Echinoderms of the Kuril-Kamchatka Trench

Journal Pre-proofs Echinoderms of the Kuril-Kamchatka Trench A.N. Mironov, A.B. Dilman, A.V. Gebruk, A.V. Kremenetskaia, K.V. Minin, I.S. Smirnov PII: DOI: Reference:

S0079-6611(19)30397-0 https://doi.org/10.1016/j.pocean.2019.102217 PROOCE 102217

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Progress in Oceanography

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30 April 2019 9 October 2019 23 October 2019

Please cite this article as: Mironov, A.N., Dilman, A.B., Gebruk, A.V., Kremenetskaia, A.V., Minin, K.V., Smirnov, I.S., Echinoderms of the Kuril-Kamchatka Trench, Progress in Oceanography (2019), doi: https:// doi.org/10.1016/j.pocean.2019.102217

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Echinoderms of the Kuril-Kamchatka Trench A.N. Mironova, A.B. Dilmana*, A.V. Gebruka, A.V. Kremenetskaiaa, K.V. Minina, I.S. Smirnovb aShirshov

Institute of Oceanology, Russian Academy of Sciences, Nakhimovsky Pr., 36, Moscow 117997, Russia bZoological Institute, Russian Academy of Sciences, Universitetskaya nab. 1, St. Petersburg 199034, Russia *Corresponding author A.B. Dilman. E-mail address: [email protected] Abstract Echinoderms collected in the Kuril-Kamchatka Trench (KKT) at depths greater than 6000 m by expeditions onboard RV Vityaz and Sonne (KuramBio II) are listed, with some taxonomic and biogeographic remarks. The echinoderm fauna of the KKT (> 6000 m) includes 44 species, of which 29 were determined to the species level and 15 to the generic level; 14 species and 6 genera (Echinosigra, Gephyrothuria, Mesothuria, Molpadia, Molpadiodemas and Sonnetrochus) were recorded for the KKT for the first time. The KKT echinoderm fauna mainly comprises species (86%) from the cosmopolitan genera. Of the 29 identified species, 11 (38%) are endemics of the hadal zone, including 6 (21%) endemics of the KKT. Species of holothurians make up more than half of all known echinoderm species in the KKT. The number of echinoderm species at depths 6000–7000 m is 29, which decreases sharply to 16 at depths 7001– 8000 m, stabilizes at 17 at depths 8001–9000 m and further declines to 8 from 9001–10000 m. The proportion of holothurians increases with depth from 48% to 87%. The number of species in the KKT echinoderm fauna is much greater than in any other trench. The second–highest echinoderm species richness (19 species) is known based on the South Sandwich Trench. Both trenches are located in areas of high primary productivity and are characterized by a high abundance of benthic invertebrates in trawl catches. Keywords: North-West Pacific, Kuril-Kamchatka Trench, hadal fauna, Echinodermata, endemic, species richness. 1. Introduction In the Pacific Ocean, most hadal trenches are located at the ocean periphery. In the NorthWest Pacific, the chain of four trenches (Izu-Bonin, Japan, Kuril-Kamchatka and Aleutian) stretches along the Japanese, Kuril and Aleutian Islands. The biogeographic classification of benthic habitats by UNESCO (2009) categorized these four trenches into the Aleutian-Japan biogeographical hadal (ultra-abyssal) province, based primarily on Belyaev (1989). The hadal endemics (unknown from depths < 6000 m) in the fauna of each of these trenches made up from 42 to 53%, and in the fauna of all four trenches 51% (Belyaev, 1989). The hadal fauna have been most fully studied in the KKT, which is also one of the deepest and largest trenches in terms of area. Deeper 6500 m, the area of the KKT is 91 692 km2 (Jamieson, 2015) with a maximum depth 9717 m (Mikhailov, 1970). Jamieson (2015) has reported an even greater depth of 10542 m. Initial investigations of the KKT hadal fauna were conducted during the 2nd cruise of RV Vityaz in 1949. The first trawl sample obtained at St. 162 from 8100 m comprised 150 benthic invertebrates belonging to 20 species from 10 different classes. Echinodermata were represented by the holothurians Pseudostichopus (Ushakov, 1952, Belyaev, 1966) and Elpidia kurilensis 1

(Belyaev, 1971). Thirty-one samples of benthic invertebrates were collected during several RV Vitjaz cruises from 1949 – 1966. Among them, 19 were obtained with the Sigsbee trawl, 7 – with the Galathea trawl and 5 with the Okean grab with a 0.25-m2 sampling area. Echinoderms were found in 25 trawls and one grab sample (Table 1). The hadal (ultra-abyssal) echinoderms from the KKT were listed by Belyaev in two monographic works (1966, 1989). More recently, the holothurian Peniagone sp. was identified as Peniagone incerta (Gebruk, 1990), the asteroids Hymenaster spp. – as Amembranaster dimidatius, Pteraster ifurus and Pteraster textius (Golotsvan, 1998), the holothurian Kolga hyalyna – as Kolga kamchatica (Rogacheva, 2012), and the sea lily Bathycrinus sp. B – as B. volubilis (Mironov, 2000). Litvinova (2010) found Amphioplus (Unioplus) cernuus (Lyman, 1879) in the Vityaz collection. A total of 32 echinoderm species were reported from the KKT. In the hadal echinoderm fauna, the most species rich are holothurians, with the family Elpidiidae being the most prominent. Belyaev (1989) considered the trench depths as the ‘kingdom of Holothuroidea’. He estimated the frequency of occurrence of holothurians in hadal trawl samples at 88%, comparable only to Polychaeta. The hadal Holothuroidea, however, were found to be less species enriched than Crustacea and Polychaeta, but similar in this regard to Gastropoda and Bivalvia (Belyaev, 1989; Jamieson, 2015). Holothurians are also among the numerically dominant taxa in the hadal zone. In trawl samples from the KKT, the abundance of specimens of the genus Elpidia reaches several thousands, with percentages up to 75-98% among all sampled individuals (Belyaev, 1989; Mironov, 2019). Excluding Elpidia, only bivalves of the genera Vesicomya and Parayoldiella are known to reach similarly high numbers in the KKT, as observed in one trawl catch (Filatova, 1971; Filatova, Shileiko, 1985). Morphology-based phylogenetic reconstruction showed that three species endemic to the Aleutian-Japan province, Elpidia kurilensis, E. birsteini and E. longicirrata, are grouped into clearly distinguished clade (Budaeva, Rogacheva, 2013). Concurrently, E. hanseni, also endemic to the Aleutian-Japan province, remains outside the NW Pacific trench clade, suggesting that evolutionary relationships among congeneric species in the KKT can differ. In 2016, vast material on hadal echinoderms was collected by RV Sonne in the framework of the KuramBio II project. Here, we investigate echinoderms both from the KuramBio II and the RV Vityaz collections. The main aim of this work is to obtain the most complete list of echinoderm species that are present in the KKT at depths exceeding 6000 m. Some biogeographic characteristics of the KKT fauna are also provided, including comparisons (by number of species) of echinoderm faunas of abyssal North-West Pacific areas and hadal trenches. 2. Material and methods The material for this study was collected during the expeditions of the RV Vityaz in 1949–1966 and German-Russian expedition KuramBio II in August–September 2016 on board the RV Sonne (SO250). On the Cruise SO250, echinoderms were sampled at 27 hadal stations at depths from 6047 m to 9582 m. The sampling gear used included the Agassiz trawl (11 stations), the Epi-Benthic Sledge (EBS) (12 stations), the Giant Box Corer (three stations) and the Multi Corer (one station) (Table 1). Specimens were preserved in 80% ethanol (Vityaz material) or 96% ethanol (Sonne material). Some asteroid specimens from the Vityaz collection were dried after fixation by ethanol. Fragments of echinoderm specimens or pieces of their tissues were dissolved in chlorinated bleach solution to study the morphology of calcareous ossicles. The crinoids were identified by A.N. Mironov, the asteroids by A.B. Dilman and A.N. Mironov, the echinoids and holothurians of the family Myriotrochidae by A.N. Mironov and K.V. Minin, other holothurians by A. V. Kremenetskaia and A.V. Gebruk, and the ophiuroids by I.S. Smirnov. The taxonomic 2

classification accepted in the Word Register of Marine Species (last visited on 2019-04-24) was used. Studies of the KKT echinoderm fauna are complicated with difficulties in the taxonomy of several taxa. In particular, several genera require a revision in the following families: Bathycrinidae, Pterasteridae, Gephyrothuriidae, Mesothuriidae, Molpadiidae, Molpadiodemidae, Psychropotidae and Myriotrochidae. We divided the results of this work into a series of publications. One paper focused on the genus Bathycrinus was published by Mironov (2019). Two other papers (on the holothurians of the family Myriotrochidae and the genus Psychropotes of the family Psychropotidae) are published in the present issue (Mironov et al.; Gebruk et al.). The present publication is the fourth in this series. Here, we provide short comments on the species described in our previous publications, preliminary comments on assumed undescribed species and more detailed comments on poorly known species. The morphology of some species has been illustrated recently in detail (Gebruk et al., this issue; Mironov, 2019; Mironov et al., 2015, Mironov et al., this issue); therefore, illustrations of those species are not presented in the present paper. Descriptions of assumed new species will be published separately. The following abbreviations were used: IORAS - Shirshov Institute of Oceanology, Russian Academy of Sciences; ZMMU – Zoological Museum of the Moscow State University; KKT – the Kuril-Kamchatka Trench, R/r – radius/interradius in asteroids. 3. Results 3.1. Systematics Class Crinoidea Miller, 1821 Order Comatulida Clark A.H., 1908 Family Bathycrinidae Bather, 1899 Bathycrinus volubilis Mironov, 2000 Fig. 1A, B Bathycrinus sp. B. – Belyaev, 1966: 118, 119, fig. 31. Bathycrinus volubilis – Mironov, 2000: 714–719, Figs. 2–4; Mironov, 2019: 402-405, Fig. 1. Type locality. KKT, RV Vityaz, cruise 39, St. 5612, 26.07.1966, Sigsbee trawl, 45º25'N, 153º07'E, 8185–8400 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 52, 1 spm; St. 77, 2 spms; St. 78, 50 spms; St. 103, fragments of the stalks. Total: 54 spms from the Kuril-Kamchatka Trench; most of them represented by aboral cup with proximal arms and proxistele. Upper diameter of radial ring from 1.8 to 3.3 mm. Remarks. In the KKT, B. volubilis was sampled by RV Vityaz at stations 2217, 3176, 5612, 5631 and 5632. These specimens were designated by Belyaev (1966) as morphospecies Bathycrinus sp. B. More recently, they were described by Mironov (2000) as B. volubilis. In addition, Belyaev (1966) distinguished morpho-species Bathycrinus C from St. 3176, which is not found. Close similarity of B. volubilis to B. kirilli from the Izu-Bonin Trench (9715–9735 m) supports a suggestion (Belyaev, 1989) that their speciation (endemism) might be driven by the topographic isolation of these two trenches from one another (Mironov, 2019). Distribution. KKT, 8175–9584 m. 3

Bathycrinus longipinnus Mironov, 2019 Bathycrinus sp. A – Belyaev, 1966: 119. Bathycrinus sp. E – Belyaev, 1966: 119. Bathycrinus longipinnus – Mironov, 2019: 409-414, Figs. 4A-J, 5A-H, 6A-K. Type locality. Aleutian Trench, RV Vityaz, cruise 20, St. 3340, Sigsbee trawl, 01.06.1955, 53°53,2' N, 166°55,6' E, 6410–6757 м. Material examined. RV Sonne, cruise SO250, Kurambio II, St. 43, 4 spms, diameter of radial circlets 2.93–3.69 mm. Distribution. KKT, 7241–7245 m. Aleutian and Japan Trenches, 6380–6865 m. Class Asteroidea de Blainville, 1830 Order Paxillosida Perrier, 1884 Family Porcellanasteridae Sladen, 1883 Eremicaster vicinus (Ludwig, 1907) Eremicaster vicinus Madsen, 1961 (synonymy up to 1961): 161–165, Fig. 30; Belyaev and Mironov, 1977a: 16, 17; Belyaev, 1985: 871–873, Fig. 1G; Dilman, 2013: 567–568; Mironov et al, 2015: 366. Type locality. Off Peru, USFC Albatross, St. 4670, 12º08'S, 79º02'W, 5869 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 28, 1 spm; St. 29, 13 spms; St. 31, 10 spms; R/r from 14/8 to 28/12 mm; St. 41, 2 spms; St. 98, 11 spms; R/r from 11/6 to 29/14 mm. Remarks. In the KKT, E. vicinus was sampled by RV Vityaz at stations 2144, 5609, 5617, 5625, 5633. A parasitic crustacean Dendrogaster sp. (Ascothoracida) was found in the coelomic cavity of Eremiсaster vicinus from St. 31. Examination of the 78 specimens of hosts revealed only one parasite (Petrunina, et al., 2016). Distribution. Kuril-Kamchatka Trench, 6090–6860 m; the Pacific, Indian, Atlantic and Southern Oceans, 2605–7250 m. The species was found below 6000 m in seven trenches: Aleutian, Izu-Bonin, Japan, Kermadec, Kuril-Kamchatka, Peru-Chile (Atakama) and SouthSandwich (Belyaev, 1989; Belyaev, Mironov, 1977a; Madsen, 1961; Suyehiro et al., 1962; Vinogradova et al., 1974) Order Velatida Perrier, 1884 Family Pterasteridae Perrier, 1875 Amembranaster dimidatius Golotsvan, 1998 Fig. 2A–C Amembranaster dimidatius Golotsvan, 1998: 1152–1153, Fig. 1. Type locality. KKT, RV Vityaz, cruise 39, St. 5625, 21.08.1966, Galathea trawl, 45º28′ N, 153º46′ E, 6225–6235 m. Material examined. Holotype. ZMMU, A–198, RV Vityaz, St. 5625, R/r = 16/8 mm. 4

