Field operations in the western CCZ were conducted from the RV Kilo Moana between May 14 and June 16, 2018, as part of the DeepCCZ project, which included assessment of the benthic and benthopelagic environments and communities across an range of size classes from microbes to megafauna as well as ecosystem function (Drazen et al., 2019). Operations for this study were conducted using the remotely operated vehicle (ROV) Lu’ukai.

Field sites were selected in each of the three APEIs in the western CCZ (APEIs 1, 4, and 7), and included a location on the abyssal plain in each, and a location on a proximate seamount in APEIs 4 and 7 (Figure 1). Practical constraints precluded the examination of a seamount in APEI-1 for this study. Similar site locations were selected across the APEIs based on several criteria: (1) Site locations were selected to be within the CCZ. (2) Central location within the APEI was preferred, with locations in the APEI perimeter “buffer zone” (as described by Wedding et al., 2013) avoided, if possible. In order to meet the other criteria in APEI 1, the study site was located in the southern buffer zone. (3) Predicted high nodule abundance on the abyssal plain based on geological modelling (International Seabed Authority, 2010; Wedding et al., 2013) was preferred, so that abyssal plain sites would be comparable to those in areas to be mined. (4) The presence of a suitable seamount with an area of abyssal plain outside the presumed zone of seamount influence was required. The zone of influence was estimated to extend outwards from the seamount base for 15 km, based on a maximum influence of 30 km from the summit for very large seamounts with shallow summit depths (Gove et al., 2016). (5) Suitable seamounts each had a summit rising a minimum of 1,000 m above the abyssal plain estimated from satellite bathymetry (SRMT30+), with a “flat” area likely containing soft sediment near the top. Similar summit water depths and geomorphologies between seamounts were preferred, confirmed with shipboard multibeam.

Seabed images were captured for the assessment of both the seabed environment and the megafaunal community (Table 1). ROV transects were undertaken at a randomly selected heading within the range of headings allowed by environmental conditions (surface winds and currents, bottom currents) and topography, with the aim of minimising suspended sediment from obscuring collected imagery and facilitating reasonable ship movement. Seabed transects at each location were planned to be 2 km in length on abyssal plains and 1 km on seamounts, at a target speed over ground of 0.5 knots, with a target camera altitude of 2.62 m above bottom. Still images were captured at 10-s intervals with a downward-facing Ocean Imaging Systems DSC 24000 digital camera system mounted to the front of the Lu’ukai ROV (Nikon 7100 camera set to aperture F8, shutter speed 1/60, focal distance 8.5 feet, images 4,000 × 6,000 pixels). Dedicated lighting was provided by two Ocean Imaging Systems 300 W-S remote head strobes synchronised to the camera shutter. Parallel lasers at a separation of 350 mm were used for scaling in the imagery.

Seafloor photos were examined if they met the following criteria: captured at camera altitudes up to 3 m above the seabed; ROV pitch and roll between −5 and 5 degrees; no overlap between photos; no obscuration (e.g., by large shadows or suspended sediment). The selected photos were cropped to 3,000 × 6,000 pixels to remove obscuration from ROV equipment in the image, and colour-corrected to approximate true-to-life colour in a well-lit environment. Annotations of photos was completed using the Monterey Bay Aquarium Research Institute’s Video and Annotation Referencing System (Schlining and Stout, 2006).

Four aspects of the benthic environment were examined: seabed bathymetry, estimated organic matter flux at the seabed, nodule abundance and sediment particle size. Seabed bathymetry was measured using a ship-borne multibeam echosounder (12 kHz Simrad EM120, Kongsberg Maritime), with a swath width of 12–14 km. Measured bathymetry was processed onboard using Qimera software (Quality Positioning Services), gridded at 100 m, with a correction for the sound velocity profile applied. Organic matter flux to the abyssal seabed was estimated as particulate organic matter (POC) flux for the study sites by Leitner et al. (2020a) based on a model by Lutz et al. (2007). POC flux to the seamount summits was not estimated because the coarse bathymetric model used by Lutz et al. (2007) to estimate of seafloor POC flux does not reliably capture seamount summit depth.

