Background

Deep-sea areas (>200 m and beyond) support high biodiversity, but also hold economically important reservoirs of mineral deposits, including polymetallic nodules (or manganese nodules), cobalt-rich ferromanganese crusts and polymetallic sulphides, among others. Regions of interest are often located in areas beyond jurisdiction, i.e. in ‘The Area,’ and any activities there related to the exploration and exploitation of non-living resources are managed by the International Seabed Authority (ISA). For polymetallic nodules, the Clarion Clipperton Fracture Zone (CCZ), in the north-eastern equatorial Pacific, is the area of greatest commercial interest (Fig. 1) (Wedding et al. 2015). The CCZ encompasses about six million square kilometres of abyssal (>3000 m) seafloor, an area nearly the size of Europe, and the region harbours among the world’s most economically valuable deposits of polymetallic nodules (Figs 2 and 3) and is, thus, potentially a major source of copper, nickel, cobalt and other minerals.

Fig. 1
figure 1

Location of licence areas and Areas of Particular Environmental Interest (APEIs) across the Clarion Clipperton Fracture Zone [CCZ; map courtesy of the International Seabed Authority (ISA)]

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Fig. 2
figure 2

Seabed image (using ROV) of a nodule site in the CCZ. Polymetallic nodules cover large areas of abyssal seafloor in the north-eastern central Pacific, varying greatly in size and density, even at the kilometre scale (image courtesy of GEOMAR)

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Fig. 3
figure 3

Examples of important planktonic and benthic organisms and habitats present in the CCZ: a dinophysoid dinoflagellate, Phalacroma favus Kofoid & Michener, 1911; scale bar: 10 μm (image: C. Zinssmeister); b isopod, Ketosoma ruehlemanni Kaiser & Janssen, 2017; scale bar: 500 μm (image: T.C. Kihara); c bivalve, Nucula profundorum E. A. Smith, 1885; specimens between 1 and 6 mm in length (image: A. Glover, T. Dahlgren & H. Wiklund); d hexactinellid sponge, Saccocalyx pedunculatus Schulze, 1896; specimen about 18.9 cm in length (image courtesy of GEOMAR); e nematode, Acantholaimus sp. (family Chromadoridae Filipjev, 1917); specimen about 1.4–1.7 μm in length (image: R. Singh); f polymetallic nodules represent important habitats for a diverse fauna inhabiting the surface and nodule crevices; the nodule pictured is about 8 cm in diameter (image: S. Kaiser); g wood fall collected during the JPIO EcoResponse (SO239) expedition to the CCZ (image courtesy of GEOMAR); h xenophyophore, Psammina limbata Kamenskaya, Gooday & Tendal, 2015, encrusting a manganese nodule; specimen about 2 cm in size (image: O. Kamenskaya); i seamount habitat in the CCZ (image courtesy of GEOMAR); j harpacticoid copepod, Pseudotachidius bipartitus Montagna, 1980; scale bar: 500 μm (image: U. Raschka); k polychaete, Bathyeliasona sp. nov., Polynoidae Kinberg, 1856; specimen 9.86 mm in length (image: P. Bonifacio); l antipatharian (black) coral, Abyssopathes lyra (Brook, 1889); specimen about 8 cm in length (image: T. Molodtsova); m ophiuroid, Ophiomusium cf. glabrum, scale bar: 1 cm (image courtesy of A. Hilario & P. Ribeiro)

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The CCZ is also of great interest from an ecological point of view. In fact, it displays a spatially very heterogeneous environment characterised by, for example, changes in nodule sizes and densities, large-scale productivity and depth gradients, as well as a high abundance of topographic features, such as seamounts, hills and channels (e.g. Wedding et al. 2013). The variety of habitats has been thought to promote higher diversity of associated benthic communities compared to abyssal areas elsewhere (Ramirez-Llodra et al. 2010; Janssen et al. 2015; Amon et al. 2016b; Vanreusel et al. 2016).

Nodules lie on the surface of the soft-sediment seafloor and, therefore, themselves represent important (micro-)habitats for the sessile biota (e.g. xenophyophores, antipatharian corals, sponges), as well as several meiofaunal and microbial taxa found inside the sediment-filled nodule crevices (Fig. 3; Thiel et al. 1993; Veillette et al. 2007; Vanreusel et al. 2016). A recent study by Amon et al. (2016b) suggested that more than half of megafaunal species in the CCZ depend on nodules as a hard substrate. Furthermore, nodule areas seem to contain higher densities of mobile megafauna compared to those lacking nodules (Amon et al. 2016b; Vanreusel et al. 2016). Many species appear have to have very limited geographic ranges consistent with reproduction patterns; alternatively, these species may simply be rare and undersampled (Glover et al. 2002; Janssen et al. 2015; Wilson 2017). In addition, there are quite a few common species, in some cases broadcast spawners, occurring widely across the CCZ (e.g. Glover et al. 2002; Gooday et al. 2015; Wilson 2017).