Description. The genus Amembranaster with one species, A. dimidatius, was described by Golotsvan (1998) based on the single specimen designated as the holotype. Re-examination of the holotype revealed some morphological characters that were missed in the original description. Here, we re-describe the holotype and provide remarks and photographs. R/r= 16/8 mm. Form stellate. Lateral fringe partly preserved. Five interradial grooves radiate dorsally from the centre to the margin of the disc. Supradorsal membrane lacking, with the exception of small areas above some interradial grooves. Paxillae with cruciform or lobed bases, very low pedicels having the appearance of a small boss, and fascicle of delicate spines webbed together. One papula per mesh, located between the lobes of the paxilla. Paxillae arranged in almost regular oblique intercrossing rows. Five central paxillae with up to 20 spines, other paxillae on the disc and at the base of the rays with 10–15 spines, those at the distal part of rays with 8–10 spines. Spines fragile, hyaline. Unbroken spines (approximately 0.8 mm long) remain in interradii near grooves. Three adambulacral spines (sometimes four proximally, two distally), webbed. The second and third spines representing the longest, up to 1.7–2 mm long. The furrow spine approximately 1.5–1.6 mm long. The spines of each adambulacral plate united by the membrane at their bases, rarely up to middle of their length, sometimes without a membrane. Operculum absent. Outer adambulacral spine on each plate slightly shifted away from the other spines, located at the position where the operculum is usually present in Hymenaster species. In some plates, the adambulacral web joins the actinolateral web. Oral opening unusually large, approximately 7.5 mm in diameter. Oral plates plow–shaped, with a narrow keel. Oral spines 2 or 3 sitting on the oral margin. In some cases, spine bases united by a membrane. Suboral spines absent. Lateral fringe less damaged in interradial areas. Segmental apertures in the actinolateral membrane discernible interradially, large, circular in shape. Two first pairs of actinolateral spines meeting interradially, more distally rays becoming separate. Generally, there is an additional small spine sticking out from behind the major actinolateral spine adorally, just next to its base, which is also embedded in the actinolateral membrane (Fig. 2C). In some cases, there is only a major actinolateral spine. Ambulacral furrow broad. Tube feet in two rows, positioned widely apart from each other. Remarks. According to the original description, the membrane connecting bases of adambulacral combs along the ambulacral furrow, the membrane between oral spines, the actinolateral membrane and the membrane between adambulacral spines were well–developed (Golotsvan, 1998, Fig. 1). However, membrane structures were completely or partly destroyed during preservation over the years. The membrane connecting bases of adambulacral combs along the ambulacral furrow was noted by Golotsvan (1998, Fig. 1 g, d) as a main specific feature of the genus Amembranaster, which was not observed in our examination. In contrast, Golotsvan pointed out a complete absence of the supradorsal membrane, whereas small fields of supradorsal membrane were presented in interradii. This might indicate that the supradorsal membrane is rudimentary in this species. An additional peculiarity, which was not mentioned previously, is an additional small spine next to almost each major actinolateral spine (Fig. 2C). This character was not mentioned in the original description. It is also worth mentioning the presence of a small number of adambulacral plates along each ray, only 15. For comparison, some specimens of Pteraster with a similar size (R = 16 mm), from the IORAS collection, have 20–30 adambulacral plates along each ray, with 18–19 plates in Hymenaster (R = 12 mm).

5

As Golotsvan (1998) points out, this genus combines the morphological characters of two families, Pterasteridae and Korethrasteridae, but it is more similar to Pterasteridae. Amembranaster and Korethrasteridae are similar in absence of aperture opercula. Amembranaster and Pterasteridae share the characters such as actinolateral membrane, segmental apertures in this membrane, and supradorsal membrane, if the small fields of membrane in interradii of Amembranaster is considered as rudimentary supradorsal membrane. Transversal adambulacral series of the spines united by membrane show similarity to the genus Pteraster. Distribution. Amembranaster dimidatius is known only from the holotype, found in the KKT at depths 6225–6235 m. Hymenaster aff. blegvadi Madsen, 1956 Fig. 2D–F Material examined. RV Vityaz, St. 5616, 1 spm, dried, R/r = c. 22/12 mm. Description. R/r = c. 22/12 mm. Form subpentagonal. Broad, bare interradial areas. Supradorsal membrane thin, opaque, without a visible reticulum of the muscle bands. Spiraculae numerous, uniformly scattered. Paxillae arranged in four longitudinal rows. Paxillae forming a circle around the osculum bearing 5–6 spines, the other paxillae with 5 (6) spines. Oscular valves contain approximately 10 spines and 5 spines in the back row. Adambulacral plates with 2 long (1.7–2 mm) spines in the transverse row. Furrow spine usually slightly longer than the outer spine. Membrane sacculus shrinks tightly after drying out. Paired oral plates about as broad as long, with a narrow keel. Oral margin not straight, slightly tapered. Each plate bears 2 suboral and 2 oral spines at the corner. Rays united by an actinolateral membrane from the 1st to the 11– 12th spines, which are the longest, after which the rays become separate. Tube feet in two rows. Remarks. No typical opercula or traces of their attachment were found in this specimen. Assuming that the entire loss of opercula is unlikely, we suggest that the outer spine on the adambulacral plate is a nontypical large spine of aperture operculum. The presence of four longitudinal rows of paxillae per ray, 5–6 spines per paxilla and one adambulacral spine make the specimen from St. 5616 similar to H. blegvadi, described by Madsen (1956) from the Kermadec Trench at depths from 6660–6720 m. However, this specimen differs from H. blegvadi by the absence of crisscrossed muscle bands in the supradorsal membrane (possibly due to desiccation of the specimen) and has a very different operculum appearance (Fig. 2 G). We examined a specimen of H. blegvadi (R approximately 29 mm) from the ZMUC (Zoological Museum, University of Copenhagen) collection, sampled at the type locality (Galathea expedition, St. 658) and identified by Madsen. This specimen has an operculum with a 1.2-mmlong central axis and a serrate expanded base, differing markedly from the operculum in the specimen from St. 5616. Adambulacral plates in the ZMUC specimen bear one adambulacral spine (two proximally). It also differs by having adambulacral spines and aperture papillae that are more slender, fragile and smaller in size than in the specimen from St. 5616. There is no information on the intraspecific variability of the operculum; therefore, the specimen from St. 5616 is related herein to H. aff. blegvadi. Distribution. KKT, 7795–8015 m. H. blegvadi is known from the Kermadec Trench, depths 6660–6720 m. Hymenaster sp. A Fig. 3C, D 6

Material. RV Vityaz, St. 5616, 1 spm, dried, R/r =c. 38/15 mm. Description. Rays and lateral fringe bent upward, interradius at this position approximately 15 mm. Original form was likely subpentagonal. Supradorsal membrane thin, opaque, parchment–like, without a reticulum from muscle bands. Spiraculae small, numerous, irregularly scattered. Five longitudinal rows of paxillae in the mid part of the ray, at the base of ray 5–6 rows. Paxillae forming a circle around the osculum with 7–8 spines, other paxillae also with 7–8 spines when distinguishable. Oscular valves contain approximately 10–12 spines and 5 spines in the back row. Adambulacral armature preserved on some proximal plates only. Armature consists of one adambulacral spine (3–3.8 mm) and an operculum with a calcareous part approximately 2 mm long, with a long central axis and expansion at the base. Paired oral plates a little broader than long, with a conspicuous median keel. Most suboral spines are lost, with only attachment traces visible. Each plate with 2–3 (4) oral spines near the corner of the plate and a trace of one suboral spine. The 10th actinolateral spines the longest. Up to this position, rays joined by actinolateral membrane, after that becoming separate. Remarks. Among Hymenaster species with one adambulacral spine (H. blegvadi, H. formosus, H. koehleri, H. pergamentaceus, H. rhodopeplus, H. violaceus, H. aff. blegvadi), the specimen Hymenaster sp. A is the most similar to H. pergamentaceus, known from the Atlantic, in having the same number of paxillar spines, more than four longitudinal rows of paxillae per ray and an operculum with an axis having an expansion at the base. H. pergamentaceus differs from Hymenaster sp. A in having numerous fibrous bands in the supradorsal membrane, another arrangement of spiraculae (uniformly scattered in our specimen compared to microscopic, in small groups of 6 to 10 in H. pergamentaceus), and two suboral spines (one spine in our specimen). The specimen of Hymenaster sp. A was dried, resulting in its contraction and complicating the examination of some structures. Thus, we could not identify it with certainty to the species level. Distribution. KKT, 7795–8015 m. Hymenaster sp. B Fig. 3A, B Material examined. RV Vityaz, St. 5612, 3 spms. Remarks. Three specimens with R from 21 mm to 70 mm. This is a tentatively new species that differs from all known species of the genus by a complex of characters: stellate form of the body; one adambulacral spine; spine of the operculum similar to adambulacral spine, without base expansion in the small specimen and with a slightly expanded base in larger specimens; four rows of paxillae in the middle part of the ray. The full description will be published later. Two specimens from RV Vityaz St. 2144 are similar to Hymenaster sp. B in having one adambulacral spine and 6-8 spines in the paxilla. Similarities in other features are uncertain due to a very poor condition, thus these specimens could not be identified with certainty. Distribution. KKT, 8185–8400 m. Pteraster ifurus Golotsvan, 1998 7

Pteraster ifurus Golotsvan, 1998: 1155–1156, Fig. 3. Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 39, St. 5617, 06.08.1966, Sigsbee trawl, 45º32′ N, 153º46′ E, 6700–6740 m. Remarks. The species was established for five specimens: holotype from RV Vityaz St. 5617, three more specimens from the type locality, and one specimen from RV Vityaz, cruise 14, St. 2208, 49º29′ N, 158º41′ E, 7210–7230 m. The specimens were described by Golotsvan (1998) from the Kuril-Kamchatka Trench. All specimens have been lost. According to the original description, the species Pteraster ifurus has the following characters: form stellate; ventrolateral membrane narrow, not developed at the distal part of the rays; paxillae with 6–9 spines; spines thin, tri-ridged, glassy, five times higher than the pedicel, no central paxilla; supradorsal membrane thin, perforated, lacking in interradial areas, and in the central area (in one specimen); membrane without muscle fibres and deposits; spiraculae not numerous; adambulacral plates with three unwebbed spines; aperture papillae small, free along one side (as typical for Pteraster); oral plates plow-shaped; two thin oral spines of equal size (in one specimen there is one more small spine displaced towards the base of the jaw); no suboral spines, each pair of oral spines webbed at their bases, these spines not webbed across the interradius; tube feet in two rows. The species name is Latin for devoid, meaning the absence of membrane between adambulacral spines. One young damaged specimen (R= 8 mm, r=approximately 3.5 mm), similar to P. ifurus, was sampled in the KKT by RV Vityaz at St. 2144. It differs from P. ifurus by less numerous spines on the paxilla (4–7) and spines of the adambulacral plates (2). It is likely that these differences reflect age variability. Distribution. KKT, 6700–7230 m. Pteraster texius Golotsvan, 1998 Fig. 3E, F Pteraster texius – Golotsvan, 1998: 1153–1155, Fig. 2. Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 39, St. 5617, 06.08.1966, Sigsbee trawl, 45º32′ N, 153º46′ E, 6700–6740 m. Material examined. RV Vityaz, St. 5633, paratype, EtOH, R max = 14 mm, r max =7 mm. RV Sonne, cruise SO250, KuramBio II, St. 86, 45°0,435'–45°1,371' N, 151°6,010'–151°6,001' E, 5493.4–5529.5 m, 2 spms, R/r 23/13 and 8/6 mm. Description. The species was described by Golotsvan (1998) based on two specimens. The holotype from St. 5617 was lost. The specimen from St. 5633 was not designated as a type specimen in the publication but was labelled “paratype”. We present herein a more detailed description of the paratype specimen, including photographs. Form stellate. Lateral fringe lacking almost everywhere around the ambitus, but it was originally present according to Golotsvan (1998). Five interradial grooves extending from the centre to the margin on the dorsal side of the disc. Supradorsal membrane thin, without calcareous deposits and visible muscle fibres. Membrane lacking (or damaged) in the centre of the disc and in some places above the interradial grooves. Spiraculae uniformly scattered. Five central paxillae bear approximately 20 spines, paxillae not modified in valves. Madreporite 1.6 mm in diameter, located close to the ring of the central paxillae. Paxillae in 8

