The CIOB has a northward opening in the Bay of Bengal. The approximate dimensions of CIOB are 6,000 km long and 1,600 km wide, and its depth reaches 6,090 m (MoES, 2020). Indian Ocean Nodule Field (IONF) is situated in the CIOB, extending approximately from 9°S to 16°30′S and 72°E to 80°E, stretched across 739,260 km², and refers to a commercially exploitable portion of the nodule field (Mukhopadhyay et al., 2008).

In the IONF, a number of seamounts of various characteristics and dimensions were identified, along with abyssal hills. Afanasiy Nikitin seamount in the central part of the basin is well known for cobalt crust deposits (Glasby, 2010), which reach a maximum height of 1,549 m (Krishna, 2003). These seamounts and abyssal hills make the habitat more complex for the faunal component and enhance the diversity of the region. The Ganges–Bramhaputra fan appears to be a long distance from the CIOB, but the fan extends up to about 10°S and influences the nutrient chemistry in the IONF (Krishna, 2003; Mukhopadhyay et al., 2008).

The average primary productivity in CIOB is 3.72 mg C m⁻³ day⁻¹ at the surface and a mean of 51.23 mg C m⁻² day⁻¹ in the water column (0–150 m), respectively (Matondkar et al., 2005), which is considerably lower than in other oceans. Subsequently, the low organic content in surface sediments (0.07%–0.41%) is attributed to low sediment accumulation rates in the CIOB (Pattan et al., 2005). CIOB exhibited a high microbial diversity (Shah et al., 2024), and abundance was found to be three times greater than the earlier reported range of 10⁸ to 10⁹ CFU g⁻¹ dry sediment in other oceanic waters around the globe (Raghukumar et al., 2001). Thus, the IONF and adjacent area in CIOB are very complex, and it is important to understand their full biological resources with respect to geomorphology and geophysical parameters.

In the year 2013, a block of ~ 13.875 km × 13.875 km (7.5° × 7.5°) was designated as the Impact Reference Zone (IRZ) and Preservation Reference Zone (PRZ) for conducting the trials of mining in CIOB (MoES, 2020). Within this area, the sediment samples were collected in July 2019 on board R/V Sindhu Sadhana (cruise No. SSD062) at three sites in the licensed area for PMN exploration, approximately 100–120 km apart ( Figure 1A ; Supplementary Table S2 , panel A). The PRZ represents a reference site and should not be influenced by any mining activities or impacts in the future. The IRZ is designated as an Impact Reference Zone, where the benthic impact experiments and possible mining will take place. Site BC20 will also be used as a reference site located in the north of both sites.

Two sampling gears were used to collect seafloor sediment, viz. multi-corer (Bowers & Connelly Corer, USA; 10 cm inner diameter; Figures 1B, C ) and USNEL spade box corer (0.25 m²; Figure 1D ). The multicorer with eight core liners and box corer were deployed thrice and twice at each site, respectively. Due to time constraints and logistical limitations, the number of samples collected had to be restricted. The lowering speed of the spade core was about 0.6 m s⁻¹ until the 100-m altimeter reading, then after the spade corer was lowered at 0.1 m s⁻¹ to minimize the impact (i.e., bow wave). Each deployment of gear retrieved relatively undisturbed sediment from the sea floor. The depth of vertical sediment penetration varied between ~ 15 cm and 35 cm, depending on the sediment surface and nodules present. The upper 0–10 cm of sediment layer only was used. The overlaying water in multi- and box corer was also added to the topmost layer of the sediment. Each box corer (0–10 cm) was completely used for macrobenthic faunal extraction according to ISBA guidelines (ISBA/25/LTC/6/Rev.1, 2020). Among the three operations of multicorer deployment, two core liners from the first two operations and a single core liner from the third operation were used for macrobenthic faunal extraction, bringing it all together five core liners from each site ( Supplementary Table S2 , panel A). Thereafter, two core liners taken from the same operation were analyzed separately, but the data were merged (averaging), making them single core liner data as per the ISBA guidelines (ISBA/25/LTC/6/Rev.1, 2020). Therefore, the used replicates will be three and two per site for multi- (0.039 m² area coverage) and box corers (0.5 m² area coverage), respectively. Another two core liners were taken for estimation of environmental parameters. The remaining core liners were used for separate biological, chemical, and geological studies (not included in this study). Thereafter, the core liner samples for macrobenthic fauna and environmental analysis were vertically sectioned at depths of 0–1 cm, 1–2 cm, 2–3 cm, 3–4 cm, 4–5 cm, 5–6 cm, 6–8 cm, and 8–10 cm below the sediment surface. The polymetallic nodules brought with gears were carefully removed with minimum disturbance and washed to remove any associated fauna. The final abundance data merged these faunal contents with the 0–1-cm layer of multicorer samples. The macrobenthic samples of the single core liner from each operation (n = 3) and the first box corer were fixed and stored in 10% borax-buffered formalin and 0.5% Rose Bengal. Other samples of the single-core liner from the first two operations (n = 2) and the second box corer were directly preserved in 80% denatured ethanol and stored in a − 80°C deep freezer along with all samples of environmental parameters until further laboratory analysis.