The density, seabed cover, and individual sizes of nodules were assessed in a randomly selected subset of 1090 seabed photographs (Table 1). A single annotator enumerated and measured all nodules in the central portion of each image in the subset (2,000 × 1,500 pixels; representing ∼0.92 m² seabed), a size selected for the practicality of annotating images with high densities of nodules.

Sediment was collected in pushcores (70 mm internal diameter) deployed by the ROV at locations near the seabed photograph transects (Table 1). Particle size distributions were determined by laser diffraction using a Malvern Mastersizer on the refrigerated collected sediment (0–50 mm horizon) suspended in reverse osmosis water (i.e., without digestion), following the removal of any nodules. Particle size distributions were divided into three size ranges for analysis, to divide the major modes in the distributions: fine particles (<17.5 μm), intermediate-sized particles (17.5–174 μm) and coarse particles (>174 μm).

Megafauna (generally > 10 mm in dimension, sensu Grassle et al., 1975) were enumerated in the photos and classified to the most detailed taxonomic level possible based on previously published studies of megafauna in the CCZ (Figures 2, 3; Simon-Lledó et al., 2019a,b,c; Cuvelier et al., 2020; Kahn et al., 2020). A six-digit alphanumeric code was assigned to each morphotype. Highly mobile scavengers and predators, and monothalamous Foraminifera (Xenophyophoroidea) were treated separately from the remaining “invertebrate megafauna” (see below), and additional investigations were made of key taxonomic groups (i.e., echinoderms, porifera and cnidarians).

Prepared seabed photos (3,000 × 6,000 pixels) were randomly assigned to two experts for megafaunal annotation. In addition to randomising the collections and order of images annotated by each expert, the following steps were taken to reduce annotator bias: (1) Prior to annotation of the main photo set, a subset of images was annotated by both experts, and their detections and classifications of megafauna in this “training” image set were discussed. These annotation data were removed and the images returned to the image pool. (2) Subsets of photos were randomly selected for repeated annotation for assessment and reduction of both inter- and intra-annotator bias in detection and classification of specimens. (3) Following annotation, all identified specimens were reviewed by a single expert on a morphotype basis, and reclassified where necessary to ensure consistency. Annotated counts are provided in Supplementary Material 1.

Individual body size (mm) was measured in each specimen in the images. Taxon-specific measurements (Durden et al., 2016a) were made for morphotypes with at least four individuals at each of a minimum of two sites. Fresh wet weight biomass was estimated for echinoderms (Holothuroidea, Crinoidea, Ophiuroidea, Asteroidea, and Echinoidea) from these measured dimensions in the seabed photographs and established length-to-wet weight relationships (Durden et al., 2016a). In the case of echinoids, fresh wet weight was estimated based on a spherical test volume of measured diameter, with an estimated tissue volume of 25% of test volume (Ebert, 2013) converted to fresh mass, assuming a density of 1 g cm^–3, a method similar to that employed by Durden et al. (2019).

Megafaunal xenophyophores were enumerated in the seabed photographs (as described above), with identification based on published literature on xenophyophores from the CCZ (Gooday et al., 2017a,b,c, 2018a,b). New morphotypes have been observed in seabed images (Gooday et al., 2020a), and new genera and species have been identified from specimens collected at these sites (Gooday et al., 2020b). The maximum diameter of a subset of 8750 specimens were measured in randomly selected images.

Highly mobile scavengers and predators of interest consisted of highly mobile benthopelagic morphotypes, including all fishes, shrimp-like arthropods and octopus. Individuals of these morphotypes were enumerated in seabed photographs (as described above), and classified based on (Drazen et al., in review). These morphotypes were treated separately to facilitate comparisons with other studies, since many megafaunal assessments omit these taxa, while studies using bait, including those from the wider DeepCCZ project, focus on them (Leitner et al., 2020a; Drazen et al., in review).

