Title Observations of organic falls from the abyssal Clarion-Clipperton Zone in the tropical eastern Pacific Ocean

Authors Amon, Diva; Hilario, A; Arbizu, PM; Smith, CR

Date Submitted 2020-09-11 Observations of organic falls in the abyssal Clarion-Clipperton Zone, tropical eastern Pacific Ocean

Diva J. Amon a*, Ana Hilario b, Pedro Martinez Arbizu c, Craig R. Smith a a Department of , University of Hawai’i at Mānoa, 1000 Pope Road, Honolulu, HI

96822 USA b Departamento de Biologia & CESAM, Universidade de Aveiro, 3810-125 Aveiro, Portugal c German Center for Marine Biodiversity Research, Senckenberg am Meer, Südstrand 44, D-26382 Wilhelmshaven, Germany

*Corresponding author - D.J. Amon, [email protected]

Acknowledgements The authors thank the National Research Foundation of Singapore, Ocean Mineral Singapore and the National University of Singapore for supporting this work done in partnership with the Keppel-NUS Corporate Laboratory, as well as UK Seabed Resources Ltd. Cruise SO239, EcoResponse was financed by the German Ministry of Education and Science BMBF as a contribution to the European project JPI-Oceans “Ecological Aspects of Deep-Sea Mining”. PMA acknowledges funding from the European Union Seventh Framework Programme (FP7/2007–2013) under the MIDAS project, grant agreement n° 603418 and funding from BMBF contract 03 F0707E. This work has also received funding from FCT (Apoios Especiais 2015 - Fundo de Apoio à Comunidade Científica) under the framework of JPI Oceans. Thanks to the Masters, crew and scientists of the RV Thompson (TN319, ABYSSLINE 02), the RV Sonne (SO239, EcoResponse) and the ROV Kiel 6000 (GEOMAR) for their support during fieldwork in the Clarion-Clipperton Zone. We are grateful to the AUV REMUS6000 team from Woods Hole Oceanographic Institution for the collection of the imagery during the UKSRL-funded AYBSSLINE 02 cruise. Further thanks to Dr. Jeffrey Drazen from the University of Hawai’i at Manoa for checking fish identifications, Christopher Mah from the Smithsonian Museum for identifying the asteroids, and Kai George from Senckenberg am Meer for help in identifying the laophontodid copepods. The conclusions put forward reflect the views of the authors alone. This is UH SOEST publication number XXXX.

1 1 Abstract 2 Organic falls can form nutrient-rich, ephemeral hotspots of productivity and biodiversity at the 3 deep-sea floor, especially in food-poor abyssal plains. We report here the first wood falls and 4 second carcass fall recorded from the Clarion-Clipperton Zone in the tropical eastern Pacific 5 Ocean, an area that could be mined for polymetallic nodules in the future. A small cetacean fall 6 in the mobile-scavenger stage likely recently arrived on the seafloor was observed, whereas most 7 of the wood falls were highly degraded. There were multiple species in attendance at the wood 8 falls including organic-fall specialists such as Xylophagaidae molluscs. Many of the taxa 9 attending the carcass fall were known mobile scavengers that regularly attend bait parcels in the 10 Pacific Ocean. These results further confirm that wood falls can occur at large distances (> 1450 11 km) from major land masses, providing an adequate supply of wood to the abyssal seafloor for 12 colonization by wood-boring molluscs and associated fauna. Organic falls may be regionally 13 abundant and are likely to influence species and habitat diversity in the abyssal areas of the 14 CCZ. 15 16 Keywords: whale fall, wood fall, scavenger, Clarion-Clipperton Zone, deep-sea mining, 17 Xylophagaidae

