Linley TD, Stewart A, Mcmillan P, Clark M, Gerringer ME, Drazen JC, Fujii T, Jamieson AJ

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Linley TD, Stewart A, McMillan P, Clark M, Gerringer ME, Drazen JC, Fujii T, Jamieson AJ. Bait attending fishes of the abyssal zone and hadal boundary: community structure, functional groups and species distribution in the Kermadec, New Hebrides and Mariana trenches. Deep Sea Research Part I: Oceanographic Research Papers (2016) DOI: http://dx.doi.org/10.1016/j.dsr.2016.12.009

DOI link to article: http://dx.doi.org/10.1016/j.dsr.2016.12.009

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14/12/2016

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21 December 2017

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Bait attending fishes of the abyssal zone and hadal boundary: community structure, functional groups and species distribution in the Kermadec, New Hebrides and Mariana trenches.

Linley, T.D.1†, Stewart, A.L.2, McMillan, P.J.3, Clark, M.R.3, Gerringer M.E.4, Drazen, J.C. 4, Fujii, T. 1 and Jamieson, A.J.1†*

1 Oceanlab, University of Aberdeen, Institute of Biological and Environmental Sciences, Main Street, Newburgh, Aberdeenshire, AB41 6AA, UK. 2 Museum of New Zealand Te Papa Tongarewa, 169 Tory St, PO Box 467, Wellington, New Zealand. 3 National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington 6021, New Zealand. 4Department of Oceanography, University of Hawaii, Honolulu, HI 96822, USA. †Present address: School of Marine Science and Technology, Newcastle University, Newcastle Upon Tyne, NE1 7RU, UK.

*Corresponding author Dr Alan Jamieson: [email protected]

Running head: Abyssal and hadal fish communities

Keywords: Deep-Sea Fish, Deep sea, Zonation, Kermadec Trench, New Hebrides Trench, South Fiji Basin, Mariana Trench, Pacific Ocean, Abyssal Hadal Transition Zone, AHTZ.

Highlights:

 Three Pacific hadal trench system’s fish communities are systematically sampled  Differences in the fish fauna of hadal trenches and surrounding plain are identified  Scavenging feeding behavior in fishes is lost deeper than ~7000 m  The interaction of temperature and primary production may inhibit fish scavengers  A predatory ophidiid fish community may be more common globally than thought

Abstract

Baited landers were deployed at 83 stations at four locations in the west Pacific Ocean from bathyal to hadal depths: The Kermadec Trench, the New Hebrides Trench, the adjoining South Fiji Basin and the Mariana Trench. Forty-seven putative fish species were observed. Distinct fish faunal groups were identified based on maximum numbers and percentage of observations. Both analyses broadly agreed on the community structure: A bathyal group at <3000 m in the New Hebrides and Kermadec trenches, an abyssal group (3039 – 4692 m) in the Kermadec Trench, an abyssal-hadal transition zone (AHTZ) group (Kermadec: 4707 – 6068 m, Mariana: 4506 – 6198 m, New Hebrides: 2578 – 6898 m, South Fiji Basin: 4074 – 4101 m), and a hadal group of endemic snailfish in the Kermadec and Mariana trenches (6750 – 7669 m and 6831-8143 m respectively). The abyssal and hadal groups were absent from the New Hebrides Trench. Depth was the single factor that best explained the biological variation between samples (16%), the addition of temperature and average surface primary production for the previous year increased this to 36% of variation.

The absence of the abyssal group from the New Hebrides Trench and South Fiji Basin was due to the absence of macrourids (Coryphaenoides spp.), which defined the group. The macrourids may be energetically limited in these areas. In their absence the species of the AHTZ group appear released of competition with the macrourids and are found far shallower at these sites.

