The deep sea is the largest, but the least explored environment on Earth. However, less than 0.0001% of the deep sea (deeper than 200 m) has been explored so far, making it the least explored environment on Earth (Brandt and Malyutina 2015; Danovaro et al. 2017). Deep-sea macrobenthos are speculated to comprise about 25 million species; meiofauna seems to be composed of 20 to 30 million species (Adrianov 2004), in contradiction with what was estimated previously for deep-sea macrofauna (between 0.5 and 10 million species) (Brandt et al. 2007a; Brandt et al. 2007b; Grassle and Maciolek 1992). Appeltans et al. (2012) predicted that 100,000 species will be described in the next 50 years if the rate of 2,000 new species description per year maintains in the future (Appeltans et al. 2012). Recent expeditions in the framework of the Census of Marine Life have documented that deep-sea biodiversity is very unevenly distributed in different oceans and ocean basins (Ramirez-Llodra et al. 2010). However, some mechanisms have been identified that can cause high local species concentrations (e.g., productivity and isolation) (Brandt et al. 2007a; Gibson et al. 2002; Levin et al. 2001; Saeedi et al. 2019a; Smith and Stockley 2005).

The NW Pacific is one of the most productive, nutrient rich, and diverse regions of the World Ocean, and includes several deep-sea basins diverging in depths, hydrology, and isolation (Grebmeier et al. 2006; Leprieur et al. 2016; Renema et al. 2008; Saeedi et al. 2019b). The Sea of Japan, the warm temperate zone of the NW Pacific, and the Kuril-Kamchatka Trench (KKT) receive two major currents: the warm Tsushima and cold Oyashio currents (Fujikura et al. 2010; Su et al. 1990) (Map 1). The Tsushima Current splits from the Kuroshio Current and flows from off Kyushu into the Sea of Japan. The Oyashio Current flows southward through Japanese waters from off Hokkaido along the Pacific coast (Fujikura et al. 2010; Su et al. 1990). The tropical and subtropical parts of the western Pacific host the highest number of marine species around the world (Asaad et al. 2018; Crame 2000; Jablonski et al. 2006; Krug et al. 2008; Sanciangco et al. 2013). Some areas of the Sea of Japan, Sea of Okhotsk, and Kuril-Kamchatka Trench (KKT) have been specifically sampled and studied by deep-sea scientists (Brandt et al. 2020; Brandt and Malyutina 2015; Malyutina and Brandt 2013; Malyutina et al. 2018). In the next few paragraphs we review the geography and hydrology of these three ecosystems.

Map 1.

Study area, sampling locations, and currents in three areas from four expeditions. Colored circles show 418 sampling effort coordinate points (per sampling gear per station per cruise) during four deep-sea cruises including Sea of Japan Biodiversity Study (SoJaBio, 2010), Kuril-Kamchatka Biodiversity Study (KuramBio I and II, 2012–2016), and Sea of Okhotsk Biodiversity Study (SokhoBio, 2015). The area KuramBio is represented by the two expeditions KuramBio I in 2012 (open abyssal of NW Pacific) and Sea of Okhotsk; SoJ: Sea of Japan; KKT: Kuril-Kamchatka Trench; EKC: East Kuril Current; ESC: East Sakhalin Current; KC: Kuroshio Current; NPC: North Pacific Current; OC: Oyashio Current; OG: Okhotsk Gyre; SC: Soya Warm Current; WKC: West Kuril Current. ArcMap 10.5.1 was used to create this figure.

Several biological expeditions to the deep NW Pacific onboard the Russian RV Vityaz were performed between 1949 to 1966. The data found on faunal taxonomic description, hydrology, topography, chemical characteristics, and organic matter of the Kuril-Kamchatka Trench (KKT) and the adjacent abyssal plain area were reported in many publications (Belyaev 1989; Belyaev 1983; Ushakov 1953; Vinogradova 1977; Zenkevitch et al. 1955). Moreover, Sirenko et al. (2013) published a check list of species of free-living invertebrates of the Russian Far Eastern Seas. Isopoda (Crustacea) was one of the dominant taxa in species richness of the abyssal KKT area and one of the most well studied (Birstein 1970; 1971; Birstein 1963; Kussakin 1971; 1988; 1999; Kussakin and Vasina 1990; Kussakin 2004).

In the past decade, the biology of the bathyal, abyssal, and hadal faunas of all size classes (meio- macro-, and megabenthos) of the NW Pacific have been intensively investigated based on a Memorandum of Understanding (2007) between Russian and German partners. A total of four Russian-German and German-Russian expeditions (with the RV Akademik M.A. Lavrentyev and RV Sonne) have provided a wealth of data on the systematics, evolution, and biogeography of the deep-sea faunas of the Sea of Japan (SoJaBio 2010) (Malyutina and Brandt 2013), Sea of Okhotsk (SokhoBio 2015) (Malyutina et al. 2018), the Kuril-Kamchatka Trench (KKT), and the NW Pacific open abyssal plain adjacent to the KKT (KuramBio I and II, 2012–2016) (Brandt et al. 2019; Brandt et al. 2018; Brandt et al. 2020; Brandt and Malyutina 2015) (Map 1). All specimens were collected using standard gear (CTD, MUC (multicorer), GKG (giant box corer), EBS (epibenthic sledge), AGT (Agassiz Trawl), WT (bottom trawl) (Brandt and Malyutina 2014), BC (box corer) (Brandt et al. 2010), and PN (plankton net) (Chernyshev and Polyakova 2018)being deployed following a standardized method (Brandt and Malyutina 2014).

The goals of these expeditions were to study the biodiversity, biogeography, and evolution of the benthic organisms in different NW Pacific deep-sea environments. We aimed to compare more isolated deep-sea basins with more easily accessible ones (Sea of Japan vs. Sea of Okhotsk) and to test whether the hadal bottom of the trench of the KKT isolates the fauna from the Sea of Okhotsk to the fauna of the open NW Pacific area. The faunal composition of these areas comprising systematic, ecological, and biogeographical data, as well as evolution of protists, selected invertebrate taxa and fish, has been published in four scientific volumes, and includes the formal descriptions of many species, some genera, and one family (Brandt et al. 2020; Brandt and Malyutina 2015; Malyutina and Brandt 2013; Malyutina et al. 2018; Saeedi et al. 2020).

Based on these expeditions, the Beneficial project (Biogeography of the northwest Pacific fauna. A benchmark study for estimations of alien invasions into the Arctic Ocean in times of rapid climate chance) was designed. The main aims of the Beneficial project were 1- digitizing the biodiversity and environmental data collected during our expeditions, 2- discovering the deep-sea biogeography and biodiversity patterns in the NW Pacific, 3- predicting the potential future distribution range shifts of key species from the NW Pacific to the Arctic Ocean under rapid climate change, and 4- compiling a novel book on the taxonomy and biogeography of the highly abundant key species. All the data, publications, and the book arising from this project provide crucial benchmarks and datasets for any deep-sea biodiversity assessment, and help predict the future status of the Arctic marine ecosystem in a changing environment (Brandt et al. 2020; Canonico et al. 2019; Saeedi et al. 2019b; Saeedi et al. 2019c; Saeedi et al. 2019d; 2020; Saeedi et al. 2019e).

We mined and mobilized 7,042 unique deep-sea benthos taxa records, with 1,723 records at the species level (more than 50% at the speciesand genus level, the rest were at the higher taxa level, mostly family, order, and class) from our four deep-sea cruises including SoJaBio, SokhoBio, and KuramBio I and II (Saeedi et al. 2019e). Using this dataset and data collected from the open-access databases, we analysed species richness, endemicity, and community composition in the shallow and deep NW Pacific and its adjacent Arctic Ocean (Saeedi et al. 2020, Brandt et al. 2020; Canonico et al. 2019; Saeedi et al. 2019b; Saeedi et al. 2019c; Saeedi et al. 2019d; Saeedi et al. 2019e). Our findings supported the hypothesis that biodiversity, while highest in the tropics and coastal depths (mostly in the Philippines), decreases at the equator and decreases at depths below ca. 2,000 m (Saeedi et al. 2019b; Saeedi et al. 2019d). Despite high species richness in the eastern Philippines, the Yellow Sea and Gulf of Tonkin had the highest benthic species endemicity rates (ca. 70%), while the Aleutian Islands had the highest pelagic endemicity rate (ca. 45%) among all different ecoregions (Saeedi et al. 2019b; Saeedi et al. 2019d). Our generalized linear models (GAMs) showed that the combined effects of all environmental predictors produced the best model driving species richness in both shallow and deep sea in the NW Pacific (Saeedi et al. 2019b; Saeedi et al. 2019d; 2020). However, among all predictors, dissolved oxygen, bottom temperature, and salinity were the most important environmental drivers of the deep-sea species richness in the NW Pacific and the adjacent Arctic Ocean (Saeedi et al. 2019b; Saeedi et al. 2019d; 2020). These investigations should strengthen and inform marine protection plans as species richness and endemicity hotspots could be profoundly compelling in helping to pinpoint and prevent biodiversity loss.

This book is designed as a guide, synthesis, and review of the current knowledge of the benthic fauna in the NW Pacific. This book includes benthic species that are distributed in the bathyal and abyssal zones (below 2,000 m) of the NW Pacific (latitude ca. 30 to 60°N, longitude ca. 120 to 180°E) (Map 2). This book consists of 21 chapters, with an introduction followed by 20 chapters on taxonomy and biogeography of different deep-sea taxa including Porifera, Cnidaria, Brachiopoda, Entoprocta, Nemertea, Solenogastres, Bivalvia, Sipuncula, Polychaeta, Echiura, Nematoda, Kinorhyncha, Pygnogonida, Ascothoracida, Ostracoda, Decapoda, Amphipoda, Isopoda, Ascothoracida, Tanaidacea, Echinoidea and Asteroidea. The editors worked closely with the authors and ensured that the work being produced has suitable literary merit and is free from inconsistencies, including grammatical and spelling errors. However, all the chapters proofread by the authors and all authors must take public responsibility for the content of their chapter. A total number of 2,503 distribution records belonged to 503 deep-sea taxa were used in the 20 chapters of this book (Map 2). A list of species represented in this book is given in supplementary table 1 (https://doi.org/10.3897/ab.e51315.suppl-1). The highest numbers of distribution records used in this book were from latitudes 40–48°N belonging to the materials were collected during the four joint Russia-Germany expeditions (Map 3).

Map 2.

Distribution records of all 503 deep-sea taxa used in 20 chapters of this book (see supplementary table 1).

Map 3.

Total number of distribution records used in 20 chapters of this book calculated per ca. 50,000 km² hexagonal cells. The highest numbers of distribution records (100 to 362 records) used in this book were from latitudes 40–48°N belonging to the materials were collected during our four expeditions.

In this book, we aimed to include chapters on the major groups of benthic fauna that are especially species rich (or being very diverse). However, for some taxa including Bryozoa, Harpacticoida, Crinoidea, Ophiuroidea, Holothuroidea, and Tunicata we were not able to find expert taxonomists with the time and/or the necessary skills to complete chapters.

In times of rapid climate change and increasing anthropogenic impact, a compilation of life at the seafloor in the deep sea, where environmental parameters resemble those of the Arctic Ocean, is urgently needed. To date; however, there has been no compilation and synthesis of deep-sea biodiversity in the deep NW Pacific excluding Sirenko’s (2013) species list of free-living invertebrates of the Russian Far Eastern seas (Sirenko 2013). It is thus necessary to accurately assess deep-sea areas to establish biodiversity conservation plans and Marine Protected Areas (MPAs).

This book could not be published without the financial support of “Biogeography of the NW Pacific deep-sea fauna and their possible future invasions into the Arctic Ocean project“ (BENEFICIAL project)” funded by Federal Ministry for Education and Research (BMBF: Bundesministerium für Bildung und Forschung) in Germany (grant number 03F0780A). We also gratefully acknowledge the generous funding of this book publication by the Open Access Publication Fund of Goethe University. We would like to thank the captains and crews of RVs ‘Akademik M.A. Lavrentyev’ and ‘Sonne’, student helpers, and anyone else who contributed to sample management and sorting the specimens. Special thanks to Mark Costello for his advice and Marianna Simões for her help in this book project. A very special thanks to James Reimer for editing and English proofreading of this chapter and to Rachel Downey for English proofreading most of the chapters of this book.

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The main objectives of this study were to analyse our current knowledge of deep-sea sponge species, genera, and family diversity, endemism and richness, sponge abundance, and latitudinal and depth gradients in the NW Pacific Ocean, as well as to investigate deep-sea faunal biogeographical patterns within this region and with adjacent deep-sea regions utilising new and previously identified sponge collections.

The deep NW Pacific sponge fauna was first sampled over a century ago (Okada 1932); however, it was systematically investigated during the 1940s to the 1960s during several campaigns from the RV Vitiaz (Koltun 1955a, 1955b, 1958, 1959, 1962, 1966, 1967, 1972). Until the most recent Russian-German expeditions, only 33 species and one subspecies, were known from this region. This figure has now more than doubled to 70 species morphotypes, two subspecies, with a high-likelihood of increased species numbers from the Sea of Japan and the Kuril-Kamchatka Trench, which have yet to be fully identified (Table 2). The number of genera known to this entire region has increased by a third from these recent expeditions (10 additional genera). The KuramBio (Kuril–Kamchatka Biodiversity Studies) expedition to the Kuril-Kamchatka abyssal plain found an additional eight species and two new genera (Downey and Janussen 2015). During the SokhoBio (Sea of Okhotsk Biodiversity Studies) expedition, new specimens close to tripled the number of known species for this region and increased the number of genera by c. 50% (six new genera to region) (Downey et al. 2018). Only five sponge specimens were collected from stations deeper than 2,000 m during the 2010 SoJaBio (Sea of Japan Biodiversity Studies), and these are yet to be investigated, but are believed to represent one hexactinellid morphotype. No known sponges have been previously found below 2,000 m in the Sea of Japan (OBIS 2019), and so these would be the first sponges from this region. Thirteen species were previously known from the Kuril-Kamchatka Trench, and with an additional 87 sponge specimens from KuramBio II in 2016, it is likely that many new species will be described from this region. This expedition also collected specimens of both demosponges and hexactinellids at deeper depths than ever before, with one hexactinellid found at a depth of 9,301 m, and one demosponge at 9,013 m, several hundred metres deeper than the previous global records of depth for sponges (Koltun 1959). Early Russian expeditions also explored the Bering Sea, and so far 15 spp. are known from this and later expeditions (Koltun 1967; Reiswig and Stone 2013; OBIS 2019); and in the Emperor Seamounts, five spp. are known as well from earlier Russian expeditions (Koltun 1959, 1962, 1972). A new expedition in 2019 to the Emperor Seamounts (Nintoku Seamount) has recorded many species from depths of 2,100-2,400 m that are not known from previous expeditions in the NW Pacific Ocean, with several morphotypes noted from the families of Farreidae, Euplectellidae (particularly the genus Walteria Schulze, 1886), Rossellidae and some encrusting demosponges (pers. com L. Watling, August 2019) (Figure 2). These initial results indicate that this seamount ridge has families, genera, and species that are unusual compared to the rest of the NW Pacific deep-sea region.