2 18 1. Introduction 19 Organic falls are ephemeral habitat islands, which include two important types: wood falls and 20 cetacean falls. These habitats are often nutrient-rich compared to the food-poor deep-sea floor, are 21 known to support a high diversity of both specialist and opportunistic species, are sources of 22 evolutionary novelty, and may also act as stepping stones between chemosynthetic deep-sea habitats 23 (Baco and Smith 2003; Distel et al. 2000; Smith et al. 2015). Whale falls, and to a lesser extent, wood 24 falls, pass through several decompositional stages on the deep-sea floor (Bienhold et al. 2013; Smith 25 and Baco 2003). Shortly after a cetacean carcass arrives on the seafloor, the mobile-scavenger stage 26 begins; during this stage, which may last weeks to years, the soft tissue is removed by large 27 scavenging organisms such as fish, galatheids and amphipods (Smith and Baco 2003; Smith et al. 28 2015). The enrichment-opportunist stage follows, supporting a different faunal assemblage that is 29 attracted to the detrital enrichment resulting from the previous stage, and can last for over 7 years 30 (Smith and Baco 2003; Smith et al. 2014a). This is followed by the sulfophilic stage, which occurs 31 when anaerobic microbial degradation of the -rich skeleton creates reduced compounds that fuel a 32 chemosynthetic community that can last for decades (Amon et al. 2013; Schuller et al. 2004; Smith 33 and Baco 2003; Smith et al. 2015; Smith et al. 1998). Wood falls that are colonized by wood-boring 34 bivalves show a similar pattern to that of cetacean falls (Bienhold et al. 2013). The feeding activity of 35 wood-boring bivalves results in the deposition of detrital matter enriching the sediment surrounding 36 the wood fall (Bernardino et al. 2010; Bienhold et al. 2013). This enrichment stage can last for years 37 and leads to a sulfophilic stage as seen at most whale falls (Bernardino et al. 2010; Bienhold et al. 38 2013). 39 The Clarion-Clipperton Zone (CCZ) is a region of the tropical eastern Pacific Ocean where 40 deep-sea mining may take place in the near future (Ramirez-Llodra et al. 2011; Wedding et al. 2013). 41 High-grade polymetallic nodules, which could provide a commercial source of copper, cobalt, nickel, 42 and manganese (among other metals), are abundant in this six-million km2 region (Amon et al. 2016; 43 Clark et al. 2013). Thus far, there have been 16 leases (each up to 75,000 km2 in area) granted by the 44 International Seabed Authority for exploration in the CCZ with those for exploitation expected to soon 45 follow (https://www.isa.org.jm/). It is expected that nodule mining will drastically alter this unique 46 habitat and yet, very little is known about the ecology and biogeography of the fauna inhabiting the 47 region (Amon et al. 2016; Vanreusel et al. 2016). 48 Discoveries of organic-fall habitats have been serendipitous. It has been hypothesized that 49 cetacean falls are most abundant along cetacean migration routes, and wood falls in higher densities in 50 terrestrial conduits to the deep sea, e.g., estuarine areas, submarine canyons and on the continental 51 slope (Roman et al. 2014; Smith and Baco 2003; Vetter and Dayton 1998). Few organic falls have 52 been found in abyssal regions or even at great distances from land, likely not because they do not exist 53 but rather due to the low levels of deep-sea research in these remote areas. There has only been one 54 record of a natural modern organic fall in the CCZ: a large odontocete whale fall in the sulfophilic

3 55 stage, which was observed at 4850 m (Smith et al. 2015). Blackened areas of sediment and white 56 bacterial mats indicated the presence of sulfides resulting from the anaerobic decay of whale biomass 57 (Smith et al. 2015). The only recognizable fauna attending this whale fall were squat lobsters 58 (galatheids), likely from the genus Munidopsis (Smith et al. 2015). There have also been several 59 records of abyssal manganese-encrusted fossil whale falls in the CCZ (Heezen and Hollister 1971; 60 Smith et al. 2015)(Amon and Smith, unpublished data). However, no wood falls or wood-fall 61 associated fauna have been recorded from the vast region of the CCZ. Here, we present the first 62 records of wood falls, including one collected with associated fauna, and the second record of a likely 63 cetacean fall in the CCZ. We also discuss the ecology of these organic falls and the potential impacts 64 of polymetallic-nodule mining in the region on these unique habitats. 65 66 2. Methods and study sites 67 2.1. Locations of the organic falls 68 Seven organic falls were found serendipitously in the CCZ (Fig. 1). Four wood falls and a 69 cetacean fall were located during the second cruise of the ABYSSal baseLINE (ABYSSLINE) project, 70 AB02 (RV Thompson TN-319). This cruise conducted benthic baseline surveys in the exploration 71 contract area leased to UK Seabed Resources Ltd. (UK-1), the exploration contract area leased to 72 Ocean Minerals Singapore (OMS-1), and the International Seabed Authority’s Area of Particular 73 Environmental Interest 4 (APEI-4). Three wood falls (“A”, “B” and “C”) were observed during 74 megafaunal and geophysical surveys conducted by the AUV REMUS in the UK-1 exploration contract 75 area (Fig. 1 and Table 1). The cetacean fall and one wood fall “D” were observed during megafaunal 76 and geophysical surveys conducted by the AUV REMUS in the OMS-1 contract area (Fig. 1 and Table 77 1). Two more wood falls (“E” and “F”) were discovered during the SO239 EcoResponse cruise 78 onboard the RV Sonne, as part of the JPI Oceans (JPIO) “Ecological Aspects of Deep Sea Mining” 79 project. The falls were observed during two surveys using the ROV Kiel 6000. Wood fall “E” was 80 observed in an exploration contract area leased to IFREMER (IFREMER-East), and wood fall “F” was 81 observed in APEI-3, another of the areas targeted by the ISA for preservation (Fig. 1 and Table 1) 82 (Wedding et al. 2013). 83 84 2.2. Analysis of imagery and samples 85 Wood falls “A” to “D” and “F”, and the cetacean fall were visually inspected with underwater 86 imagery but not sampled. Visible fauna attending these organic falls in images from AB02 and SO239 87 were identified to the lowest taxon possible and enumerated. The high altitude of the AUV REMUS 88 yielded poor image resolution which prevented the fauna from being identified to species level and in 89 most cases, generic level, at the cetacean fall and wood falls “A” to “D”. Abundances of were 90 calculated using the laser scale markers to estimate view area, and then counting animals in each 91 image.