The fish groups had distinct feeding strategies while attending the bait: The bathyal and abyssal groups were almost exclusively necrophagous, the AHTZ group comprised predatory and generalist feeders, while the hadal snailfishes were exclusively predators. With increasing depth, predation was found to increase while scavenging decreased. The data suggest scavenging fish fauna do not extend deeper than the hadal boundary.

1. Introduction

The abyssal zone (3000 - 6000 m) accounts for the majority of the world’s surface (Vinogradova, 1997) with the average global ocean depth of ~4200 m (Danovaro et al., 2014; Thurber et al., 2014). With the exception of some small, isolated basins (e.g. in the Mediterranean sea, parts of the Indo-West Pacific and the Guatemalan Basin), deep water corridors are found between the majority of the world’s abyssal basins (Briones et al., 2009).

It is assumed that abyssal fish species possess large geographical ranges unimpeded by depth barriers (Vinogradova, 1997).

Despite assumed wide distribution, the bathymetric and geographic extent of abyssal fish species is only known at the resolution of which sampling has occurred (Carney, 2005). The abyssal, and in particular the lower-abyssal to abyssal-hadal transition zone (AHTZ; Jamieson et al., 2011), are seldom studied due to the technical challenges of sampling at increasing distance from the vessel, and hydrostatic pressures. Furthermore, the geographic extent of the abyssal zone is such that even geographically wide studies are performed in relatively small areas and there are few that unequivocally define geographic boundaries. This is especially the case in the Pacific Ocean due to its immense size (165.2 million km², mean depth = 4280 m). The expense of sea-time has led to a tendency for research to be carried out in relatively close proximity to land masses (Kintisch, 2013), specifically around nations actively involved in deep-sea research (e.g. Japan, New Zealand, USA). Studies seldom addressed the distribution of fishes on greater geographic scales and across areas of contrasting environmental conditions. Fish beyond the abyssal-hadal transition zone have rarely been studied (Fujii et al., 2010; Jamieson et al., 2010; Nielsen, 1964) although the bathymetric range of fishes is known to exceed 8000 m (Linley et al., 2016; Yancey et al., 2014).

In addition to the geographic and bathymetric expanse, the deep Pacific Ocean seafloor underlies highly variable surface productivity, with two very large oligotrophic gyres in the northern and southern hemispheres. The distribution and community structure of deep fishes is often observed to change in response to variation in overlying surface productivity resulting in a non-homogeneous distribution. This effect is usually explored through comparison of a high and low productivity location (e.g. Cousins et al., 2013a; Priede et al., 2003; Sulak, 1982). Determining the drivers of community structure and thereby disentangling the effects of depth, location, food supply and other environmental parameters requires standardised data over large areas and greater depth ranges.

Conventional sampling of abyssal fish using trawls is problematic. It is very time consuming and requires specialised and expensive vessels and equipment, which reduces opportunity and restricts access (Kintisch, 2013). These challenges have prompted extensive use of free- fall baited landers as an alternative to trawls, which offer increased opportunity for access

from vessels of a wide range of size and capabilities, relatively unrestricted by depth. With baited landers, the results are limited to scavenging species and species that prey upon scavengers (necrophagivores), collectively referred to as ‘bait-attending’.

The baited lander methodology emulates a natural process. Large food falls, often in the form of the carcasses of shallower living fauna but also including wood and macroalgae, represent a local and highly concentrated organic input to the deep sea. The scale of such food falls can range from fishes and birds (mesocarrion ~1 kg), to seals and dolphins (macrocarrion ~100 kg) and the largest cetaceans (megacarrion >100 000 kg) (Bailey et al., 2007; Britton and Morton, 1994; Higgs et al., 2014; Kemp et al., 2006; Smith et al., 2015; Stockton and DeLaca, 1982). Particulate material from surface productivity decreases rapidly with depth (Lutz et al., 2007) whereas carrion-falls should in theory occur irrespective of depth. It is hypothesised that direct scavenging (or indirect predation through ‘bait-attending’) may have an increasingly important role in maintaining deep-water fish communities with increasing depth (Yeh and Drazen, 2011).

