with annular facet of small crenulae and large meshed galleried stereom bordering a wide lumen (Fig. 17 B–C). Wb Wrt Hc Hrr Hri Htga Htgo Dc Drb Dp Dm 2.7 4.2 8.1 3.7 4.5 3.8 1.7 6.2 5.2 1.3 0.95 Hc/Dc Hri/Hc Hrr/Hc Htgo/Hc Hri/Wrt Wrt/ Wb Dc/Dp Dc/Drb Drb/Dp Dp/Dm 1.30 0.56 0.46 0.21 1.07 1.57 4.89 1.19 4.10 1.34 Remarks. The Eltanin specimen is substantially smaller than the holotype (Dp 2.4 mm) and likely immature. Its mesistele symplexies differ in having short crenulae and large axial canal and claustrum (Fig. 17 B–C) corresponding to juvenile characters. New columnals appearing below basal ring (i.e., later during ontogeny) display symplexial pattern (Fig. 17 A) similar to those of proxistele and mesistele columnals in the holotype (Mironov & Sorokina 1998, pl. 5-5 and 6-1). The species is now known from two specimens only. The holotype was found in NW Pacific at a shallower depth (22°20.0’ N – 161°31.6’ E at a depth between 1,292 and 1,334 m). T. alvinae and T. clausus share both the same anal cone feature (Roux & Lambert 2011) and a large axial canal in the mesistele of juvenile specimens (see Roux 2002, fig. 2A–E). They mainly differ in proximal arm morphology. T. alvinae displays a greater Wr/Wb ratio than T. clausus. Mironov & Sorokina (1998) attributed to a new species, i. e., T. depauperatus, a poorly preserved specimen with broad proximal arms, anal cone substantially higher than oral cone and orals without finger-like terminal projection. It differs from Eltanin specimen in having larger size (Dp 1.65 mm), three interbasal sutures, aboral cup more elongated (Hc/Dc 1.38), stalk symplexies with 7 crenular units and proximal brachials without lateral wings. Such differences could be accounted for by intraspecific variation, and/or correspond to ontogenetic changes. Moreover, T. depauperatus was collected at a slightly greater depth (5,020 m – 5,130 m) off Sunda Strait (11°49.6’ S – 102°54.6’ E). Additional material is required to know if the two species are distinct or synonym. Occurrence. North western Pacific and South Central Pacific, depth 1,334 m to 4,831 m (possibly 1,292 m to 4,851 m). This depth range could be extending to 5,020 m (possibly 5,130 m) and the geographical distribution to NW Indian Ocean if T. clausus and T. depauperatus become synonym.

Published as part of Roux, Michel, 2015, Crinoids of the stalked family Hyocrinidae (Echinodermata) collected by the USNS Eltanin in the Southern Ocean, pp. 381-409 in Zootaxa 3990 (3) on pages 405-407, DOI: 10.11646/zootaxa.3990.3.4, http://zenodo.org/record/243049