Culeolus tenuis Vinogradova, 1970

Culeolus tenuis Vinogradova, 1970 View in CoL

( Figs 1A View Fig , 2 View Fig ; Tables 1–3)

Culeolus tenuis Vinogradova, 1970: 491–493 View in CoL , text-figs 1, 2 (part); Sanamyan and Sanamyan 2006: 329–331 View Cited Treatment , text-fig. 12A, B ( syntype description, 22 mm body length, designated here as lectotype); Nishikawa and Kuramochi 2023: 75–80, text-fig. 2 (excluding incorrect records given in text-fig. 1, and table 1).

Excluded records. Culeolus tenuis Vinogradova, 1970: 491–493 View in CoL , text-figs 1, 2 (part; specimens each having a Culeolus View in CoL -ridge composed of a series of triangular papillae issuing from a low flat membrane); Sanamyan and Sanamyan 1998: 214.

Material Examined (identified by molecular analysis as cluster A). NSMT-Pc 6894, 6905, and 6906 (total 3 specimens), continental slope of Erimo Seamount , between Chishima-Kamchatka and Japan Trenches, from 41°20.79′N, 144°56.46′E to 41°20.52′N, 144°57.30′E, 6116–6131 m depth, R / V “Hakuho-maru” (KH-22-8-A3), 3 October 2022, Y GoogleMaps . Ise coll GoogleMaps .; CMNH-ZV 112 to 116 (total 5 specimens), seaward slope of S end of Chishima-Kamchatka Trench , from 41°28.41′N, 146°06.80′E to 41°28.52′N, 146°07.63′E, 6176– 6200 m depth, R / V “Hakuho-maru” (KH-23-5-C5), 21 September 2023, K GoogleMaps . Yanagi coll GoogleMaps .

Designation of lectotype. As shown in Fig. 1A–C View Fig , the Culeolus -ridge structure (a ring of conical projections vs. a continuous crest) was sufficient for distinguishing between the two molecularly-separated clusters (species). The original description of C. tenuis , based on 40 syntypes from several localities in the NW Pacific, from depths of 5027 to 6282 m, indicated the ridge as a ventral series of triangular papillae issuing from a common low flat membrane ( Vinogradova 1970: 493, fig. 1). On the other hand, a syntype was redescribed, by Sanamyan and Sanamyan (2006: 330, text-fig. 12A) as having the crest “consisting of a low thick lamella, not reaching the dorsum”, without any papillae issuing from the lamella. That syntype, with a 22 mm and 126 mm long body and stalk, respectively, is one of 10 syntypes from A/ V “Vityaz” St. 5621 ( 5030–5031 m depth) in the Chishima-Kamchatka Trench. It follows that the original 40 syntypes of C. tenuis included at least the Sanamyan and Sanamyan’s (loc. cit.) crest-furnished specimen, plus specimens “intermediate” between crested and cone-furnished forms (see below). For taxonomic clarity, the syntype redescribed by Sanamyan and Sanamyan (2006) and deposited in the Institute of Oceanology, Moscow, is here designated as the lectotype of C. tenuis ; a nomenclatural act consistent with the subsequent descriptions of C. tenuis by Sanamyan and Sanamyan (2006) and Nishikawa and Kuramochi (2023). Thus, C. tenuis is defined here as characterized by a Culeolus -ridge (a ventral continuous crest not reaching the dorsum), the proximal end of the stalk running posteriad along the anteriormost part of the body mid-ventral line (as depicted by Sanamyan and Sanamyan’s text-fig. 12A), six branchial folds and three gonads on each side, the latter four- or five-lobed, with one on the left being located inside the first intestinal loop.

Remarks. The lectotype was similar to the Japan-Trench specimen (NSMT-Pc 5353) described by Nishikawa and Kuramochi (2023), and the cluster A specimens identified here from the Erimo Seamount (NSMT-Pc 6894, 6905, and 6906) and Chishima-Kamchatka Trench (CMNH-ZV 112 to 116), having a white to light brown tunic, body prop- er length 23.0–72.0 mm, a stalk up to 555 mm long and to 2.5 mm thick, and lacking branches at the midpoint. In spite of the frequent occurrence of four gonads on the right side in the specimens examined here (see Table 3), all of the latter were thus unequivocally identified as C. tenuis . Dense horny polygonal spicules were found in the mantle and viscera, but no parasitic nemerteans (as detected in C. uschakovi below).

Sanamyan and Sanamyan (1998: 214) described a specimen of C. tenuis from the Aleutian Trench, 5300–6074 m deep, as having “two gonads on the right and three on the left side of the body” with “two of the left gonads above gut loop and one in the gut loop”, and “6 [branchial] folds on each side”, without any reference to the morphology of the Culeolus -ridge. The identity of this specimen therefore remains uncertain, and the record is excluded here.

Distribution. Chishima-Kamchatka Trench, 5030– 5031 m depth ( Vinogradova 1970; Sanamyan and Sanamyan 2006) and 6176–6200 m (present study); Erimo Seamount, 6116–6131 m (present study); Japan Trench, 5784 m ( Nishikawa and Kuramochi 2023).