4 92 ROV collections were made of two wood fragments with associated invertebrates from wood 93 fall “E” only during the SO239 cruise. Both samples were placed into the same sealed ROV biobox. 94 Upon recovery of the ROV, samples were immediately placed in 96% ethanol. The water in the 95 biobox was sieved on a 300-µm mesh to ensure all animals were collected. Fauna present on the 96 surface of the wood was sorted under a stereomicroscope and identified to the lowest possible 97 taxonomic level. Both wood fragments were carefully sectioned to examine the presence of boring 98 invertebrates. Ethanol used to fix and process the wood samples was carefully sieved through a 32-µm 99 mesh and meiofauna was sorted from the material retained. 100 101 3. Results 102 3.1. Observations of the cetacean fall from the OMS-1 exploration contract area 103 The cetacean carcass was found on the abyssal seafloor in an area known for high abundances 104 of polymetallic nodules, with the immediate vicinity (<60 cm) around the carcass having the highest 105 visible nodule densities in the image (Fig. 2e). The surface sediments in this area are flocculent and 106 thus may have been swept away by the intense scavenging activity or by the impact of the sinking 107 carcass, exposing the polymetallic nodules below. The carcass was approximately 82 cm long and 44 108 cm at its widest point (Fig. 2e). Skin appeared to be missing on the top part of the carcass but soft 109 tissue was visible (Fig. 2e). At least 12 intact mammalian rib bones could be seen on the left of the 110 carcass (one was displaced from the carcass), implying the carcass was lying with the left side facing 111 upwards (Fig. 2e). In the immediate vicinity (< 60 cm) of the carcass, the highest densities of fauna 112 were noted, with 26 individuals from five morphospecies observed (Fig. 2e and Table 2). The most 113 abundant phylum was the Chordata with Zoarcidae being the most abundant family (7.47 ind. m-2) 114 (Fig. 2e and Table 2). Other fish included a morphospecies of ophidiid (potentially Bassozetus sp.) and 115 one individual that could not be identified (Fig. 2e and Table 2). Ophiuroids, the second most 116 abundant morphotype (2.67 ind. m-2), and munidopsid squat lobsters were also observed in close 117 proximity to the carcass (Fig. 2e and Table 2). At distances greater than 60 cm from the carcass, the 118 benthic habitat mainly consisted of sediment. There were also low densities of small nodules, large 119 nodules and Mn crusts. This area had much less fauna than in the immediate vicinity of the carcass, 120 with morphotypes mainly consisting of ophiuroids (0.84 ind. m-2), as well as low densities of sponges, 121 fish and holothurians (Table 2). 122 123 3.2. Observations of the wood falls from the UK-1 and OMS-1 exploration contract areas 124 Wood fall “A” was observed on sediments and appeared to be a piece of trunk or branch of a 125 tree (Fig. 2a). The wood fall was approximately 30 cm long and the dark color could indicate that bark 126 was still present (Fig. 2a). There was a patch of lighter sediment at the top of the wood fall which may 127 have been, wood detritus, bacterial mats or bioturbated sediment (Fig. 2a). The abundance of fauna 128 was difficult to decipher given the poor resolution of the image, although there were several white dots