The analysis of the sediment-water interface at the three analyzed sites yielded several noteworthy findings. Temperature remained consistent at 1.45°C among all sites with no variability ( Supplementary Table S2 , panel B), while DO levels were highest at PRZ (7.50 ± 0.95 mL L⁻¹), followed by IRZ (6.04 ± 0.56 mL L⁻¹) and BC20 (5.61 ± 0.40 mL L⁻¹; Supplementary Table S2 , panel B). The mean salinity displayed minimal variation, with values ranging from 31.49 ± 0.31 psu (IRZ) to 31.76 ± 0.08 psu (PRZ). The highest pH (8.17 ± 0.04) was recorded at PRZ, while the lowest was observed at BC20 (8.01 ± 0.03), with subsurface pH exhibiting lower values in sediment layers below 6–8 cm vertical depth ( Figure 2 ). Subsurface salinity, measured from pore water analysis, showed relatively low variation except for the IRZ’s bottom layer, where salinity ranged between 30.5 psu and 31.1 psu. Nutrient profiles varied considerably between sites and vertical depths. Phosphate values at the sediment–water interface were similar in PRZ and BC20 (2.36 µmol L⁻¹), but much lower than IRZ (5.68 µmol L⁻¹). Subsurface pore water phosphate (PO₄³^--P) profiles indicated the highest values at BC20’s 8–10 cm depth (4.18 µmol L⁻¹) and less variation (1.59–2.36 µmol L⁻¹) throughout vertical depths in PRZ, whereas IRZ’s bottom sediment showed the highest values at surface layers and lower values at 8–10 cm depth (2.84 µmol L⁻¹). Nitrite (NO₂ ⁻-N) concentrations were similar throughout the vertical profile of IRZ and BC20; however, higher concentrations were reported in the subsurface layers of PRZ. Nitrate (NO₃ ⁻-N) concentrations were higher at surface layers than subsurface layers. Contrastingly, silicate (SiO₂ ⁻–Si) values gradually increased with the depth sediments at all the sites ( Figure 2 ).

TOC levels in sediments were relatively consistent across sites, with the lowest at PRZ (0.35%) compared to IRZ (0.40%) and BC20 (0.47%). TOC decreased with depth ( Figure 3 ). CHO, protein, and LOM exhibited similar distribution patterns, with higher values in surface sediment layers, while lipid content was higher in subsurface layers around 2–4 cm sediment depth ( Figure 3 ). Surface sediments were primarily composed of clay, ranging from 65.06% to 74.02% in all three sites ( Supplementary Table S2 , panel B), with silt being the second most dominant grain size (22.96% to 31.7%). Sand particles played a minor role in sediment texture. The sediments were predominantly characterized by pelagic dead radiolarian remains, composed of clay, fine silt, and very coarse silt particles, which exhibited similar properties in BC20 and IRZ compared to PRZ. Subsurface sediments (0–10 cm vertical) displayed greater variability in texture properties, indicating that BC20 and IRZ shared a similar sediment particle origin compared to PRZ. Clay content was notably higher in the upper fractions (0–2 cm, 2–4 cm, and 4–6 cm), while the bottom fractions (6–8 cm and 8–10 cm) exhibited less clay dominance in all three sites.