almost regular oblique rows. Paxillae in the median rows on rays with 3–4 spines, lateral paxillae with 5–6 spines, paxillae on disc with 10 spines. Pedicel of the paxillae very short, bulb-shaped. Spines short, slightly protruding supradorsal membrane. Adambulacral plates with 4 (3) spines almost equal in size, arranged in slightly crescent rows oblique to the furrow. Spines glassy, fragile, with three vertical ridges. Membrane between spines visible on some plates only. Aperture papilla placed outwards from each comb of the adambulacral spines, as in other Pteraster species. Aperture papillae small, narrow, with a free aboral side and adoral side embedded in the actinolateral membrane. Most papillae are free, without a membranous sheath as the actinolateral membrane is almost destroyed. Oral plates long, with narrow lateral lobes. True suboral spines absent, 3 or 4 oral spines of equal size crowded at the corner of each plate, slightly shifted on the plate. Remnants of membrane between some spines can be seen. Actinolateral spines and membrane greatly destroyed. Tube feet in two rows. Remarks. P. texius is similar to P. ifurus, but it differs by webbed adambulacral spines and more numerous oral spines (4 instead of 2–3 in P. ifurus). The holotypes of both species were sampled from the same station; therefore, differences can be related to morphological variability and preservation condition. Two specimens (R1/r1=23/13, R2/r2=8/6) similar to P. texius were sampled in the KKT during the KuramBio II expedition from a depth slightly less than 6000 m (St. 86). They are in good condition, all with a membranous structure, including a lateral fringe and supradorsal membranes, being well-preserved, in contrast to the paratype of P. texius. A detailed description of these specimens will be published later. Distribution. KKT, 6120–6740 m. Western slope of KKT, 5493.4–5529.5 m. Order Forcipulatida Perrier, 1884 Family Pedicellasteridae Perrier, 1884 Hydrasterias sp. Material examined. RV Vityaz, St. 5633, seven rays; RV Sonne, cruise SO250, KuramBio II, St. 7, 18.08.2016, Agassiz trawl, 43° 49,814' N, 151° 44,787' E – 43° 48,076' N,151° 45,255' E, 5210.1–5103.3 m, 5 rays; St. 9, 19.08.2016, Agassiz trawl, 43° 48,439' N, 151° 44,351' E – 43° 47,643' N, –151° 44,513' E; 5134.2–5101.5 m, 9 rays. Ray length from 31 to 56 mm. Remarks. Specimens from the KuramBio II collection are similar to the specimen from Vityaz St. 5633, particularly in having one adambulacral spine on most plates (some rays with two adambulacral spines on the most proximal plates). They share this character with species H. sacculata McKnight, 2006 and H. tasmanica McKnight, 2006 from New Zealand. McKnight’s species and our specimens share monocanthid adambulacral plates. Our specimens differ from H. sacculata in having abactinal spines without conspicuous sheaths of skin, and in much shorter abactinal spines (up to 0.6 mm instead of 2 mm long in H. sacculata). The differences from H. tasmanica are the possession of much shorter abactinal spines (up to 0.6 mm instead of 2 mm long), one inferomarginal spine (instead of one–two spines), and the lack of a straight pedicellaria on the abactinal surface of the rays. It is likely that the differences in spine length and in the number of inferomarginal spines are not species-specific features but related to the age variability because McKnight’s specimens are larger than our specimens (R/r = 90/6.5 mm in H. sacculata, R/r = 66/5 mm in H. tasmanica). 9

In addition to the differences noted above, the species H. sacculata and H. tasmanica differ from each other in the number of rays, with 6 rays in H. sacculata and 5 rays in H. tasmanica. Our specimens lack a disc, and accordingly, we cannot refer them to either of McKnight’s species with certainty. Distribution. KKT, 6160–6120 m. East of KKT, 5101–5210 m. Order Brisingida Fisher, 1928 Family Freyellidae Downey, 1986 Freyella kurilokamchatica Korovchinsky, 1976 Freyella kurilokamchatica – Korovchinsky, 1976: 165–169, Figs. 2, 3. Type locality. KKT, RV Vityaz, cruise 14, St. 2144, 01.06.1953, Sigsbee trawl, 48º25' N, 156º34' E, 6860 m. Material examined. RV Vityaz, St. 2144, holotype with 5 intact rays and two dried arms, IO RAS ECH00393; St. 5625, 5 paratypes, IO RAS ECH00388; St. 5624, 45°26’N, 154°12’ E, 5200 m, paratype, IO RAS ECH00390. RV Sonne, cruise SO250, KuramBio II, St. 9, 19.08.2016, Agassiz trawl, 43° 48,439' N, 151° 44,351' E – 43° 47,643' N, 151° 44,513' E; 5134.2–5101.5 m, 3 spms. Remarks. The detailed description of the holotype and remarks concerning the morphological variability of the species have been provided by Korovchinsky (1976). All the specimens listed above have 7 arms (based on the discs). Distribution. KKT, 6205–6860 m. NW Pacific and North-West Pacific Basin, 4890–6282 m. Class Ophiuroidea Gray, 1840 Order Ophiurida Müller et Troschel, 1840 sensu O'Hara et al., 2017 Family Ophiopyrgidae Perrier, 1893 Amphiophiura bullata pacifica Litvinova, 1971 Fig. 4B Amphiophiura bullata pacifica – Litvinova, 1971: 303, pl. I. 1–4; Paterson, 1985: 131, 132. Amphiophiura bullata convexa – Litvinova, 2010 (part): 29–31 (Non: Amphiophiura bullata convexa (Lyman, 1878)). Amphiophiura pacifica – Stöhr, O’Hara, Thuy, 2019. Type locality. North-West Pacific Basin, RV Vityaz, cruise 19, St. 3156, Okean grab, 28.09.1954, 39º57'N, 165º07.8'E, 5535 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 97, 3 spms; St. 98, 4 spms. Disc diameter 3.5–12 mm. Remarks. In the KKT, A. bullata pacifica was sampled by RV Vityaz at stations 5609 (Litvinova, 1971 as Amphiophiura bullata pacifica) and 5625 (Litvinova, 2010 as Amphiophiura bullata convexa). Among the KuramBio II specimens examined, the smallest ones (disk diameter 3.5–8 mm) show similarity to A. bullata convexa in dorsal disk plating: they have a distinct rosette of primary plates and contiguous radial shields. The presence of these features only in small specimens likely reflects the growth variability of the disk plating. Species delimitation analyses, performed using the COI sequence data, showed that all the specimens examined belong to a single molecular species (Eichsteller, 2017). 10

As indicated by Paterson (1985), the Amphiophiura bullata complex comprises A. bullata bullata, A. bullata pacifica, A. convexa and A. vitjazi, taxa which have the same organization of the ventral and oral plates but differ mainly in the plating of the dorsal surface of the disk. Paterson (1985) noted that the range of variation described by Litvinova (1971) for A. bullata pacifica appears to encompass the range of disk plate patterns described for A. bullata bullata to A. vitjazi. Patterson reduced A. convexa and A. vitjazi to the rank of subspecies of A. bullata together with A. bullata pacifica. The ascothoracidan crustacean Cardiosaccus pedri lives as a mesoparasite in permanent cysts within the bursae of A. bullata pacifica from the Kuril-Kamchatka Trench area: RV Sonne, St. 7, 4348'N, 15145.2'E, 5200 m, and St. 64, 4509.8'N, 15346.2'E, 5700 m (Kolbasov et Petrunina, 2018). Distribution. KKT, 6090–6235 m. North-West and North-East Pacific, 5027–6380 m; St. 4138, RV Vityaz, north of Vancouver Island, at depths from 2507–2608 m (Litvinova, 1971). Ophioplinthus madseni (Belyaev et Litvinova, 1972) Fig. 4D Homalophiura madseni – Belyaev, Litvinova, 1972: 11–13, Figs. 3.3–4; 4; Litvinova, 2010: 39. Type locality. KKT, RV Vityaz, cruise 14, St. 2208, 22.06.1953, Sigsbee trawl, 4929'N, 15841'E, 7210–7230 m. Remarks. Litvinova (in Belyaev, 1989) suggested that two species of Ophioplinthus occur in the KKT: Ophioplinthus madseni (St. 2208) and Ophioplinthus sp. n. 2 (St. 3457). More recently, Litvinova (2010) referred to the specimen from St. 3457 as O. madseni. In the KKT, O. madseni was also sampled at station 5617 (Belyaev, Litvinova, 1972). Distribution. KKT, 6475–7230. Japan Trench, 6156–6571 м. Ophioplinthus cf. madseni was found in Mariana, Palau and Ryukyu trenches, 7000–7459 m (Belyaev, Litvinova, 1972; Litvinova, 2010) Family Ophiuridae Müller et Troschel, 1840 Ophiura bathybia H.L. Clark, 1911 Ophiura bathybia – H.L. Clark, 1911: 58, Fig. 14; Djakonov, 1954: 117; Baranova, 1957: 205–206; Belyaev, Litvinova, 1972: 13–14; Litvinova, 2010: 42–43. Type locality. Bering Sea, USFC Albatross, St. 4766, 52°38' N, 174°49' W, 3231 m (Clark, 1911). Remarks. In the KKT, Ophiura bathybia was sampled by RV Vityaz at stations 5633 (Belyaev, Litvinova, 1972; Litvinova, 2010). Distribution. KKT, 6090–6135 m. North Pacific, 2870–6328 m. Order Ophiurida incertae sedis Abyssura brevibrachia Belyaev et Litvinova, 1976 Fig. 4C

11

Abyssura brevibrachia – Belyaev, Litvinova, 1976: 128–129, Fig. 1, 5.1–2; Litvinova, 2010: 28. Type locality. KKT, RV Vityaz, cruise 14, St. 2208, 22.06.1953, Sigsbee trawl, 4929'N, 15841'E, 7210–7230 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 40, 2 spms, diameter of disk 1,2 and 3 mm. Remarks. In the KKT, A. brevibrachia was sampled by RV Vityaz at stations 2208, 5608 and 5617 (Belyaev, Litvinova, 1976; Litvinova, 2010). Distribution. KKT, 6710–7300 m. Aleutian and Japan trenches, 6156–7000 m (Belyaev, Litvinova, 1976; Litvinova, 2010) Perlophiura profundissima Belyaev et Litvinova, 1972 Perlophiura profundissima Belyaev, Litvinova, 1972: 7–11, Fig. 2, 3.1–3, Litvinova 1975: 198-199. Paterson, 1985:145, Fig. 55; Litvinova, 2010: 50–52. Type locality. South of Aleutian Islands, RV Vityaz, cruise 45, St. 6088, 04.05.1969, Sigsbee trawl, 5358'5 N, 15736'0 E, 5740 m. Remarks. In the KKT, P. profundissima was sampled by RV Vityaz at stations 5615 and 5616 (Belyaev, Litvinova, 1976; Litvinova, 2010). Distribution. KKT, 7795–8060 m. Pacific Ocean including Aleutian, Izu-Bonin, Volcano trenches, North-West Pacific Basin. Indian and Atlantic Oceans. Depths 2265–8135 m. Order Amphilepidida O'Hara et al., 2017 Family Amphiuridae Ljungman, 1867 Amphioplus (Unioplus) cernuus (Lyman, 1879) Amphiura cernua Lyman, 1879: 28, Pl. XII, Figs. 323–325; Lyman, 1882: 138, Pl. XVII, Figs. 13–15. Amphioplus (Unioplus) cernuus – Clark A. M., 1970: 40, 42, 44. Unioplus cernuus – Litvinova, 2010: 11–12. Amphioplus cernuus – Martynov, 2010: 26, 43, 121, Figs. 15L, 31L. Type locality. SE of Japan, HMS Challenger, St. 241, 23.06 1875, 35º41'N, 157º42'E, 4209 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 29, 4 spms; St. 31, 5 spms, disk diameter 9–14 mm; St. 98, 1 spm, disk diameter 12 mm. Remarks. In the KKT, A. (Unioplus) cernuus was sampled by RV Vityaz at stations 5609, 5617, 5625 and 5633. For additional information, see remarks on Amphioplus (Unioplus) cf. daleus. Distribution. KKT, 6090–6710 m. According to Litvinova (2010), this species is widely distributed in the Pacific Ocean, from the Aleutian Trench to the Palau Trench, and from the Gulf of Alaska to the Peru-Chile Trench; 2880–7170 m. Amphioplus (Unioplus) cf. daleus Lyman, 1879 12