Figure 2.

Examples of two hexactinellids sponges from the deep Emperor Seamounts (2019), Nintoku Seamount. Schmidt Ocean Institute. Les Watling.

Currently, over 20% (14 spp.) of species collected by these new Russian-German expeditions are new to science, which is similar to other deep–sea expeditions, such as the ANDEEP I–III (Janussen and Tendal 2007), and the abyssal Clarion Clipperton Zone in the mid–Pacific (Kersken et al., 2017), which both found 20–33% of species were new to science. However, with many specimens not fully identified, this is likely to increase substantially, indicating that the deep NW Pacific Ocean is likely richer in species but also limited in terms of sampling.

More than half of sponge species found on the Kuril-Kamchatka abyssal plain, and nearly two-thirds of species from the Sea of Okhotsk, appear to be geographically limited, only being found at one station (Downey and Janussen 2015; Downey et al. 2018). Close to 60% of species have apparent restricted geographical ranges and are represented by few specimens (Table 2). This is a known characteristic of deep-sea fauna, with many species either found in few samples, or found as singletons, indicating a low population density (Carney 1997; Ebbe et al. 2010; Hardy et al. 2015; Costello and Chaudhary 2017; Danovaro et al. 2017). Sponge growth and reproductive regimes in the deep sea are not well known (Hogg et al. 2010); however, it is likely that most sponges reproduce both sexually by releasing larvae and asexually by budding and fragmentation episodically. As reproduction is likely seasonal and recruitment episodic, these small, patchily rich areas of sponge singletons, could be due to rare reproductive events.

As well as a low population density, localised habitat features could be driving habitat-specific rarity (Ramirez-Llodra et al. 2010). The deep seafloor is formed of hundreds of millions of kilometres of continental slope and abyssal plain; however, small geological features within these are varied, such as seamounts, trenches, and ridges, which are common in the NW Pacific Ocean, and could be driving localised habitat specific rarity, potentially seen in a number of sponge species, such as K. flabelliformis, H. (C.) ternerum vitiazi Koltun, 1967, and S. cantharellus. Sponges tend to be more abundant and diverse on harder substrates, particularly seamounts, canyon heads and walls, ridges, and banks (Ramirez-Llodra et al. 2010; van Soest et al. 2012). The diversity of hard habitats in the NW Pacific could explain some of the localised higher diversity and/or abundance observed closer to the Kuril Islands ridge, trench and on the Emperor Seamounts (Table 2). Theseregions also tend to be areas of localised upwelling, generating greater food availability for sponges in these regions (Ramirez-Llodra et al. 2010), which is potentially another factor determining the greater numbers of sponges found at some stations in the Sea of Okhotsk and around the Kuril-Kamchatka ridge and trench. These unique abiotic attributes and biotic processes have likely helped to create varied levels of patchy sponge presence and diversity across the NW Pacific region.

Demosponges dominate the deep NW Pacific region, accounting for two-thirds (46 spp.) of all species currently found (Table 2). Many of these newly added species were found in both the Sea of Okhotsk, as well as several from the Kuril-Kamchatka abyssal plain (Downey and Janussen 2015; Downey et al. 2018). Cladorhizid sponges dominate the faunal composition in the NW Pacific region, accounting for more than three quarters (35 spp.) of known demosponge species, and accounting for more than a third (6) of all demosponge genera. Cladorhizid sponges are typical deep-sea sponge fauna, having likely evolved carnivory in the food–poor depths of the ocean (Vacelet and Boury–Esnault 1995). More than 10% (20 spp.) of globally described cladorhizids (173 spp., WORMS 2019) are found from this region, and with the addition of new species in the future (currently 13 new spp. and 2 spp. aff.), this will increase. Currently, two-thirds of cladorhizid species (23 spp.) are endemic to the deep NW Pacific Ocean. This region contains 100% of Koltunicladia species (1 sp.), 43% of all Asbestopluma (A.) species (13 spp.), 22% of all Chondrocladia (C.) species (3 spp.), 18% of all Cladorhiza species (8 spp.), 10% of all Lycopodina species (3 spp.), and 8% of all Abyssocladia species (2 spp.). It is likely that with an increase in deep-sea exploration of the North Pacific, and identification of new specimens from the Kuril-Kamchatka Trench, that many more new cladorhizid species and genera will be found in the NW Pacific, which could have been an important region for the evolution of this family, potentially due to the variety of deep-sea habitats and numerous currents and upwelling areas (Lundsten et al. 2014; Downey and Janussen 2015).

Glass sponges (Hexactinellida) are not as rich as demosponges at the species (23), genera (10) or family (6) level in the deep NW Pacific (Table 2). However, glass sponges outweigh demosponges in abundance, accounting for nearly 60% of all specimens found in this region (Table 1), and were found to be particularly rich in the Sea of Okhotsk, accounting for three-quarters of all specimens found (Downey et al. 2018). Many species of hexactinellid, including Hyalonema, Caulophacus, Eurete Semper, 1868 and Bathydorus are found across many stations (Maps 7–9), highlighting their broad distributions across the NW Pacific (Table 2). Nine species morphotypes of hexactinellids were found in the Kuril Basin of the Sea of Okhotsk, indicating the richness of hexactinellids in this semi-enclosed deep-sea region (Downey et al. 2018). Hexactinellid species richness also occurs in the Bering Sea (12 spp.), the Kuril-Kamchatka abyssal plain (8 spp. and 1 subsp.), and the Kuril-Kamchatka Trench (6 spp.) (Table 2). Hexactinellid diversity is found to be at its least in the Bussol Strait (4 spp.) and the Emperor Seamounts (1 spp.). However, with a new expedition to the Emperor Seamounts, it is likely that hexactinellid richness will increase (pers. com L. Watling, August 2019). Hexactinellid species account for a third (23 spp.) of known speciesin this deep-sea region (Table 2). The SokhoBio expedition has added six new hexactinellid species to the deep Sea of Okhotsk, Bussol Strait and NW Pacific fauna, which has nearly doubled the number of previously known hexactinellid species and added in one new genus (Bathydorus) to this region. One new to science species has been found: Hyalonema (Paradisconema) sp. Nov. 1. Hexactinellid abundance is exceptional in the Sea of Okhotsk, which is found to be at least fourteen orders of magnitude greater than the adjacent Kuril–Kamchatka region (Downey et al. 2018). Most hexactinellid families and genera found in the NW Pacific are found globally in all deep-sea environments (Tabachnick 1994), and the results from these studies indicate the presence of both new species, high levels of abundance, and the broad distributions of many hexactinellid species.

A hypothesised poleward decrease in diversity of deep benthic communities has been proposed, although it remains controversial (Gray 2001; Ramirez-Llodra et al. 2010). Latitudinal bands were compared within this study region, and superficially, diversity does appear to be greater at the southerly sector of the NW Pacific (Table 3). However, large differences in depth range, longitudinal range and sampling intensity, make it difficult to discern if there was a real decrease in diversity with increased latitude. Areas of semi-enclosed seas, such as the Sea of Okhotsk, regional diversity is found to be relatively high (Downey and Janussen 2015; Downey et al. 2018), compared to the adjacent Kuril-Kamchatka abyssal plain, and this high level of richness and diversity of species is likely influencing this poleward trend in reduced diversity above 50°N. What could be detected in this latitudinal analysis was a decrease in demosponge species and an associated increase of hexactinellids with increasing latitude. Hexactinellids are known to be more abundant and diverse in the high latitudes of the North and NE Pacific (Beulieu 2001; Leys et al. 2004; Reiswig 2014), which could be influencing this pattern. This is also found in the high latitudes of the Southern Ocean, with hexactinellids often found in greater numbers due to likely higher levels of dissolved silica, upwelling bottom waters, reduced runoff, and highly seasonable food availability (Downey et al. 2018).

Energy availability (Woolley et al. 2016) and major ocean currents (Lembke–Jene et al. 2017) could partly explain the variations observed in diversity and abundance throughout the NW Pacific Ocean. Large sectors of the NW Pacific Ocean are in areas of upwelling, such as the Sea of Okhotsk, the Kuril-Kamchatka Trench, Kamchatka Peninsula, Sea of Japan, and the Bering Sea (Zhabin et al. 2017), which increases the amount of nutrients available for a diverse range of species to thrive in these regions. Results from the Sea of Okhotsk indicate that sponges in this region are generally richer and more abundant than other areas sampled (Downey et al. 2018). The Sea of Okhotsk is a highly productive region (Sorokin and Sorokin 1999, Arzhanova and Zubarevich 1997, Radchenko et al. 2010, Sahling et al. 2003), and is located within a high carbon import and export latitude (Hays and Morley 2004). High levels of productivity are likely to be important in not only maintaining the high numbers of species in the Sea of Okhotsk, but also in the remarkable abundance of specimens found, which were at least eight orders of magnitude greater than theadjacent Kuril–Kamchatka region (e.g. Downey and Janussen 2015; Downey et al. 2018). Major currents that flow in and out of this region could also explain differences in faunal abundance and composition. The East Kamchatka Current, Soya Warm Current, and Kuroshio Current are warmer, saline, nutrient poor currents, whereas the East Sakhalin Current and Oyashio Current are colder, denser, less saline, nutrient and oxygen–rich currents (Lembke–Jene et al. 2017). Food-rich and highly oxygenated currents flow into NE sector of the Kuril Basin, the Bering Sea, and the northern sector of the Emperor Seamounts, which could partly explain why we find more sponge species and greater abundance in many of these regions.

The source-sink hypothesis predicts that abyssal species distributions are sinks regulated by source populations in bathyal regions (Rex et al. 2005). However, sponge species depth distributions (abyss and shallower depth zones within their entire distribution: 17 spp.) are not common in this region (Table 4), with only a quarter of species with this distribution. Therefore, the source-sink hypothesis does not explain much of abyssal species distributions in the NW Pacific Ocean. A recent study using global data, found that it is unlikely that abyssal faunal populations are sustained only by larvae from bathyal and shelf populations, and that in productive regions, such as the Sea of Okhotsk, Bering Sea, and Emperor Seamounts, reproduction is local, and could actually be important in sustaining smaller populations in less productive regions of the deep sea (Hardy et al. 2015), such as the adjacent Kuril-Kamchatka abyssal plain. Results indicate that there is strong relationship between abyssal and hadal depths, with two-thirds (6 spp.) of species in the hadal, also found in the abyss in the NW Pacific (Table 4). A transition zone between the abyss and hadal has been indicated in previous studies (Belyaev 1966, 1989; Kamenskaya 1981, 1995; Jamieson et al. 2011), and could be due to abyssal populations sustaining hadal depths. However, a third of species in the hadal are not known from any other depth, and thus, hadal regions could be geological features that have evolved their own self-sustaining populations in the NW Pacific Ocean (Ramirez-Llodra et al. 2010).

In the NW Pacific Ocean, emergence and submergence processes are important to consider in understanding the evolution of deep-sea fauna in semi-enclosed seas, such as the Sea of Okhotsk, which has only limited deep-sea straits allowing access of species between this region and adjacent deep-sea regions (e.g. Brown and Thatje 2014), and the Bering Sea, which has multiple deep-sea straits. In the Bering Sea, two hexactinellid species (Acanthascus profundum Koltun, 1967 and H. (C.) apertum simplex) could be classed as pseudo-abyssal, as they are found to be deeper in the Bering Sea than elsewhere in their distribution. Two other species, demosponge P. pacifica, and hexactinellid C. (C.) elegans, are potentially undergoing emergence, as they are found to be shallower in the Bering Sea than elsewhere in their range. The Sea of Okhotsk is connected to the Kuril–Kamchatka abyssal plain through the Krusenstern Strait (1,920 m) and the Bussol Strait (2,318 m) (Radchenko et al. 2010). Analysis of the Sea of Okhotsk sponges indicates that six demosponge species are undergoing submergence, which indicates they could be classed as ‘pseudo-abyssal’, such as Megaciella ochotensis (Koltun, 1959) and Forcepia uschakowi (Burton, 1935), whereas one hexactinellid species, C. (C.) lotifolium Ijima, 1903, could be undergoing emergence from this typically deeper abyssal and hadal ranged species (Downey et al. 2018). These results suggest that some Sea of Okhotsk and Bering Sea sponges are undergoing submergence from bathyal populations within and outside the Sea of Okhotsk, allowing the development of ‘pseudo-abyssal’ species, as well as emergent fauna from deeper distributed species, and therefore deep-sea straits could have been key in connecting these sponge communities. The lower proportion of submergent, pseudo-abyssal fauna in the Bering Sea could be due to limited sampling, but also the greater numbers of deep-sea connections to the NW Pacific abyssal plain enabling improved connectivity. However, the presence of a small number of eurybathic sponge species both within and outside of the Sea of Okhotsk and Bering Sea could indicate that emergence and submergence processes are important in understanding some of the sponge distributions in the semi-enclosed seas of the NW Pacific Ocean.

Utilising all depth information from the NW Pacific deep sea, stenobathy is found to be a common characteristic, with more than three-quarters (55 spp.) of sponges in this sector found with this trait (Table 4). Demosponges are more likely to have restricted depth distributions (39 spp., about 85%), compared to hexactinellids (15 spp., c. 65%) in the NW Pacific. Diversity in species is found to be greatest in the NW Pacific abyss, with 56 spp. found in this depth zone, compared to 20 spp. in the lower bathyal, and 13 spp. in the hadal. A known large-scale diversity pattern of deep-sea benthic communities is a unimodal relationship between diversity and depth, which peaks at intermediate depths (2,000–3,000 m) (Gray 2001). This study found the abyss to be richer in species than the lower bathyal; however, only sponges with deeper depths were analysed, and so a re-analysis of sponges found at shallower depths is needed to confirm these findings. However, abyssal depths could still be found to be richer in the NW Pacific, as there are a number of semi-enclosed basins, seamounts, and ridges at this depth range, which all have unique oceanographic features that could be enabling this abyssal diversity (Ramirez-Llodra et al. 2010). Stenobathy could be also be an artefact of sampling at a limited depth range, or it could be driven by absence of substrate, reduced food-supply at greater depths, and greater distances from deeper or shallower sites, such as in the broad abyssal plain in the NW Pacific. Less common, broader depth ranges found in both demosponges and hexactinellids are potentially driven by their ability to construct root tufts and reproduce both sexually and asexually (Tabachnick 1994), enabling them to colonise a broader range of deep-sea environments in the NW Pacific.