Mobility and sediment reworking traits exhibited distinct differences in sampling gear based on the total abundances of morphospecies associated with specific traits. However, the community-weighted mean (CWM) of traits did not significantly differ in gear types ( Table 2 ). The multicorer method predominantly captured a high abundance of traits, whereas the box corer exhibited considerably less abundance of these traits ( Figure 7 ). This discrepancy could be attributed to the consideration of abundance-weighted traits. However, the variation is much less pronounced or comparable when only the count is considered without weighting. Moreover, relying solely on counts could introduce bias to the results, as abundance serves as a crucial measure of intensity per unit area. Therefore, we considered an abundance-weighted total for each trait. In mobility traits, less motile and discretely motile modalities were dominant among all. Sessile fauna very rarely occurs only in multicorer (PRZ and IRZ) and box corer (BC20). Surficial modifiers emerged as the dominant factor in the PMN region, closely followed by biodiffusors and upward/downward conveyors. Epifauna, on the other hand, was exclusively observed in the PRZ of multicorer samples. In the IRZ multicorer samples, surficial modifiers held sway. These findings illustrate the prevailing presence of fauna with slow and limited mobility, particularly those capable of surface diffusion that moves particles horizontally across the surface in polymetallic nodule fields.

The examination and comparison of available literature data from similar surveys were pivotal in our quest to understand metazoan abundance and distribution within the abyss nodule fields, particularly in different PMN regions. One notable observation was the varying mesh sizes used in macrofaunal studies conducted in the CIOB and the Clarion–Clipperton Fracture Zone (CCFZ) regions. In the CIOB, where studies focused on nodule fields employed large mesh-sized sieves of 500 μm, a unique scenario emerged. This choice of sieve size, while capturing macrobenthic organisms, also contributed to relatively low recorded animal abundance in comparison to studies employing smaller mesh sizes (Ingole et al., 2005b; Pavithran et al., 2007). The utilization of large mesh sizes is not uncommon in the deep-sea research community, as it can aid in the collection of larger and more conspicuous organisms. On the other hand, macrofaunal surveys conducted in the Central North Pacific and Peru Basin within the CCFZ region typically adopted a smaller mesh size sieve of 300 and 250 μm (De Smet et al., 2017; Wilson, 2017; Stratmann et al., 2018; Yu et al., 2018; Sweetman et al., 2019; Chuar et al., 2020; Francesca et al., 2021). This choice was consistent with established deep-sea sampling practices and allowed for the collection of a broader spectrum of metazoan taxa. However, it’s important to note that these surveys opted to exclude meiofaunal taxa, i.e., Nematoda, Copepoda, Ostracoda, Acari, and Kinorhyncha, from their analyses to ensure meaningful comparisons with most previous studies in the CCFZ region (De Smet et al., 2017; Wilson, 2017; Francesca et al., 2021; Washburn et al., 2021). In essence, the mesh size utilized in deep-sea sampling can significantly influence the recorded abundance and diversity of metazoan communities. The contrasting mesh sizes used in the CIOB and CCFZ studies reflect the diverse approaches employed by researchers in different PMN regions, each driven by unique research objectives and considerations. Understanding these variations in sampling methodologies is crucial when interpreting and comparing metazoan abundance data from various abyss nodule fields, ultimately advancing our knowledge of these complex deep-sea ecosystems. Few CIOB studies have reported 336 ind. m⁻², 376.39 ind. m⁻², and 507 ind. m⁻², which was the highest faunal abundance among other studies from CIOB, including “meiofaunal” taxa, i.e., Nematoda (Ingole et al., 2001; Ingole and Koslow, 2005; Ingole et al., 2005a). The notion that the inclusion of meiofaunal taxa in the macrofaunal community (macrobenthos) is extremely useful and commendable and is gaining ground recently (Ingels et al., 2021). In the present study, both densities (macrobenthos and macrofauna) were mentioned for a clear understanding of benthic community structure and comparison. It has been recommended that the whole box corer should be used for macrofauna analysis (De Smet et al., 2017). Nevertheless, pseudoreplicates were used in studies from CIOB in the past (Ingole et al., 2005a; Pavithran et al., 2007) and also compared in Supplementary Table S1 , thus creating discrepancies in macrobenthic faunal density. Hence, meaningful comparisons between CIOB and Pacific Ocean studies have been challenging so far. In 2009, a similar study reported a total of 66 species in 120 individuals (> 500 μm) from PMN, Indian Ocean (Pavithran et al., 2009a). Contrastingly, 5,199 metazoans (> 250 μm) were collected from 12 box corers in CCFZ and encountered 33 higher taxa (Chuar et al., 2020). The present study encountered 117 metazoan (> 300 μm) individuals in the box corer. Therefore, the available data strongly suggest that the CIOB nodule field has the relatively lowest macrobenthic faunal abundance and higher taxa diversity.