Fig. 4A Material examined. RV Vityaz, St. 3457, 1 spm, disc diameter 15 mm; RV Sonne, cruise SO250, KuramBio II, St. 98, 6 spms; disk diameter 6–12 mm. Remarks. Specimens from RV Vityaz, St. 3457 and KuramBio II, St. 98 differ from A. (Unioplus) cernuus in having smaller tentacle scales and in the primary plate arrangement on the dorsal side of the disk: only a central primary plate conspicuous (all primary plates conspicuous in A. (Unioplus) cernuus). Similar features are known in A. (Unioplus) daleus Lyman, 1879. A.M. Clark (1970) suggested that A. cernuus and A. daleus may be synonymous despite the wide separation of their type-localities, North-West Pacific and South-West Atlantic, respectively. Eichsteller (2017) identified all Amphioplus specimens from the KuramBio II collection as A. (Unioplus) daleus. Using the COI sequence data, she showed that all these specimens belong to a single molecular species. More detailed morphological and genetic examinations are required to distinguish or synonymize A. (Unioplus) cernuus and A. (Unioplus) daleus. Distribution. KKT, 6442–6445 m. A. (Unioplus) daleus is known from Atlantic and East Pacific, 1170–5870 m (Smirnov et al., 2014; Amon et al., 2017). Order Ophiacanthida O'Hara, Hugall, Thuy, Stöhr et Martynov, 2017 Family Ophiacanthidae Ljungman, 1867 Ophiacantha bathybia H.L. Clark 1911 Ophiacantha bathybia – Clark, 1911: 233, Fig. 110; Djakonov, 1954: 39; Baranova, 1957: 192; Belyaev, Litvinova, 1972: 13–14 ; Imaoka et al., 1991: 129, Fig. 52; Lambert and Boutillier, 2011: 40, Fig. 26; Mironov et al. 2018: 7. Ophiacantha pacifica – Litvinova (2010) (partim.): 16, RV Vityaz, St. 3184 (Non: Ophiacantha pacifica Lütken and Mortensen, 1899). Type locality. Off Alaska, USFC Albatross, St. 2859, 55°20' N.; 136°20'W, 2871 m. Remarks. Belyaev and Litvinova (1972) reported O. bathybia from St. 5609 (KKT, 6090– 6235 m). More recently, Litvinova (2010) identified the specimens from St. 5609 and 5633 as Ophiacantha cosmica. Distribution. KKT, 6090–6235 m. From Bering Sea south to California, Sea of Okhotsk, and Japan Trench; 800–6440 m. Class Echinoidea Leske, 1778 Order Echinothurioida Claus, 1880 Family Kamptosomatidae Mortensen, 1934 Kamptosoma abyssale Mironov, 1971 Kamptosoma abyssale – Mironov, 1971: 321–323, Figs. 2–4; 1997b: 75; Mironov et al., 2015: 358–360, Fig. 1A–C, E–L. Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 39, St. 5609, 23.07.1966, Sigsbee trawl, 46º06' N, 153º18' E, 6090–6235 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 9, 19.08.2016, Agassiz trawl, 43°48,439' N, 151°44,351' E – 43°47,643' N, 151°44,513' E, 5134.2–5101.5 m, 6 spms, diameter of test 26–78 mm. 13

Remarks. The morphology of K. abyssale from the KKT area was recently illustrated in detail (Mironov et al., 2015). Globiferous pedicellariae of the genus Kamptosoma were considered by Mironov (2015) to be better described as claviform due to their rudimentary, nonfunctional valves. K. abyssale differs from the type species of the genus (K. asterias) in having a smaller number of valves and saccules in the claviform pedicellariae: 2 valves and 1–2 saccules in the former, and 3 valves and 3 saccules in the latter. However, one of the specimens from St. 9 has claviform pedicellariae of both types. The variability of diagnostic characters of K. abyssale is low and insufficient to synonymize this species with K. asterias. Among 80 specimens collected by RV Vityaz in the Pacific and Indian Oceans (17 stations), only two specimens had claviform pedicellariae of two types, both with two and three saccules (Mironov, 1971, Fig. 3). Claviform pedicellariae of K. abyssale collected during the KuramBio expedition (51 specimens from 14 stations) had two saccules. Anderson (2016) examined 97 specimens of Kamptosoma from the Tasman Basin. He noted that claviform pedicellariae were readily found, but not common; they have 2 valves as described for K. abyssale. Mortensen (1935) studied K. asterias from the South-East and Central Pacific. He did not mention if all the examined specimens from both localities had three-sacculate pedicellariae. Re-examination of the specimen of K. asterias from the Central Pacific (H.M.S. Challenger, St.272) showed that it had pedicellariae with three saccules (Mironov et al., 2015, Fig. 3D). However, no clear information is yet available on the form of claviform pedicellariae of the type specimen from the South-East Pacific (H.M.S Challenger, St. 299). Recent field work in Antarctica has provided additional K. asterias material (Heterier, 2006; Mooi et al., 2004). However, the number of saccules in these specimens is not known. The status of K. abyssale will remain unconfirmed until additional material on the variability of claviform pedicellariae in K. asterias from the type locality becomes available. Distribution. KKT, 6090–6235 m. From Aleutian Islands south to Tasman Sea and from Madagascar eastward to Hawaii; 4374–5998 m. Order Holasteroida Durham et Melville, 1957 Family Pourtalesiidae A. Agassiz, 1881 Echinosigra (Echinogutta) amphora amphora Mironov, 1974 Echinosigra amphora amphora – Mironov, 1974: 246–247, Figs. 1, d, g, f, m, 2 g, k, pl. I, d, e, g. Echinosigra (Echinogutta) amphora amphora – Mironov, 1997: 181, Fig.7; Mironov et al., 2015: 363, Figs. 2D, H–K, P, 5A–H. Type locality. East of Kuril-Kamchatka Trench, RV Vityaz, cruise 19, St. 3114, 27.08.1954, Sigsbee trawl, 48º51' N, 160º01' E, 5511 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 42, 5 juveniles; St. 97, 1 young spm; 98, 1 adult spm. Remarks. The morphology of Echinosigra amphora from the KKT area was recently illustrated in detail (Mironov et al., 2015). The juvenile specimens (test length 0.4–0.8 mm) are identified by the presence of ophicephalous pedicellaria with broad short valves. Pedicellaria of this type are always positioned on the juveniles of E. (Echinogutta) amphora amphora at the centre of the aboral side of the test (Mironov et al., 2015). 14

Distribution. KKT, 6440–7119 m. North-West Pacific, 4650–6282 m. The subspecies E. (Echinogutta) amphora indica Mironov, 1974 is known from the Java Trench, 6433–6850 m. Fragments of E. (Echinogutta) amphora spp. were collected in the Palau Trench, 7000–7170 m (Mironov, 1997). Class Holothuroidea de Blainville, 1834 Order Elasipodida Théel, 1882 Family Elpidiidae Théel, 1882 Elpidia birsteini Belyaev, 1971 Fig. 5B Elpidia birsteini – Belyaev, 1971: 336–338, Fig. 4; Hansen, 1975: 181; Stepanov, 2015: 58– 59. Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 39, St. 5612, 27.07.1966, Sigsbee trawl, 45°25’N, 153°07’E, 8185–8400 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St.18, 20 spms, length 30–45 mm; St. 20, 6 spms, length 35–45 mm; St. 52, 4 spms, length 11–35 mm; St. 55, 1 spm, length 35 mm; St. 56, 10 spms, length 18–36 mm. Remarks. In the KKT, E. birsteini was sampled by RV Vityaz at stations 2120, 2216, 3176, 5612, 5615 and 5631. Elpidia birsteini was often found together with Elpidia longicirrata. The species differ from each other by the length of the papillae: E. birsteini has one long anterior pair of papillae while two other pairs are short, whereas E. longicirrata has three long pairs of papillae. Belyaev (1971) noted that E. longicirrata has more gracile ossicles than E. birsteini. Examination of KuramBio II specimens showed no clear differences in ossicle morphology between these species. Distribution. KKT, 8060–9345 m. Izu-Bonin (Izu-Ogasawara) Trench, 8530–8540 m. According to Belyaev (1971), E. birsteini is very common species in this area and often forms aggregations at depths 8060–8400 m. Elpidia hanseni hanseni Belyaev, 1971 Fig. 5C Elpidia hanseni – Belyaev, 1971: 339–342, Figs. 6–7. Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 14, St. 2217, 29.06.1953, Sigsbee trawl, 44°08’N, 150°32’E, 9000–9050 m Material examined. RV Sonne, cruise SO250, KuramBio II, St. 54, 80 spms, length 12–25 mm; St. 78, 31 spms, length 14–27 mm; St. 102, 58 spms; length 8–23 mm. Remarks. In the KKT, E. hanseni was sampled by RV Vityaz at stations 2216, 2217, 2218, 3176, 5613, 5627, 5628 and 5631. According to Belyaev (1971) this subspecies is a characteristic representative of the maximum depths of the Kuril-Kamchatka trench where it was found in large numbers. Distribution. KKT, 8610–9582 m. KuramBio II records slightly extended the deeper limit of this subspecies from 9530 to 9582 m. In the KuramBio II cruise E. hanseni hanseni was found at depths from 8729–9582 m. E. hanseni idzibonensis is endemic to the Idzu-Bonin Trench, 8800– 9735m. 15

Elpidia kurilensis Baranova et Belyaev in Belyaev, 1971 Fig. 5E Elpidia glacialis kurilensis (nomen nudum) – Baranova, 1962: 2. Elpidia kurilensis – Belyaev, 1971: 333–336; Stepanov, 2015: 59. Elpidia glacialis kurilensis – Hansen, 1975: 180–181. Type locality. Kuril-Kamchatka Trench, RV Vityaz, Cruise 2, St. 162, 10.10.1948, Sigsbee trawl, 44°56’N, 152°24’E, 8100 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 43, 1 spm, length 50 mm; St. 97, 1 spm, length 38 mm. Remarks. In the KKT, E. kurilensis was sampled by RV Vityaz at stations 162, 5608 and 5617. Distribution. KKT, 6440–8100 m. Japan and Aleutian trenches, 6156–7587 m. Elpidia longicirrata Belyaev, 1971 Fig. 5D Elpidia longicirrata – Belyaev, 1971: 338–339, Fig. 5; Hansen, 1975: 181; Stepanov, 2015: 59. Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 39, St. 5629, 31.08.1966, Galathea trawl, 43°54’N, 149°43’E, 8035–8120 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 18, 6 spms, length 32–43 mm; St. 20, 1 spm, length 40 mm; St. 90, 6 spms, length 25–50 mm; St. 102, 5 spms, length 15– 18 mm. Remarks. In the KKT, E. longicirrata was sampled by RV Vityaz at stations 5629 and 5632. Distribution. KKT, 8035–9537 m. KuramBio II records have extended the deeper depth limit of this species from 8345 to 9537 m. Kolga kamchatica Rogacheva, 2012 Fig. 6 Kolga kamchatica – Rogacheva, 2012: 1190, Fig. 8; Stepanov, 2015: 60. Kolga hyalina – Belyaev, 1989: 225, Tab. 22; Gebruk, 1990: 121–122 (partim.) Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 39, St. 5625, 21.08.1966, Galathea trawl, 45° 28' N, 153° 46' E, 6225–6236 m. Material examined. RV Akademik M.A. Lavrentiev, cruise 75, Station LV75-16, sample 1, ROV Comanche-18, 55° 34.6’N, 167°19.5’E, depth 4278 m, 5 spms, body length 23–35 mm. Remarks. This species has long been known based on a single record in the KKT. Recently, aggregations of this species were found in the Bering Sea (Fig. 6). Distribution. KKT, 6225–6236 m. Bering Sea, 4278 m. Peniagone cf. incerta (Théel, 1882)