The current study examined trends in the species composition and function (whether scavenging or predatory) of bait-attending fish communities in three hadal trench systems in order to explore:

1. The occurrence of distinct fish communities from bathyal to hadal depths. 2. If the same pattern is common to all trenches. 3. Drivers of community change, bathymetrically and/or geographically.

2. Materials and Methods

2.1. Study sites

The four study locations (Figure 1) in the west Pacific can be characterised as follows:

1. The Kermadec Trench (Kerm) is a deep (10,047 m), cold trench which underlies the most productive surface waters within this study. The trench is formed by the subduction of the Pacific Plate under the Australian Plate resulting in a trench 1500 km long and on average 60 km wide (Angel, 1982).

2. The New Hebrides Trench (NHeb) is 1000 km to the northwest of the Kermadec Trench. It is a shallower trench of ~7156 m. Relative to the other trenches in this study it is the warmest, and underlies intermediate productivity. The trench is formed by the Australian plate subducting north-eastwards under the overriding Vanuatu archipelago resulting in a trench ~ 1200 km long. 3. The South Fiji Basin (SFB) partitions the Kermadec and New Hebrides trenches. The basin is a uniform abyssal plain (~4100 m) with similar water temperature and average surface productivity as the New Hebrides Trench. 4. The Mariana Trench (Mar) is located southeast of the island of Guam in the Central Pacific. It is of intermediate bottom temperature. It underlies the lowest surface productivity of the areas studied. It is the deepest trench in the world, with a maximum depth of ~11,000 m (Gardner et al., 2014) and is 2550 km long with a mean width of 70 km (Angel, 1982). The trench is formed as the Pacific Plate subducts beneath the Mariana Plate to the west.

When discussed in the text the names of the trenches refer to an area wider than just the trench and include the surrounding bathyal and abyssal depths.

2.2. Equipment

Data were collected by the 6000 m rated Abyssal-lander, and the full-ocean depth Hadal- lander, part of the Oceanlab fleet of autonomous landers at the University of Aberdeen. The systems are negatively buoyant on release from the vessel and free-fall to the seafloor. Upon acoustic command from the surface, ballast is jettisoned via dual acoustic releases, and the systems return to the surface by virtue of a positively buoyant moored glass sphere (Nautilus, Germany) floatation array above the landers.

The Abyssal-lander is described in Linley et al. (2015). The Abyssal-lander’s 5 megapixel digital still camera is suspended 2 m above the seabed and is optimised to this focal length. The camera faces vertically downwards to a steel ballast clump, a scale cruciform (50 cm axis length, 10 cm markers) and 500 g of bait, resting on the seafloor. All systems were baited with a locally sourced ungutted oily fish; mackerel (Scomber spp.) or jack mackerel (Trachurus spp.).

The area of seabed recorded by the Abyssal-lander was 2 x 1.5 m, and an image was taken every 60 seconds throughout the deployment.

The Abyssal-lander also included a Seaguard recording platform (Aanderaa, Norway) equipped with a Doppler current meter (RDCM) and conductivity, temperature and pressure (CTD) probes recording at 30 second intervals, ~2.5 m above the seabed.

The Hadal-lander (described in Jamieson, 2015) lands directly onto the seabed, resting on the frame feet and records video in a near-horizontal orientation. A 120 cm long tubular arm secured the bait within view of the camera and included a horizontal scale bar at the point of bait attachment. The field of view was approximately 40° giving ~75 cm scene width at 120 cm in front of the camera. The basic delivery system was the same as the Abyssal-lander, but rated to 11,000 m operational depth. The scientific payload comprised a 3CCD Hitachi colour video camera (800 TV lines), controlled and logged autonomously by a custom built control system (NETmc Marine, UK). Illumination was provided by two LED lamps within glass vacuum spheres. The camera was pre-programmed to take 1 min of video in every 5 min, and was powered by a 12V lead acid battery (SeaBattery; DSP&L, US). An SBE-39 pressure and temperature sensor (Sea- Bird Electronics, US) logged at 30 s intervals throughout.