In the current investigation, our examination of the Shannon–Wiener index (H’) revealed a lower diversity compared to other PMN areas within the CCFZ (> 2.0 along the GSR [De Smet et al., 2017]). Polychaetes, widely acknowledged as prominent members of deep-sea macrofaunal communities (Sanders, 1968; Hessler and Jumars, 1974; Wilson, 2017), emerged as the most dominant macrofaunal taxon (macrofauna) in our study, encompassing 17 distinct morphospecies. Surprisingly, tanaidaceans, traditionally recognized as the predominant taxon in the CCFZ located in the northeastern Pacific Ocean (Yu et al., 2018), took a secondary role in our findings. The present study, incorporating nematodes and copepods into the macrobenthos category, witnessed a shift in dominance, with nematodes securing the top position, followed by copepods. This contrasts with the conventional hierarchy, where polychaetes and tanaidaceans typically hold sway. Our observations align with similar dominance patterns reported in the easternmost region of the CCFZ (Chuar et al., 2020; Kaiser et al., 2024).

For vertical distribution, Montagna et al. (2017) reported that more multicorer samples from the top 5 cm sediment depths in various locations of the northern Gulf of Mexico revealed that approximately 90% of faunal communities were concentrated in this uppermost layer. Meiofaunal communities dominated the top 2 cm, while macrofauna extended further down to 10 cm. Similar findings were documented in the Clarion–Clipperton Fracture Zone (CCFZ), with 67% observed in the upper 2 cm, 25% up to 5 cm, and 8% down to 10 cm depths (Chuar et al., 2020). The current study, conducted in the CIOB, aligns with these patterns, highlighting that maximum communities were harbored in the top 5 cm sediment layer across all three sites. Interestingly, previous research in the CIOB contradicted this trend, revealing that benthic fauna was more abundant in the middle (2–5 cm) and lower layers, with traces even in the 20 to 30 cm layer (Pavithran et al., 2009b). This discrepancy may be attributed to the scarce food and surface competition, which may cause the benthic fauna to migrate downward, forming protective tubes that, during sampling, serve as escape routes, leading to inadvertent transport through the sediment layers (Ingole et al., 2005b; Pavithran et al., 2009b).

The methods employed for collection may significantly influence the results of benthic analysis (Jensen, 1981, 1983; Bett, 1994; Hughes and Gage, 2004; Shields and Hughes, 2009). Since 1970, box corers have been used primarily for quantification of deep sediment (Gage and Tyler, 1991); however, few studies have tackled the issue of inefficiency in box corer sampling for macrofauna (Hughes and Gage, 2004; Shields and Hughes, 2009; Blomqvist et al., 2015; Montagna et al., 2017). In this study, we attempt to investigate the differences in methodologies in the abyssal plain of the world’s lowest productive zone (Pavithran et al., 2007). The box corer area coverage at each station was 12.82 times larger than the combined multiple corer sample area. However, a higher mean density of macrofauna (“sensu stricto”) was reported in multicorer ( Supplementary Table S6 ). Polychaetes were dominant in the present study, similarly observed in CCFZ (Bonifácio et al., 2024). Furthermore, total biomass estimates were higher in multicorer samples with less variation between the stations ( Figures 5A, B ), owing to two possibilities: (1) box corer may have led to a loss of the fauna due to a bow-wave effect before settling on surface sediment and (2) multicorer captured all sized range effectively with much smoother approach near sediment than box corer.