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Peniagone incerta – Gebruk, 1990: 108–109 [partim.], Fig. 42(8–11); Stepanov, 2015: 57 [partim.]. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 29, 45 spms, length 10–30 mm; St. 31, 12 spms, length 30–75 mm; St. 86, 6 spms, length 43–75 mm; St. 98, 9 spms, 28–65 mm. Remarks. In the KKT, P. cf. incerta was sampled by RV Vityaz at stations 2144, 2208 and 5609. KuramBio II specimens are thought to be conspecific with specimens from the North-West Pacific assigned to Peniagone incerta by Gebruk (1990). Examination of the type specimens of Peniagone incerta [Natural History Museum London, UK, Cat. Nr 83.6.18.7] revealed noticeable differences in the dorsal ossicle morphology, suggesting that the North-West Pacific specimens belong to a separate species. Distribution. KKT, 6065–7230 m. Bering Sea, Kuril-Kamchatka and Japan trenches and adjacent abyssal areas. Depth 4820–7230 m. Scotoplanes hanseni Gebruk, 1983 Fig. 5A Scotoplanes hanseni – Gebruk, 1983: 1366–1367, Figs. 2(14–29); 1990: 129–132, Figs. 53(7–9), 57; Stepanov, 2015: 60. Scotoplanes globosa – Hansen, 1975: 167–169 [partim.: Fig. 83(5–8)]. Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 39, St. 5633, 06.09.1966, Galathea trawl, 44°07’N, 149°34’E, 6090–6135 m. Material examined. RV Sonne, cruise SO250, KuramBio II, St. 29, 5 spms, body length 45– 70 mm; St. 64, 1 spm, length 70 mm; St. 98, 2 spms, length ~75 mm. Remarks. In the KKT, S. hanseni was sampled by RV Vityaz at stations 2144 and 5633. Distribution. KKT, 6090–6860 m. KKT area, Japan, Aleutian, Bougainville, New Hebrides and Kermadek trenches, 4650–7660 m. Family Psychropotidae Théel, 1882 Psychropotes moskalevi Gebruk et Kremenetskaia, this issue Psychropotes moskalevi – Gebruk et Krementskaia in Gebruk et al., this issue. Type locality. North-West Pacific, RV Sonne, cruise SO223, KuramBio, St. 223/01-12, 30.07 2012, Agassiz trawl 43°58.19' N, 157°19.11' E–43°57.81'N, 157°21.58'E, 5422–5379 m. Material examined. RV Vityaz, St. 5625, 1 spm, body length 100 mm. Remarks. Belongs to the group of species of Psychropotes with a long dorsal appendage (“tail”). This group currently includes eight species, four of which occur in the North-West Pacific. Psychropotes moskalevi has been described by Gebruk et al. (this issue). Distribution. KKT, 6205–6215 m. East of KKT, Aleutian Trench, 5020–5502 m. Psychropotes pawsoni Gebruk et Kremenetskaia, this issue Psychropotes pawsoni – Gebruk, Krementskaia, in Gebruk et al., this issue.

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Type locality. North-West Pacific, RV Vityaz, cruise 19, St. 3156, 28.09.1954, Sigsbee trawl, 39°57’N, 165°07.8'E, 5535 m. Material examined. RV Vityaz, St. 5633, 1 spm, incomplete. Remarks. Belongs to the group of species of Psychropotes with long dorsal appendage (“tail”). This group currently includes eight species, four of them occur in the North-West Pacific. Psychropotes pawsoni is described in Gebruk et al. (this issue). Distribution. KKT, 6090–6135 m. North-West Pacific, 2515–5738 m. Psychropotidae gen. sp. Psychropotidae gen. sp. – Belyaev, 1989: 226 (Table 22). Material examined. None. Remarks. In the KKT, Psychropotidae gen. sp. was sampled by RV Vityaz at St. 5627. Distribution. KKT, 9170–9335 m. Order Holothuriida Miller, Kerr, Paulay, Reich, Wilson, Carvajal et Rouse, 2017 Family Mesothuriidae Smirnov, 2012 Mesothuria sp. A Material examined. RV Sonne, cruise SO250, KuramBio II, St. 18, 1 spm, body length 50 mm. Remarks. This is a tentative new species that is distinct from all known species of the genus by unusually large triradiate tables with numerous perforations. The table ossicles were up to 400 µm in diameter with several tens of perforations (up to 60–70), a spire up to 250 µm in length, consisting of three rods and two connecting beams, and bifurcated spire rods at the ends. This is the deepest known record of the genus Mesothuria. Distribution. KKT, depth 8185–8200 m. Order Molpadida Haeckel, 1896 Family Molpadiidae J. Müller, 1850 Molpadia cf. granulata (Ludwig, 1893) Fig. 5G Material examined. RV Sonne, cruise SO250, KuramBio II, St. 31, 12 spms, body length 25– 70 mm. Remarks. Examined specimens are most similar to Molpadia granulata (Ludwig, 1893) in ossicle morphology: phosphatic ossicles absent, body wall table ossicles ranging in diameter from 200–250 µm with up to eight perforations with an oval to slightly angular shape. According to a description provided by Pawson (1977), KuramBio II specimens differ by dominance in the body wall skin of ossicles with three perforations, as well as numerous ossicles with three large perforations and 1–2 smaller perforations. Tail ossicles were elongated rod-like tables, usually with 4–5 (up to 8) perforations. Distribution. KKT, depth 6185–6202 m. Molpadia sp. A 18

Material examined. RV Sonne, cruise SO250, KuramBio II, St. 98, 4 spms, body length 30– 65 mm. Remarks. The species is characterized by the absence of phosphatic bodies, a robust body wall table ossicles usually less than 180 µm in diameter with 3–10 perforations (usually more than 4) and a spire of 3 (rarely 2) pillars, and tail ossicles elongated rod-shaped tables up to 320 µm in length with 4–7 perforations and a spire of usually 3 pillars. This is the deepest known record of the genus Molpadia (GBIF.org, 2019). Distribution. KKT, depth 6442–6446 m. Order Persiculida Miller, Kerr, Paulay, Reich, Wilson, Carvajal et Rouse, 2017 Family Gephyrothuriidae Koehler et Vaney, 1905 Gephyrothuria sp. Fig. 5H Material examined. RV Sonne, cruise SO250, KuramBio II, St. 9, 2 spms, length 25–32 mm; St. 31, 1 spm, 40 mm. Remarks. The record from Station 31 at 6185–6221 m is the deepest published record for this genus. Thus far, representatives of Gephyrothuria have been found at depths from 732–5379 m. Distribution. KKT, depth 5102–6221 m. Hadalothuria sp. Fig. 5I ?Hadalothuria sp. – Belyaev, 1989: 226 (Table 22). Material examined. RV Sonne, cruise SO250, KuramBio II, St. 20, 1 spm, body length 45 mm; St. 29, 2 spms, body length 65–68 mm; St. 31, 17 spms, length 35–120 mm; St. 98, 1 spm, 50 mm. Remarks. Genus Hadalothuria with a single species, Hadalothuria wolffi, was designated by B. Hansen based on two records in the New Britain Trench at depths from 8810–8940 m (Hansen, 1956). KuramBio II specimens of Hadalothuria probably belong to a separate species. Representatives of this genus have been recorded twice from the Kuril-Kamchatka trench at depths from 9070–9530 m (Belyaev, 1989). It is possible that these specimens are conspecific with those from KuramBio II. Distribution. KKT, depth 6183–8199 m. Family Molpadiodemidae Miller, Kerr, Paulay, Reich, Wilson, Carvajal et Rouse, 2017 Molpadiodemas Heding, 1935 Remarks. Pseudostichopus sp. found by Belyaev (1989) at St. 162 of RV Vityaz could possibly belong to the genus Molpadiodemas. Examination of Molpadiodemas was performed in collaboration with Mark O’Loughlin, Allison Miller, Melanie Mackenzie and Greg Rouse. Preliminary results based on both morphological and genetic data suggest the presence of three species in the examined area; all are probably new to science. Their descriptions will be published separately. Here, a list of preliminary identifications is presented. 19

Molpadiodemas sp. A Material examined. RV Sonne, cruise SO250, KuramBio II, St. 20, 5 spms, body length 20– 50 mm. Remarks. The species is characterized by a yellowish colour, short tube feet on the dorsum, and tentacle ossicles rods that are often enlarged at their ends. Distribution. KKT, depth 8191–8199 m. Molpadiodemas sp. B Material examined. RV Sonne, cruise SO250, KuramBio II, St. 29, 8 spms, body length 40– 65 mm; St. 31, 11 spms, body length 45–70 mm. Remarks. This species was recognized mainly based on genetic data. Specimens of this species were very different in morphology, i.e., skin colour, tube feet arrangement, tentacle ossicles, etc. A more detailed morphological examination is required to detect species borders. Distribution. KKT, depth 6183–6221 m. Molpadiodemas sp. C Fig. 5J Material examined. RV Sonne, cruise SO250, KuramBio II, St. 56, 4 spms, body length 40– 70 mm; St. 78, 1 spm, body length 55 mm. Remarks. The species is characterized by a purple colour and dark purple tube feet. Distribution. KKT, depth 8404–9582 m. Order Apodida Brandt, 1835 Family Myriotrochidae Théel, 1877 Myriotrochus longissimus Belyaev, 1970 Fig. 7 Myriotrochus longissimus – Belyaev, 1970: 471–474, Figs. 6, 7; Table II. 3, 4; Belyaev, Mironov, 1977b: 168–169, Fig. 4; 1982: 105; Panina, Stepanov, 2014: 89. Type locality. South end of the Kuril-Kamchatka Trench, RV Vityaz, cruise 22, St. 3457, 21.09.1955, Sigsbee trawl, 41°17'N, 145°50'E, 6475–6571 m. Material examined. RV Vityaz, St. 3457, 36 spms; RV Sonne, KuramBio II, St. 37, 36 spms; St. 40, 8 spms; St. 41, 6 spms; St. 42, 3 spms; St. 43, 33 spms; St. 86, 1 spm The single complete specimen (St. 43) 68 mm long, with calcareous ring diameter 6 mm. Other specimens are represented by body fragments; calcareous ring diameter ranges from 4 to 8 mm. Remarks. The type series from St. 3457 includes 36 short fragments with 6–8-mm-diameter calcareous rings (IORAS catalogue numbers ECH00301) and 33 fragments without a ring, up to 58 mm in length (IORAS ECH00304). Tentacle crown colourless. Most anterior fragments with rings are lacking wheels. In some anterior fragments, the wheels are few, up to 190 μm in diameter, with regular distributed teeth and straight spokes. Two anterior fragments, each with a single wheel of 240 and 260 μm, with some teeth fused with each other. Wheels are numerous on the posterior ends of the fragments devoid of a ring; wheel diameter up to 291 μm. In some fragments, the wheels often have branched spokes (Fig. 7A, B), and in other fragments, all the 20

spokes are straight and unbranched (Fig. 7D–G). Among the specimens from St. 43, RV Sonne, anterior body fragments are usually lacking wheels. In the posterior fragments, the wheels are large (up to 315 μm), with curved branched or unbranched spokes. The majority of the wheels have 3–4 fused teeth; sometimes most of the teeth are fused (Fig. 7H–I). Some anomalous wheels are lacking teeth or have a disrupted rim (Fig. 7K, L). The specimens from St. 42, 86 and 97 are characterized by a slight purple colour of the tentacle crown, relatively small wheels (up to 220 μm), and absence of branched spokes and fused teeth (Fig. 7M). Anomalous wheels are very rare (Fig. 7N). The wheels at the anterior and posterior body ends are numerous and similar. It is likely that these specimens belong to undescribed species that is very similar to M. longissimus. The range variation in M. longissimus appears to encompass the range of wheel size, form and distribution pattern described for M. giganteus and M. macquariensis (Belyaev & Mironov, 1981, 1982; Gage, Billett, 1986). A more detailed morphological and genetic examination is required to distinguish these species. Distribution. KKT, 6475–7300 m. Kuril-Kamchatka Trench area, North-East Pacific Basin, Japan, Palau, Philippine Trenches, 5422–7370 м Prototrochus zenkevitchi zenkevitchi (Belyaev, 1970) Figs. 5F, 8A–E Myriotrochus zenkevitchi zenkevitchi – Belyaev, 1970: 462–468, Figs. 2, 3, Pl. 1.1–4; Belyaev, Mironov, 1977b: 167–168, fig. 3; Pl. 1; 1981: 170–171, Fig. 5; pl. 1. Prototrochus zenkevitchi zenkevitchi – Belyaev, Mironov, 1982: 94, Fig. 6; Panina, Stepanov, 2014: 90. Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 14, St. 2217, 29.06.1953, Sigsbee trawl, 44°07'N, 150°32'E, 9000–9050 m. Material examined. RV Sonne, cruise So250, KuramBio II, St. 17, 370 spms; St. 19/1, 1 spm; St. 28, 30 spms; St. 30, 21 spms; St. 52, 1270 spms; St. 54, 75 spms; St. 55, 360 spms; St. 67, 3 spms; St. 77, 420 spms; St. 78, 3 spms; St. 97, 40 spms; St. 100, 25 spms; St. 102, 90 spms; St. 103, 21 spms. Total: 2929 spms; calcareous ring diameter from 0.35 to 3.0 mm. Remarks. In the KKT, P. zenkevitchi zenkevitchi was sampled by RV Vityaz at stations 2217, 2218, 3176, 5612, 5628, 5631. In addition, the specimens from St. 5615 were identified as subspecies P. zenkevitchi exiguus (Belyaev, 1970). Distribution. KKT, 8175–9530 m. Izu-Bonin Trench, 8800–9735 m; Japan Trench, 7500 m; Peru-Chile (Atacama) Trench, 7720 m. The subspecies P. zenkevitchi exiguus was reported from the KKT, 8060–8135 m; P. zenkevitchi atlanticus Belyaev et Mironov, 1978 – from Romanche trench fault, 7430–7600 m; P. zenkevitchi rockallensis Gage, Billett, 1986 – from Rockall Trough, NE Atlantic, 1000–2921 m. Prototrochus kurilensis (Belyaev, 1970) Fig. 8F–N Myriotrochus kurilensis – Belyaev, 1970: 468–471, Figs. 4, 5, pl. II. 1, 2. Prototrochus kurilensis – Belyaev, Mironov, 1982: 88–89, Figs. 4, pl. I. 4, IV. 6; Panina, Stepanov, 2014: 90.