Statistical comparisons of conditions on the abyssal plain were made between APEIs by creating sample units from the set of pooled photos across all transects in an APEI. For statistical comparisons of nodule parameters, annotation data from the abyssal plain locations were aggregated into a minimum of four sample units of 73 randomly selected photographs each, the maximum sample unit size for the smallest nodule image set per APEI (Table 1; 295 photographs for nodule assessment at APEI-7). Mean fine, medium and coarse sediment content was computed across cores collected from a site. For statistical comparisons of density, diversity, echinoderm biomass, and community composition, annotation data from the abyssal plain locations were aggregated into a minimum of four sample units of 312 randomly selected photographs, the maximum sample unit size for the smallest image set per APEI (Table 1; 1250 seabed photos for megafaunal assessment at APEI-1). Comparisons of individual body size were conducted on a specimen basis. Although the limited photography and sampling on seamounts precluded statistical comparisons, all photographs captured at each seamount were aggregated into a single unit to facilitate computation of ecological metrics.

Nodule and faunal abundance, and echinoderm biomass were standardised to unit seabed area. Univariate Hill’s diversity indices (Nq), morphotype richness (q = 0), exponential of the Shannon index (q = 1), and the inverse of the Simpson’s index (q = 2; Magurran, 2013) were computed along with the rarefied number of morphotypes (Hurlbert, 1971) at the minimum number of individuals in any sampling unit (65). Environmental and univariate ecological metrics on the abyssal plains were compared between APEIs using ANOVA, with significance reported at α = 0.05 and post hoc testing using Tukey’s honest significant difference. Normality was tested by visual inspection of normal quantile plots, and the Shapiro-Wilk normality test. Percentage data were arcsine transformed before assessment. Accumulation curves were computed for median values of density and diversity indices over aggregated images, repeated 500 times.

Differences in apparent assemblage composition were assessed using multivariate statistics (Bray-Curtis dissimilarity measure and 2-dimensional non-metric multidimensional scaling ordination), with the sample unit-like groupings of photographic data on the seamounts included in multidimensional analyses for illustration. Statistical comparisons between the community data on the abyssal plains were tested using ANOSIM and SIMPER routines (Clarke, 1993). All ecological metrics were calculated using the vegan package in R (Oksanen et al., 2012).

We found low density, high diversity benthic megafaunal communities on the abyssal plains of the western CCZ, with significant differences between the study areas in the three APEIs in both the megafaunal communities and habitat. These community differences were observed in terms of density, diversity and assemblage composition, with few common morphotypes between sites and many rare morphotypes observed only once. Metazoan invertebrate density and diversity were lower to the southeast, at APEI-7, where estimated POC flux was highest, sediments were finest and no nodules were observed. Conversely, metazoan invertebrate density and diversity were higher at APEIs-1 and 4 to the northwest, where nodules were observed on the coarser sediment, and POC flux was estimated to be lower. These environmental factors (nodule abundance, sediment texture, POC flux) are likely to have impacted the observed megafaunal density, diversity and assemblage composition, both directly and in combination.