5 129 to the right of the wood fall which could be animals, such as crustaceans, that were too small to 130 resolve (Fig. 2a). 131 Wood fall “B” was comprised of two pieces of wood that appeared to be highly degraded. The 132 larger piece was 58 cm in length and appeared to be covered in white bacterial mats (Fig. 2b). The 133 smaller piece (33 cm) also had some white areas present which could represent animals too small to 134 resolve or bacterial mats (Fig. 2b). It was not possible to positively identify any fauna at wood fall “B” 135 due to the poor image resolution. This wood fall was observed in an area of high polymetallic-nodule 136 density (Fig. 2b). Wood fall “C” appeared to be a wood fragment that was heavily degraded, given the 137 jagged edges (Fig. 2c). There was little evidence of reduced sediment or bacterial mats but there 138 appeared to be two white structures on the surface of the wood (Fig. 2c). Apart from these elongated 139 structures, which could be the calcified tubes of serpulid worms, little else can be said about the fauna 140 due to the poor image resolution. 141 Wood fall “D” was observed on the sedimented and nodule-laden abyssal seafloor (Fig. 2d). It 142 appeared to be a trunk of a small tree that terminated in the beginnings of the root structure, but with 143 most of the roots missing (Fig. 2d). The wood fall was approximately 100 cm long and the diameter of 144 the trunk was 8 cm (Fig. 2d). There was little evidence of degradation of the wood (no reduced 145 sediment or bacterial mats) apart from divot-like structures in the wood surface (Fig. 2d). The 146 abundance of fauna seen inhabiting this wood fall was low: there were several white dots on the trunk 147 as well as to the right of the root structure, likely animals too small to resolve (Fig. 2d). There was also 148 a holothurian (possibly a juvenile of the family Psychropotidae) to the right of the root structure (Fig. 149 2d). 150 151 3.3. Observations of the wood falls from the IFREMER exploration contract area and APEI-3 152 Wood fall “E” was observed in a flat area of soft sediment amid very low densities of 153 polymetallic nodules (Fig. 3a). Several fragments of different types of wood, measuring from 6 to 49 154 cm in length, were found in a rectangular depression of approximately 0.8 m2. This depression showed 155 more signs of bioturbation than the surrounding seafloor, but there was no evidence of reduced 156 sediment or bacterial mats. All pieces of wood were highly degraded and had visible boring cavities 157 (Fig. 3a-c). Within the depression, there was a high abundance of chaetopterid polychaetes living in 158 the sediments surrounding the wood (50 ind. m-2, Fig. 3c), but low abundances of all other groups 159 present, including sponges and aspidodiadematid echinoids (Table 3). Wood colonizers included 160 sponges, anemones, hydrozoans, munidopsid lobsters, caymanostellid asteroids, polychaetes from the 161 families Dorvilleidae (Fig. 3b-d), Ampharetidae and Hesionidae, gastropods from the subclasses 162 Vetigastropoda and Cocculiniformia (Fig. 3f-g), and xylophagid bivalves (Fig. 3e), all of which were 163 present in low abundances (Table 3). The meiofauna fraction contained at least seven species of 164 harpacticoid copepods including the donsiellinids Xylora bathyalis and a new species which was 165 morphologically similar to both Xouthous and Donsiella, the canthocamptid Mesochra, two species of

6 166 Laophontodinae, Laophontodes sp. and a new species of Calypsophontodes, a new genus of 167 Cerviniidae, and one species of Argestidae. 168 Wood fall “F” occurred in an area with high nodule density and consisted of a single wood 169 fragment ~13 cm long (Fig. 3h). There was no evidence of reduced sediment or bacterial mats and no 170 fauna could be observed in association with the wood, although there was an unidentifiable 171 (maybe a bivalve or tunicate) living on a nodule near the wood parcel. It was clear that the wood had 172 previously been degraded due to the presence of boring cavities (Fig. 3h). 173 174 4. Discussion 175 4.1. Ecology of the cetacean fall 176 The robust ribs of the carcass indicate this is a mammal rather than a fish. The small size (<1 177 m) allows us to further conclude that this could be a pygmy sperm whale or dwarf sperm whale 178 (Kogiidae). It could also be an adult or subadult of the Delphinidae or Phocoenidae. There are nearly 179 25 cetacean species in total from these respective families inhabiting the tropical eastern Pacific Ocean 180 (Ballance et al. 2006). This cetacean fall is the 9th natural whale fall to be studied in situ with only one 181 other previously studied in the CCZ and central abyssal Pacific Ocean (Amon et al. 2013; Fujioka et 182 al. 1993; Goffredi et al. 2004; Lundsten et al. 2010; Smith et al. 2015; Smith et al. 1989; Smith et al. 183 2014b; Sumida et al. 2016), although there are at least 38 records of natural cetacean remains, e.g., 184 single bones, at the deep-sea floor (Smith et al. 2015). This is the third whale fall to be recorded from 185 deeper than 4000 m (Smith et al. 2015; Sumida et al. 2016). 186 The species assemblage was dominated by scavenging fauna such as ophidiids, zoarcids, and 187 munidopsids, which are typical of the first mobile-scavenger successional stage of both natural and 188 implanted whale falls worldwide (Goffredi et al. 2004; Jones et al. 1998; Smith et al. 2015; Smith et 189 al. 2014b; Sumida et al. 2016). It is likely that there were also smaller scavengers present, such as 190 lysianassid amphipods (Smith and Baco 2003), but that they could not be resolved in the imagery. 191 Zoarcids were the most abundant taxon in the immediate vicinity of the carcass (7.47 ind. m-2) and yet 192 their abundance was low when compared with densities observed at a shallower whale fall off the 193 western Antarctic Peninsula (22.6 ind. m-2) (Smith et al. 2014b). This disparity likely reflects the 194 significant difference in carcass size or reduced scavenger abundance under relatively oligotrophic 195 abyssal waters compared to bathyal depths in the Southern Ocean. These scavengers have also been 196 observed attending bait parcels at similar depths in the CCZ during the ABYSSLINE project (J. 197 Drazen, in prep.). While the only other whale fall previously observed in the CCZ was at a different 198 successional (sulfophilic) stage, munidopsids also attended that carcass (Smith et al. 2015). In 199 contrast, scavenging experiments at similar depths in the abyssal Pacific off the Hawaiian Islands 200 observed five morphotypes, only one of which (Ophidiidae sp.) resembled those observed during this 201 study (Yeh and Drazen 2009). Zoarcids were observed off Hawai’i but only at depths < 3000 m (Yeh