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Type locality. Kuril-Kamchatka Trench, RV Vityaz, cruise 14, St. 2120, 26.05.1953, Sigsbee trawl, 46°13'N, 154°11'E, 8330–8430 m. Material examined. RV Vityaz, cruise 39, St. 5616, 15 spms, cat. no. ECH00313 and ECH00316; St. 5632, 1 complete spm and 22 fragments with calcareous ring, cat. no. ECH00314 and ECH00315. RV Sonne, cruise SO250, KuramBio II, St. 8, 2 spms; St. 16, 1 spm; St. 17, 11spms; St. 19, 7 spms; St. 28, 3 spms; St. 55, 7 spms; St. 89, 11 spms; St. 90, 6 spms. Total: 86 spms; calcareous ring diameter from 1.7 to 6.0 mm. Remarks. In the KKT, P. kurilensis was sampled by RV Vityaz at stations 2120, 5615, 5616, 5632. P. kurilensis differs from P. zenkevitchi zenkevitchi in having a short anterior process on the calcareous ring plates (shorter than the basal height of plate) (compare Fig. 7A, B and 7G, H), larger body size, more numerous tentacle digits, less numerous wheels, and a larger wheel diameter (Fig. 7 D, E and 7 J–N). However, there are numerous transitional forms. Differences between these species appear to be related to growth, except for the less numerous wheels in P. kurilensis. The morphology of P. zenkevitchi exiguus is transitional between P. kurilensis and P. zenkevitchi zenkevitchi: calcareous ring diameter up to 3 mm in P. zenkevitchi zenkevitchi from the KKT, 3.7 mm in P. zenkevitchi exiguus and 6 mm in P. kurilensis; maximum number of tentacle digits 10, 12 and 14; mean wheel diameter 106 μm, 113 μm and 119 μm, respectively (Belyaev, 1970). The wheels of P. kurilensis display greater variability than in P. zenkevitchi zenkevitchi. Their spokes are often branched or exhibit short lateral processes (Fig. 7 L–N). More detailed morphological and genetic examinations are required to distinguish P. bruuni, P. kurilensis, P. zenkevitchi zenkevitchi, P. zenkevitchi exiguus, P. zenkevitchi atlanticus and P. zenkevitchi rockallensis (the Prototrochus bruuni complex). It is noteworthy that the specimens of P. bruuni from 6487– 9750-m depths have wheels, whereas the specimens from 9995–10730 m lack them (Belyaev, 1970; Belyaev, Mironov, 1982). Distribution. P. kurilensis was sampled by RV Vityaz in the KKT at depths 7795–8430 m. KuramBio II records have extended the known range to 6184–9540 m. Psilotrochus spiculifer (Belyaev et Mironov, 1981) Myriotrochus (?) sp. – Belyaev, 1970: 480–481, Fig. 12. Siniotrochus spiculifer – Belyaev, Mironov, 1981: 172–173, Fig. 6, Pl. I.6–10; 1982: 109; Panina, Stepanov, 2014: 91. Psilotrochus spiculifer – Mironov, Minin, Kremenetskaia, this issue. Type locality. East of Japan Trench, RV Vityaz, cruise 59, St. 7501, 22.06.1976, Galathea trawl, 37º32'5N, 143º22'3 E, 4800–4650 m. Material examined. RV Vityaz, St. 2120 (1 fragment). Remarks. P. spiculifer is described and illustrated by Mironov et al. (this issue). Distribution. KKT,8330–8430 m. East of Kuril-Kamchatka, Japan and Izu-Bonin trenches, Gulf of Alaska; 4650–5690 m. Sonnetrochus diaphorus Mironov, Minin et Kremenetskaia, this issue Sonnetrochus diaphorus – Mironov, Minin, Kremenetskaia, this issue.

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Type locality. Kuril-Kamchatka Trench, RV Sonne, cruise SO250, KuramBio II, St. 90, Agassiz trawl, 17.09 2016, 44°40,950' N, 151°27,347' E – 44°41,992' N, 151°26,321' E, 8254.6– 8272.9 m. Material examined. Sonne 250, KuramBio II, St. 17, one complete spm and fragments of 11 spm; St. 89, one complete spm and fragments of 17 spms; St. 90, five complete spms and fragments of nine spms; St. 97, one complete spm. Total: eight complete spms and fragments of 37 spms. Diagnosis. Myriotrochid with ten tentacles. Calcareous ring symmetrical, with dorsal and ventral plates subequal in size; each plate with a single anterior process. Wheels in adult specimens with inward-pointing teeth of two types: long scale-like primary teeth aligned with spokes, and short knob-like secondary teeth, located between spokes. Single secondary tooth between neighbouring spokes. Wheel hub simple, lacking perforations. Rods absent. Remarks. S. diaphorus is described and illustrated in Mironov et al. (this issue). The holotype is stored in the Senkenberg Research Institute and Natural History Museum, Frankfurt, RV Sonne, cruise SO250, KuramBio II expedition, St. 90, Agassiz trawl, 17.09 2016, diameter of calcareous ring 1.8 mm, length of body 8.2 mm. Distribution. KKT, 6440–8273 m. KKT area, 5106–5150 m. 4. Discussion There are 125 species of echinoderms known from the hadal zone (> 6000 m) of the World Ocean. They are attributed to 56 genera and 32 families. The hadal echinoderm fauna of the KKT includes 44 species belonging to 31 genera (55%) and 19 families (59%). Of these, 29 species are determined to the species level, 15 to the generic level and 14 are recorded from the KKT for the first time (Tables 2 and 3). Echinoids of the genus Echinosigra and holothurians of the genera Gephyrothuria, Mesothuria, Molpadia, Molpadiodemas and Sonnetrochus were found in the KKT for the first time. Earlier reports of three species from the KKT are not confirmed (Table 2). Of 31 genera found in the KKT, 25 (81%) are cosmopolitan, i.e., reported from three or more oceans. Amembranaster is endemic to the KKT, Abyssura is endemic to the Aleutian-Japan biogeographical hadal province, Hadalothuria and Sonnetrochus occur only in the Pacific Ocean, and Psilotrochus is known from the North Pacific and Atlantic (unpublished data). The KKT echinoderm fauna mainly comprises species (86%) from the cosmopolitan genera. Of 29 identified species, 11 (38%) are endemic to the hadal zone, including 6 (21%) endemics to the KKT. Among 19 echinoderm families in the KKT, Elpidiidae, Myriotrochidae and Pterasteridae are represented by three or more genera (Table 3). Species of holothurians make more than a half of all known echinoderm species in the KKT. The number of echinoderm species at depths from 6000–7000 m is 29, decreasing sharply to 16 at depths from 7001–8000 m, stabilizing at 17 from 8001–9000 m and further declining to 8 at depths from 9001–10000 m. The proportion of holothurians in the KKT fauna increases from 48% at depths from 6000–7000 m to 87% from 9001–10000 m. The number of species in the KKT echinoderm fauna is much greater than in any other trench. The second highest echinoderm species richness (19 species) is known from the South Sandwich Trench. Both trenches are located in areas of high primary productivity and characterized by a high abundance of benthic invertebrates in the trawl catches. There are several possible explanations for this relatively high species richness: intensive sampling effort, large area of a trench and high primary productivity in surface waters. Indeed, 23

the KKT is the most sampled trench with 37 trawl and 12 epibenthic sledge stations already taken. In the South Sandwich Trench, only five trawl stations were taken. In terms of area, the KKT is the second largest after the Izu-Bonin Trench; it is also one of the five deepest, together with the Mariana, Tonga, Philippine and Kermadec Trenches (Jamieson, 2015). Examination over the last two years of the KuramBio and KuramBio II materials as well as the collections of RV Vityaz and RV Akademik M. A. Lavrentyev from the North-West Pacific have extended the fauna list of the open-oceanic North-West Pacific. Abyssal species from this area that are new to the North-West Pacific or new to science are found in the genera Bathycrinus, Gaussaster, Mesothuria, Molpadiodemas, Myriotrochus, Pteraster, Peniagone, Prototrochus, Psychropotus, Zygothuria and some others. The species richness of abyssal echinoderm fauna in the North-West Pacific areas negatively correlates with the level of their geomorphological isolation (Mironov et al., 2018). The higher the level of isolation of the abyssal area, the lower is the species richness of its deep-sea fauna. The number of abyssal echinoderm species increases in the following order: nine in the most isolated Sea of Japan, 34 in the semi-enclosed Sea of Okhotsk, 49 in the least isolated Bering Sea, and >127 in the openoceanic abyssal area (Table 4, Fig. 9). In the hadal areas of the North-West Pacific (KKT, Japan, Aleutian trenches and North-West Pacific Basin), 58 echinoderm species are recorded, which is more than two-fold less than the species number in the open oceanic abyssal area. The openoceanic abyssal area of the North-West Pacific with the highest number of species serves as a species donor for other abyssal and hadal regions of the North-West Pacific, except of the abyssal region of the Sea of Japan. The large number of pseudo-abyssal species in the latter suggests that the main species donors for the abyssal region of the Sea of Japan are the shelf and the slope (Mironov et al., in press). The number of trawls obtained in the neighbouring Aleutian, Japan and Izu-Bonin Trenches is significantly lower than in the KKT: 8, 13 and 10, respectively. The information on vertical and geographical species ranges is insufficient to confirm the Aleutian-Japan hadal province. Particularly controversial is the inclusion of the Izu-Bonin Trench in the province. The distribution pattern of some genera suggests that different congeneric species in the KKT and the Izu-Bonin Trench are the result of population isolation in one trench owing to the topographic barrier (Belyaev, 1989; Mironov, 2019).

Conflict of interest No conflict of interest.