Nodules are an important source of habitat heterogeneity on abyssal plains, and exert a major influence on megafaunal assemblages across the CCZ. At the western CCZ study sites, there was a 40% increase in density and an increase of 30 morphotypes at the study sites with nodules compared to the site without (Table 3). At a local scale, nodule presence has been observed to increase densities twofold, with nodules harbouring specific megafauna, such as antipatharians and alcyonaceans, while some fauna have been observed only in nodule-free areas (Amon et al., 2016; Vanreusel et al., 2016; Simon-Lledó et al., 2019b). Metazoan and foraminiferal densities have been observed to increase substantially, and metazoan assemblage composition to vary, with nodule cover at low nodule coverage (∼1–3%; Simon-Lledó et al., 2019b). Similarly, metazoan diversity was higher at lower nodule densities (<15% seabed cover; Amon et al., 2016) than at higher nodule densities. These community-level variations were a result of changes in the densities of both mobile and non-mobile taxa. The low nodule coverages are comparable to those found at the study sites in APEIs 1 and 4, suggesting that the presence or absence of nodules may have influenced the assemblages observed there. Since nodule cover was patchy at these two study sites (Figure 3), the megafauna observed likely include a combination of morphotypes suited to both nodules and nodule-free areas. These habitat niches are favoured by different taxa, leading to switches in major groups dominant in areas with contrasting nodule presence. For example, while densities of holothurians and (largely nodule-attaching) actiniarians at the high nodule density APEI-1 site were double and triple those observed at the nodule-free APEI-7 study site, densities of (largely non-nodule attaching) hexactinellids and sponges of indeterminate class were halved (Table 4). Docossacus sp., a plate-shaped glass sponge that is supported on soft sediments by spicules (Kahn et al., 2020), contributed significantly to the high density of hexactinellids at APEI-7. The preference of some taxa for the presence or absence of nodules (latter for hexactinellids) at these sites were confirmed through the analysis of environmental DNA (Laroche et al., 2020). Thus, the habitat heterogeneity provided by the patchy abundance of nodules at APEIs 1 and 4 is likely a substantial driver of variation in the megafaunal communities in the western CCZ.

Nodule-related seabed heterogeneity is accompanied by differences in the soft sediment particle size, which likely also influences the megabenthic assemblage. Taxa that interact with the sediment, such as deposit feeding and burrowing fauna, are sensitive to its granulometric characteristics (Hudson et al., 2005). Nodule-free areas were observed at all three APEIs (Figure 3), and few soft-sediment interacting taxa were common to all sites (though the lack of overlapping morphotypes may be related to the small sample size). However, these contributed only a small proportion of all morphotypes observed at the nodule-free APEI-7 study site. Morphotypes that interact with soft sediments (e.g., holothurians, annelids, some asteroids) were observed at all abyssal plain study sites, but the morphotypes present and their densities both varied. Densities of echiurans (ANN020), deposit feeding burrowing annelids that leave large feeding traces on the sediment surface (Bett and Rice, 1993), were highest at APEI-7, with 3.5- and 5.4-fold higher densities than in APEIs 1 and 4, respectively. Differences in densities and observed morphotypes may be related to differences in sediment particle size distributions between the sites (Figure 3), with the APEI-7 having the coarsest sediment. Similar relationships between soft-sediment adapted megafaunal morphotypes and sediment texture have been observed with small changes to sediment texture at the nodule-free abyssal Porcupine Abyssal Plain in the Atlantic Ocean (Durden et al., 2020b). Other sediment conditions not measured at these sites, such as porosity and biogeochemical parameters (e.g., Volz et al., 2018), may also influence the assemblage in areas of soft sediments. Finally, the observed sediment particle size distribution is related to other environmental factors important to the faunal assemblage, including the near surface current regime, and deposition and bioturbation of organic material (Turnewitsch et al., 2015), which are influenced by seabed bathymetry. Even small changes in seabed water depth (12.5 m), smaller than those in the study areas (18–50 m), can result in changes in sediment particle size, with corresponding alterations to megafaunal assemblage related to deposit feeders and burrowing organisms (Durden et al., 2020b). While the effects of sediment texture on megafaunal density and diversity may not be comparable to those of nodules (Simon-Lledó et al., 2019a), our results suggest that particle size may influence the presence and densities of morphotypes in the western CCZ.

Invertebrate megabenthic densities in the three APEIs in the western CCZ were low compared to other study locations in the central Pacific, without a straightforward relationship to POC flux. Two other studies provide east/west end members to our study locations, with an east-to-west increase in POC flux and megafaunal density between them: the Kiribati Area C to the west, where estimated POC flux (1.5 gC m^–2 y^–1) was 125% and total density (0.12 ind m^–2) was 200% of those observed at APEI-1 (Simon-Lledó et al., 2019c); and a central equatorial Pacific site (5°N, 140°W) to the east, where POC flux (1.9 gC m^–2 y^–1; Washburn et al., 2021a) was similar and density (0.18 ind m^–2; Smith et al., 1997) was more than four times those at APEI-7. The lower POC flux and difference in megafaunal densities observed at APEIs-1 and 4 may be related to latitude, as more labile organic matter has observed closer to the equator in the Pacific, where upwelling leads to much greater primary production (Smith et al., 1997). Both the Kiribati Area C and the central equatorial Pacific that bookend our study sites east-to-west are at 5°N, similar to APEI-7, and also used photographic methods for estimating megafaunal densities. Thus, it appears that the relationship between estimated POC flux and metazoan densities across the area is not simple.