The deep NW Pacific is found to be a distinct biogeographic region, as it is overwhelmingly endemic, both in terms of sponge species and in terms of genera, which had been previously proposed by Menzies et al. (1973). However, the presence of large-scale oceanographic features, such as the similar levels of moderate particulate flux and the dominance of sub-Polar Gyre currents, have made a greater number of researchers to propose a single North Pacific biogeographic region (Vinogradova 1959, 1997; Kussakin and Mezhov 1979; Belyaev 1989; Watling et al. 2013). It is likely that smaller differencesgenerated by localised currents, and geological features (seamounts, trenches, ridges, and semi-enclosed basins), are far more important in the evolution of NW Pacific deep-sea sponge fauna (McClain and Mincks Hardy 2010), and these smaller features are likely important in the adjacent NE Pacific. The apparent richness of fauna surrounding the Kuril Islands, has led other researchers to hypothesise that this region could have been a recent centre of diversification for North Pacific boreal fauna (Kussakin and Mezhov 1979). So far, 15% of species found in the Kuril-Kamchatka Trench are endemic; however, the majority of samples have not been fully identified, and so these lower levels of endemism are likely to increase. Previously, exploration of trenches in the North Pacific found high numbers of endemic species, c. 40–50%, which led researchers to propose a distinct Aleutian-Japan biogeographic hadal province (Belyaev 1989; Vinogradova 1997). New species found around the Emperor Seamounts, which have not yet been found in the NW Pacific (Figure 2), indicate that seamounts are also likely to have distinct sponge communities in the NW Pacific, potentially being more faunistically connected to Hawaiian seamount chain, and they could also be a barrier between the NW Pacific and NE Pacific abyssal sponge fauna.

Moderate abyssal faunal connectivity is found between the NW Pacific region and other sectors of the Pacific Ocean (18 spp.), and this faunal connection is stronger with the rest of North Pacific (Table 2). Several species, which are new to this region, including Lycopodina globularis (Lévi, 1964), Chondrocladia (C.) koltuni Vacelet, 2006, and Cladorhiza mirabilis (Ridley & Dendy, 1886), highlight this faunal connection, as they were previously only known from the N and NE Pacific abyss (Lévi 1964; Koltun 1970; Vacelet 2006). The deep-sea Aleutian Island Archipelago sponge fauna, distributed both in the Bering Sea and abyssal North Pacific, has previously found to have strong taxonomic affinities with the Sea of Okhotsk sponge fauna (30% of species in common) (Stone et al. 2011). In this study, at least 12 spp. and 2 sub. spp. are found to be distributed in the Bering Sea/Aleutian Island area, as well as elsewhere in the NW Pacific. The majority of these species are hexactinellids, which could have come from the North Pacific Bering Sea region, which appears to be richer in glass sponges from latitudinal analysis in this study, and could have been distributed via the anticlockwise currents associated with the sub-Polar Gyre, to the Kamchatka Peninsula, the Sea of Okhotsk and the remainder of the region. Most deep-sea sponges are likely to be hermaphrodites, reproducing both sexually and asexually, and likely producing lecithotrophic (non-feeding) larvae. In one sponge study, researchers found high levels of genetic variability within populations, which indicated contributions from both highly dispersed sexually produced larvae and asexually produced budding/fragmentation (Maldonado and Uriz 1999). Despite vast distances and depths, some sponge species in the deep sea are likely connected through long-distance sexual and asexual dispersal through currents, across broad, often fragmented, distributions.

We would like to thank our DFG (Deutsche Forschungsgemeinschaft), project JA-1063/17-1 and the joint Russian–German SokhoBio andKuramBioII expeditions for funding the field research for Melanie Fuchs and Rachel Downey. Rachel Downey would also like to acknowledge her Australian Government Research Training Grant for funding the synthesis of this project. We would also like to especially thank Hanieh Saeedi for the production of maps and the project management of this book, and Angelika Brandt and Marina Malyutina for organising workshops and expeditions, as well as the crew of the RV Akademic M.A. Lavrentyev and RV Sonne for their help and guidance during these expeditions.

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The marine region encompassed between 40°N and 60°N and 120°E to 180°E contains a large area of the NW Pacific Ocean, including the northern half of the Sea of Japan, the Sea of Okhotsk, the southwestern part of the Bering Sea, and the Pacific Ocean, as well as the Kamchatka Peninsula, the Kuril Islands and the northern Japanese Islands. Much of this marine region is deep sea (>200 m in depth). Economically, the region is among the most productive marine fisheries regions in the world, including not only open-water fisheries but also commercially important crab fisheries (Spiridonov 2005). Despite its obvious importance to the global fishery, research on many marine taxa in the region is lacking, due to the relative remoteness of the region as well as other logistical issues such as severe weather. The lack of data and research in the region thus hampers our knowledge of many marine taxa, making conservation and protection planning difficult.

In this chapter, we will examine the state of knowledge in the NW Pacific Ocean for two phyla of marine invertebrates, the Cnidaria and Ctenophora, at depths below 2,000 m, in the lower bathyal (2,000–3,000 m), abyssal (3,000–6,000 m) and hadal (>6,000 m) zones. These two phyla are among the most ancient of metazoans (Simion et al. 2017), and are often grouped together as “coelenterates”, although the scientific validity of such a group has now been debunked (e.g. Whelan et al. 2015; Simion et al. 2017; Nielsen 2019). Although superficially similar morphologically, this similarity is only at the macromorphological level, with both groups being often fragile and gelatinous. For example, ctenophores have rotational symmetry while cnidarians have diverse symmetries (including bilateral and radial symmetry). In all probability, the Cnidaria and Ctenophora are sister groups that evolved independently from the last common ancestor of Eumetazoa (Nielsen 2019).

Cnidaria (Figure 1) include the various corals, true jellyfish, and related animals, and can be broadly split into three major groupings; 1) the class Anthozoa, primarily benthic corals and allies, 2) the subphylum Medusozoa of true jellyfish (classes Scyphozoa, Cubozoa, Staurozoa) and hydrozoans (class Hydrozoa), and the classes 3) Myxozoa and Polypodiozoa, marine parasites. Cnidaria can be united by cnidocytes (“polar capsules” in Myxozoa), which are specialized cells used for prey capture, defense, and/or attachment. Many Cnidaria have asexual and sexual life-history stages, with two basic body forms, benthic polyps with a single oral opening facing upwards, and a planktonic medusa with the oral opening facing downwards. Around these oral openings, tentacles are aligned in various arrangements.

Figure 1.

Some Cnidaria and Ctenophora species observed in the deep sea of the NW Pacific. (A) Stone with numerous benthic animals, including light pink alcyonarians, dark brown polychaete tubes, and light brown broken-off hydrozoan colony (?) from station 8–5 EBS, depth 2,327-2,330 m. Scale = 5 mm. (B) Actinarian Hormathia spinosa from station XR-10 off Kushiro, depth 5,509-5,561 m. (C) Actinarian Galatheanthemum sp. from station XR-12 off Kushiro, depth 5,471-5,514 m. (D) Octocoral Aspera rosea from station 8-4 EBS, depth 2,333-2,336 m. Scale = 5 mm. (E) Halicreas minimum from 1,043 m in Sagami Bay, Japan; this species has also been reported from >2,000 m within the study area of the NW Pacific. Images A and D by Anna Lavrentieva, B and C by Kensuke Yanagi, and E by Dhugal Lindsay.

Cnidarians are common components of the deep-sea plankton and benthos (e.g. Hunt and Lindsay 1999; Cairns 2007; Dautova 2018a,b), and can often be considered as ecosystem engineers, providing habitat for other marine organisms (Ohtsuka et al. 2009; DiCamillo et al. 2013). As such, their diversity, abundance, and distribution in the water column and on the seafloor are important datasets in establishing conservation and protection planning. Deep-sea Cnidaria contain some of the most well-known and spectacular organisms from these ecosystems, including examples such as Pandearubra, a hydrozoan anthomedusan species that illustrates the run-on effect that ocean acidification can be expected to have on deep-pelagic ecosystems (Lindsay et al. 2008), as well as the huge, red, tentacle-less “fingerfoot medusa” Tiburonia granrojo (Matsumoto et al. 2003), Epizoanthus zoantharian species symbiotic with hermit crabs (Muirhead et al. 1986; Ryland and Ward 2016; Kise et al. 2019), and the enigmatic Relicanthus anemone, a sea anemone with unusual phylogenetic placement (Rodriguez et al. 2014; Xiao et al. 2019).

Ctenophores are also common components of the deep-sea plankton, sometimes being almost as abundant as cnidarians (e.g. Fig. 7 in Lindsay et al. 2000), with members of the benthic ctenophoran order Platyctenida commonly observed as epibionts on other benthic deep-sea animals (DJL, pers. obs.).

The main objective of this chapter is to provide an overview of the state of current knowledge of the phyla Cnidaria and Ctenophora at depths below 2,000 m in the NW Pacific by examining records and occurrences from the Ocean Biogeographic Information System (OBIS) database combined with additional records from the literature and unpublished data.

In this review of knowledge of the Cnidaria and Ctenophora of the NW Pacific, we searched for records and occurrence data for each phylum in the Ocean Biogeographic Information System (OBIS 2020) on January 4, 2020. We limited our data search to depths below 2,000 m, and within 40°N to 60°N and 120°E to 180°E. Resulting datasets were downloaded, and assessed by taxonomic experts of each group (Anthozoa: JDR, KY, Medusozoa + Ctenophora: DJL). The OBIS Cnidaria dataset initially included 184 occurrence records, while that of the Ctenophora included three occurrence records. We subsequently removed eight records from the Cnidaria dataset as the species identities and depths were clearly erroneous, and corrected the taxonomic nomenclature of several other records. We also added Actiniaria records from two research cruises that were not previously published; the Japan Agency for Marine Science and Technology’s (JAMSTEC) KH01-2 cruise by the RV Hakuho-Maru in October to November 2001 around Chishima and the Japan Trench (n=38), and the KT08-27 cruise by the RV Tansei-Maru in October 2008 off the Sanriku Coast of northeastern Honshu, Japan (n=5) (both datasets and identifications from KY). We additionally added Octocorallia occurrence records (n=21) from Dautova (2018a) and Hydrozoa and Scyphozoa occurrence records (n=16) from Naumov (1971). After these checks, edits, and additions, the Cnidaria dataset contained 256 records, while the Ctenophora record numbers remained unchanged. Taxonomy of both phyla followed the World Register of Marine Species (WoRMS 2020). It should be noted that the higher taxonomy of the phylum Ctenophora is in desperate need of revision (e.g. Podar et al. 2001; Lindsay and Miyake 2007). Datasets are available as Supplemental Material 1.

We then examined the datasets in detail, and assessed 1) numbers of species (counting only records identified to species), 2) numbers of records by lowest taxonomic rank, 3) numbers of species for each larger taxonomic grouping (class/subclass/order), 4) numbers of records by depths, and 5) numbers of records reported by year. After this, we then reviewed and discussed the state of knowledge for both phyla in the NW Pacific Ocean, and propose ways forward to broaden our knowledge of these taxa in the deep sea of this region.

Images of some Cnidaria and Ctenophora species observed in the deep sea of the NW Pacific are shown in Figure 1.

4.1 Cnidaria

Within Cnidaria, of 256 records and occurrences, 86 were for planktonic taxa (=33.6% of Cnidaria records), and 166 (=64.8%) were for benthic anthozoan taxa. Four records (=1.6%) were only noted as “Cnidaria” and could not be placed into either group. No Myxozoa or Polypodiozoa were reported. Within the non-Anthozoan cnidarians, there were 28 Scyphozoa (=10.9% of Cnidaria, 32.6% of Medusozoa), 58 Hydrozoa (=22.6%, 67.4%, respectively), and no Staurozoa or Cubozoa. Within Anthozoa, there were 28 Octocorallia (=10.9% of Cnidaria, 16.9% of Anthozoa), seven Scleractinia (=2.7%, 4.2%), and 83 Actiniaria (=32.3%, 50.0%), with an additional 48 records for “Anthozoa” only (=18.7%, 28.9%).

By lowest taxonomic rank, four records were to phylum (=Cnidaria; 1.6% of Cnidaria), 92 to class (=35.9%), one to subclass (=0.4%), 19 to order (=7.4%), 13 to superfamily (=5.1%), 12 to family (=4.7%), 29 to genus (=11.3%), and 86 to species (=33.6%). The 86 species records consisted of 23 different species: eight Hydrozoa, one Scyphozoa and 14 Anthozoa (eight Actiniaria, five Octocorallia, and one Scleractinia). By numbers of records, the anemone Edwardsia sojabio Sanamyan N. & Sanamyan K., 2013 was by far the most common, with 33 occurrences, followed by the octocoral Radicipes sakhalinensis Dautova, 2018 (n=9), the hydrozoans Opercularella angelikae Stepanjants, 2012 (n=7), Botrynema brucei Browne, 1908(n=5), the anemone Hormathia spinosa (Hertwig, 1882) (n=5), the hydrozoans Halicreas minimum Fewkes, 1882 (n=3), and Pantachogon haeckeli Maas, 1893 (n=3), the scleractinian Fungiacyathus (Bathyactis) marenzelleri (Vaughan, 1906) (n=3), the scyphozoan Atolla wyvillei Haeckel, 1880 (n=2), the octocoral Aspera rosea Dautova, 2018 (n=2), and the actiniarian Bathydactylus kroghi Carlgren, 1956 (n=2, tentative identification). All other species only occurred once in our dataset.

By depth, there were nine records from >6,000 m, 90 from 5,000 to 6,000 m, 29 from 4,000 to 5,000 m, 73 from 3,000 to 4,000 m, and 55 from 2,000 to 3,000 m (Figure 3). The deepest records were for the hydrozoan Pectis profundicola (Naumov, 1971) between 6,800 to 8,700 m (Naumov 1971), an unidentified Anthozoa from 7,366 m, the actiniarians Bathydactylus kroghi from 7,141 m, and Sicyonis sp. (tentative identification), Galatheanthemum sp. and Mesomyaria sp. E from 7,139 m, as well as an unidentified Cnidaria at 6,985 m.

Figure 2.

Records from the NW Pacific below 2,000 m of Cnidaria and Ctenophora by lowest taxonomic rank of identification (total n=256).

Figure 3.

Records from the NW Pacific below 2,000 m of Cnidaria and Ctenophora by depth in m (total n=256).

By year, the earliest records of Cnidaria within the region were from 1906 (specimens now housed in the Smithsonian, USNM), but these were removed from our dataset as described in the methods due to their capture depths being inferred as being at the seafloor, presumably during their import into the OBIS dataset, even though they were obviously from the pelagic zone closer to the surface (DJL, pers. obs.). Thus, subsequently, the next records were from 1966 (Naumov 1971) and 1981, while the large majority of Cnidaria records from the region were from 2000 or later, from only two cruises by K. Yanagi in 2001 and 2008 and a few publications, namely Brandt et al. (2013), Sanamyan and Sanamyan (2013), Stepanjants (2013), Trebukhova et al. (2013), Brandt et al. (2015), Fischer and Brandt (2015), Schwabe et al. (2015), and Dautova (2018a,b).

4.3 Distributional patterns

Although distribution maps for various taxonomic groupings of cnidarians have been provided in this chapter (Maps 1–11), we consider the state of knowledge to still be too nascent to make serious hypotheses on distributional patterns of Cnidaria or Ctenophora in the NW Pacific.