Acknowledgements We thank Angelika Brandt and Marina Malyutina for the opportunity to examine the KuramBio II collection of echinoderms. We are grateful to Anastassya Maiorova, Anna Lavrentyeva and Ulrike Minzlaff-Weber for the photographs of freshly caught specimens. We are also grateful to Angelina Eichsteller and the anonymous reviewers whose constructive comments and suggestions helped to improve this paper. This work was supported by the State assignment of Minobrnauki of Russia, theme № 0149-2019-0009, also this work was partly funded by the German Ministry for Science and Education, KuramBio II BMBF grant 24

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Mironov, A.N., 1971. Soft sea urchins of the family Echinothuriidae collected by the R/V “Vityaz” and the “Academician Kurchatov” in the Pacific and Indian Oceans. Trudy Instituta Okeanologii AN SSSR 92, 317–325. (In Russian) Mironov, A.N., 1974. Pourtalesiid sea urchins of the Antarctic and Subantarctic Echinoidea: Pourtalesiidae). Trudy Instituta Okeanologii AN SSSR 98, 240–252. (In Russian) Mironov, A.N., 1997. Holasteroid echinoids. 4. Echinosigra. Zoologicheskij Zhurnal 76 (2), 173–186 (In Russian) Mironov, A.N., 2000. New taxa of stalked crinoids from the suborder Bourgueticrinina (Echinodermata, Crinoidea). Zoologicheskij Zhurnal 79 (6), 712–728. (In Russian) Mironov, A.N., 2019. See lilies of the genus Bathycrinus (Echinodermata, Crinoidea: Bathycrinidae) from the North-West Pacific hadal trenches. Zootaxa, 4604 (3), 401–427. https://www.mapress.com/j/zt /. Mironov, A.N., Minin, K.V., Dilman, A.B., 2015. Abyssal echinoid and asteroid fauna of the North Pacific. Deep-Sea Research II 111, 357–375. https://doi.org/10.1016/j.dsr2.2014.08.006 Mironov, A.N., Minin, K.V., Dilman, A.B., Smirnov, I.S., 2018. Deep-sea echinoderms of the Sea of Okhotsk. Deep-Sea Research Part II 154, 342–357. https://doi.org/10.1016/j.dsr2.2017.10.003 Mironov, A.N., Dilman, Minin, K.V., Malyutina M.V., 2019. Submergence of the lower limits of species vertical distribution in the Sea of Japan Oceanology 59 (6), in press. Mironov, A.N., Minin K.V., Kremenetskaia, A.V.,???. Two new genera of the family Myriotrochidae (Echinodermata, Holothuroidea). This issue. Mooi, R., Constable, H., Lockhart, S., Pearse, J., 2004. Echinothurioid phylogeny and the phylogenetic significance of Kamptosoma (Echinoidea: Echinodermata). Deep Sea Research II 51 (14-16), 1903–1919. https://doi:10.1016/j.dsr2.2004.07.020. Mortensen, T., 1934. New Echinoidea. Preliminary Notice. Videnskabelige Meddelelser fra Dansk naturhistorisk Forening i København 98, 161–167. Mortensen, T., 1935. A Monograph of the Echinoidea.Vol. 2. Bothriocidaroida, Melonechinoida, Lepidocentroida and Stirodonta. Reitzel, Copenhagen, 647 pp. Müller, J., 1850. Anatomische Studien über die Echinodermen. Archiv für Anatomie, Physiologie und wissenschaftliche Medicin 1850, 129–155. Müller, J., Troschel, F.H., 1840.Über die Gattungen der Ophiuren. Archiv für Naturgeschichte 6 (1), 326–330. O’Hara, T.D., Hugall, A.F., Thuy, B., Stöhr, S., Martynov, A.V., 2017. Restructuring higher taxonomy using broad-scale phylogenomics: The living Ophiuroidea. Molecular Phylogenetics and Evolution 107, 415–430. https://doi.org/10.1016/j.ympev.2016.12.006 Panina, Е.G., Stepanov, V.G., 2014. List of species of the sea cucumbers in the Far-Eastern seas of Russia: the order Synaptida (=Apodida) Cuénot, 1891 (Holothuroidea: Synaptida). Kamchatka State Technical Univ., Bulletin 30, 88–99. (In Russian) Paterson, G.L.J., 1985. The deep-sea Ophiuroidea of the north Atlantic Ocean. Bulletin of the British Museum (Natural History) Zoology 49 (1), 1–162. Pawson, D. L., 1977. Marine flora and fauna of the northeastern United States. Echinodermata: Holothuroidea. NOAA technical report NMFS circular 405, 1–15. Perrier, E., 1875. Revision de la Stellerides du Museum d'Historie Naturelle de Paris. Archives du Zoologie Experimentale et Generale 4, 265–450. Perrier, E., 1884. Me´moire sur les e´toiles de mer recueillis dans la Mer des Antilles et la Golfe de Mexique durant les Expe´ditions de dragage faite sous la Direction de M. Alexander 29

Agassiz. Nouvelles Archives et Me´moires, Muse´um d’Histoire Naturelle de Paris, ser.2 6, 127-276. Perrier, E., 1893. In: Masson, G., (Ed.), Traité de Zoologie. Fasc 3. Arthropodes. Libraire de l’Académie de Medécine, Paris, pp. 781–864. Petrunina, A., Marinez Arbizu P., Tanaka, H., Yoo H., 2016. Parasitic Crustaceans of the KurileKamchatka trench. In: Brand A. and shipboard scientific party. RV Sonne SO-250. Cruise Report / Fahrtbericht Tomakomai - Yokohama (Japan), University of Hamburg, pp. 65–69. Rogacheva, A., 2012. Taxonomy and distribution of the genus Kolga (Elpidiidae: Holothuroidea: Echinodermata). Journal of the Marine Biological Association of the United Kingdom 92 (5), 1183–1193. https://doi.org/10.1017/S0025315411000427 Sladen, W.P., 1883. The Asteroidea of H. M. S. Challenger Expedition. (Preliminary notices.) 2. Astropectinidae. Journal of the Linnean Society of London, Zoology 17, 214–269. Smirnov, A.V., 2012. System of the Class Holothuroidea. Paleontological Journal 46 (8), 793– 832. https://doi.org/10.1134/s0031030112080126 Smirnov, I.S., Piepenburg, D., Ahearn, C., Juterzenka, K.V., 2014. Deep-sea fauna of European seas: An annotated species check-list of benthic invertebrates living deeper than 2000 m in the seas bordering Europe. Ophiuroidea. Invertebrate Zoology 11(1), 192–209. Stepanov, V.G., 2015. List of species of the sea cucumbers (Holothuroidea) in the Far-Eastern seas of Russia, V. The order Elasipodida Théel, 1882 (Echinodermata: Holothuroidea). Bulletin оf Kamchatka State Technical University 33, 54–66. (In Russian). Stöhr, S., O’Hara, T., Thuy, B. (Eds), 2019. World Ophiuroidea Database. Suyehiro, Y., Okada, Y., Horikoshi, M., Iwai, E., 1962. A brief note on the benthic animals on the Fourth Cruise of Japanese Expedition of Deep Seas (JEDS-4). Oceanographical Magazine 13 (2), 149–153. Théel, H, 1877. Note sur quelques Holothuries des mers de la Nouvelle Zemble. Nova Acta Regiae Societatis Scientiarum, Upsaliensis 3 (17), 1–18. Théel, H., 1882. Report on Holothurioidea. Pt. I. Report of the Scientific Results of the Voyage of HMS Challenger. Zoology 4 (13), 1–176. UNESCO, 2009. Global Open Oceans and Deep Seabed (GOODS) – Biogeographic Classification. IOC Technical Series 84, UNESCO-IOC, Paris. Ushakov, P.V., 1952. Study of deep-sea fauna. Priroda 6, 100–102. (In Russian) Vinogradova, N.G., Kudinova-Pasternak, R.K., Moskalev, L.I., Muromtseva, T.L., Fedikov, N. F., 1974. Some regularities of quantitative distribution of bottom fauna of the Scotia Sea and the deep-sea trenches of the Atlantic sector of the Antarctic. Trudy Instituta Okeanologii AN SSSR 98, 157–182. (In Russian)

30

Figure captions Figure 1. Sea lily Bathycrinus volubilis, RV Vityaz, St. 5612, holotype: (A) aboral cup with arms and proxistele; (B) mesistele. Specimen photographed after ethanol fixation. Figure 2. Sea stars of the family Pterasteridae. (A–C) Amembranaster dimidatius, RV Vityaz, St. 5625, holotype: (A) dorsal view; (B) oral view; (C) armature of adambulacral and oral plates, segmental apertures and additional small spine next to each actinolateral spine viewed from oral side. (D–F) Hymenaster aff. blegvadi, RV Vityaz, St. 5616: (D) dorsal view; (E) oral view; (F) opercular spine; (G) H. blegvadi, Galathea expedition, St. 658: opercular spine. Specimens photographed after ethanol fixation. Figure 3. Sea stars of the families Pterasteridae, viewed from dorsal and oral side: (A–B) Hymenaster sp. B, RV Vityaz, St. 5612, the largest specimen; (C–D) Hymenaster sp. A, RV Vityaz, St. 5616; (E–F) Pteraster texius, RV Vityaz, St. 5633 paratype. Specimens photographed after ethanol fixation. Figure 4. Ophiuroids of the families Amphiuridae (A), Ophiopyrgidae (В, D) and Ophiurida incertae sedis (C): (A) Amphioplus (Unioplus) cf. daleus, RV Vityaz, St. 3457; (B) Amphiophiura bullata pacifica, KuramBio II, RV Sonne, cruise SO250, St. 97; (C) Abyssura brevibrachia, RV Vityaz, St. 2208, holotype; (D) Ophioplinthus madseni, RV Vityaz, St. 5617. Specimens photographed before (B) and after (A, C–D) ethanol fixation. Image courtesy: (B) Anastassya Maiorova, National Scientific Center of Marine Biology, Far Eastern Branch, Russian Academy of Sciences. Figure 5. Holothurians of the families Elpidiidae (A–E), Myriotrochidae (F), Molpadiidae (G), Gephyrothuriidae (H–I) and Molpadiodemidae (J): (A) Scotoplanes hanseni, KuramBio, RV Sonne, cruise SO223, St. 9-9; (B) Elpidia birsteini, KuramBio II, RV Sonne, cruise SO250, St. 41; (C) Elpidia hanseni, KuramBio II, RV Sonne, cruise SO250, St. 100; (D) Elpidia longicirrata, KuramBio II, RV Sonne, cruise SO250, St. 90; (E) Elpidia kurilensis, KuramBio II, RV Sonne, cruise SO250, St. 97; (F) Prototochus zenkevitchi, KuramBio II, RV Sonne, cruise SO250, St. 102; (G) Molpadia cf. granulata; (H) Gephyrothuria sp., KuramBio II, RV Sonne, cruise SO250, St. 31; (I) Hadalothuria sp., KuramBio II, RV Sonne, Cruise SO250, St. 31; (J) Molpadiodemas sp. C, KuramBio II, RV Sonne, cruise SO250, St. 54. Specimens photographed before (A–C, F, I–J) and after (D–E, G, H) ethanol fixation. Image courtesy: Anastassya Maiorova (A, C) and Anna Lavrentyeva (B, I–J), National Scientific Center of Marine Biology, Far Eastern Branch, Russian Academy of Sciences; (F) Ulrike Minzlaff-Weber. Figure 6. Aggregation of Kolga kamchatica at the abyssal depth of the Bering Sea. Image courtesy: National Scientific Center of Marine Biology, Far Eastern Branch, Russian Academy of Sciences. Figure 7. Wheels of Myriotrochus longissimus, RV Vityaz, St. 3457, type locality (A–G), KuramBio II, RV Sonne, cruise SO250, St. 43 (H–L), St. 86 (M, N). (A, B) wheels from posterior end of body fragment without ring, length of fragment 50 mm; (C) single wheel in the anterior end of the same fragment; (D–F) wheels from posterior end of body fragment without ring, length of fragment 30 mm; (G) wheel in the anterior end of the 31

same fragment; (H–L) wheels from posterior end of body fragment with calcareous ring, length of fragment 40 mm, diameter of ring 7.5 mm; (M, N) wheels from posterior end of body fragment without ring, length of fragment 23 mm. Scale bar length is equal for all wheels. Figure 8. Calcareous ring plates and wheels of Prototrochus zenkevitchi zenkevitchi from KuramBio II, RV Sonne, cruise SO250, St. 77 (A–E) and P. kurilensis, from KuramBio II, RV Sonne, cruise SO250, St. 55 (F–N). Ring diameter 3.0 and 2.6 mm respectively. (A, G) inner views of radial plates; (B, H) inner views of interradial plates; (C, F) anterior views of radial plates; (I) lateral view of plate; (D, J) largest wheels of usual form; (E, K) smallest wheels of usual form; (L–N) wheels with anomalies. Figure 9. Number of echinoderm species in deep-sea areas of the North-West Pacific (north of 30ºN and west of 180ºE). For the Bering Sea and the Aleutian Trench, species occurring also east of 180ºE were considered. Diameter of circles corresponds to the species number. White circles show the number of species in abyssal regions (2000–6000 m), dark circlets – the number of species in hadal areas (> 6000 m).