Intriguingly, only multicorer samples were deemed suitable for discerning the densities of small-body-sized fauna (especially Meiofauna). Particularly, nematodes and copepods showed high abundance in the multicorer samples, and many morphospecies captured by the multicorer exclusively did not occur in the box corer samples ( Supplementary Table S3 ; Figure 4 ). These results indicate the suitability of multicorer usage for macrobenthic fauna collection in the deep sea. Nevertheless, the contribution of smaller organisms retained on a 300 µm mesh to community biomass is relatively minor (Gage et al., 2002). Despite this, there are 11 morphospecies of polychaetes, including motile subsurface deposit feeders, i.e., Cirratulidae sp., Lacydoniidae sp.2, Lumbrineris sp., Opheliidae sp., Amphicorina bansei, and Spiophanes sp. exclusively occurred in multicorer samples and were associated with increased biomass. In contrast, sessile or less motile and large-sized polychaetes, i.e., Glyphanostomum sp., Capitellidae sp.1, Brada sp., Glycera sp., and Sabellidae sp. were found in the box corer samples. Multicorer showed inefficiency in capturing isopods and tanaids, as represented by the low contribution of these taxa to total abundance. Box corer showed the presence of all isopod and tanaids species listed except Leptognathiidae sp.3.

Morphospecies richness and diversity were higher in the box corer compared to the multicorer samples ( Supplementary Table S6 ). This is not surprising, as diversity tends to increase with the sampling area (Lomolino, 2000), and the area covered by the box corers is larger than that of the multicorer. Margalef richness (d) is three times higher in box corer than in multicorer samples. While Shannon–Wiener diversity (H’) was also higher in box corer.

The findings highlighted that smaller fauna, particularly meiofauna, were more likely to be captured by the multicorer, suggesting a potential escape from the box corer. Utilizing box corers for benthic sampling presents a notable advantage by allowing the collection of a more extensive and quantifiable representation of individual organisms over a relatively large area. This comprehensive sampling approach facilitates a nuanced exploration of the diverse benthic community, enhancing the understanding of species diversity within the study area (Montagna et al., 2017). However, it is crucial to recognize the potential drawbacks associated with the use of box corers. One significant concern stems from the generation of bow waves during the sampling process, which can disturb the seabed, resulting in the resuspension of surface sludge and the potential loss of sediments (Blomqvist et al., 2015). This disturbance may result in the displacement or loss of individual organisms within the sampled area, affecting the accuracy of biodiversity assessments and compromising the integrity of ecological data. Furthermore, the present study underscores the advantages of multicorer sampling in some aspects for investigating the vertical distribution of benthic communities and capturing small-sized faunal fractions (Meiofauna). Particularly, where small-sized fractions could be an important component to understanding the whole ecological perspective in the ecosystems characterized by low food availability, such as open water of deep sea zones and potential mining sites. However, the box corer yielded scant additional information regarding the macrobenthic faunal community compared to the insights gained from the much smaller multiple corer samples. The considerable variation in density and diversity responses to the two sampling methods poses a challenge to confidently recommending one technique over the other (Montagna et al., 2017). Thus, the use of both sampling gears will offer more detailed insights into biodiversity and ecosystem functioning.