Map 1.

Map of occurrence records of phylum Cnidaria from the NW Pacific below 2,000 m. Note these records are for occurrences not identified to any taxonomic level below Cnidaria.

Map 2.

Map of occurrence records of class Anthozoa from the NW Pacific below 2,000 m. Note these records are for occurrences not identified to any taxonomic level below Anthozoa.

Map 3.

Map of occurrence records of subclass Octocorallia from the NW Pacific below 2,000 m.

Map 4.

Map of occurrence records of order Scleractinia from the NW Pacific below 2,000 m.

Map 5.

Map of occurrence records of order Actiniaria from the NW Pacific below 2,000 m. Note these records are for occurrences not identified to any taxonomic level below Actiniaria.

Map 6.

Map of occurrence records of actiniarian superfamilies Actinostoloidea and Actinioidea and the family Galatheanthemidae from the NW Pacific below 2,000 m.

Map 7.

Map of occurrence records of actiniarian suborder Anenthemonae from the NW Pacific below 2,000 m.

Map 8.

Map of occurrence records of actiniarian superfamily Metridioidea from the NW Pacific below 2,000 m.

Map 9.

Map of occurrence records of class Hydrozoa from the NW Pacific below 2,000 m.

Map 10.

Map of occurrence records of class Scyphozoa from the NW Pacific below 2,000 m.

Map 11.

Map of occurrence records of class Ctenophora from the NW Pacific below 2,000 m.

The area examined in this paper between 40°N to 60°N and 120°E to 180°E covers over nine million square kilometers of the Earth’s surface, with the majority being marine. Of this marine area, a large portion is below 2,000 m in depth (e.g. see maps 1–11), and thus it is easy to state that the deep-sea area we examined in this study is vast. Despite this, records indicate less than 250 records of cnidarians, and only three records of ctenophores from this region. These records are almost completely from within the last 20 years. It is not an overstatement to say that research on these basal metazoan groups is almost completely lacking for the region, and that this research is in its infancy.

Within the region examined in this study, there are undoubtedly more specimens and records than we have reported here. For example, other planktonic Cnidaria records from the NW Pacific include those from four deep dives by the Mir submersibles in the Kurile-Kamtchatka Trench that were analyzed by Vinogradov and Shushkina (2002), but the majority of the pelagic cnidariantaxa reported below 2,000 m depth were unidentifiable to species level, with the exception of Pantachogon haeckeli Maas, 1893 (reported as >2,500 m depth) and Botrynema brucei Browne, 1908 (deepest record at 3,400 m depth). Even generic identifications in Vinogradov and Shushkina (2002) were marked with question marks (e.g. Poralia?, Turritopsis?) or were for genera that have never previously been reported from depths below 2,000 m (e.g. Colobonema), illustrating the lack of information in general on bathyal and abyssal gelatinous zooplankton (Vinogradov and Shushkina 2002). These records have not been included in the dataset of the current study.

As well, Brandt et al. (2018) reported that there were 43 Cnidaria specimens collected in the2015 SokhoBio Expedition in the Sea of Okhotsk, but asides from 21 records mentioned in Dautova (2018a) already included in our current dataset, we could not find detailed information on these specimens.

It is well known that the marine environment becomes more similar with surrounding regions as depth increases (Vecchione et al. 2015), and looking at the deepest records worldwide of various cnidarian and ctenophore taxa may provide a guideline for what species we may expect to find in future surveys of the NW Pacific. According to Malyutin (2015), the deep-sea coral communities of the Aleutian Islands, the eastern Bering Sea and the Gulf of Alaska (northestern Pacific) are similar to the coral communities of the NW Pacific. All the occurrence records in Malyutin’s 2015 paper had depths shallower than 1,000 m. Although we might expect similar trends at deeper depths, the investigation of additional deep-water communities in those areas is crucial to further corroborate such prediction.

For example, for Anthozoa, there are records of the orders Actiniaria (sea anemones), Scleractinia (hard corals), and the subclass Octocorallia (soft corals, sea pens) from the NW Pacific area examined in this study. However, from the presence and depths of anthozoans from other regions, we may also expect to find the subclass Ceriantharia (deepest report of Cerianthus valdiviae Carlgren, 1912, from 5,248 m, Indian Ocean south of Sumatra, OBIS dataset), and the orders Corallimorpharia (Corallimorphus sp. reported from 5,274 m, South Orkney Islands, USNM catalogue number 61003), Zoantharia (Abyssoanthus convallis, 5,362 m, Japan Trench, Reimer & Sinniger, 2010), and Antipatharia (Stichopathes variabilis van Pesch, 1914, 7,000 m, Sunda Trench, ZMUC Gal-II-1248) within the NW Pacific.

Similarly, regarding planktonic data, the class Staurozoa has been reported down to 2,694 m (Lucernaria janetae Collins & Daly, 2005, East Pacific Rise). The deepest record for a cnidarian is currently a small red rhopalonematid hydromedusa (DJL, pers. obs.), observed at 9,970 m depth in the Mariana Trench by a drop camera (Gallo et al. 2015). The deepest scyphozoan record is for an ulmarid medusa at 8,200 m depthin the New Britain Trench (Gallo et al. 2015), identified at the time as tentatively belonging to the subfamily Poraliinae but now believed to warrant the erection of a new subfamily to contain it (DJL, pers. obs. based on new material collected in 2019 in the Gulf of Alaska). The deepest record for a ctenophore is 7,217 m depth in the Ryukyu Trench, for an enigmatic tentaculate ctenophore that anchors itself to the sediment by two filaments, while trailing two tentacles that lack tentillae (Lindsay and Miyake 2007). On the other hand, the cnidarian classes Cubozoa, Myxozoa, and Polypodiozoa have not been reported from depths of >2,000 m worldwide, and thus we would not expect them to be present in the NW Pacific.

Another way to predict more about the deep-sea Cnidaria and Ctenophora fauna of the NW Pacific would be to examine occurrence records and information from neighboring regions that are more well examined. For example, the waters around Japan have long been surveyed and there are comparatively many more records, other data, and specimens from the deep sea of this area (e.g. Kitamura et al. 2008a, b). There are also deep-sea records of anthozoans to the east from the Aleutian Islands of Alaska (e.g. Cairns 1994; Heifetz et al. 2005; Herrera et al. 2010; Cairns 2011; Cairns and Lindner 2011; Thoma 2013; Stone and Cairns 2017).

From marine regions to the south of the current area of interest, there are many additional Cnidaria and Ctenophora records. For benthos, in the western Pacific, Williams (2011) reported the octocoral sea pen genera Gyrophyllum at depths ≥2,000 m, and Protoptilum and Scleroptilum at depths ≥4,000 m. Among gorgonians, Swiftia pacifica (Nutting, 1912) is a western Pacific bathyal species whichhas been found at depths ≥2,000 m (Horvath 2019). Similarly, Imahara (1996) noted the gorgonian Bathygorgia profunda Wright, 1885 from 4,200 m, and the sea pen Kophobelemnon stelliferum (Müller, 1776) from down to 3,650 m in Sagami Bay. Additionally, from older records, Kölliker (1880) reported two more sea pen species from the deep sea of the North Pacific Ocean; an Umbellula (Gray, 1870) species from approximately 3,750 m, and Scleroptilum grandiflorum Kölliker, 1880 from approximately 4,200 m. Among other anthozoans, two species of the deep-sea zoantharian genus Abyssoanthus (Reimer & Fujiwara in Reimer, Sinniger, Fujiwara, Hirano & Maruyama, 2007) are known from the Japan Trench (5,347-5,360 m; Reimer andSinniger 2010) and the Nankai Trough (3,259 m; Reimer et al. 2007), respectively.

Regarding planktonic species, between 2,000-4,000 m depths in the Japan Trench, Lindsay (2005) reported the occurrence of unidentified narcomedusan species belong to the genera Solmissus (Haeckel, 1879) and Aegina (Haeckel, 1879). The most commonly observed medusa was Botrynema brucei Browne, 1908, which was most abundant in the 2,000-2,500 m layer but occurred down to 3,500 m depth (Lindsay 2005). The Bathocyroe (Madin & Harbison, 1978) species observed below 4,000 m depth was also most abundant in the 2,000-2,500 m depth layer (Lindsay 2005). The only other pelagic cnidarians identified to species level by Lindsay (2005) were: the trachymedusae Halicreas minimum Fewkes, 1882 (2,439 m depth) and Crossota aff. millsae (3,668 m depth); the physonect siphonophoresMarrus antarcticus pacificus Stepanjants, 1967 [=Marrus orthocanna (Kramp, 1942)] (2,869 m depth) and Tottonia contorta Margulis, 1976 (=Apolemia sp.) (2,810 m depth); and the ulmarid scyphomedusa Poralia rufescens Vanhöffen, 1902 (2,522 m depth). An extremely large (ca. 80 cm diameter) Aulacoctenid cydippid ctenophore was observed at 3,562 m depth (Lindsay 2005).

However, even in surrounding regions, some taxa still display an apparent lack of data. For example, very little information has been published on the abyssal planktonic cnidarian and ctenophoran fauna of the entire NW Pacific Ocean with the notable exceptions of several works by Naumov and Lindsay (e.g. Naumov 1971; Lindsay 2005). Lindsay (2005) recorded a coronate medusa belonging to the genus Periphylla (F. Müller, 1861) at 6,464 m depth in the Japan Trench, though they were hesitant to identify it as the species Periphylla periphylla (Péron & Lesueur, 1810) due to the spiralling morphology and white colour of the tentacles. Below 4,000 m depth, Lindsay (2005) recorded an undescribed lobatectenophore species belonging to the genus Bathocyroe, as well as at least two species of physonect siphonophores (as “Agalmidae”), four species of hydroidomedusae (non-Narcomedusan), one “cydippid” ctenophore species and one other lobate ctenophore species.

Formal taxonomic species descriptions can help collate past data, and thus provide important information. This can be clearly seen for the sea anemone species Edwardsia sojabio, which is by far the species with the most numerous numbers of records for both phyla examined here, directly as a result of its formal species description (Sanamyan and Sanamyan 2013). Similarly, the octocorals I. rubeus, A. rosea, and R. sakhalinensis were recently described from specimens collected in 2015 (Dautova 2018a), greatly increased our knowledge of deep-sea octocorals in the region, increasing the numbers of records by four times (7 to 28 records). Species descriptions by their nature will provide taxa identified to the species level, something that is sorely lacking from our current understanding of the gelatinous fauna of the NW Pacific. Similarly, the previously unpublished data from two cruises newly reported in this paper (KH01-2, KT08-27 datasets) almost doubled the amountof sea anemone data available, increasing the number of records of Actiniaria from 43 to 85. Thus, it can be easily concluded that even single studies can greatly increase our knowledge on Cnidaria and Ctenophora in the NW Pacific given the current state of our understanding.

Due to the paucity of data, conclusions on the patterns of distribution within the NW Pacific remain to be made. For example, there are almost no data for many Cnidaria or Ctenophora taxa for the deep sea below 2,000 m in the northern Japan Sea (e.g. Maps 4–14). Examining data from further south, a high diversity of deep-sea ctenophores has been reported on the Pacific side of the Japanese archipelago (Lindsay and Hunt 2005), while only the beroid ctenophore Beroe abyssicola Mortensen, 1927, the “cydippid” ctenophore Euplokamis sp. and the lobate ctenophore Bolinopsis infundibulum F.O. Müller, 1776 have been recorded in the deep Japan Sea at the present time, albeit above 1,000 m depth (Lindsay and Hunt 2005). As the water mass in the deep Japan Sea (Japan Sea Proper Water) remains homogeneous below the thermocline, with extremely low temperatures, the above three ctenophore taxa can be inferred to be secondary deep-sea species that have invaded the deep Japan Sea to take advantage of the niche space made available by primary deep-sea taxa not being able to survive at such temperature-pressure combinations (Lindsay and Hunt 2005). The Ctenophora of the deep NW Pacific Ocean would be expected to contain both primary and secondary deep-sea forms. Future work should not only focus on the NW Pacific, but particularly on areas shown by this review to be neglected thus far.

Due to the lack of Cnidaria and Ctenophora data in the NW Pacific, it is far too premature to make any conclusions on abundances, total diversity, endemicity, or ecology of the various cnidarian and ctenophore species in the region. One positive aspect that can be discerned from the general lack of Cnidaria and Ctenophora deep-sea data from the NW Pacific is that any future surveys or research on the region will almost assuredly gather important new information.

Although many oceanographic cruises and research expeditions have been undertaken over the last two centuries with the aim of exploring the marine diversity of the NW Pacific and Far East, such as the Pacific expedition led by Mortensen in 1914 and the Galathea Expedition organized by the Zoological Museum University of Copenhagen (ZMUC), most of these expeditions were further south from the NW Pacific region between 40°N and 60°N and 120°E to 180°E examined in the current paper. Expeditions and cruises make the collection of a variety of marine invertebrates possible, which are then deposited in museums and research institutes. However, even with extensive and massive sampling campaigns, the identification of material, especially at the species level, is often hampered by a lack of taxonomic specialists. Thus, we strongly recommend future research cruises investigating the deep sea of the NW Pacific involve relevant Cnidaria and Ctenophora taxonomy experts to help increase the current paucity of data for these two phyla (as seen in the 2015 SokhoBio survey; Brandt et al. 2018; Dautova 2018a). Due to theextreme fragility of many taxa within these two gelatinous phyla, future surveys should include a crewed submersible or remotely-operated vehicle in order to collect both in-situ photographic and video records and the physical specimens needed for a complete morphological and molecular characterization of the fauna. Once this high-quality baseline data has been collected and vetted by taxonomic experts it will become possible for future surveys to be carried out at lower cost using such tools as towed cameras and environmental DNA probing, but our present state of knowledge about these taxa in the study area prohibits such approaches at the present time.

JDR thanks Dr. Takuma Fujii (Kagoshima University) for his assistance in searching for relevant data. DJL expresses his appreciation to Ms. Kumiko Oshima for assisting with the transcription of published data into a digital, Darwin Core-compatible format. This book could not be published with a financial support of “Biogeography of the NW Pacific deep-sea fauna and their possible future invasions into the Arctic Ocean project“ (BENEFICIAL project)” funded by Federal Ministry for Education and Research (BMBF: Bundesministerium für Bildung und Forschung) in Germany (grant number 03F0780A).

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The brachiopods or lamp shells are a diverse group of exclusively marine invertebrates. Their bodies are enclosed in two bilaterally symmetrical valves. The ventral (pedicle) valve is usually larger than the dorsal (brachial) valve. The shell morphology, and its various skeleton structures, and its soft body impression, are well preserved in fossil states and allow tracing of the evolution of these animals. Brachiopods are known from the early Cambrian with the greatest diversity found during the Palaeozoic. The modern brachiopods comprise only 5% of the total number of species that ever existed on Earth (Carlson 2016).

