Introduction

The Northwest (NW) Pacific is one of the most extensively studied regions of our world’s oceans. Notably, the research led by Lev Zenkevich aboard the RV Vityaz (e.g., Zenkevich et al. 1955) greatly contributed to our understanding of this area. Moreover, over the past decade, there have been a total of four collaborative German-Russian expeditions to the NW Pacific, specifically aimed at investigating the patterns and processes of biodiversity and biogeography, species connectivity, range extensions, and species evolution in this region (Brandt 2016, Brandt et al. 2020).

The deep-water properties (alt: characteristics) and geomorphology of the NW Pacific’s hadal trenches have been extensively discussed in publications by Johnson (1998) and Jamieson and Stewart (2021). Recent studies have also documented the composition of the isopod and crustacean macrofauna sampled in the NW Pacific, including the Kuril-Kamchatka Trench (KKT) and adjacent area, as highlighted in Brandt et al. (2020) and Knauber et al (2022a). Depth zonation in this region has also been addressed by Brandt et al. (2019). For the Sea of Japan, comprehensive compilations of its deep-sea fauna can be found in works by Brandt et al. (2010), which note that the deep-sea macrofauna of this area is impoverished. A thorough record of deep-sea mega- and macrofauna surveyed in the southern part of the Sea of Japan is compiled in Fujita (2014).

The geography and biology of the NW Pacific have also been comprehensively summarized by Jamieson (2015) and Jamieson and Stewart (2021), which include extensive lists of species occurrences in hadal tranches worldwide. Historically, benthic isopod samples were primarily collected through large bottom trawls aboard the RV Vityaz (Zenkevich et al. 1955), and more recently using an epibenthic sledge (Brandt and Barthel 1995; Brenke 2005; Brandt et al. 2019). However, the Japan Trench has not been investigated using modern fine-meshed EBS in the past years with success. The use of plankton nets in large bottom trawls was introduced by Akiyama et al. (2008). Given the efficiency of this method, we also employed it during the KH-23–5 expedition aboard the RV Hakuho Maru to the KKT and JT for capturing isopod crustaceans.

Material and methods

The KH-23–5 expedition, conducted aboard the RV Hakuho Maru, spanned from September 12th to October 2nd, 2023, with a focus on exploring the KKT and the JT. The primary objectives of this expedition were to collect samples from 28 stations positioned along four transects traversing the trenches (Table 1; Fig. 1). Deployments involved the use of an ORI-type 4-m beam trawl and an Agassiz-type 3-m beam trawl, accompanied by plankton nets either fixed or suspended from the main frame (Fig. 2). Samples from the fixed plankton nets primarily contained pelagic animals from the water column and were fixed immediately upon retrieval of the gear; however, these samples are not referred to in the present paper. The suspended, smaller plankton nets, referred to as the “inner net”, were deployed specifically to sample benthic macrofaunal Peracarida (Crustacea, Malacostraca) from abyssal and hadal depths (Table 1; Fig. 1). The plankton nets used in this study had a mesh size ranging from 330 to 500 µm. This method of combining inner nets with beam trawls (Fig. 2) and employing the “decantation method” (the method to collect small organisms nondestructively by stirring bottom sediment in seawater and sieving suspended materials) is commonly used in Japan and is known to be highly effective in collecting small crustaceans (Akiyama et al. 2008). Collected sediments were sieved using a sieve basket (mesh size: 450 µm). Specimens were sorted under cold condition using stereomicroscopes (SZ61, Olympus, Tokyo, Japan) and identified into the family level on board. Identified specimens were then fixed in 99.5% ethanol.

Table 1 Station list of beam trawl (BT) and Agassiz trawl (AGT) deployments during the KH-23–5 expedition onboard the RV Hakuho Maru
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Fig. 1
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Station map of the RV Hakuho Maru expedition KH-23–5

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Fig. 2
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Illustration of the trawls and the plankton-net fixation. a View of plankton nets in the ORI-type trawl; b ORI-type 4-m beam trawl; c Agassiz-type 3-m beam trawl

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Results

During the KH-23–5 expedition aboard of the RV Hakuho Maru, plankton nets attached to bottom trawls deployed sampled 2656 isopod specimens from at least 16 families (Table 2) across 28 stations at abyssal and hadal depths (3428–7510 m). However, further differentiation within the suborder Epicaridea was hindered as all specimens collected were larvae and could not be identified to family level. The isopod families identified from this material include Antarcturidae, Arcturidae, Desmosomatidae, Haplomunnidae, Haploniscidae, Ischnomesidae, Janirellidae, Janiridae, Macrostylidae, Mesosignidae, Munnidae, Munnopsidae, Nannoniscidae, and Paramunnidae. Amongst these, the most frequently encountered families, with more than 100 specimens, are illustrated in Fig. 3.

Table 2 Isopod family records from the RV Hakuho Maru expedition KH-23–5
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Fig. 3
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© Shoki Shiraki; b Ischnomesidae—© Andreas Kelch; c Haploniscidae—© Henry Knauber; d Desmosomatidae—© Keiichi Kakui; e Macrostylidae—© Anchita Casaubon

Isopods of the families occurring in the plankton nets most frequently. a Munnopsidae—

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Isopods collected with plankton nets attached to both types of bottom trawls were comparable in number. The isopod family Munnopsidae stood out as the most abundant isopod family across all stations (1123 ind.). In contrast, all other isopod families are either epibenthic or possess the ability to burrow. The second most frequently observed family was Haplonsicidae (564 ind.), followed by Macrostylidae (430 ind.), Ischnomesidae (245 ind.), and Desmosomatidae (188 ind.). All other families were much rarer; Nannoniscidae were recorded with 36 individuals, Janirellidae (28 ind.), Antarcturidae (10 ind.), Janiridae (8 ind.), Arcturidae (2 ind.), Haplomunnidae (2 ind.), the Mesosignidae (2 ind.), Munnidae (2 Ind.), Paramunnidae (1 ind.), and Thambematidae. (1 ind.).