The composition of functional traits did not differ between the sites and gear types, indicating that there was no discernible influence of spatial structure or gear effects. This lack of differentiation could stem from the fact that many species share similar functions across these traits, highlighting redundancy in their functional roles. However, these functions may still impact the local flux of available nutrients in the sediment. Macrobenthos contributes to increased organic matter inputs through biodeposition, a process where organic materials are released into the sediment as metabolic by-products (Graf and Rosenberg, 1997). Simultaneously, these organisms engage in bioturbation, stirring and mixing the sediments, thereby distributing and incorporating organic materials into deeper strata (Kristensen, 2000; Welsh, 2003). These fundamental aspects of the functional traits, including mobility and sediment reworking, of macrobenthic fauna lie in their ability to enhance organic matter (OM) flux within sediments, influencing nutrient cycling, sediment structure, and overall benthic community dynamics. In the present study, the less motile and discretely motile macrobenthic faunal communities were dominant along all the sites, while sessile fauna very rarely occurred in both the sampling gears (multi- and box corer). These findings can be attributed to the burrowing behavior exhibited by a majority of species, namely polychaete families (Ampharetidae, Capitellidae, Cirratulidae, Glyceridae, Lacydoniidae, Lumbrineridae), tanaid families (Leptognathiidae, Neotanaidae, Tanaellidae), and other taxa. This behavior classifies them as less motile, with their movements primarily confined to the sediment. Less motile macrobenthic faunal communities in the deep sea typically include organisms with limited mobility, relying on structural adaptations or passive transport for movement (Jones et al., 1998; Billett et al., 2001). Furthermore, less motile organisms may contribute to sediment stability and long-term habitat structure, while discretely motile species actively participate in bioturbation processes, influencing nutrient availability and community composition (Ruhl and Smith, 2004). Discretely motile macrobenthic faunal communities comprise organisms such as mobile predators, scavengers, and deposit-feeders like crustaceans and agile polychaetes, capable of more pronounced and intentional movements within the sediment and across the seafloor to locate food resources and suitable habitats (Thistle et al., 2008). Biodiffusors, organisms that enhance the vertical mixing of sediments through their burrowing and feeding activities, are significant contributors to sediment reworking (Aller, 1982; Aller and Aller, 1992; Aller, 1994), which dominated the macrobenthos population in the present study. Upward and downward conveyors refer to transporting sediment particles vertically within the sediment column. For example, upward conveyors such as certain polychaetes (Capitellidae sp.1 and Capitellidae sp.2) exhibit upward conveyance by ingesting deeper sediments and excreting processed material at the surface, influencing nutrient distribution (Miri et al., 2023). Furthermore, organisms exhibiting upward and downward conveyance, such as certain burrowing fauna (e.g., Glyphanostomum sp., Brada sp., and Spiophanes sp., see Supplementary Table S3 ), transport surface sediments into deeper layers and vice versa, affecting sediment structure and biogeochemical cycling (Shull and Yasuda, 2001).