Trapping systems were deployed alongside the imaging landers to collect voucher specimens to verify the image identifications. Two traps were used, both were deployed and recovered using the same method as the imaging landers. The Small Fish Trap; was described in Jamieson et al. (2013). The Large Fish Trap was a 1 x 1 x 2 m netting covered frame. Two entrance configurations were used: four tapering 20 cm2 funnel entrances or two wide openings 70 x 15 cm close to the seabed and four 15 x 30 cm on the alternate sides. Mesh size was 1.5 cm at widest point in all systems.

2.3. Data processing

Each Abyssal-lander image and Hadal-lander video was analysed manually and all observed fish were identified to the lowest possible taxonomic level. Anderson et al. (1998) provided a general overview of bathyal species known in the New Zealand Exclusive Economic Zone (EEZ) which helped in identifications. Specific taxonomic texts were consulted for each of the major groups. The macrourids were identified following Cohen et al. (1990), with Jamieson et al. (2012) allowing distinction between Coryphaenoides armatus and C. yaquinae from in situ

images. The ophidiids were identified following Nielsen et al. (1999) and Nielsen and Merrett (2000). Additional sources consulted included: Anderson (1994); Castle (1968); Froese and Pauly (2016); Gon and Heemstra (1990); Iwamoto and Sazonov (1988); Karmovskaya and Merrett (1998); Sulak and Shcherbachev (1997).

The Hadal-lander’s horizontally oriented camera imaged fish in a lateral view that provided more taxonomic detail. Deinterlaced stills were taken from the video using VLC media player (VideoLAN, 2013) when the target animal was best positioned. The open-source image analysis software package ‘Fiji’ (Schindelin et al., 2012) was used to measure relative body proportions of distinct individuals (identified through differences in colouration, size or scarring). If the animal was parallel to the scale bar and at the same distance from the camera as the scale bar, length estimation was also possible. The cell counter plugin (De Vos and Rueden, 2015) was used to count fin rays and scale rows.

The video recorded by the Hadal-lander allowed for the observation of feeding behaviour. A feeding event was defined as either suction feeding directed at, or a fish’s jaw closing on, the bait (necrophagy) or another animal (predation). No distinction was made between successful and unsuccessful strikes, as this could not often be determined. Multiple feeding events by the same animal were all counted. Fish suction feeding at the surface of the bait would often take bait into their mouth also. These feeding events were considered necrophagous. When being compared, the feeding events were standardised to events per fish (of that species) per minute.

In addition to the environmental data from the Seaguard platform, the biological pump model outlined in Lutz et al. (2007) provided an estimate of particulate organic carbon (POC) transport to the seabed at the sampling locations. The Ocean Productivity Datasets (Behrenfeld and Falkowski, 1997) were used to estimate surface primary production at each location during the month of the deployment and an average value for the previous year. The time of day that the lander arrived at the seabed, expressed as ‘quarter of day’ (6 hr blocks from midnight) was included to detect any diurnal variation. In the absence of samples the seabed sediments were interpreted from lander images using a modified classification based on Wentworth (1922) and Folk, (1954). Six categories were identified: bedrock, cobbles and pebbles, muddy gravel, gravelly fine-grained sediment, slightly gravelly fine-grained

sediment, and fine grained-sediment. It was not possible to categorise the 5254 m site within the Kermadec Trench as the seabed was not clearly visible in the image.

2.4. Analysis

All non-permutation based statistical analysis was produced using R (R Development Core Team, 2005), figures were produced using ggplot2 (Wickham and Chang, 2007). Relationships between feeding type and location and depth were explored through fitting of generalised linear models (GLM) using the Gaussian family. The significance of explanatory variables, residual plots and Akaike Information Criterion (AIC) values were referenced to adjust the models to best fit the data. The resulting models were assessed through Analysis of Variance (ANOVA) F-tests using type-II sum of squares.