The brachiopods have a pelago-benthic life cycle with larval or juvenile planktonic and benthic adult stages (Lüter 2007; Kuzmina et al. 2019). Most brachiopods are epifaunal animals with a few exceptions. The adults attach to the substrate with a pedicle, which can regulate the position of the shell relative to water currents or may cement with their ventral valves. All brachiopods are filter feeding animals and filter using a special tentacle-organ, the lophophore. The lophophore is “a tentaculated extension of the mesosoma (and its cavity, the mesocoelom) that embraces the mouth but not anus” (Hyman 1959; Emig 1976). The presence of lophophore characterizes Lophophorata, a group that includes Bryozoa, Phoronida and Brachiopoda. In brachiopods, the lophophores may vary from the simple ring shape (trocholophous) to the curved shape consisting of three arms (plectolophous) (Emig 1992; Kuzmina and Temereva 2019). The brachiopod lophophores are located within the mantle cavity, a space between the valves. Tentacles are covered by ciliary bands that create the water currents.

The phylum Brachiopoda Duméril, 1805 consists of three subphyla: Linguliformea Williams, Carlson, Brunton, Holmer & Popov, 1996, Craniiformea Popov, Basset, Holmer & Laurie, 1993, and Rhynchonelliformea Williams, Carlson, Brunton, Holmer & Popov, 1996. The shell linguliforms is organophosphatic and lacks articulatory structures. The linguliforms have a U-shaped gut with an anus that is located anterior on the right side. Recent linguliforms are represented in only two families, Lingulidae Menke, 1828, and Discinidae Gray, 1840, which strongly differ in their biology and the morphology of their shells and soft bodies (Emig 1997). The lingulides are infaunal, they have a long muscle pedicle and burrow into soft sediments. The discinides are epifaunal and live on hard substrates usually fixed by a short muscle pedicle.

The craniiforms is a minor group of brachiopods and comprise only one class with a recent family Craniidae Menke, 1828, with three extant genera. All craniiforms have organocalcitic shell without articulatory structures. The gut is open with the anus located in the posterior midline of the body. Recent craniiforms lack the pedicle to cement themselves to substrate by the ventral valve (Williams et al. 1997).

The rhynchonelliforms are the most advanced group of brachiopods. This subphylum includes five classes but only the class Rhynchonellata Williams, Carlson, Brunton, Holmer & Popov, 1996, retains extant species, which includes threeorders: Rhynchonellida Kuhn, 1949, Thecideida Elliot, 1958, and Terebratulida Waagen, 1883. The rhynchonelliform shell is calcitic and has well-developed articulation structures and the calcitic lophophore supports the brachidium. The gut of the recent rhynchonelliforms is blind, so the fecal pellets are eliminated through the mouth (Williams et al. 1997).

Recent brachiopods are found in all seas and oceans from the Arctic to the Southern Oceans (Zezina 2008). The north subtropical and tropical zones contain the largest number of species. Each of these zones includes almost a third of the entire fauna of extant brachiopods (see table 6 in Zezina 2008). The rhynchonelliforms include a large number of endemic species, while the recent linguliforms lack endemic species (Richardson 1997). This is due to the presence of planktotrophic stages of linguliformes that can float up to six weeks (Chuang 1959; Paine 1963), while the craniiform and rhynchonelliform larvae swim for only 4 days.

The extant brachiopods occur in all depths from littoral to abyssal except ultra-abyssal (Bitner et al. 2013). However, most of the recent brachiopods prefer to live on the seaward edge of the shelf and the upper part of the slope, which can be explained by the following factors (Zezina 2008):

The current chapter is a review of published data on deep-sea brachiopod Pelagodiscus atlanticus (King, 1868) found during four Russian-German deep-sea expeditions in the NW Pacific Ocean.

Two specimens of P. atlanticus were collected in the Kuril-Kamchatka Trench (45° 12 02′N, 151°60 08′E) during the German-Russian expedition Kurambio II on RV Sonne (16 August 2016–26 September 2016) (Map 1). Specimens were obtained at 5,571.6 m depth using the Agassiz Trawl at station number SO-250-86.

Map 1.

Distribution of Pelagodiscus atlanticus.

P. atlanticus is a small deep-water representative of family Discinidae, a linguliform brachiopod. Its soft body is enclosed by dorsal and ventral chitinophosphatic valves, which are very small and thin (Figure 1). The dorsal valve (about 4 mm in diameter) is much larger than the ventral valve (about 3 mm in diameter). The valves grow holoperipherally, so the apex (first-formed region) is near the center of the valve (Figure 1a). The body wall forms mantle folds that line the inner surface of both valves. Marginal long and short setae emerge from the edge of both mantles. The short muscle pedicle attaches the ventral valve to the hard substrate (Figure 1c) (Kuzmina and Temereva 2019). P. atlanticus has a small simple horseshoe-shaped lophophore with two arms directed posteriorly (Figure 1b) (Kuzmina and Temereva 2019). The lophophoral arms are located symmetrically about the mouth. The basal part of the lophophore is attached to the anterior body wall. The distal portions of the arms are separated from the body wall and are raised into the mantle cavity. The arms are fringed with a double row of tentacles. Two coelomic canals, large and small, extend inside each arm of the lophophore. Hydrostatic pressure in the large coelomiccanals supports the arms of lophophore. The small canals connect with the coelomic canals of tentacles and perform transport functions. The lophophore of P. atlanticus was regarded as a modified zygolophous type and, apparently, demonstrated the distinct part of the lophophore evolution of brachiopods (Kuzmina and Temereva 2019). This simple form of the lophophore is a transition form in the ontogenesis of other representatives of discinides (Zezina 2015).

Figure 1.

Dorsal and ventral chitinophosphatic valves of deep-water of Pelagodiscus atlanticus (Kuzmina & Temereva, 2019). (A) Dorsal view of the fixed animal: the dorsal valve (dv) with long setae (ls) is visible. (B) Ventral view of the fixed animal: the ventral valve (vv) is partly open, and the mantle cavity (mc) contains two lophophoral arms (lam). (C) View of the ventral valve (vv) with short setae (ss) and a round pedicle (p). first-formed region of the shell.

We studied the spermatogenesis and ultrastructure of sperm in P. atlanticus (Temereva and Kuzmina 2018). The spermatozoon has a large acrosome, a small compact nucleus, eight mitochondria around the nucleus, two orthogonal centrioles, and a long tail. Comparative evaluation of all data on the structure of brachiopod spermatozoa indicates that P. atlanticus retains the most ancestral type of spermatozoon among all brachiopods.

Thera are 63 species of deep-sea recent brachiopods that live at a depth of more than 2,000 meters and can be divided into two groups (Zezina 1989, 1994). The first group contains eurybathic species, the second is exclusively deep-sea animals (Zezina 1994). The most deep-sea species of recent brachiopods is P. atlanticus, whose empty shells were found at a depth of 7,460-7,600 meters in the Romanche Fracture Zone (Zezina 1994). This species is eurybathic with a range of depths for live specimens from 336-5,530 meters and is also the deepest among recent brachiopods (Zezina 1994; Emig 1997). On the marginal ridge of the Kuril-Kamchatka Trench, the density of this species is about 12 individuals per square meter (Zezina 1981). Stones of various diameters (Foster 1974), shells of brachiopods and molluscs (Cooper 1975), bones of whales and manganese nodules (Zezina 1981) can serve as a substrate for the attachment of this species in the deep sea.

Deep-water conditions are unfavourable for brachiopods, leading to dwarfism, paedomorphosis, and homeomorphy (Zezina 2008; Bitner et al. 2013). Thus, the small size of the shell and the underdeveloped simple lophophore of P. atlanticus are interpreted as paedomorphic features (Zezina 2015), which is characteristic of the deep-sea brachiopod fauna (Zezina 1994, 2015; Bitner et al. 2013). A large number of mitochondria in the spermatozoon of P. atlanticus may also be considered as an adaptation for deep-water occurrence in this species since spermatozoon needs a lot of energy for external fertilization at extreme depths (Temereva and Kuzmina 2018).

This paper was part of the “Biogeography of the NW Pacific deep-sea fauna and their possible future invasions into the Arctic Ocean project (Beneficial project)”. Beneficial project (grant number 03F0780A) was funded by Federal Ministry for Education and Research (BMBF: Bundesministerium für Bildung und Forschung) in Germany. This work was supported by a grant from the Russian Science Foundation (#18-14-00082). I would like to thank Hanieh Saeedi forcreating the distribution maps and to Rachel Downey for a language editing of the text.

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Entoprocta , or Kamptozoa, is a phylum of invertebrate animals including about 200 species. Entoprocts are solitary or colonial animals living as epibionts of different animals (sponges, cnidarians, polychaetes, sipunculans, echiurids, bryozoans, echinoderms, and hemichordates) or as foulers of various substrates (stones, algae, mollusk shells, arthropod cuticle, tunic of ascidians) (Nielsen 1964, 2008, Soule and Soule 1965, Borisanova 2018). Two species of Entoprocta were found in fresh water (Leidy 1851, Wood 2005), all the others are marine animals. The majority of species live in the shelf zone, most often at shallow depths down to several tens of meters. Only one colonial species, Barentsia gracilis (Sars, 1835), and six solitary species of genus Loxosomella Mortensen, 1911 were described from the deep-sea floor (Borisanova 2018, Borisanova and Chernyshev 2019). All deep-sea Loxosomella species were described in recent years, and all of them were found in NW Pacific.

This chapter represents a brief review of the biogeography of the deep-sea Entoprocta found during Russian-German deep-sea expeditions in the NW Pacific.

Six deep-sea species of the genus Loxosomella (family Loxosomatidae Hincks, 1880) were found in the NW Pacific region during three German–Russian deep-sea expeditions (Map 1).

Map 1.

Distribution of Entoprocta species in the NW Pacific.

Loxosomella profundorum Borisanova, Chernyshev, Neretina & Stupnikova, 2015, was collected during the German–Russian deep-sea expedition KuramBio aboard RV Sonne to the abyssal plain adjacent to the Kuril-Kamchatka Trench in the summer of 2012; Loxosomella marcusorum Borisanova, Chernyshev, 2019 was collected during the German–Russian deep-sea expedition KuramBio II aboard the RV Sonne to the Kuril-Kamchatka Trench region in August–September 2016; Loxosomella aeropsis Borisanova, Chernyshev & Ekimova, 2018, L. cyatiformis Borisanova, Chernyshev & Ekimova, 2018, L. malakhovi Borisanova, Chernyshev & Ekimova, 2018, L. sextentaculata Borisanova, Chernyshev & Ekimova, 2018 were collected during the German-Russian deep-sea expedition SokhoBio aboard RV Akademik Lavrentyev to the Kuril Basin of the Sea of Okhotsk, Bussol Strait, and the adjacent open Pacific abyssal area in July - August 2015. Five species were found in abyssal area (L. aeropsis, L. cyatiformis, L. malakhovi, L. profundorum, L. sextentaculata) at depths ranging from 3,206 to 5,223 m, one species in hadal zone at depth 6,202-6,204 m (L. marcusorum) (Table 1). The samples containing entoprocts were collected with a box corer (BC) (L. malakhovi) or dredged with a modified camera-epibenthic sledge (EBS) (L. cyatiformis, L. malakhovi, L. profundorum), and Agassiz trawl (AGT) (L. aeropsis, L. marcusorum, L. sextentaculata).

Table 1.Download as CSV 

List of deep-sea loxosomatid entoprocts from the NW Pacific.

Species	Point of species detection	Coordinates	Depth, (m)	Locality	Host species	References
Loxosomella profundorum		43.029667 N 152.976833 E	5,222–5,223	NW Pacific, east of the Kuril Islands	Corallimorpharia Carlgren, 1943	Borisanova et al. 2015
Loxosomella aeropsis	1	48.05 N 150.005 E	3,348	Kuril Basin, Sea of Okhotsk	Aeropsis fulva (Agassiz, 1898) (Aeropsidae Lambert, 1896, Echinoidea)	Borisanova et al. 2018
	2	46.261667 N 152.051667 E	3,580	Open Pacific abyssal area between the Bussol Strait and the Kuril-Kamchatka Trench.
Loxosomella cyatiformis	1	45.588333 N 146.411667 E	3,206	Kuril Basin, Sea of Okhotsk	Catillopecten squamiformis (Bernard, 1978) (Propeamussiidae Abbott, 1954, Bivalvia)	Borisanova et al. 2018
	2	46.91 N 151.088333 E	3,296
Loxosomella malakhovi	1	45.625 N 146.373333 E	3,216	Kuril Basin, Sea of Okhotsk	Aglaophamus sp. (Nephtyidae Grube, 1850, Polychaeta)	Borisanova et al. 2018
	2	46.91 N 151.088333 E	3,296
	3	46.95 N 151.083333 E	3,300
	4	48.09 N 150.026667 E	3,347
	5	47.203332 N 149.611666 E	3,366
Loxosomella sextentaculata		48.05 N 150.005 E	3,348	Kuril Basin, Sea of Okhotsk	Laonice sp. (Spionidae Grube, 1850, Polychaeta)	Borisanova et al. 2018
Loxosomella marcusorum	1	45.942883 N 152.904267 E	6,201.7	Kuril-Kamchatka Trench	Thalassema sp. (Thallassematidae Forbes & Goodsir, 1841 (Echiura)	Borisanova and Chernyshev 2019
	2	45.943117 N 152.904183 E	6,203.9

All species were fixed in 95% ethanol (for light microscopy, scanning electron microscopy, and for molecular analyses). Several specimens of two species (L. aeropsis, L. malakhovi) were also fixed in 4% paraformaldehyde solution in 0.1 M phosphate-buffered saline (for confocal laser scanning microscopy).

The brief descriptions of the deep-sea entoproct species of the NW Pacific are given below. The main morphological characteristics are listed in Table 2.

Table 2.Download as CSV 

Main morphological characteristics of deep-sea Entoprocta.

Species	Average body length, μm	Tentacle number	Sensitive papilla	Shape of stomach	Foot	Budding area
Loxosomella cyatiformis	498	14	One unpaired	triangular	No	latero-frontal, lower level of stomach
Loxosomella malakhovi	190	8	No	roundish	No; stalk ends with concaved disk	frontal, middle level of stomach
Loxosomella sextentaculata	834	6	No	roundish	Present	latero-frontal, upper level of stomach
Loxosomella aeropsis	558	9-10	No	roundish or slightly triangular	No	latero-frontal, middle level of stomach
Loxosomella profundorum	3,200	10-12	1 pair	heart-shaped	No	latero-frontal, upper level of stomach
Loxosomella marcusorum	596	10-12	No	roundish-triangular	No; stalk ends with star-shape plate	latero-frontal, bottom of stomach

Loxosomella cyatiformis Borisanova, Chernyshev, Ekimova, 2018

(Figure 1A)

Figure 1.