7 202 and Drazen 2009). First-stage scavenger assemblages at whale falls at bathyal depths off California 203 tended to be dominated by hagfish, amphipods, sharks and ophiuroids (Smith and Baco 2003). 204 In the immediate vicinity of the CCZ carcass, ophiuroid densities were more than ten times 205 background levels (2.67 versus 0.23 ind. m-2) observed on surveys nearby in the eastern CCZ (Amon 206 et al. 2016). This suggests that ophiuroids were attracted to carcasses to feed, as has been seen at a 207 whale carcass in the Southern Ocean where ophiuroids were as abundant as 3.5 ind. m-2 (Smith et al. 208 2014b), and for other types of nekton falls (Smith 1985). It is not clear from this image which of the 209 attending fauna were consuming the carcass rather than preying on the amphipods and other animals 210 in attendance (Jones et al. 1998). Other invertebrates (two poriferans and one holothurian) observed in 211 the vicinity of the whale fall were likely background fauna that were not actively drawn towards the 212 carcass. 213 The presence of soft tissue and abundant scavenging fauna indicate that this cetacean fall is in 214 the mobile-scavenger stage of succession (Smith and Baco 2003). This is only the second natural 215 whale fall to be studied during this stage, although numerous experimentally-implanted cetacean falls 216 show that this stage is widespread in the deep sea (Smith et al. 2015; Smith et al. 2014b). During this 217 stage, large mobile scavengers are attracted to the carcass by its odor plume and potentially by 218 mechanosensory cues (Klages et al. 2002). Via sloppy eating, these scavengers are responsible for 219 making food available to smaller organisms on the surrounding seafloor (Smith 1985; Smith and Baco 220 2003). The scavenging activity appears to have been vigorous as a rib bone is displaced from the 221 carcass and a high abundance of nodules may have been uncovered in the immediate vicinity. These 222 feeding aggregations could play a role in removing sediment from the tops of nodules, helping to 223 maintain polymetallic nodules at the sediment-water interface (Fig. 2). 224 Cetacean carcasses of a similar size that were implanted at abyssal depths (4000-4800 m) in 225 the northeast Atlantic Ocean were skeletonised within five days (Jones et al. 1998). Scavengers were 226 initially dominated by macrourids and replaced by zoarcids after the first 18 hours (Jones et al. 1998). 227 However, munidopsid lobsters were only observed 106 hours after deployment at the experiments in 228 the Atlantic (Jones et al. 1998). If scavenging processes are similar in the abyssal Atlantic and Pacific 229 Oceans, given the presence of tissue and this particular scavenger assemblage, the carcass in the CCZ 230 could be approximately four days old. 231 232 4.2. Ecology of the wood falls 233 The stages of the six wood falls from the CCZ reported here are varied. Wood falls “A” and 234 “D” appear to be relatively fresh and show little degradation whereas “B”, “C”, “E”, and “F” are 235 highly degraded with bacterial materials and evidence of enhanced bioturbation near the falls. This 236 suggests that these pieces of wood likely reached the seafloor at different times. Levels of degradation 237 could be related to time since arrival at the seafloor (with “B”, “C”, “E”, and “F” having arrived