The two landers are not directly comparable due to their different fields of view and sampling frequencies. Data collected by the Abyssal-lander lends itself better to statistical analysis due to its fixed field of view, discrete sampling method and larger suit of environmental sensors. Two metrics are commonly extracted from lander data (Farnsworth et al., 2007; Fleury and

Drazen, 2013): 1) The time of first arrival (Tarr) is the time from the arrival of the lander at the seabed until the first arrival of each species; and 2) The maximum number (MaxN) of individuals of each species observed simultaneously. Tarr can be difficult to work with due to its high variability and inverse relationship with fish abundance (Farnsworth et al., 2007;

Priede et al., 1990). MaxN is a more reliable proxy of local fish density than Tarr (Stoner et al., 2008; Willis and Babcock, 2000; Yeh and Drazen, 2011). The MaxN data from the Abyssal- lander formed the initial basis of the quantitative multivariate analysis. A second “percent of observations” (%Ob) dataset included both the Abyssal-lander and Hadal-lander data: Each image or video sequence was considered a single observation and fish species were characterized by what proportion of observations they were present in, for each camera deployment. In order to validate combining the data in this way, a one-way analysis of similarities (ANOSIM) for location and lander with up to 999 permutations was performed on the %Ob dataset for the depth range sampled by both landers.

Community structure analysis was performed in Primer 6.1.14 (Clarke and Gorley, 2006). A square root transformation was applied to the MaxN Abyssal-lander data and the %Ob data from both lander systems. Before proceeding a RELATE test using Spearman rank correlation

on 999 permutations was used to evaluate if the duration of the deployments had a significant effect on the biological data.

Bray-Curtis similarity was applied pairwise to all deployments to produce a resemblance matrix of fish community distance between deployments. CLUSTER (hierarchical agglomerative/bottom-up clustering) group average analysis was then performed. The structure formed through clustering was assessed with a similarity profile analysis (SIMPROF) test of 999 permutations at the 5% significance level. This was used to group deployments into statistically distinct groups based on their community structure which were then displayed using non-metric multidimensional scaling (MDS). Similarity percentages analysis (SIMPER) was performed to quantify the similarity/dissimilarity among the identified groups, and to identify those species most responsible (up to 90% cumulative within-group similarity) for defining those groups.

Environmental predictors were pairwise plotted against each other (Draftsman plots), to identify any skew or inter-correlation. A heavy right-skew to temperature was corrected through natural log transformation. All environmental data were then normalised to make the different measurement scales comparable.

Distance-based linear models (DISTILM) as part of the PERMANOVA (Gorley and Clarke, 2008) expansion to PRIMER 6 were used to explore the relationship between fish community structure and the potential environmental predictors. BEST procedure (all possible combinations of predictors) was performed with Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) selection criteria. The two measures differ in their ‘penalty’ towards the addition of predictors; AIC has a lower penalty and is therefore more generous in the addition of predictors than BIC (Gorley and Clarke, 2008). Marginal tests were used to remove predictors not significantly (α = 0.05) correlated with the biological data. Heavily inter-correlated and therefore interchangeable predictors were explored. Variables were selected over their correlates if they had more significant p-values and explained more of the biological variation when combined with other variables. The models were then visualised using Distance Based Redundancy Analysis (dbRDA).

Across all study locations, 83 lander and 84 trap deployments were achieved (Figure 1). Full deployment details are included in supplementary material (lander deployments: Table S1,

trap deployments: Table S2). When specific deployments are referred to it will be by their region and depth as defined in the supplementary material.