The extraction of deep-sea minerals will alter the structure and functioning of ecosystems targeted for mining. Although biological research on the nodule fauna began in the 1970s (Jones et al. 2017 and citations therein), many fundamental ecological questions (e.g. related to levels of biodiversity, species ranges, connectivity among populations and habitats) remain unanswered, making the impacts of mining difficult to predict. It does seem to be clear, though, that nodules have both significant economic and ecological value (Fritz 2016). In particular, the removal or burial of nodules by mining activities will erase the biota that depend on nodules for habitat, and will also affect the soft-sediment fauna through sediment compression and disruption of near-surface sediment layers (Miljutin et al. 2011; Vanreusel et al. 2016; Jones et al. 2017; Gollner et al. 2017). Due to the slow growth rates of nodules (ca. 10 mm/My) and overall very low sedimentation rates, short-term recovery is unlikely; the nodules and nodule-dependent fauna may take millions of years to recover, and even the partial recovery of the motile sediment-dwelling fauna may take hundreds to thousands of years (Smith et al. 2008b; Miljutin et al. 2011; Wedding et al. 2013; Gollner et al. 2017). Additionally, mining impacts may be far-reaching, beyond the actual mining block, that would affect benthic and pelagic communities largely through the dispersion of sediment plumes, as well as (potentially toxic) discharge water from mine tailings (ECORYS 2014; Gollner et al. 2017).

Recent biological findings have led to a plea to develop and improve environmental regulations and agreements to better balance conservation and exploitation needs (e.g. Wedding et al. 2015; Fritz 2016; Le et al. 2017; Vanreusel et al. 2016). Following a precautionary approach, the ISA protected a network of nine 160,000-km2 no-mining areas, now called Areas of Particular Environmental Interest (APEIs), to safeguard biodiversity and ecosystem function in the CCZ (Smith et al. 2008a; Wedding et al. 2013, 2015). Although the original network design included APEIs in the core of the CCZ (Smith et al. 2008a; Wedding et al. 2013), the APEIs are now located mainly in the outer regions of the CCZ and were selected based on environmental proxies; until 2015, no biological sampling had been undertaken in these areas to directly evaluate the degree of connectivity between faunal assemblages of APEIs and areas targeted for mining (Martínez Arbizu and Haeckel 2015; Amon et al. 2016b; Vanreusel et al. 2016). Nevertheless, Vanreusel et al. (2016) concluded that APEIs are crucial to protect and preserve regional-scale diversity, but also recommended protected areas (i.e. preservation reference areas, PRA) within each licence area using criteria (e.g. sufficient nodule coverage) to facilitate recolonisation of nearby impacted seafloor areas.

The first 15-year licences to explore polymetallic nodule occurrences in the CCZ were issued in 2001 and, since then, ISA has approved 15 exploration contracts (by March 2017, http://www.isa.org.jm), of which six expired or were extended in 2016. The ISA is currently framing regulations for the exploitation of deep-seabed minerals in ABNJ. At the same time, the UN is developing recommendations for ecosystem-based management of mining activities in ‘The Area’ to better protect its biodiversity (ISA and ITLOS Preparatory Commission 1990; Danovaro et al. 2017). Thus, it is timely to improve the characterisation of the biodiversity of the CCZ and the understanding of underlying processes, including taxonomic analyses and inventories, to provide a sound basis to address crucial ecological and biogeographic questions.

Highlights in this special issue

This special issue of Marine Biodiversity presents a series of papers illustrating different facets of the biodiversity of the CCZ region, encompassing a wide range of taxonomic groups, size classes and habitats (see Fig. 3). New species descriptions, in particular for previously neglected groups, illustrate a taxonomically more balanced view and represent one of many smaller steps for diversity estimations (Kamenskaya et al. 2016; Markhaseva et al. 2017; Molodtsova and Opresko 2017; Zinssmeister et al. 2017). In situ observations from video and still imagery using manned and unmanned (ROV, AUV) submersibles have led to discoveries of new habitats and also yielded valuable biogeographic information (Amon et al. 2016a; Durden et al. 2017; Vecchione 2016), while community analyses provide important insights into the patterns and processes driving (macro-)faunal diversity and turnover at local to regional scales (Wilson 2017).