8 238 earlier than “A” and “D”) as well as to the type of wood or the faunal assemblages present (e.g. the 239 presence/abundance of wood borers) (Bienhold et al. 2013; Voight 2007). 240 The occurrence of wood is expected to decline, as well as the size increase, with distance from 241 land (Voight 2015); nonetheless, we observed six wood falls within approximately 1.4 km2 of 242 surveyed seafloor, indicating an abundance of 4.29 wood falls per km2. Ocean circulation patterns 243 suggest that the wood may have been transported from the west coast of North or Central America in 244 the North Equatorial (Tomczak and Godfrey 1994). Wood fall “E” is the furthest recorded 245 from land with the Mexican coast 2250 km away, wood fall “F” was 1770 km from the coast of 246 Mexico, whereas wood falls “A”, “B”, “C” and “D” were 1450 km from the Mexican coast. Clarion 247 and Clipperton Islands were not included as potential sources of wood as there are only a handful of 248 trees on each of these islands. However, this wood may also have been deposited from passing ships, 249 as is suspected for wood fall “E” which appeared to be of anthropogenic origin (perhaps a pallet) 250 given the regular structure and shape of the depression. 251 Because all wood falls were degraded to some degree, this suggests there is an active and 252 diverse wood-consuming fauna, which capitalizes on the occasional arrival of terrestrially-derived 253 material to the seafloor in the CCZ. The presence or past presence of fauna is nearly certain on all 254 wood falls excepting “B”. Many of the animals observed at wood fall “E” have been observed 255 previously at wood falls. Munidopsid lobsters have been recorded consuming wood (Hoyoux et al. 256 2009; Macpherson et al. 2014), and other crustacea such as copepods and isopods have also been 257 recorded at deep-sea wood-fall habitats (Amon et al. 2015a). Many of these colonizing species were 258 suspension feeders (serpulid polychaetes, poriferans, actiniarians, corallimorpharians, cerianthids, and 259 hydrozoans) likely using the wood falls as a hard substrate (Amon et al. 2015a). Amon et al. (2016) 260 have reported that more than 50% of megafaunal morphotypes observed in the eastern CCZ were 261 directly dependent on hard substrate, and thus the wood may have been playing this ecological role. 262 There were also several echinoderm and molluscan taxa noted at wood fall “E”; Caymanostella 263 asteroids and several ophiuroids have been observed on natural wood falls in the Caribbean, as have 264 Pseudococculinidae and Cocculiniformia gastropods (D. Amon, in prep.). 265 Xyloredo bivalves were the only confirmed wood-fall specialists noted at wood fall “E”. 266 Xylophagaidae molluscs, such as Xyloredo, are completely reliant on wood as a source of food as well 267 as habitat; thus it seems likely that this family would decline with distance from land and sources of 268 wood (Voight and Segonzac 2012). The Xylophagaidae is comprised of several keystone species that 269 convert energy in refractory wood and other plant material into more labile organic matter such as 270 fecal matter consumed by detritus feeders, and animal tissue consumed by predators and scavengers. 271 This more labile organic matter may also facilitate development of the anaerobic conditions, 272 producing chemosynthetic habitats (Bienhold et al. 2013; Turner 1973; Turner 2002). The role of 273 these specialized organisms is likely to be of even higher importance on food-poor abyssal plains 274 worldwide (Smith et al. 2008; Voight and Segonzac 2012).

9 275 The Xylophagaidae have been found in all oceans except the Southern Ocean (Glover et al. 276 2013). There are four known species of Xyloredo bivalves: Xyloredo ingolfia, Xyloredo naceli, 277 Xyloredo nooi (Turner 1972) and Xyloredo teramachii (Haga and Kase 2008). Xyloredo ingolfia and 278 X. naceli have been recorded off Iceland at 1783 m and California at 2073 m respectively (Turner 279 1972). Xyloredo nooi was described from the Bahamas at 1737 m and the Cayman Trench at 4773 m 280 (Amon et al. 2015b; Turner 1972). The specimen from this study was most similar in morphology to 281 X. teramachii, which was recorded at depths of up to 580 m off Japan and Vanuatu (Haga and Kase 282 2008). Although the morphology does not definitively match any of these species, this would 283 represent a large range extension if it was confirmed as any of the known species (Turner 1972). This 284 also represents the furthest record of Xylophagaidae from land (Amon et al. 2015a; Voight and 285 Segonzac 2012). Voight and Segonzac (2012) collected Xylophaga alexisi on the abyssal plain 1600 286 km from the Subsaharan coast of Africa and concluded that despite paucity in the supply of wood to 287 the deep sea, the larvae of some Xylophagaidae are planktonic and likely to survive in the water 288 column for long periods and disperse great distances. The mechanism by which these larvae locate 289 these relatively tiny settlement substrates is still unknown however. 290 The anaerobic degradation of sunken wood can support chemosynthetic communities similar 291 to those seen at deep-sea hydrothermal vents and cold seeps (Bernardino et al. 2012; Bernardino et al. 292 2010; Duperron et al. 2008; Samadi et al. 2010). The presence of possible bacterial mats at some of 293 the wood falls indicate that this process may have been underway. Ophryotrocha polychaetes were 294 likely attracted to the wood falls with bacterial mats as this genus includes known bacteriovores, 295 which occur frequently at organic falls and in areas of organic enrichment (Amon et al. 2015a; 296 Gaudron et al. 2010; Wiklund et al. 2009). It is likely that the high abundance of deposit-feeding 297 chaetopterid polychaetes (cf. Spiochaetopterus) was due to the presence of bacterial mats also; high 298 abundances of such chaetopterids have been observed off Sweden in bacteria-laden pockmarks (H. 299 Wiklund, pers. comm.). 300 Harpacticoid copepods of the family Cerviniidae and Argestidae found in wood fall “E” may 301 be considered bycatch, as these are common and very diverse inhabitants of oligotrophic abyssal 302 sediments (George et al. 2014). However, the Laophontodinae and Mesochra are typical “shallow- 303 water” representatives that are rarely found in oligotrophic deep-sea habitats (George and Gheerardyn 304 2015). Mesochra has already been found as a frequent component of the harpacticoid communities at 305 hot vents (Gollner et al. 2015) and cold seeps (Plum et al. 2015). The donsiellinid species Xylora 306 bathyalis was described from decaying wood off Castlepoint, New Zealand at 1200 m depth, and 307 subsequently also found in other chemosynthetic habitats including hot vents and cold seeps (Gollner 308 et al. 2015; Gollner et al. 2006; Plum et al. 2015), while it has never been found in oligotrophic deep- 309 sea sediments. 310 Organic falls can create seafloor habitat heterogeneity, enhancing biodiversity (including beta 311 and gamma diversity) through the support of specialized “island” communities across the region