Figure 1: Sampling locations. Triangles represent Abyssal-lander deployments, stars the Hadal-lander deployments, squares the Large Fish Trap and diamonds the Small Fish Trap. Deployments that did not record fish are hollow outlines. Global overview is adapted from Google Earth (Google, 2016), the location maps are produced from GEBCO bathymetry data (GEBCO, 2015). Isobaths have been added at 1000 m intervals.

2 3. Results

3 3.1. Environmental characteristics

4 The depth related trends of the recorded or estimated environmental variables at each of the 5 study locations are illustrated in Figure 2.

6 7 Figure 2: Estimated surface primary production for the previous year (PPYr), the month of 8 the deployment (PPMo) and flux to the seabed (Bottom POC) in mg C/m2/day, Salinity (Sal) 9 in PSU, Current speed (Cur. Speed; cm/s) and temperature (Temp) in °C measured by the 10 Abyssal-lander (circles) and Hadal-lander (triangles) in the Kermadec Trench (Kerm), South 11 Fiji Basin (SFB), New Hebrides (NHeb) and Mariana (Mar) trenches.

12 3.2. Species diversity

13 Of the 83 Abyssal-lander and Hadal-lander deployments 77 included fish observations 14 (Supplement Table S1). A total of 47 fish Operational Taxonomic Units (OTUs) were identified. 15 Nine categories could only be resolved to family level, five were resolved to genus level and 16 the remaining categories (33) could be given a likely species identification. Twelve species 17 were confirmed through the capture of voucher specimens from the trap systems 18 (Supplement Table S2). A full species list is presented in Table 1 with example images in Figure 19 3 to 6.

20 There are two known species of the genus Spectrunculus which are not possible to 21 differentiate visually. Spectrunculus crassus have not been recorded from the Central or South 22 Pacific. The otolith morphology, and the ratios of dorsal fin rays to vertebrae of captured 23 specimens appear to conform to S. grandis (see Uiblein et al., 2008). The observed species 24 are thereby designated S. grandis.

25 The zoarcid OTU is known to contain Pachycara moelleri (Shinohara, 2012) as specimens were 26 captured in the South Fiji Basin and New Hebrides Trench at 4100 m depth. Specimens from 27 the Kermadec Trench are currently under assessment and include one specimen of Pyrolycus 28 cf. moelleri Anderson, 2006. Visually distinct zoarcids in the lander images support that there 29 are multiple species which cannot be resolved from images (Figure 3).

31 Figure 3: Examples of the Zoarcidae recorded at A) the Kermadec Trench at 3940 m, B) The 32 South Fiji Basin at 4074 m, C) the New Hebrides Trench at 6056 m and D) the Mariana 33 Trench at 6142 m.

35 Identifying eels in the Family Synaphobranchidae can be extremely difficult as they are very 36 similar (Sulak and Shcherbachev, 1997). No more than two species of this family are known 37 to co-occur within a single bathymetric or geographical zone (Sulak and Shcherbachev, 1997). 38 Ilyophis robinsae was captured in the New Hebrides Trench at 5180 m depth. However, it is 39 very possible that other species are present. In the southern Kermadec Trench region, 40 Histiobranchus australis and H. bruuni have been recorded, the latter from 3000–4974 m 41 depth (Roberts et al., 2015). The OTU “Large Synaphobranchidae” will be used to differentiate 42 the large deeper-occurring eels (Figure 4) from the visually distinct Diastobranchus capensis, 43 Simenchelys parasitica and Synaphobranchus brevidorsalis (Figure 6).

45 Figure 4: Large Synaphobranchid A) 5254 m Kermadec Trench, B) 4100 m South Fiji Basin, C) 46 4123 m New Hebrides Trench.

48 A large cusk-eel featured prominently in the observed fish fauna. Discussion with J.G. Nielsen 49 indicated that the fish was a member of the genus Bassozetus. These were never caught in 50 the fish traps, which precluded species identification. When viewed laterally meristic and 51 morphometric measurements suggested Bassozetus robustus and B. levistomatus (Figure 5) 52 however it was not possible to distinguish between these tentative identifications in most 53 images. Hence, they were categorised as Bassozetus spp.