Dinoflagellate protists are a common and diverse component of the marine plankton and, yet, the study of Zinssmeister et al. (2017) represents the first on the diversity of dinophysoid dinoflagellates in the CCZ. The authors could identify 66 species and 11 genera, which greatly exceeds numbers previously reported from elsewhere in the eastern Pacific. The sample collection contained a number of species new to the area, some of which may represent undescribed species.

Kamenskaya et al. (2016) report novel data on xenophyophores, a group of large, agglutinated foraminiferans that are typically among the most dominant megafaunal taxa in the CCZ. Xenophyophores also appear to be particularly diverse in nodule areas of the eastern Pacific (Gooday et al. 2017) and Kamenskaya and co-authors (2016) add to this diversity by describing three new species from the Russian licence area of the CCZ. This is likely reflects just a fraction of the species present, because there are still several xenophyophore species awaiting formal description (Kamenskaya et al. 2016).

Until now, there has been a paucity of data on benthopelagic calanoids from abyssal waters of the CCZ. Markhaseva et al. (2017) provide a description of a new species within the cosmopolitan family Aetideidae, which is complemented by biogeographic information. While this is the first record of the genus Pseudeuchaeta from the equatorial Pacific, the few records of benthopelagic calanoid taxa in general points towards great undersampling of the deep benthopelagic realm (Markhaseva et al. 2017).

Antipatharian corals, or black corals, are commonly found in nodule areas of the abyss, while they are absent from nodule-poor sites, making them potentially very susceptible to mining impact (Vanreusel et al. 2016; Molodtsova and Opresko 2017). In their study, Molodtsova and Opresko (2017) present a checklist of abyssal antipatharian species, as well as a morphological assessment of all species known from the CCZ. Furthermore, they describe a new genus and species from the CCZ, both of which were previously recorded from the Indian Ocean and, thus, have rather broad distributions.

Paleodictyon nodosum represents a trace fossil found in deep-sea areas of the Atlantic and Pacific oceans. Durden et al. (2017) provide the first in situ observation from the CCZ. Although Paleodictyon nodosum creates a distinct symmetric pattern and is commonly found on the abyssal seabed, its origin and function remains elusive. Therefore, Durden et al. (2017) discuss the potential use of time-lapse imagery, among other techniques, to elucidate unresolved questions.

Cirrate (finned) octopods are confined to the deep sea, where they are distributed globally. In the CCZ, they belong to the larger, potentially rarer megafauna, and data on their distribution are scarce. Vecchione (2016) reports records of two families of cirrate octopods, based on identification from video images, from the French licence areas. It is likely that other cephalopod taxa are present in the CCZ, such as big-fin squid and incirrate octopods (Vecchione 2016; see also Purser et al. 2016).

Organic falls, such as wood and whale carcasses, are a significant source of habitat heterogeneity in the deep sea. On the seafloor, they represent an important hard-substrate and organic-rich habitat, supporting unique microbial and faunal communities. Amon et al. (2016a) present the first study of wood falls and associated faunal assemblages examined from different licence areas and APEIs in the CCZ. Their findings suggest that wood falls are, despite the great distance from land, a common phenomenon in the CCZ, with the potential to enhance local and regional diversity.

Understanding of the drivers that constrain species distributions and the degree of connectivity between communities is important to assess ecosystem resilience to mining impacts. Wilson (2017) analysed variation in macrofaunal community composition and diversity under different productivity regimes in the CCZ and in relation to reproductive mode. He found contrasting patterns in diversity and distributional ranges among the taxa investigated. This leads to the assumption that diversity patterns should not be inferred from a single taxon, but should include a large array of taxa representative for the CCZ that encompass a variety of life-history traits and functions.

Summary of highlights

This special issue of Marine Biodiversity includes:

  • Among the first biological data from APEIs (Amon et al. 2016a; Durden et al. 2017)

  • The first study of dinophysoid dinoflagellates from the CCZ (Zinssmeister et al. 2017)

  • The first record of wood falls from the CCZ (Amon et al. 2016a)

  • The first record of Paleodictyon nodosum from the eastern equatorial Pacific (Durden et al. 2017)

  • New species and/or new distributional records of xenophyophores, benthopelagic calanoids, antipatharian corals and cirrate octopods (Kamenskaya et al. 2016; Markhaseva et al. 2017; Molodtsova and Opresko 2017; Vecchione 2016)