The observed megafaunal densities in the western APEIs were similar to or lower than those previously observed in the eastern CCZ. Metazoan megafaunal density in APEI-3 (∼0.05 ind m^–2) (Vanreusel et al., 2016) was similar to those observed in APEIs 1, 4 and 7. However, the APEI-3 densities are likely an underestimate, as organisms less than 30 mm in size were excluded based on image resolution. In contrast, observed megafaunal densities from the easternmost APEI-6 and eastern CCZ contract areas (BGR, GSR, IFREMER, IOM, TOML, UK-1 contract areas) were substantially higher than those found in the APEIs of the western CCZ: reported metazoan densities ranged from 0.11 to 1.5 ind m^–2 (Amon et al., 2016; Vanreusel et al., 2016; Simon-Lledó et al., 2019a, 2020; Jones et al., in review), values that may also be underestimates because of similar exclusion of small organisms. Where values were reported, the locations with higher megafaunal densities than the western APEIs also had higher nodule abundance (134 m^–2 in APEI-6) (Simon-Lledó et al., 2019a) or greater seabed nodule cover (13–37% in TOML contract areas) (Simon-Lledó et al., 2020), and POC flux similar to or lower (0.5–1.6 gC m^–2 y^–1) than those observed in the western APEIs. These comparisons suggest that environmental and the megafaunal density variations are substantial across the CCZ, and that the relationships between environmental factors and the megafaunal assemblage are complex.

These spatial variations in megafaunal densities across the CCZ may be substantially confounded by temporal variation. The temporal mismatch between studies, often with seasons and years separating observations (e.g., observations from 2013 in Amon et al., 2016; observations from 2018 in this study), makes differentiating spatial from temporal variations difficult. Metazoan megafaunal assemblages have been observed to vary with seasonal and interannual changes to organic matter inputs at other abyssal sites, with densities of some taxa changing by orders of magnitude over periods of 1–2 years (Billett et al., 2001, 2010; Kuhnz et al., 2014). Shorter-term factors altering behaviour of some abyssal organisms, such as movements in sponges that may impact their consistent identification in images, have only just been discovered (Kahn et al., 2020), and longer-term changes to megabenthic biomass and biodiversity are anticipated as a result of climate change (Smith et al., 2008; Jones et al., 2014; Levin et al., 2020).

Diversity of metazoan megafauna on the abyssal plain is high across the CCZ and central Pacific, as evidenced by the high numbers of morphotypes observed (e.g., 136 morphotypes at APEI-6, Simon-Lledó et al., 2019a; 145–189 invertebrate taxa at three TOML sites, Simon-Lledó et al., 2020; 126 morphotypes, this study). The combination of high diversity with low density results in substantial proportions of rare morphotypes. At APEI-6 in the eastern CCZ, 40% of morphotypes were observed fewer than 3 times (Simon-Lledó et al., 2019a), while in the APEIs of the western CCZ, 27% of morphotypes were observed only once. Although direct comparisons of beta-diversity across the region using images is currently precluded by a lack of standardisation of taxonomic identifications and methodological differences including sampling effort, some studies using consistent methodologies have found few overlapping morphotypes between the multiple areas they examined. At the three sites near Kiribati, nearly half (49%) of all morphotypes were detected only at a single location, with only 33% of morphotypes found across all study sites (Simon-Lledó et al., 2019c). Similarly, in the APEIs of the western CCZ, 48% of morphotypes were found only at one of the APEIs, and only 18% of morphotypes were found at all locations. In reviewing taxonomic identifications used in previous studies (see “Megafaunal Community Assessment”), approximately 100 of those observed in this study were not observed in those previous studies. However, the small seabed areas surveyed precludes robust assessments of endemicity from morphotype presence in photographs. Genetic techniques have been used to examine the connectivity of individual species, with connectivity between APEI-6 and adjacent contract areas in the eastern CCZ found for a common demosponge (Taboada et al., 2018).