Deep-sea entoprocts of the NW Pacific. (A) Loxosomella cyatiformes, lateral view, (B, C) Loxosomella malakhovi: (B) two specimens on a gill of parapodia, (C) specimen with a bud, lateral view, (D) Loxosomella sextentaculata, frontal view, (E, F) Loxosomella aeropsis: (E) two specimens on a spine of sea urchin, (F) lateral view of specimen, (G, H) Loxosomella profundorum: (G) frontal view of specimen, (H) lateral view of calyx with a bud, (I) Loxosomella marcusorum, lateral view. Abbreviations: b, bud; e, embryo; f, foot; g, gill of parapodia; ht, host tissue; pl, star-shaped plate; sp, spine of sea urchin; st, stalk; t, tentacles. Scale bars: (A, B, D–I) 200 µm, (C) 100 µm.

Loxosomella cyatiformis was found in the Kuril Basin of the Sea of Okhotsk, at depths 3,206 m and 3,296 m. Specimens were found living on scallop valves. L. cyatiformis is a medium-sized species. The total body length is from 383 μm to 596 μm. The stalk is longer than the calyx. Foot is absent in adults. Calyx bears 14 tentacles. One unpaired papilla is present on the abfrontal side of calyx. Stomach is triangular. Buds originate from the latero-frontal areas located at the lower level of the stomach. Full-developed buds with a conspicuous foot. Species was collected in late July-early August 2015, and some specimens were found with developing embryos, up to seven embryos in the calyx.

Loxosomella malakhovi Borisanova, Chernyshev, Ekimova, 2018

(Figure 1B, 1C)

Loxosomella malakhovi was found in the Kuril Basin of the Sea of Okhotsk, at depths 3,216–3,366 m. Specimens were attached to the gills of parapodia of nephtyid polychaetes. L. malakhovi is a small-sized species. The total body length is from 160 μm to 225 μm. The stalk is very short. Foot is reduced, and the stalk is ended with a roundish concaved disk which grasps part of the host tissue. The calyx bears eight tentacles. Sensitive papillae are not found. The stomach is roundish. Buds originate from the frontal area located at the middle level of the stomach. Full-developed buds with a prominent foot. Specimens were collected from mid-July to early August 2015, and many specimens were found with developing embryos, usually with two large embryos in the calyx.

Loxosomella sextentaculata Borisanova, Chernyshev, Ekimova, 2018

(Figure 1D)

Loxosomella sextentaculata was found in the Kuril Basin of the Sea of Okhotsk, at a depth of 3,348 m. Specimens were found attached to parapodia of spionid polychaetes. L. sextentaculata is a large-sized species. The total body length is from 705 µm to 938 µm. The stalk is long, and ends with a large foot. Calyx bears six tentacles. Sensitive papillae are not found. The stomach is roundish. Buds originate from the latero-frontal areas located at the upper level of the stomach. Full-developed buds were not observed. Species was collected in late July 2015, and no embryos were observed.

Loxosomella aeropsis Borisanova, Chernyshev, Ekimova, 2018

(Figure 1E, 1F)

Specimens of Loxosomella aeropsis were collected in the Kuril Basin of the Sea of Okhotsk at a depth of 3,348 m and in open Pacific abyssal area between the Bussol Strait and the Kuril-Kamchatka Trench at a depth of 3,580 m. Specimens were found living attached to the anterior spines of sea urchins. L. aeropsis is a medium-sized species. Total body length is from 417 µm to 925 µm, the stalk is longer than the calyx. The foot is absent in adults. Calyx bears 10 or, more rarely, nine tentacles. Sensitive papillae are not found. The stomach is roundish or slightly triangular. Buds originate from the latero-frontal areas located at the middle level of stomach. Full-developed buds bear eight tentacles and have a short foot. Species was collected in late July 2015, and no embryos were observed in the calyxes of any specimens.

Loxosomella profundorum Borisanova, Chernyshev, Neretina & Stupnikova, 2015

(Figure 1G, 1H)

Loxosomella profundorum was found in the abyssal plain adjacent to the Kuril-Kamchatka Trench, at depths of 5,222–5,223 m. Specimens were attached to the oral disc of the corallimorpharian polyp. L. profundorum is one of the largest species among entoprocts. The total body length is from 1.3 mm to 4 mm, the stalk is long, up to 3.5 mm. Foot is reduced in adults. Calyx bears 10–12 tentacles. One pair of sensitive papillae is present. Stomach is heart-shaped. Buds originate from the latero-frontal areas located at the upper level of the stomach. Full-developed buds were not observed. Species was collected in mid-August 2012, and no embryos were observed.

Loxosomella marcusorum Borisanova, Chernyshev, 2019

(Figure 1I)

Loxosomella marcusorum was found in the Kuril-Kamchatka Trench, in the hadal zone, at depths of 6,202-6,204 m. L. marcusorum is an epibiont of echiurids. It is a medium-sized species. Total body length is from 449 µm to 685 µm, the stalk is shorter than the calyx. Foot is absent, the stalk is ended with an expanded star-shaped plate immersed in the host body. Calyx bears 10–12 tentacles. Sensitive papillae were not found. Stomach is roundish-triangular. Buds originate from the latero-frontal areas located at lower level of stomach. Full-developed buds were not observed. The species was collected in late August 2016, and many specimens had embryos developing in the calyx, usually with four embryos at a time.

Eight new species of Entoprocta were found in the NW Pacific region during three deep-sea expeditions in recent years: Loxosomella aeropsis, L. cyatiformis, L. malakhovi, L. marcusorum, L. profundorum, L. sextentaculata, and two species, that have not yet been described (one species is an epibiont of polychaetes from the family Scalibregmatidae Malmgren, 1867, another species is associated with Sipuncula (Borisanova et al. 2018)). The discovery of eight new species in three expeditions suggests that the biodiversity of Entoprocta in the abyssal and hadal zone may be quite high in this region, and future investigations will contribute to our knowledge of the deep-sea entoproct species diversity.

Deep-sea Entoprocta are associated with different deep-sea animals, including those that are not characteristic for shallow-water entoproct species: corallimorpharians, sea urchins, and bivalve molluscs. The new symbiotic associations in the abyssal zone could have evolved due to the shortage of firm substrata and the low density of benthic animals, which results in an extremely limited choice of available hosts for epibiotic species. Although five species (Loxosomella malakhovi, L. marcusorum, L. sextentaculata, and two undescribed Loxosomella species) were found in association with annelids, the most typical hosts for Entoprocta (Nielsen 1964, Borisanova 2018).

Molecular genetics analysis of four species of deep-sea Entoprocta indicates that these species are not close to each other and cluster with different species of shallow-water entoprocts from different habitats. L. cyatiformis forms a single clade with L. vancouverensis Rundell & Leander, 2012 from the western coast of the Pacific Ocean (Vancouver Island in British Columbia, Canada). L. malakhovi clustered together with L. varians Nielsen, 1964 which was found in the Atlantic Ocean, and L. murmanica (Nilus, 1909) which was found in the Atlantic, Antarctic and Arctic Ocean, but not in the Pacific (Nielsen 1989, Emschermann 1993). L. aeropsis forms a sister clade to L. malakhovi, L. murmanica, and L. varians. L. profundorum is genetically different from all solitary entoproct species and forms a sister clade to all Loxosomatidae.

This paper was part of the “Biogeography of the NW Pacific deep-sea fauna and their possible future invasions into the Arctic Ocean project (Beneficial project)”. Beneficial project (grant number 03F0780A) was funded by Federal Ministry for Education and Research (BMBF: Bundesministerium für Bildung und Forschung) in Germany. The author is deeply thankful to Hanieh Saeedi for her help with the preparing of the manuscript. The author is grateful to Rachel Downey for reviewing and English proofreading this chapter. This work was financially supported by the Russian Science Foundation (grant 18–14–00082) and Moscow State University Grant for Leading Scientific Schools “Depository of the Living Systems” in frame of the MSU Development Program.

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Nemertea is a phylum of the invertebrates known as nemerteans, or ribbon worms, which includes about 1300 valid species (Kajihara et al. 2008). These worms are found from the supralittoral to the abyssal zone on different bottoms, including silt, sand, algae, sponges, and dead corrals. Nemerteans are predators and scavengers feeding on small crustaceans, annelids, and molluscs. About 50 species are symbionts of decapods, barnacles, bivalve mollusks, ascidians, star-fishes, sea urchins, sea anemones, and echiurids. Most of the bathypelagic nemerteans (about 100 species) known to date are commonly found within a depth range of a few hundred to a few thousand meters, reaching their highest abundance at 600–3,000 m. To date, 22 species of fresh-water nemerteans and 13 species of land nemerteans living have been described (Gibson 1995; Sundberg and Gibson 2008).

Available information on deep-sea benthic nemerteans is even less compared to the shallower waters. By 2013, only seven of the described species of benthic nemerteans have been collected from depths exceeding 2,000 m, with none truly abyssal among them. In the last five years, we have had the opportunity to create a diverse and abundant collection of benthic nemerteans from abyssal and hadal depths. The data obtained indicate a high species diversity of benthic nemerteans inhabiting depths greater than 3,000 m. Nine species of the abyssal, pseudoabyssal and hadal nemerteans have been described quite recently (Chernyshev 2013; Chernyshev et al. 2015; Chernyshev and Polyakova 2018a; Chernyshev and Polyakova 2018b, Chernyshev and Polyakova 2019).

The present chapter summarizes published data on deep-sea benthic nemerteans found during four Russian-German deep-sea expeditions in the NW Pacific.

During four Russian-German deep-sea expeditions (SoJaBio 2010, KuramBioI 2012, SokhoBio 2015, and KuramBioII 2016) benthic nemerteans were collected at depths from 470 to 9,577 m. Specimens were sampled using a camera epibenthic sledge (EBS), Agassiz trawl (AGT), and giant box-corer (BK). We divided the collected nemerteans into two groups. The first one included nemerteans studied live and fixed for both morphological (in 4% paraform) and genetic (in 96% ethanol) analyses. Due to the rapid technique for examination of the internal structure using confocal laser scanning microscopy, the taxonomic affiliation of these nemerteans was determined, and some specimens were identified to the genus or species level. However, these nemerteans in each sample were represented by one, rarely two specimens. The second group includes individuals collected using EBS and fixed in chilled (−20°C) 96% ethanol and kept in a −20°C freezer. They are of little use for morphological studies, and thus their systematic position has not been determined as accurately as for individuals studied live.

Deep-sea benthic nemerteans are frequently found damaged in hydrobiological samples: in most cases, with the epidermis and the posterior or anterior parts of the body missing, making them unsuitable for description. Most of collected specimens could be identified down to the family or order level only. Animals collected using EBS are best preserved for genetic studies (Chernyshev and Polyakova 2018a), but a quick fixation in cold 95% ethanol makes specimen less suitable for histological studies; a fixation in formol does not allow genetic analysis of material. The optimum way is fixation of a largest portion of living nemertean’s body in 4% formalin for morphological investigation and a small portion in 95% ethanol for genetic studies. This approach was applied in description of the first abyssal heteronemertean Sonnenemertes cantelli (Chernyshev et al. 2015).

In samples from the abyssal and hadal zones, a vast majority of nemerteans belonged to four groups: (1) carininid palaeonemerteans (Carininidae); (2) tubulanid palaeonemerteans (Tubulanidae s. str.); (3) heteronemerteans; (4) eumonostiliferous hoplonemerteans. Archinemerteans (cephalotrichid palaeonemerteans) and reptantian hoplonemerteans occurred much more rarely; carinomid palaeonemerteans and cratenemertid hoplonemerteans were not found in the samples from the abyssal and hadal zones (Chernyshev 2013, 2015; Chernyshev and Polyakova 2018a, Chernyshev and Polyakova 2018b, 2019; Chernyshev et al. 2015) (Figure 1 and Map 1). Deep-sea palaeonemerteans are represented by new species from known genera (Carinina, Cephalothrix, and Tubulanus s.l.).

Figure 1.

NW Pacific nemerteans. (A) Cephalothrix iwatai; (B) Sonnenemertes cantelli; (C) Proamphiporus crandalli.

Map 1.

Distribution of NW Pacific nemertean species.

All deep-sea Carinina collected in the NW Pacific form a highly supported clade, which is sister group to a clade of the shallow-water species (Chernyshev and Polyakova 2019). The most basal position in this clade is occupied by Carininidae KuramBio II 103, collected in the Kuril-Kamchatka Trench at a depth of 9,301 m. A preliminary study of the internal structure of some of the hadal carininids showed their structure typical of Carininidae: they had intraepidermal lateral nerve cords, well-developed inner circular musculature of the body wall, rhynchocoel wall with thick circular muscle layer, and inner position of the lateral blood vessels. A distinguishing feature of the collected deep-sea Carinina (except for Carininidae KuramBio II 103) is that they had relatively large body sizes (with the length of the anterior fragment of the body being 4–5 cm; the total body length was apparently 1.5–2 times larger). It should specially be noted that, according to DNA analysis, the specimens collected at the hadal stations, did not belong to the same species as those collected at abyssal depths in the Kuril-Kamchatka Trench, near the Kuril Islands, and from the abyssal plain near the Kuril-Kamchatka Trench.

Cephalothrix iwatai Chernyshev, 2013 (Figure 1a) is a single known deep-sea archinemertean and obviously common species in depth of 1,500–3,300 m in the Sea of Japan (Chernyshev 2013). Most nemerteans of genus Cephalothrix occur in the intertidal zone; whereas, only few have been found in depths from 20 to 40 m. In the Sea of Japan, several shallow-water Cephalothrix species have been recorded, but they all differ from C. iwatai and do not live in soft bottoms. Other deep-sea species for the Sea of Japan, Micrura bathyalis Chernyshev, 2013 from depths 2,670–3,426 m, is closely related to the sublittoral Cerebratulus species. The lack of typical abyssal species of nemerteans in the deep Sea of Japan is connected with isolation of this basin from the Pacific abyssal depths.

Basal heteronemertean Sonnenemertes cantelli Chernyshev, Abukawa & Kajihara, 2015 (Figure 1b) was collected in samples from four stations in the Sea of Okhotsk and on both sides of the Kuril-Kamchatka Trench. Sonnenemertes cantelli has not been found in samples from depths shallower than 3,000 m and deeper than 6,000 m. For this reason, the species can be considered a marker of abyssal fauna. Our records of S. cantelli, originally described from a single abyssal station adjacent to the Kuril-Kamchatka Trench (Chernyshev et al. 2015) and reported from the Sea of Okhotsk (Chernyshev and Polyakova 2018b), further support relationships between the abyssal fauna of the Kuril Basin and parts of the NW Pacific fauna via straits between the Kuril Islands. Sonnenemertes cantelli apparently feeds on small sipunculid worms (Chernyshev and Polyakova 2018b).

Abyssal Proamphiporus crandalli Chernyshev & Polyakova, 2019 (Figure 1c) found in the Kuril-Kamchatka Trench from depth of 5,496 m is very similar in external traits and internal morphology to Amphiporus rectangulus Strand et al., 2014, described from the coastal waters of Norway from a depth of 220 m. Deep-sea benthic nemerteans do not have any specific color pattern, and live worms are unicoloured: mostly in whitish, pinkish, yellowish or, rarely, reddish tones. It is noteworthy that P. crandalli is the only abyssal nemertean with specific color pattern on the head, much resembling that in A. rectangulus.