32

(49)

(34) (9)

20

00

(127)

Table 1. List of stations in the Kuril-Kamchatka Trench, where the echinoderms were collected. at depths greater than 6000 m. AGT – Agassiz Trawl, DO – Grab Ocean 0.25 m2, EBS – Epi-Benthic Sledge, GKG – Giant Box Corer, KUD – Kudinov Trawl, MUC – MultiCorer, TS – Sigsbee Trawl

Station

Gear

Date

Latitude, N

Longitude, E

162 2120

TS TS

2144 2208

TS TS

01.06.1953 22.06.1953

48° 25' 49° 29'

156° 34' 158° 41'

2216

TS

26.06.1953

45° 41'

153° 23'

2217

TS

29.06.1953

44° 7'

150° 32'

2218

TS

01.07.1953

43° 47'

149° 54'

3176

TS

06.10.1954

44° 08.5'

150° 22.0'

3457

TS

21.09.1955

41° 17.3'

145° 50.2'

5607 5607

DO-0.25 TS

18.07.1966 18.07.1966

46° 12' 46° 11'

153° 13' 153° 17'

5608

TS

22.07.1966

46° 0'

153° 27'

5609

TS

23.07.1966

46° 6'

153° 18'

5611

TS

25.07.1966

45° 48'

153° 21'

5612

TS

27.07.1966

45° 25'

153° 07'

5613

TS

02.08.1966

45° 25'

152° 45'

5615

TS

03.08.1966

45° 56'

153° 28'

5616

TS

04.08.1966

45° 40'

153° 33'

5617

TS

06.08.1966

45° 32'

153° 46'

5625

TG

21.08.1966

45° 28'

153° 46'

RV Vityaz, cruises 2, 14, 19, 22, 39 10.10.1948 44° 55.5' 152° 24.0' 26.05.1953 46° 13' 154° 11'

33

Depth, m 8100 8330– 8430 6860 7210– 7230 8610– 8660 9050– 9000 9700– 9950 8840– 8175 6475– 6571 6225 6080– 6185 7265– 7295 6090– 6235 7600– 7710 8185– 8400 9030– 9530 8060– 8135 7795– 8015 6710– 6675 6215– 6205

5627

TG

27.08.1966

44° 15'

150° 46'

5628

TG

30.08.1966

43° 54'

149° 57'

5629

TG

31.08.1966

43° 54'

149° 43'

5630

KUD

01.09.1966

43° 59'

149° 39'

5631

TG

04.09.1966

43° 47'

149° 43'

5632

TG

05.09.1966

43° 44'

149° 52'

5633

TG

06.09.1966

44° 7'

149° 34'

16 17 18 19 20 28 29 30 31 37 40 41 42 43 52 54

RV Sonne, Cruise SO250, KuramBio II expedition MUC 21.08.2016 45° 50.877' 153° 47.996' EBS 22.08.2016 45° 52.036' – 153° 51.390'– 45° 51.401' 153° 50.406' AGT 22.08.2016 45° 50.861' – 153° 49.568' – 45° 51.954' 153° 51.259' EBS 23.08.2016 45° 52.023' – 153° 51.156'– 45° 51.412' 153° 50.215' AGT 23.08.2016 45° 51.327' – 153° 50.083' – 45° 52.203' 153° 51.435' EBS 26.08.2016 45° 54.438' – 152° 47.025'– 45° 54.520' 152° 47.204' AGT 26.08.2016 45° 56.731'– 152° 52.545' – 45° 56.570' 152° 54.499' EBS 26.08.2016 45° 56.468'– 152° 55.593'– 45° 56.834' 152° 50.943' AGT 27.08.2016 45° 56.688' – 152° 52.785' – 45° 56.544' 152° 54.667' GKG 28.08.2016 45° 38.604' 152° 55.911' EBS 29.08.2016 45° 39.976'– 152° 55.953'– 45° 40.839' 152° 57.687' AGT 29.08.2016 45° 39.232'– 152° 56.687' – 45° 40.114' 152° 58.366' EBS 30.08.2016 45° 39.620'– 152° 56.391'– 45° 40.263' 152° 57.638' AGT 30.08.2016 45° 38.514'– 152° 56.775' – 45° 39.358' 152° 58.377' EBS 05.09.2016 45° 29.779'– 153° 12.160'– 45° 29.187' 153° 11.138' AGT 06.09.2016 45° 28.502'– 153° 11.539' – 45° 28.125' 153° 10.109' 34

9170– 9335 9520– 9530 8035– 8120 6710– 6435 9070– 9345 8240– 8345 6090– 6135 8255.2 8185.7– 8183.7 8200.3– 8185.2 8192.7– 8187 8191.4– 8199.3 6050.2– 6047.1 6183.1– 6202.2 6228.3– 6163.7 6184.5– 6220.6 7135.5 7300.3– 7055.2 7154.4– 7163.9 7110.6– 7119.6 7241.1– 7245.4 8704.1– 8698.7 8728.9– 8734.8

55

EBS

56

AGT

06.09.2016– 07.09.2016 07.09.2016

67 77

GKG EBS

10.09.2016 13.09.2016

78

AGT

13.09.2016

89

EBS

90

AGT

16.09.2016– 17.09.2016 17.09.2016

97

EBS

98

AGT

18.09.2016– 19.09.2016 19.09.2016

100 102

GKG EBS

20.09.2016 20.09.2016

103

AGT

21.09.2016

45° 29.240'– 45° 29.580' 45° 29.630'– 45° 30.086' 45° 12.944' 45° 13.719'– 45° 14.219' 45° 13.979'– 45° 14.482' 44° 40.124'– 44° 39.053' 44° 40.950'– 44° 41.992' 44° 5.680'– 44° 6.942' 44° 5.538' – 44° 6.253' 44° 12.378' 44° 11.996'– 44° 12.003' 44° 12.499'– 44° 12.502'

35

153° 13.460'– 153° 12.240' 153° 12.028' – 153° 10.369' 152° 42.844' 152° 51.219'– 152° 49.956' 152° 48.980' – 152° 47.736' 151° 27.350'– 151° 27.343' 151° 27.347' – 151° 26.321' 151° 24.880'– 151° 24.888' 151° 24.258' – 151° 25.935' 150° 39.053' 150° 34.077'– 150° 32.745' 150° 39.035'– 150° 37.258'

8743.6– 8735.4 8725.9– 8403.8 9494.6 9427.8– 9582.8 9581.7– 9581.3 8227.4– 8216.5 8254.6– 8272.9 6440.4– 6560.7 6445.6– 6442.1 9304.9 9547.2– 9473.9 9292.9– 9430.8

Table 2. List of echinoderm species reported from the Kuril-Kamchatka Trench at depths greater than 6000 m. Belyaev, 1989 (review until 1989); Gebruk, 1990; Golotsvan, 1998; Litvinova, 2010; Mironov, 2000; Rogacheva, 2012 Crinoidea Bathycrinus volubilis Mironov, 2000 Bathycrinus sp. C Belyaev, 1966 – Asteroida Amembranaster dimidatius Golotsvan, 1998 Eremicaster vicinus (Ludwig, 1907) Freyella kurilokamchatica Korovchinsky, 1976 – Hymenaster spp. – – Pteraster ifurus Golotsvan, 1998 Pteraster texius Golotsvan, 1998 Ophiuroidea Abyssura brevibrachia Belyaev et Litvinova, 1976 Amphiophiura bullata convexa Amphioplus (Unioplus) cernuus (Lyman, 1879) – Ophioplinthus madseni (Belyaev et Litvinova, 1972) Ophiacantha cosmica Lyman, 1878 Ophiura bathybia H.L. Clark, 1911 Perlophiura profundissima Belyaev et Litvinova, 1972 Echinoidea – Kamptosoma abyssale Mironov, 1971 Holothuroida Elpidia birsteini Belyaev, 1971 Elpidia hanseni Belyaev, 1971 Elpidia kurilensis Baranova et Belyaev in Belyaev, 1971 Elpidia longicirrata Belyaev, 1971 – Hadalothuria sp.

Present study; Mironov, 2019; Gebruk et al , this issue; Mironov et al., this issue Bathycrinus volubilis Mironov, 2000 – Bathycrinus longipinnus Mironov, 2019 Amembranaster dimidatius Golotsvan, 1998 Eremicaster vicinus (Ludwig, 1907) Freyella kurilokamchatica Korovchinsky, 1976 Hydrasterias sp. Hymenaster aff. blegvadi Madsen,1956 Hymenaster sp. A Hymenaster sp. B Pteraster ifurus Golotsvan, 1998 Pteraster texius Golotsvan, 1998 Abyssura brevibrachia Belyaev et Litvinova, 1976 Amphiophiura bullata pacifica Litvinova, 1971 Amphioplus (Unioplus) cernuus (Lyman, 1879) Amphioplus (Unioplus) cf. daleus (Lyman, 1879) Ophioplinthus madseni (Belyaev et Litvinova, 1972) Ophiacantha bathybia H.L. Clark 1911 Ophiura bathybia H.L. Clark, 1911 Perlophiura profundissima Belyaev et Litvinova, 1972 Echinosigra (Echinogutta) amphora Mironov, 1974 Kamptosoma abyssale Mironov, 1971 Elpidia birsteini Belyaev, 1971 Elpidia hanseni Belyaev, 1971 Elpidia kurilensis Baranova et Belyaev in Belyaev, 1971 Elpidia longicirrata Belyaev, 1971 Gephyrothuria sp. Hadalothuria sp. 36

Kolga kamchatica Rogacheva, 2012 – – – – – – Myriotrochus longissimus Belyaev, 1970 Peniagone incerta (Théel, 1882) Peniagone spp. Prototrochus kurilensis (Belyaev, 1970) Prototrochus zenkevitchi (Belyaev, 1970) Pseudostichopus sp. (St. 162) Psychropotes spp. – Psychropotidae gen. sp. Scotoplanes hanseni Gebruk, 1983 Siniotrochus spiculifer Belyaev et Mironov, 1981 –

Kolga kamchatica Rogacheva, 2012 Mesothuria sp. A Molpadia cf. granulata Molpadia sp. A Molpadiodemas sp. A Molpadiodemas sp. B Molpadiodemas sp. C Myriotrochus longissimus Belyaev, 1970 Peniagone cf. incerta (Théel, 1882) – Prototrochus kurilensis (Belyaev, 1970) Prototrochus zenkevitchi (Belyaev, 1970) – Psychropotes moskalevi Gebruk et Kremenetskaia, this issue Psychropotes pawsoni Gebruk et Kremenetskaia, this issue Psychropotidae gen. sp. Scotoplanes hanseni Gebruk, 1983 Psilotrochus spiculifer (Belyaev et Mironov, 1981) Sonnetrochus diaphorus Mironov, Minin, Kremenetskaia, this issue

37

Table 3. Number of hadal (> 6000 m) genera and species in the echinoderm families, with maximum known depths. Class, family Crinoidea Antedonidae Bathycrinidae Hyocrinidae Septocrinidae Asteroidea Astropectnidae Caymanostellidae Freyellidae ?Goniasteridae* Pedicellasteridae Pterasteridae Porcellansteridae Ophiuroidea Amphiuridae Ophiacanthidae Ophiodermatidae Ophiomyxidae

Ophiopyrgidae Ophiuridae Incertae sedis Echinoidea Calymnidae Kamptosomatidae Pourtalesiidae Urechinidae Holothuroidea Caudidae Elpidiidae Gephyrothuriidae Mesothuriidae Laetmogonidae Molpadiidae Molpadiodemidae Myriotrochidae Pelagothuriidae* Psychropotidae Total (32 families)

Number of taxa World Ocean Genera Species 4 9 1 1 1 6 1 1 1 1 15 >27 1 2 1 1 1 3 1 1 1 1 4 >8 6 11 13 17 1 2 2 2 1 1 1 3 2 3 3 4 3 3 6 7 1 1 1 1 3 4 1 1 23 64 1 1 7 30 3 4 1 2 1 1 1 2 1 6 5 13 1 1 3 5 56 125

Max. depth, m 9735 6865 9735 6290 6785 9990 6096 6780 6860 8042 6160 9990 8720 8135 7170 6440 6150 7880 7459 7340 8135 7335 6940 6235 7335 6740 10730 6650 10000 9530 8200 6730 6446 9582 10730 6779 9335 10730 38

Kuril-Kamchatka Trench Genera Species Max. depth, m 1 2 9345 1 2 9345 6 9 8400 1 1 6860 1 1 6160 3 6 8400 1 1 6860 7 8 8060 1 2 7170 1 1 6235 2 2 7230 1 1 6135 2 2 8060 2 2 7119 1 1 6235 1 1 7119 15 23 9582 5 7 9582 2 2 9530 1 1 8200 1 2 6446 1 3 9582 4 5 9530 1 3 9335 31 44 9582 (56%) (35%)

*Known from underwater photographs (Lemche et al., 1976) Table 4. Numbers of echinoderm species in the deep-sea areas of the North-West Pacific (north of 30ºN and west of 180ºE). Areas

Depth, m

Bering Sea Sea of Okhotsk Sea of Japan Open-oceanic abyssal Aleutian Trench Kuril-Kamchatka Trench Japan Trench North-West Pacific Basin North-West Pacific

2000–4150 2000–3657 2000–3800 2000–6000 6001–7669 6001–10542 6001–8412 6001–7374 2000–10542

Number of species identified to species level to generic level 48 1 31 3 9 0 113 14 11 3 29 15 14 3 5 4 170 34

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Highlights

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44 echinoderm species occur in the Kuril-Kamchatka Trench at depths > 6000 m 14 species were recorded from the Kuril-Kamchatka Trench for the first time Species richness of echinoderms in KKT is the highest among all hadal trenches

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