Despite the low densities of megafauna generally in the western CCZ, there was one highly abundant group: agglutinated monothalamous benthic foraminifera (xenophyophores). These giant protists dominated the megafauna at the abyssal plain sites, and were found in the highest densities and largest test sizes at APEI-4. Why this group was particularly successful at this site, characterised by intermediate depth, nodule cover and estimated POC flux, is difficult to assess. Although it was impractical to discriminate between morphotypes among all the specimens (>58,000 were observed), the assemblages are clearly quite diverse. At least 22 morphotypes, some attached to nodules, were recognised in a subset of specimens from the abyssal plain sites (Gooday et al., 2020a). The lack of nodules observed at APEI-7 may result in a lower foraminiferal diversity at the site, similar to the nodule effect on metazoan invertebrates (Amon et al., 2016; Simon-Lledó et al., 2019b). Many of the morphotypes project well above the sediment surface and have morphologies that suggest suspension-feeding or particle trapping trophic strategies (Levin and Gooday, 1992; Gooday et al., 2020b). However, those that are flat-lying are more likely to feed directly on the sediment surface. The combination of species represented may also explain the differing sizes of specimens observed between sites, the large test sizes at APEI-4 possibly being related to a prevalent morphotype.

Although found in high densities and with large test size, the relatively small proportion of test volume occupied by cytoplasm means that megafaunal foraminifera are unlikely to contribute substantially to megafaunal biomass (Levin and Gooday, 1992; Gooday et al., 2018b, 2020a). Estimates were made using a hemisphere to represent the test volume (reasonable for those with reticulated and folded plate-like morphologies) and the method described by Gooday et al. (1993). Mean foraminiferal biomass was estimated at 2.2 × 10^–5 g m^–2 on the abyssal plain in APEI-1 and 4.6 × 10^–8 g m^–2 at APEI-4, a tiny fraction of the observed echinoderm biomass at these sites (Table 3). Thus, this group may contribute substantially to biodiversity in the western CCZ, but their contributions to megafaunal biomass appears to be small in these areas.

Xenophyophores dominate the megafauna across much of the CCZ. High densities of these protists were observed from the west, in the Kiribati Area C (0.5 ind m^–2) (Simon-Lledó et al., 2019c), to the extreme east of the CCZ, in APEI-6 (2 ind m^–2), with lower absolute densities reported in the Russian contract area (0.16 ind m^–2) (Kamenskaya et al., 2013) and UK-1 contract area (0.08–0.16 ind m^–2) (Amon et al., 2016). The diversity of xenophyophores is often difficult to estimate from seafloor imagery, partly due to the morphological variability of some morphotypes and genetic evidence that morphologically similar specimens may encompass several distinct species (Gooday et al., 2018a). Nevertheless, observations in APEIs 1, 4 and 7 have contributed substantially to the number of observed morphotypes across the CCZ (Gooday et al., 2020a,b). At least one species, Aschemonella monilis, has been observed at multiple locations including APEI-4 east to UK-1 contract area. Morphotype diversity is discussed in more detail in Gooday et al. (2020a).

The seabed environment at the soft sedimented study areas on the seamounts in APEIs 4 and 7 differ substantially from those found on the adjacent abyssal plain. The white sediments, rich in planktonic foraminiferan shells, are characteristic of many Pacific seamount summits above the carbonate compensation depth (Smith and Demopoulos, 2003). This depth is approximately 4,500 m across the CCZ, but is likely deeper to the west (Mewes et al., 2014). Sediments at these two seamount sites appeared similar in terms of composition and particle size (Table 2 and Supplementary Material 3), but differed from the thin coccolithophore-rich sediment observed at seamount in APEI-6 (Jones et al., in review). Although the seamount study sites in APEIs 4 and 7 are located at low latitudes, the depth of their summits suggest that they likely do not benefit from surface chlorophyll enhancements (Leitner et al., 2020b), but organic matter may be enhanced by resuspension and increased horizontal flux. These alterations to the seabed environment likely influence the megafaunal communities observed.