Station C8 yielded the highest number of isopods, with 502 individuals in the western KKT, while station F11 yielded the lowest number of isopods, where only a single individual was found in the northern JT.

Discussion

The North Pacific is one of the best investigated deep-sea regions, largely due to the Russian efforts involving eleven expeditions with RV Vityaz led by Lev Zenkevich (Zenkevich et al. 1955). This extensive exploration has resulted in the description of new isopod species (Birstein 1957; 1963; Kussakin 1971; 1988; 1999), accompanied by a wealth of additional publications by Belyaev (1983; 1989), Belyaev and Vilenkin (1983), and many other authors (e.g., Golovan et al. 2018). In recent decades, Malyutina has played a significant role in describing numerous new species of Munnopsidae, obtained during Russian-German and German-Russian expeditions aboard the RVs Sonne (Brandt and Malyutina 2015; Brandt et al. 2019) and M.A. Akademik Lavrentyev (Malyutina and Brandt 2013; Malyutina et al. 2018). Munnopsid isopods are known for their swimming capabilities (Hessler and Strömberg 1989), making this family a frequent encounter in the suprabenthos (Frutos et al. 2017).

The macrofauna (including Isopoda) of the NW Pacific has been comprehensively examined by Brandt et al. (2019), particularly focusing on deep-sea macrofauna based on epibenthic sledge catches from the Sea of Japan, Sea of Okhotsk, the KKT, and the abyssal plain of the NW Pacific. Across these regions, isopods were found to be more prevalent at abyssal depths compared to both bathyal (Sea of Japan and Sea of Okhotsk) and hadal zones (KKT). Data on isopod biogeography and bathymetry are also provided (Brandt et al. 2019; Malyutina and Brandt 2020; Golovan et al. 2018; Elsner et al. 2013a). In the Japan trench, some abyssal and hadal bottom-trawling research aboard the RV Hakuho-maru reported catches using inner plankton nets called “Gamo’s net”, which also yielded small and rare peracarids (Horikoshi and Ohta 1988; Ohta 1989). However, the numbers were small and their identification was vague, thus no quantitative data were available. Furthermore, no EBS samples are available to date from this trench, as an EBS has not yet been successfully deployed in this trench. Moreover, while for EBS samples the exact trawling times can be calculated, allowing for the calculation of the occurrence and number of isopod specimens over a sampled distance of 1000 m2, this was not feasible for the RV Hakuho Maru. Isopod numbers (occurrences) can only be provided on a presence/absence basis, as the exact trawling distance cannot be estimated. Moreover, the plankton nets remained open throughout the retrieval of the bottom trawls while they were holstered through the water column. In contrast, the EBS has an opening and closing device that only opens at bottom contact, ensuring that no animals are sampled in the water column. The deployment of the EBS involved using 1.5 times the cable length to the water depth is deployed in a standardized way throughout all expeditions. However, on board of RV Hakuho Maru, there was not enough wire available to deploy the benthic gear in a comparable way. This might be the reason why the numbers of isopods collected with plankton nets during the RV Hakuho Maru expedition were much lower than the numbers of isopods collected by means of an epibenthic sledge deployed during the abovementioned expeditions in the Northwest Pacific, possibly due to the smaller opening of the plankton nets. Nonetheless, the general composition of families is comparable to previous results, as demonstrated by studies such as Brandt et al. (2019) for four expeditions on the NW Pacific, where an abundance of data is presented in the electronic supplement. More specifically, 27,931 isopods were collected in the Sea of Japan due to the bathyal mass occurrence of the munnopsid Eurycope spinifrons Gurjanova, 1933 (Elsner et al. 2013b)). Additionally, 5625 isopods were collected in the Sea of Okhotsk and the Bussol Strait, 4006 in the abyssal Northwest Pacific and at the rim of the KKT, and 4949 isopods were collected from hadal and abyssal depths in the KKT (Brandt et al. 2019).

In addition to Munnopsidae, Haploniscidae, and Ischnomesidae are frequently sampled in the NW Pacific. These families were not only frequently sampled in the KKT and JT with the plankton nets, but they also occurred in the Sea of Okhotsk, in the KKT, and the abyssal North Pacific (Bober et al. 2017; 2019; Johannsen et al. 2019; Knauber et al 2022b).

Peracarid isopods captured by the beam trawl’s plankton nets aboard the RV Hakuho Maru are described at family level in the present paper, highlighting that even a large-sized beam trawl net can be equipped with fine-mesh sized gear to increase sampling results for macrofaunal organisms, including Isopoda (Akiyama et al. 2008). In the case of epicaridean isopods, we refrain from identification at the family level because all listed specimens were in the larval stage. This study represents the first comprehensive publication of isopods obtained using the “inner net” introduced by Akiyama et al. (2008). Our data demonstrates the efficiency of these additional plankton nets, and we therefore recommend more frequent deployment of bottom trawls with plankton nets in future investigations.