10 312 (Bernardino et al. 2010). This study provides the first hints of this in the abyssal CCZ. Twelve of the 313 morphospecies observed at wood fall “E” have not been seen elsewhere in the CCZ: cf. 314 Spiochaetopterus sp., Ophryotrocha sp., Xylora sp., Pseudotachidiidae gen. sp., Laophontodes sp., 315 Calypsophontodes sp., Mesochra sp., cf. Xyloredo sp., cf. Caymanostella sp., Cocculiniformia spp., 316 and Pseudococculinidae sp. 317 318 4.3 Potential effects of polymetallic-nodule mining on organic-fall habitats 319 Nodule mining will likely be very destructive; machinery will crush animals and the 320 redeposition of sediments over large seafloor areas from mining plumes has the potential smother 321 fauna and dilute benthic food resources (Ramirez-Llodra et al. 2011). Furthermore, over 50% of 322 megafaunal species in nodule regions may rely on polymetallic nodules as a source of hard substrate 323 (Amon et al. 2016; Vanreusel et al. 2016). Thus, mining activities will likely contribute to the 324 reduction of biodiversity on a large scale. The nodule-obligate fauna may require millennia or longer 325 to recolonize a mined area because the formation of new polymetallic nodules may take millions of 326 years (Amon et al. 2016; Bluhm 2001; Ramirez-Llodra et al. 2011; Vanreusel et al. 2016). However, 327 since organic falls are ephemeral communities that rely on little other than sunken bone or wood 328 substrates at the sediment-water interface, and these can sink from the sea surface at any point, the 329 organic-fall fauna may be among the first animals to recolonize an area after mining. 330 331 5. Conclusions 332 We find that organic falls may be regionally abundant and are likely to influence species and 333 habitat diversity in the abyssal CCZ. There clearly is a diverse scavenging community that attends 334 carcass falls at abyssal depths in the CCZ. These wood falls are the furthest recorded from land (over 335 1450 km) yet support highly specialized wood-boring bivalves in the family Xylophagaidae. The 336 assemblages occurring on these abyssal wood falls are comprised of both specialists and more 337 generalized opportunistic species. 338 339 6. Compliance with Ethical Standards 340 The TN-319 cruise and all subsequent work by authors were partially funded by UK Seabed 341 Resources Ltd. and Ocean Minerals Singapore. This included salary for Dr. Amon and partial 342 salaries for Drs. Martinez-Arbizu and Smith. Research support was also provided for these authors in 343 the form of equipment, supplies, and travel costs to an international conference to present results. 344 However, UK Seabed Resources Ltd. and Ocean Minerals Singapore had no role in the study design, 345 data collection analysis and interpretation, decision to publish, or preparation of the manuscript. 346

11 Table 1. The locations of the organic falls observed in the Clarion-Clipperton Zone. Wood fall “E” was the only organic fall that was sampled. Crui se Nu Date AUV or Organic Latit Longit Dept Exploration mbe Observe AUV or ROV* Fall ude ude h (m) Contract Area r d ROV* Dive Altitude TN- - 319/ Wood 12.58 116.72 AB0 Fall A 1262 1552 4226 UK-1 2 09.03.15 6 7.92 TN- - 319/ Wood 12.50 116.64 AB0 Fall B 0284 8094 4253 UK-1 2 04.03.15 5 8.28 TN- - 319/ Wood 12.49 116.65 AB0 Fall C 8419 1255 4254 UK-1 2 18.03.15 9 5.09 TN- - 319/ Wood 12.13 117.20 AB0 Fall D 865 3331 4027 OMS-1 2 11.03.15 7 7.82 - Wood 14.03 130.09 SO2 Fall E 427 4055 5038 IFREMER-East 39 15.04.16 12* 0* - Wood 18.79 128.30 SO2 Fall F 8141 4413 4917 APEI-3 39 21.04.16 13* 0* TN- - 319/ Cetacean 12.01 117.39 AB0 Fall W 6523 6942 4142 OMS-1 2 16.03.15 8 9.08