Uniporus alisae Chernyshev & Polyakova, 2018 collected in the Sea of Okhotsk from depth of 3,301 m is morphologically close to Uniporus hyalinus Brinkmann, 1914–1915 described from the bathyal (depth 1,000–1,200 m) of the Norwegian Sea. Externally both species are very similar and have gelatinous translucent body.

The Nemertovema is a single known hadal genus with two described species: Nevertovema hadalis Chernyshev & Polyakova, 2018, collected in the Puerto Rico Trench from a depth of 8336–8,339 m (Chernyshev and Polyakova 2018a), and Nemertovema norenburgi Chernyshev & Polyakova, 2019, found in the Kuril-Kamchatka Trench from depths of 8,220 and 8,271 m (Chernyshev and Polyakova 2019). The pairwise COI-gene sequence distances between N. hadalis and N. norenburgi are 8.1–8.5%, which seems unusual because of the huge geographic gap between the Puerto Rico Trench and the NW Pacific localities. For comparison, the p-distances between COI-gene sequences of the sympatric cryptic species of the shallow water nemerteans are 8–14%.

Galathenemertes giribeti Chernyshev & Polyakova, 2019, found in the tube of sea anemone Galatheanthemum sp. in the Kuril-Kamchatka Trench from depth of 7,256 m, is the deepest known symbiotic nemertean and the second known species associated with Actinia. This species is closely related to ascidian-associated nemertean Gononemertes parasita Bergendal, 1900 (Chernyshev and Polyakova 2019). Deep-sea symbiotic nemerteans comprise species from the genera Ovicides and Gononemertes s.l. At least four undescribed species of Gononemertes-like nemerteans have been obtained from the peribranchial cavity of deep-sea carnivorous ascidians of the genus Culeolus (Herdman, 1881): (1) “Gononemertes” sp.1 from Culeolus nadejdi found in the Sea of Okhotsk (depth 1,040–1,050 m) (Sanamyan 1992); (2) “Gononemertes” sp. 2 from Culeolus sp. found in the abyssal plain adjacent to the Kuril–Kamchatka Trench (depth 4,869 m) (Chernyshev et al. 2015); (3) “Gononemertes” sp. 3 from Culeolus sp. found in the Kuril Basin of the Sea of Okhotsk (depths 3,301-3,347 m) (Chernyshev and Polyakova 2018b); (4) “Gononemertes” sp. 4 from Culeolus barryi, off the coast of California (depth 1,200 m) (Sanamyan et al. 2018). The phylogenetic analysis has confirmed the previously stated assumption that deep-sea Gononemertes-like nemerteans associated with Culeolus cannot be attributed to the genus Gononemertes with the type species G. parasita associated with the shallow-water ascidian Phallusia (Chernyshev and Polyakova 2018b).

The taxonomic diversity of the abyssal and hadal nemerteans is quite high, though sequences have been obtained for less than a third of the species collected during deep-sea expeditions in the NW Pacific. Of particular interest is the finding of genetically close species (Nemertovema hadalis and N. norenburgi) in the Puerto Rico and Kuril-Kamchatka Trenches, which may indicate the relationships in the hadal nemertean fauna from different regions of the World Ocean. Among the deep-sea heteronemerteans and hoplonemerteans, species that could not be assigned to any of the known genera seem to be predominant. The genetic and morphological similarity between Amphiporus rectangulus and Proamphiporus crandalli is the first proven case of close phylogenetic relationships between sublittoral and real abyssal nemertean species (Chernyshev and Polyakova 2019).

The species diversity of nemerteans in the samples from the abyssal zone is usually quite high (about 50–60 species in the NW Pacific Ocean); however, in the Sea of Japan only two nemertean species, Cephalothrix iwatai (Chernyshev, 2013) and Micrura bathyalis, were found at a depth of over 3 km. With the rare exceptions, abyssal and hadal nemerteans are genetically well distinguished from shallow-water species. The p-distances between the COI sequences of the C. iwatai and shallow-water Cephalothrix sp. 4 TCH-2015 from northeast Pacific are 6.4–6.5%, indicating their close relationship. Accordingly, this fact indicates that C. iwatai is a ‘young’ eurybathic species.

Another interesting finding of our research was the different species compositions of nemerteans in the abyssal and hadal zones of the Kuril-Kamchatka Trench and adjacent abyssal depths. Moreover, we found no species present in the samples from both the abyssal and bathyal zones. The exceptions were Cephalothrix iwatai and Micrura bathyalis, but it should be taken into account that both species live in the Sea of Japan, which lacks real abyssal fauna (Andriashev 1979). Apparently, among nemerteans eurybathic species with a wide range of habitat depths are not as common. There is no doubt that the abyssal and hadal zones of the World Oceans are inhabited by a large number of species and genera of nemerteans new to science.

This paper was part of the “Biogeography of the NW Pacific deep-sea fauna and their possible future invasions into the Arctic Ocean project (Beneficial project)”. Beneficial project (grant number 03F0780A) was funded by Federal Ministry for Education and Research (BMBF: Bundesministerium für Bildung und Forschung) in Germany. I am grateful to Hanieh Saeedi for help with the map and the English proofread.

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Solenogastres (= Neomeniomorpha) are exclusively marine, vermiform molluscs. Together with Caudofoveata (= Chaetodermomorpha), they form the clade Aplacophora, a name referring to their body, which lacks a shell. Solenogastres is a comparatively species poor and understudied class of Mollusca with currently 293 described species organized in 24 families and four orders. Most Solenogastres are minute and reach only a few millimeters in body length and are thus usually collected with sampling gear designed for benthic meiofauna (only few giant species reach exceptional body lengths of up to 30 cm, and can be retrieved through macrobenthic sampling).

In Solenogastres, the molluscan foot is reduced to a narrow ciliary gliding sole, usually visible as a fine median line running along the ventral side of the animal and they lack a head shield (both characters help to distinguish them from equally worm-shaped Caudofoveates). Aragonitic sclerites protrude from the chitinous cuticle surrounding the entire body. These sclerites (comprising the so-called scleritome) are highly diverse, ranging from solid or hollow needles to solid scale-like elements. Depending on the composition of the scleritome, Solenogastres often appear smooth and shiny, shaggy, or very spiny. Together with the organization and histology of the digestive and reproductive system, the scleritome serves as one of the main taxonomic characters required to differentiate and identify solenogaster species. Most scientific work conducted on this group focuses on traditional taxonomy, but recent phylogenomic studies have begun investigating internal evolutionary relationships and rendered several parts of the current classificatory system (i.e. the order Cavibelonia Salvini-Plawen, 1978) paraphyletic (Kocot et al. 2019). Solenogastres systematics will thus likely receive major revisions in the near future.

1.2. Habitat

Solenogastres inhabit a wide range of sediments from coarse shell gravel and volcanic sands to fine, silty sediments. Several species have been found living epizoically on cnidarians (Figure 1) or in association with sponges (Kocot et al. 2019).

Figure 1.

Dondersiidae sp. SB-2 (Pholidoskepia), from the Kuril Basin of the Sea of Okhotsk. Found wrapped around a cnidarian. Head to the right. Scale bar: 1 mm.

While a few species can be collected in knee-deep waters of the shallow intertidal zone, the lower continental shelf is currently assumed to harbor the highest species diversity (Todt 2013). The solenogaster fauna of the world’s vast abyssal plains and the hadal zone of oceanic trenches still remain largely unexplored.

The present chapter aims to compile the current knowledge on the diversity and distribution of Solenogastres in the investigated area of the NW Pacific, recorded from the deep sea below 2,000 m. Based on this data, we explore putative patterns of species richness and distribution comparing the open NW Pacific and the semi-isolated adjacent Sea of Okhotsk.

Within the last couple of years, the number of deep-sea species of Solenogastres (below 2,000 m) recorded from the NW Pacific has risen from zero to 66 candidate species, with the majority new to science and still pending formal descriptions.

It is generally assumed that solenogaster diversity is the highest on the continental shelf (Todt 2013) and decreases with increasing depth, which is a general trend in benthic deep-sea diversity (Rex et al. 1990). The comparably high number of abyssal species in the NW Pacific is most likely result from sampling bias, as the solenogaster fauna of the adjacent bathyal zone currently remains largely unexplored.

The currently known species recorded in the NW Pacific and summarized in this chapter present only a fraction of the actual diversity of deep-sea Solenogastres in the region, and we expect them to continuously rise with increasing sampling efforts.

We wish to express our gratitude to the editors, Hanieh Saeedi and Angelika Brandt, for the opportunity to join this compilation on benthic NW Pacific deep-sea fauna. Thanks to Hanieh Saeedi for creating the maps of the present chapter and the proofread of the manuscript, Peter C. Kohnert (ZSM Munich) for providing pictures of the amphimeniid Solenogastres, and Bastian Brenzinger (ZSM Munich) for suggestions on the manuscript. We also wish to thank all crew members and scientists participating in the SokhoBio, KuramBio I and II expeditions for their support during the cruises and their sampling efforts. This chapter was part of the “Biogeography of the NW Pacific deep-sea fauna and their possible future invasions into the Arctic Ocean project (Beneficial project)”. The Beneficial project (grant number 03F0780A) was funded by Federal Ministry for Education and Research (BMBF – Bundesministerium für Bildung und Forschung) in Germany.

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Bivalve mollusks occur from the intertidal zone to the greatest depths of the World Ocean (Filatova 1968, 1971; Knudsen 1970; Scarlato 1981; Belyaev 1989; Coan et al. 2000; Jamieson 2015). Along with peracarid crustaceans, polychaetes and echinoderms, they are the most common and widespread animals in the abyssal zone and have played a key role as a model group in deep-sea biology research (Filatova 1958, 1971, 1976, 1982; Knudsen 1970; Brandt et al. 2005, 2007; Allen 2008; Tittensor et al. 2011). Bivalves are also found in the hadal zone of all investigated deep-water trenches at all depths down to the maximum depth in the Mariana Trench (Filatova 1960; Knudsen 1970; Belyaev and Mironov 1977; Filatova 1985; Belyaev 1989; Ramirez-Llodra et al. 2010; Jamieson 2015; Kamenev 2019). In the hadal zone, they are one of the richest groups of animals, after polychaetes and isopods, in terms of the number of species (Jamieson 2015), and rank second to holothurians in terms of average abundance on the bottom of various trenches, forming sometimes populations with very high densities (Zenkevitch et al. 1955; Filatova 1971; Belyaev 1989). Bivalves are currently one of the best-known animal groups in the northern Pacific (Scarlato 1981; Higo et al. 1999; Coan et al. 2000; Okutani 2000). However, the species composition and distribution of bivalves in the NW Pacific (north of 40°N) at depths in excess of 2,000 m remain insufficiently studied thus far. The NW Pacific is one of the most productive regions of the Pacific Ocean (Sokolova 1981) with high diversity of its benthic fauna (Zenkevitch and Filatova 1958; Filatova 1960, 1968; Belyaev 1989). In this regard, the study of the composition and distribution patterns of the deep-sea fauna in this region of the Pacific Ocean is of particular interest.

The investigated region of the NW Pacific includes several deep-water ecosystems that are connected with one another to a varying degree: deep-sea basins (maximum depths greater than 3,000 m) in the Sea of Japan, the Sea of Okhotsk, and the Bering Sea; oceanic slopes of the Kuril, Commander, and Aleutian Islands, as well as eastern coast of Kamchatka Peninsula (2,000-6,000 m); abyssal oceanic plain (5,000-6,000 m) adjacent to the Kuril-Kamchatka and Aleutian trenches; the northernmost part of the Japan Trench, the Kuril-Kamchatka Trench, and the deepest western part of the Aleutian Trench (depths in excess of 6,000 m). These deep-sea regions differ in the time of origin, geomorphology, depth, hydrological and hydrochemical regimes, bottom sediment structure, and consequently, the habitat conditions of benthic animals. In addition, they differ in the state of study of the deep-sea bivalve fauna.

The main objectives of this work are (1) to investigate the species composition and richness of bivalve fauna of the deep NW Pacific areas (north of 40°N) at depths in excess of 2,000 m; (2) to analyze the geographic distribution of species founded in these areas; (3) to study the change in the species composition and richness of bivalves in relation to depth.

For this study I designated 8 deep-sea areas within the NW Pacific, differing in the time of origin, geomorphology, depth, hydrological and hydrochemical regimes, bottom sediment structure: the Central Basin of the Sea of Japan; the Kuril Basin of the Sea of Okhotsk; the Commander, Aleutian, and Bowers basins of the Bering Sea; oceanic slopes of the Kuril, Commander, and Aleutian Islands, as well as the eastern coast of Kamchatka Peninsula; the abyssal oceanic plain adjacent to the Kuril-Kamchatka and Aleutian trenches; the northernmost part of the Japan Trench; the Kuril-Kamchatka Trench; the western part of the Aleutian Trench (Map 1). For the analysis of species composition and distribution of bivalve molluscs in these areas I used all available data from relevant literature sources (Table 1).

Table 1.Download as CSV 

List of species and the depth range (in meters) of finding of bivalves recorded at depths greater than 2000 m in different deep NW Pacific areas (north of 40°N) and in eastern Pacific.