Three key environmental factors, i.e., food availability, sediment texture, and nodule distribution may significantly influence the benthic community structure assemblages in the deep sea. The deep-sea floor is distinguished by exceedingly low organic material inputs, rendering it a food-limited ecosystem (Smith et al., 2008). Similarly, CIOB exhibits low productivity both at the surface and in the water column (Matondkar et al., 2005). Additionally, the region has a diminished organic carbon content attributable to low sediment accretion in the bottom surficial sediments (Pattan et al., 2005), despite the Ganges–Bramhaputra fan has been involved in the changes in nutrient chemistry up to 10°S. The reason attributed to low productivity may be the South Equatorial Counter Current (SECC) and South Equatorial Current (SEC) between 0° and 10°S may have prevented nutrient-rich water of the north Indian Ocean from entering the CIOB (Peng et al., 2015). It progressively diminishes toward the south, leading to a decrease in the flux of particulate organic carbon (POC) from the north to the south. The limited nutrient enrichment potentially holds influential capabilities in shaping benthic communities (Bonifácio et al., 2020, 2024) in the southern region of the CIOB. In the current study, the IRZ and PRZ are situated to the south, while BC20 is positioned to the north within the CIOB. This spatial arrangement may contribute to a slightly higher TOC value in BC20 compared to that in the IRZ and PRZ. The previous studies from CIOB evidently showed that a relatively higher TOC ranging from 0.52% to 1.31% near 10°S latitude (Nair et al., 2000), whereas the present (12°S –13°S) study reported 0.35%–0.47% of TOC. The presence or absence of TOC exerts a significant (positive/negative depending on feeding behavior) influence on the diversity, population dynamics, and interspecies interactions of macrobenthic fauna. The highest Margalef richness (d) and Shannon–Wiener diversity (H’) of macrobenthos at BC20 could have been associated with increased TOC and CHO compared to IRZ and PRZ ( Supplementary Table S6 ). Contrastingly, a negative correlation was observed between food availability and macrofaunal community structure (Kaiser et al., 2024). Polychaete species Capitellidae sp.2, Opheliidae sp., Sabellidae sp., Lumbrineris sp., and Glycera sp. are exclusively found in BC20, of which Capitellidae sp.2, Opheliidae sp., and Sabellidae sp. were highly dependent on suspended or deposited organic food on surface sediment. Similarly, deposit feeder polychaetes dominated Ifremer and APEI No. 3 sites associated with high POC flux (Bonifácio et al., 2024). However, Lumbrineris sp. and Glycera sp. were carnivorous in life habits (Jumars et al., 2015), which could potentially be attributed to complex interspecific relationships within the ecosystem. This has been supported by a notable contrast that emerged in the abundance of nematodes at BC20, IRZ, and PRZ sites, with BC20 displaying lower numbers compared to PRZ and IRZ ( Supplementary Table S6 ). This variance in nematode abundance could also support interspecific relationships due to the dependence of macrofauna on meiofaunal components. Interestingly, copepods (including harpacticoid) were more pronounced in BC20, and it is noteworthy that some of the harpacticoid species predominantly follow carnivorous feeding strategies (Bröhldick, 2005; Shimanaga et al., 2009).

Despite the limitation of sampling effort (logistical constraints) and a small sample size restricting definitive conclusions, the findings of this study highlight the significant influence of different investigative methodologies on observed macrobenthic faunal assemblages. Future research in the CIOB should indeed aim to increase sample size and use consistent sampling techniques to enhance reliability, but the current approach provides a critical foundation for understanding macrobenthic communities in the polymetallic nodule fields of the CIOB. For the first time, this research on macrobenthos provides quantitative insights into multicorer sampling within the PMN field in the Indian Ocean. The study examined differences in macrobenthic community structure, diversity indices, and functional composition between sites. The results revealed a relatively homogeneous nature at the PMN mining sites, encompassing both PRZ and IRZ areas. Relationships between the macrobenthic faunal community and the environment were analyzed to gain a comprehensive understanding of the ecological drivers. The findings highlight the importance of environmental relationships, particularly the influence of total organic carbon and related components, such as carbohydrates, in extremely low-productive zones. Furthermore, we advocate for the combined use of both multi- and box corers to achieve more accurate estimates of diversity and abundance, ultimately enhancing our knowledge of species richness and functional understanding. Through this study, we aimed to provide general recommendations for the collection of high-quality data that can be utilized for monitoring purposes. The present research contributes valuable insights into seabed community patterns, encompassing both structural and functional aspects by including large-sized meiofaunal taxa in the macrofauna, which may prove instrumental in managing the mining site in the CIOB. Furthermore, understanding the relationship between benthos and environmental factors is crucial for regulating deep-sea bed mining activities, as they significantly shape community structures. By integrating these insights, the study provides valuable guidance for the sustainable management of deep-sea resources and emphasizes the need for careful consideration of environmental factors in future mining operations. Thus, the scientific endeavor is pivotal for informing and shaping the regulations that will govern future mining exploitations in the deep-sea bed, ensuring a legally sound and ecologically responsible approach to this emerging industry. As the ISA continues its regulatory development, incorporating such scientific findings becomes essential for the sustainable management of deep-sea resources and the preservation of biodiversity in these ecologically sensitive areas.