Table 2. The faunal assemblage inhabiting the cetacean fall observed in the OMS-1 exploration contract area in the Clarion-Clipperton Zone. < 60 cm from carcass 60 - 360 cm from carcass Number of Density (ind. m- Number of Density Phylum Morphospecies individuals 2) individuals (ind. m-2) Arthropoda Munidopsis sp. 4 2.13 0 0 Echinodermata Ophiuroidea sp. 5 2.67 11 0.84 Holothuroidea sp. 0 0 1 0.08 Chordata Ophidiidae sp. 2 1.07 0 0 Zoarcidae sp. 14 7.47 1 0.08 Actinopterygii sp. 1 0.53 0 0 Porifera Porifera sp. 0 0 2 0.15

12 Table 3. Taxa found on the wood fall “E” observed in the IFREMER-East exploration contract area in the Clarion-Clipperton Zone. The numbers of individuals in the wood include those retrieved from the collected pieces plus those counted in video images.

Phylum Class Order Family Genus Number of Number of (*Superclass) (&Subclass) individuals individuals in the in the wood sediment Porifera Hexactinellida 1 1 Cnidaria Hydrozoa Anthoathecata n/a Anthozoa Actiniaria 6 Corallimorpharia 8 Ceriantharia& 1 Annelida Polychaeta incertae sedis Chaetopteridae cf. Spiochaetopterus 40 Eunicida Dorvilleidae Ophryotrocha 4 Phyllodocida Hesionidae 1 Terebellida Ampharetidae 1 Arthropoda Malacostraca Decapoda Munidopsidae Munidopsis 4 Isopoda 1 Copepoda* Harpacticoida Pseudotachidiidae Xylora 3 gen. sp 1 Laophontodinae Laophontodes 1 Laophontodinae Calypsophontodes 1 Canthocamptidae Mesochra 1 Cerviniidae gen. sp. 1 Argestidae cf. Fultonia sp. 1 Xylophagaidae cf. Xyloredo 2 Gastropoda Cocculiniformia& 7 Cocculiniformia& 3 Vetigastropoda& Pseudococculinidae 1 Echinodermata Asteroidea Velatida Caymanostellidae cf. Caymanostella 7 Echinoidea Aspidodiadematoida Aspidodiadematida e 2 Ophiuroidea 5

13 Figure Captions

Figure 1. Locations of the organic falls observed in the Clarion-Clipperton Zone. (a) The locations of the UK-1 exploration contract area, the OMS-1 exploration contract area, the IFREMER exploration contract areas (both East and West) and the Area of Particular Environmental Interest 3 (APEI-3) in the Pacific Ocean. (b) The locations of wood falls “E” and “F” in the IFREMER-East exploration contract area and the APEI-3 respectively. (c) The locations of wood falls “A”, “B” and “C” in the UK-1 exploration contract area and the locations of wood fall “D” and the cetacean fall (W) in the OMS-1 exploration contract area. All maps were created using ArcGIS with bathymetric data from GEBCO.

Figure 2. Organic falls from the UK-1 and OMS-1 exploration contract areas in the Clarion- Clipperton Zone. (a), (b), and (c) wood falls “A”, “B” and “C”, respectively, observed in the UK-1 exploration contract area; (d) wood fall “D” observed in the OMS-1 exploration contract area; (e) the immediate vicinity (<60 cm) surrounding the cetacean fall “W” with associated fauna observed in the OMS-1 exploration contract area. All scale bars are 30cm. Image credits: Woods Hole Oceanographic Institution.

Figure 3. Wood falls found during the SO239 cruise. (a) to (g) Wood fall “E” in the IFREMER- East contract area; (h) Wood fall “F” in the APEI-3. (a) Overview of wood fall “E” (scale bar: 30 cm); (b) and (c) details of wood fall “E” and associated fauna (scale bars: 5 cm); (d) the polychaete Ophryotrocha sp. (scale bar: 100µm); (e) the xylophagid bivalve cf. Xyloredo sp. (scale bar: 1 mm); (f) undetermined Vetigastropoda gastropod (scale bar: 1 mm); (g) undetermined Cocculiniformia gastropod (scale bar: 1 mm); (h) wood fall “F” (scale bar: 5 cm). Image credits: a, b, c and h - GEOMAR.

14

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