Family	Species	Sea of Japan	Sea of Okhotsk	Bering Sea	Oceanic slopes of the Kuril, Commander, and Aleutian islands	Oceanic plain	Northernmost part of the Japan Trench	Kuril- Kamchatka Trench	Western part of the Aleutian Trench	Eastern Pacific	References
Nuculidae Gray, 1824	Nucula profundorum Smith, 1885					4,861-5,406				734-4,134	Coan et al. 2000; Coan and Valentich-Scott 2012; Kamenev 2015, 2019
Pristiglomidae Sanders & Allen, 1973	Pristigloma cf. alba Sanders and Allen, 1973					5,112-5,427					Kamenev 2015
	Setigloma japonica (Smith, 1885)					4,861-5,427				3,000-5,240	Schileyko 1983; Coan et al. 2000; Kamenev 2015
Nuculanidae H. Adams & A. Adams, 1858	Ledellina convexirostrata Filatova & Schileyko, 1984					4,861-5,427		6,441-6,710		4,860	Filatova and Schileyko 1984; Kamenev 2015, 2019
	Ledellina formabile Filatova & Schileyko, 1984			3,661-4,294						2,800	Filatova and Schileyko 1984; Kamenev 2019
	Microgloma sp.				3,342-3,432	4,861-5,427					Kamenev 2015
	Nuculana leonina (Dall, 1896)		307-2,850	2,622-3,034						1,690-2,500	Kamenev 2018c; Coan et al. 2000
	Parayoldiella ultraabyssalis (Filatova, 1971)							8,355-9,583			Filatova 1971; Filatova and Schileyko 1985; Belyaev 1989; Kamenev 2019
	Parayoldiella mediana (Filatova & Schileyko, 1984)							7,265-8,740			Filatova and Schileyko 1984, 1985; Belyaev 1989; Kamenev 2015
	Poroleda extenuata (Dall, 1897)		2,850		2,992-3,432					2,000-2,900	Coan et al. 2000; Kamenev 2018c
	Robaia robai (Kuroda, 1929)	83-2,900									Scarlato 1981; Okutani 2000; Kamenev 2013
Bathyspinulidae Coan & Scott, 1997	Bathyspinula calcarella (Dall, 1908)				5,220-5,572	4,861-5,752		6,000-6,860	6296-6328	4,200-5,830	Filatova 1958, 1976; Filatova and Schileyko 1984; Belyaev 1989; Coan et al. 2000; Coan and Valentich-Scott 2012; Kamenev 2019
	Bathyspinula calcar (Dall, 1908)					5,379-5,743				4,000-5,000	Filatova 1958; Coan et al. 2000; Coan and Valentich-Scott 2012; Kamenev 2015
	Bathyspinula vityazi (Filatova, 1964)						6,475-7,587	6,435-9,335	6,965-7,250		Filatova 1964, 1971, 1976; Belyaev 1989; Kamenev 2019
Malletiidae H. Adams & A. Adams, 1858	Katadesmia vincula (Dall, 1908)		3,206-3,366	1,490-4,382	1,260-5,572	4,861-5,427	7,320	6,400-7,256	6,856-7,250	590-3,585	Coan et al. 2000; Okutani and Fujiwara 2005; Coan and Valentich-Scott 2012; Kamenev 2015, 2018c, 2019
Neilonellidae Schileyko, 1989	Neilonella politissima Okutani & Kawamura, 2002					4,861-5,787					Kamenev 2015, 2019
	Neilonella profunda Okutani & Fujiwara, 2005						7,320				Okutani and Fujiwara 2005
	Neilonella sp. 1					5,216-5,427					Kamenev 2015
	Neilonella sp. 2				4,679-5,572	4,861-5,752		6,047-6,561			Kamenev 2015, 2019
	Neilonella sp. 3							7,055-7,256	7,246		Belyaev 1989; Kamenev 2019
	Neilonella sp. 4							7,055-8,740			Kamenev 2019
Siliculidae Allen & Sanders, 1973	Silicula beringiana Kamenev, 2014			4,89-4,811	4,890-4,984						Kamenev 2014
	Silicula okutanii Kamenev, 2014				5,013-5,572	5,101-5,497		6,441-6,561			Kamenev 2014, 2015, 2018c, 2019
Tindariidae Verrill & Bush, 1897	Tindaria antarctica Thiele & Jaeckel, 1931				5,220-5,572	4,861-5,427		6,441-6,561			Kamenev 2015, 2019
	Tindaria sp. 1					4,861-5,352					Kamenev 2015, 2019
	Tindaria sp. 2				4,679-5,572	4,861-5,427					Kamenev 2015, 2018c, 2019
	Tindaria sp. 3					4,861-5,427					Kamenev 2015, 2019
	Tindaria sp. 4					4,861-5,427					Kamenev 2015
	Tindaria sp. 5					4,977-4,998					Kamenev 2015
	Tindaria sp. 6		2,327-3,366								Kamenev 2018c
	Tindaria sp. 7							6,441-6,561	6,296-7,286		Belyaev 1989; Kamenev 2019
Yoldiidae Dall, 1908	Megayoldia sp.		2,850								Kamenev 2018c
	Yoldiella derjugini Scarlato, 1981	22-2,520								520-800	Scarlato 1981; Coan et al. 2000; Kamenev 2013
	Yoldiella cf. jeffreysi (Hidalgo, 1877)					4,861-5,497					Kamenev 2015, 2019
	Yoldiella kaikonis Okutani & Fujiwara, 2005						7,299-7,333				Okutani and Fujiwara 2005
	Yoldiella olutoroensis Scarlato, 1981			3,000							Scarlato 1981
	Yoldiella orbicularis Scarlato, 1981	53-2,300									Scarlato 1981; Kamenev 2013
	Yoldiella sp. 1		3,206-3,307		3,342-3,432						Kamenev 2018c
	Yoldiella sp. 2							6,441-7,256			Kamenev 2019
Mytilidae Rafinesque, 1815	Dacrydium rostriferum Bernard, 1978		3,206-3,366	3,313-4,294	4,679-5,013	4,690-5,787		6,090-6,561		2,350-2,870	Coan et al. 2000; Kamenev 2015, 2018c, 2019
	Dacrydium vitreum (Møller, 1842)	40-3,347		500-3,170						depth not specified	Scarlato 1981; Coan et al. 2000; Kamenev 2013
Arcidae Lamarck, 1809	Bathyarca imitata (Smith, 1885)		3,206-3,307			4,861-5,497				1,463-4,000	Coan et al. 2000; Coan and Valentich-Scott 2012; Kamenev 2015, 2018c, 2019
	Bentharca asperula (Dall, 1881					4,861-5,223				3,100-4,900	Coan et al, 2000; Coan and Valentich-Scott 2012; Kamenev 2007, 2015
Pectinidae Rafinesque, 1815	Delectopecten vancouverensis (Whiteaves, 1893)	730-3,435								27-4,100	Scarlato 1981; Coan et al. 2000; Coan and Valentich-Scott 2012; Kamenev 2013
	Hyalopecten abyssalis Kamenev, 2018					4,550-5,020					Kamenev 2018a
	Hyalopecten kurilensis Kamenev, 2018					4,995-5,045					Kamenev 2018a
	Hyalopecten vityazi Kamenev, 2018							6,090-8,100	6,410-7,246		Kamenev 2018a, 2019
Propeamussiidae Abbott, 1954	Catillopecten brandtae Kamenev, 2018					4,860-5,423					Kamenev 2015, 2018b
	Catillopecten malyutinae Kamenev, 2018					4,988-5,418		6,090-6,135		4,081	Kamenev 2015, 2018b, 2019
	Catillopecten natalyae Kamenev, 2018				3,342-3,432	5,112-5,497					Kamenev 2015, 2018b, 2019
	Catillopecten squamiformis (Bernard, 1978)		2,901-3,366	3,957-4,382	3,342-4,990	4,391-4,990				2,000-5,020	Coan et al. 2000; Kamenev 2015, 2018b, 2019
	Parvamussium pacificum Kamenev, 2018					4,860-5,497				5,180	Kamenev 2015, 2018b, 2019; P. Valentich-Scott, personal communication
Limidae Rafinesque, 1815	Limatula sp. 1				5,220-5,572	4,861-5,497					Kamenev 2015, 2019
	Limatula sp. 2					4,997-5,406					Kamenev 2015
Thyasiridae Dall, 1900	Adontorhina cyclia S.S. Berry, 1947		308-3,366		3,342-3,432					12-3,000	Kamenev 1995, 1996, 2013, 2018c; Coan et al. 2000; Coan and Valentich-Scott 2012
	Adontorhina sp. 1				4,679-5,013						Kamenev 2018c
	Axinodon sp. 2		1,694-3,366		4,679-5,013						Kamenev 2018c
	Axinopsida subquadrata (A. Adams, 1862)	5- 2,550									Scarlato 1981; Kamenev 2013
	Axinulus sp. 1		3,342-3,432			4,861-5,787					Kamenev 2015, 2018c
	Axinulus sp. 2							9,301-9,583			Kamenev 2019
	Axinulus sp. 3				5,220-5,572			6,047-6,221			Kamenev 2019
	Axinulus sp. 4								6,460-7,285		Belyaev 1989
	Axinulus hadalis (Okutani, Fujikura & Kojima, 1999)						6,326-7,434				Okutani et al. 1999; Sasaki et al. 2005
	Channelaxinus excavata (Dall, 1901)		2,901-3,218	2,359						800-2,520	Kamenev 2018c; Coan et al. 2000; Coan and Valentich-Scott 2012
	“Genaxinus” sp. 1				5,220-5,572	5,101-5,752		6,047-9,583			Kamenev 2019
	“Genaxinus” sp. 2				5,220-5,572	5,726-5,752		6,441-7,256			Kamenev 2019
	Mendicula sp. 1		1,694-3,366		3,342-5,572	4,861-5,787		6,047-7,256			Kamenev 2015, 2018c, 2019
	Mendicula sp. 2		2,327-3,366		3,342-5,572	4,997-5,752		6,047-6,221			Kamenev 2015, 2018c, 2019
	Mendicula sp. 3		1,694-3,366		3,342-3,432						Kamenev 2018c
	Parathyasira sp. 1		3,206-3,366		3,342-5,013	4,977-5,406					Kamenev 2015, 2018c
	Parathyasira sp. 2				4,679-5,013	5,217-5,406					Kamenev 2015, 2018c
	Parathyasira sp. 3							6,047-6,561			Kamenev 2019
	Thyasira kaireiae (Okutani, Fujikura & Kojima, 1999)						5,791-6,390				Okutani et al. 1999; Sasaki et al. 2005
	Thyasira sp. 1					4,861-5,787					Kamenev 2015
	Thyasira sp. 2	900-3,102									Kamenev 2013
	Thyasiridae gen. sp.							7,055-8,740			Kamenev 2019
Tellinidae Blainville, 1814	Macoma shiashkotanika (Scarlato, 1981)				465-4,984						Kamenev and Nadtochy 1999; Kamenev 2018c
Montacutidae Clark, 1855	Montacutidae gen. sp.					5,101-5,497		6,441-6,561			Kamenev 2015, 2019
	Mysella sp.		1,694-3,351								Kamenev 2018c
	Syssitomya cf. pourtalesiana Oliver, 2012					5,347-5,427					Kamenev 2015
Vesicomyidae Dall & Simpson, 1901	Abyssogena phaseoliformis (Métivier, Okutani & Ohta, 1986)						5,400-6,400	4,700-6,200	4,550-6,400	4,190-4,982	Métivier et al. 1986; Fujikura et al. 2002; Kojima et al. 2004; Sasaki 2005; Okutani et al. 2009; Krylova et al. 2010
	Calyptogena extenta (Krylova & Moskalev, 1996)							3,512		3,000-4,445	Okutani et al. 2009; Coan and Valentich-Scott 2012
	Calyptogena sp.				4,819						Okutani et al. 2009
	Ectenagena laubieri kurilensis (Okutani & Kato, 2009)				3,512-3,560						Okutani et al. 2009
	Isorropodon fossajaponicum (Okutani, Fujikura & Kojima, 2000)						6,248-6,809				Okutani et al. 2000; Fujikara et al. 2002; Sasaki et al. 2005
	“Vesicomya” filatovae Krylova & Kamenev, 2015					4,861-5,497					Kamenev 2015, 2019; Krylova et al. 2015
	Vesicomya pacifica (Smith, 1885)		3,299-3,366	3,957-3,978	3,342-5,572	4,861-5,787				1,200-6,200	Coan et al. 2000; Coan and Valentich-Scott 2012; Kamenev 2015, 2018c, 2019; Krylova et al. 2015, 2018
	Vesicomya profundi Filatova, 1971							6,047-9,050	7,246		Filatov 1971; Belyaev 1989; Krylova et al. 2015, 2018; Kamenev 2019
	Vesicomya sergeevi Filatova, 1971							6,090-9,530			Filatova 1971; Belyaev 1989; Krylova et al. 2015, 2018; Kamenev 2019
Xylophagidae Purchon, 1941	Xylophaga sp. 1					5,216-5,223					Kamenev 2015
	Xylophaga sp. 2					5,347-5,379					Kamenev 2015
	Xylophaga sp. 3					5,217-5,243					Kamenev 2015
	Xylophaga sp. 4					5,217-5,243					Kamenev 2015
	Xylophaga sp. 5					5,347-5,352					Kamenev 2015
Protocuspidariidae Scarlato & Starobogatov, 1983	Protocuspidaria sp.					5,236-5,406					Kamenev 2015
Cuspidariidae Dall, 1886	Bathyneaera hadalis (Knudsen, 1970)			3,957-3,978		4,418-5,752		6,047-8,740			Krylova 1993, 1997; Kamenev 2015, 2019
	Cardiomya behringensis (Leche, 1883)	31-2,900									Scarlato 1981; Kamenev 2013
	Cardiomya filatovae Scarlato, 1972		3,299-3,366	3,260-3,875	3,880-3,900						Scarlato 1972, 1981; Krylova 1997; Kamenev 2018с
	Cardiomya sp. 1				3,342-3,432						Kamenev 2018c
	Cardiomya sp. 2		3,351-3,353								Kamenev 2018c
	Cuspidaria cf. abyssopacifica Okutani, 1975		3,299-3,353		3,342-3,432						Kamenev 2018c
	Cuspidaria cf. arcoida (Okutani & Kawamura, 2002)					5,112-5,130					Kamenev 2015
	Cuspidaria buccina Bernard, 1989					5,112-5,130				3,585	Coan et al., 2000; Kamenev 2015
	Cuspidaria sp. 1					5,101-5,497					Kamenev 2019
	Cuspidaria sp. 2		3,206-3,307								Kamenev 2018c
	Cuspidaria sp. 3		3,305-3,366								Kamenev 2018c
	Cuspidaria sp. 4				2,430-2,670						Krylova 1997
	Cuspidaria sp. 5			3,875							Krylova 1997; Coan et al. 2000;
	Myonera garretti Dall, 1908		3,299-3,366	3,260-4,294						1,645-4,294	Krylova 1997; Coan et al. 2000; Coan and Valentich-Scott 2012; Kamenev 2018c
	Myonera paucistriata Dall, 1886					4,550-5,427				1,000-3,806	Krylova 1997; Coan et al. 2000; Kamenev 2015
	Octoporia sp.					5,379-5,427					Kamenev 2015
	Rengea murrayi (Smith, 1885)					4,861-5,427					Kamenev 2015
	Rhinoclama filatovae (Bernard, 1979)					4,550-5,497				3,315-5,140	Krylova 1997; Coan et al. 2000; Kamenev 2015, 2019
Poromyidae Dall, 1886	Cetoconcha sp.				3,342-3,432	5,236-5,379					Kamenev 2015, 2018c
	Poromya sp.					5,347-5,352					Kamenev 2015
Lyonsiellidae Dall, 1895	Dallicordia cf. alaskana (Dall, 1895)					5,347-5,352				450-3,570	Coan et al. 2000; Coan and Valentich-Scott 2012
	Policordia extenta Ivanova, 1977							8,185-8,400			Ivanova 1977; Belyaev 1989; Kamenev 2019
	Policordia laevigata Ivanova, 1977							8,185-8,740			Ivanova 1977; Belyaev 1989; Kamenev 2019
	Policordia maculata Ivanova, 1977							9,000-9,050			Ivanova 1977; Belyaev 1989; Kamenev 2019
	Policordia ovata Ivanova, 1977								5,740	6,040	Ivanova 1977
	Policordia rectangulata Ivanova, 1977							8,175-9,583			Ivanova 1977; Belyaev 1989; Kamenev 2019
	Policordia sp. 1		3,206-3,366								Kamenev 2018c
	Policordia sp. 2							6,047-6,221			Kamenev 2019