Differences in the seabed environments between the two seamounts likely influence the differences in the observed megafaunal densities. The relatively high density (approximately fourfold) and diversity (double the morphotype richness) of metazoan invertebrates observed at the seamount in APEI-7 in comparison to the seamount in APEI-4 is echoed in the density (approximately four times greater) and diversity (double richness) of Cnidaria, and Echinoderm density (approximately six times greater). These differences are likely related to the differences in the mean seabed depths of the surveyed areas, where the APEI-7 seamount is substantially shallower, but also the range of seabed depths surveyed, which was 5 times greater at APEI-7. Such depth differences likely influence megafaunal communities, as observed for relatively small topographic features on the plains (Durden et al., 2015, 2020b; Simon-Lledó et al., 2019a, 2020), with significant differences in megafaunal communities related to species replacement at elevation differences of only 12.5 m (Durden et al., 2020b). The differences in the depth ranges of seabed observations at each seamount has been highlighted as a limitation to comparability between seamounts in the eastern CCZ (Cuvelier et al., 2020).

The significant community differences and lack of morphotype overlap between the metazoan megafaunal communities on seamounts and the abyssal plain are unsurprising, given the > 1,400 m elevation of the study areas above the abyssal plain and significantly different sediment environments. Analysis of environmental DNA in sediments at the same sites also found that communities on the seamounts differed from the plain (Laroche et al., 2020). Dissimilarity between megafaunal communities on seamounts and nearby abyssal plains related to density and diversity was also observed in APEIs-3 and 6 (Cuvelier et al., 2020; Jones et al., in review), with some morphotypes found exclusively on the seamounts. Abyssal hills, similar but comparatively small topographic features, result in significant changes to the seabed environment (Turnewitsch et al., 2015; Morris et al., 2016), megafaunal community structure and ecosystem function (Durden et al., 2015; Mitchell et al., 2020). Depth and substrate are important to the composition and distribution of organisms on seamounts (Tittensor et al., 2009), with species replacement observed to dominate beta diversity processes in seamount communities with depth, rather than changes to species richness (Victorero et al., 2018). The Cuvelier et al. (2020) study of multiple seamounts in the eastern CCZ similarly found reduced similarity in megafaunal communities between seamounts with larger depth differences than between those of similar depth, though the comparison was non-statistical and included both hard and soft substrates. The study areas on the seamounts in the western CCZ focused on soft sediments, thus controlling for differences between hard and soft substrate, and found a high proportion of morphotypes only on the seamounts (61%, representing 89% of individuals on the seamounts), which was likely related to both sediment texture (as described above) and depth differences. Some fauna observed exclusively on the seamounts in APEIs 4 and 7 are likely depth-limited; for example, Laetmogone sp. have been observed elsewhere at bathyal rather than abyssal depths (Tyler et al., 1985). Of the morphotypes found on both the seamounts and the abyssal plain, 45% were groups only identifiable to class or order, precluding our ability to assess their ranges specifically. Thus, it may be reasonable to exclude these morphotypes, leaving only 29% of morphotypes (representing 8% of individuals on the seamounts) found in both habitats. This low species overlap between seamounts and abyssal plain, a finding shared by Cuvelier et al. (2020), suggests low connectivity between the habitats.

Low morphotype overlap (21%) between the seamounts in APEIs 4 and 7 is a source of community dissimilarity, and suggests that connectivity between seamounts may also be low. However, the assessment of morphotype overlap must be tempered by the fact that morphotypes were still accumulating at the two seamounts (Supplementary Material 5). The limited connectivity between seamounts sites in APEIs 4 and 7 was also reflected in environmental DNA analysis from the same sites (Laroche et al., 2020). These community differences in the seamounts in APEIs 4 and 7 may be related to the ∼440 m difference in water depths of the study areas contributing to range restriction of particular taxa. High variation in connectivity has been observed between seamounts, related to some taxa having wide distributions while others are more localised (Clark et al., 2009). Order and class-level differences between the seamounts in APEIs 4 and 7 are evident, with Asteroidea, Crinoidea, Echinoidea, and Decapoda only observed at the seamount at APEI-7, and Actiniaria, Ceriantharia, Alcyonacea, Pennatulacea, Holothuroidea, and Ophiuroidea at both seamounts, with large differences in abundances of each between seamounts. Similar higher taxonomic-level differences were observed between seamounts in APEI-3 and contract areas in the eastern CCZ (Cuvelier et al., 2020), with Alcyonacea found at all seamounts, but large differences in densities of Antipatharia, Pennatulacea, Porifera, and Echinodermata contributing to the significant differences in megafaunal communities observed. This apparent lack of connectivity between the seamounts observed in the western CCZ, and variation in connectivity between those studied in the east, may require additional consideration in terms of regional management and conservation planning, as seamount habitats could be impacted by plumes from mining activities on nearby abyssal plains. The high percentage of species represented by singletons, and the continuing accumulation of species at each site (Supplementary Material 5), indicate that substantially more sampling of seamounts is required to fully document biodiversity and connectivity of the CCZ seamount fauna. Undersampling is a concern at other seamounts (Cuvelier et al., 2020), which limits the evaluation of biodiversity and connectivity among seamounts and between habitats across the CCZ.

Observations of environmental and faunal conditions across the APEIs are needed to provide evidence to evaluate the design and efficacy of the regional environmental management plan for polymetallic nodule mining. The design of the APEI network aimed to conserve biodiversity and ecosystem function, and was originally planned to span the range of habitat and biological variation across the CCZ, based on modelled biophysical parameters (Wedding et al., 2013). The high diversity of megafauna observed and apparently low connectivity at the western APEIs reinforces the need for the distributed nature of the APEI network. The observations from APEIs 1, 4, and 7 provide important western end members to combine with observations from the eastern CCZ, in creating a regional picture that could be used to evaluate whether the APEI network design is suitable for its aims. Some examples of potential network evaluation perspectives using these data include:

Despite the need for baseline information on the APEIs, studies of these areas remain few, and limited in spatial and temporal scope. For example, the seabed areas photographed in each of our study areas encompass only ∼5 × 10^–7% of an APEI that is 1.44 × 10⁶ km² in area (Table 1), and whether the photographed areas represent the habitats and megafauna of the remaining portions of the APEIs is unknown. Despite the lack of areal coverage, the observed low density and high diversity of the megafaunal assemblages of the western CCZ provides important information for designing future studies across the APEIs (Supplementary Material 5), and points to the potential areal coverage required to ensure the observed area is representative of an APEI (e.g., survey design calculations in Durden et al., 2016b). The observed prevalence of rare morphotypes is particularly important to future survey design, as the detection of rare morphotypes is sensitive to the area surveyed, resulting in the need for a larger study area to document the biodiversity and assess species overlaps than in habitats with few rare morphotypes. A further factor precluding robust comparisons between APEIs and evaluation of the APEI network is the lack of consideration of temporal variation. Studies of the APEIs to date have been conducted in different seasons and years (as noted above), though some studies of APEIs have been conducted at similar times to those of adjacent exploration contract areas, facilitating comparisons (e.g., Amon et al., 2016; Vanreusel et al., 2016). Finally, the practicalities of studying the APEIs must be considered. The paucity of studies conducted at the APEIs in the western CCZ may be in part related to their inaccessibility, as they are distant from shore. Thus, a comprehensive plan to study the APEIs is needed to ensure that observations and data are captured with sufficient consideration for these spatial, temporal and practical factors to produce the necessary data for understanding patterns in the environment and fauna regionally, and for evaluating, maintaining and updating the regional management plan.