56 Figure 5: Variation within the Bassozetus observed. Bassozetus cf. robustus (A–B), Bassozetus 57 cf. levistomatus (C–D). All images were taken at 5254 m depth in the Kermadec Trench.

58 Table 1 Fish categories and the depth range (m) observed in each study area. Data from imaging landers and traps. * denotes extreme of range 59 from trap, † denotes voucher specimen captured.

Study Site

Family Species Kerm SFB NHeb Mar Neomyxine caesiovitta Stewart & Zintzen, 2015, in Zintzen Myxinidae 997 - 1013† et al. (2015) Chimaeridae Hydrolagus cf. affinis (de Brito Capello, 1868) 1527 - 2503 2087 - 2578 Centroscymnus cf. coelolepis Barbosa du Bocage & Somniosidae 997 - 1971 de Brito Capello, 1864 Centroscymnus owstonii Garman 1906 1527 - 1980 Etmopteridae Etmopterus baxteri Garrick, 1957 1473 - 1554 Rajidae Amblyraja hyperborea (Collett, 1879) 1527 - 1554 Arhynchobatidae cf. Bathyraja richardsoni (Garrick, 1961) 1971 - 2503 2087 - 2578 Halosauridae Aldrovandia affinis (Günther, 1877) 4078 - 4078 Halosauropsis macrochir (Günther, 1877) 1473 - 1473 Congridae Bassanago bulbiceps Whitley, 1948 997 - 1527 Synaphobranchidae Diastobranchus capensis Barnard, 1923 997 - 1971† Ilyophis robinsae/ 3039 - 6068 4074 - 4100 2087 - 5344† Large Synaphobranchidae Sulak & Shcherbachev, 1997 Simenchelys parasitica Gill, 1879 997 - 2503† Synaphobranchus brevidorsalis Günther, 1887 2087 - 2087 Alepocephalidae Alepocephalid spp. 1554 - 1554 2578 - 2578 Macrouridae Bathygadus sp. 1473 - 1554 Coelorinchus sp. 997 - 997 Coryphaenoides armatus (Hector, 1875) 3039 - 4692† Coryphaenoides ferrieri (Regan, 1913) 1527 - 3975

Coryphaenoides cf. filicauda Günther, 1878 4194 - 4194 Coryphaenoides lecointei (Dollo, 1900) 4953 - 4953 Coryphaenoides leptolepis Günther, 1877 3655 - 4138 Coryphaenoides longifilis Günther, 1877 1971 - 1971 2087 - 2087 Coryphaenoides rudis Günther, 1878 1971 - 1971 2087 - 2087 Coryphaenoides yaquinae Iwamoto & Stein, 1974 3655*† - 5879 4441*† - 7012 Trachyrincus longirostris (Günther, 1878) 997 - 997 Macrourid 1 1527 - 1971 Macrourid 2 4138 - 4138 Macrourid 3 1527 - 1527 Macrourid 4 5281 - 5281 Macrourid 5 4158 - 4953 Moridae Antimora rostrata (Günther, 1878) 1473 - 2503 2087 - 2578 Lepidion microcephalus Cowper, 1956 997 - 997 Mora moro (Risso, 1810) 997 - 997 Carapidae Echiodon cryomargarites Markle, Williams & Olney, 1983 1473 - 1473 Ophidiidae Barathrites iris Zugmayer, 1911 5254 - 5254 4074 - 4100† 5044 - 5641 cf. Bassogigas sp. 4100† 4700 - 5300*† Bassozetus spp. 4519 - 6750 4074 - 4100 2087 - 6898 4506 - 6198 cf. Bassozetus glutinosus (Alcock, 1890) 2578 - 2578 Bassozetus cf. compressus (Günther, 1878) 4506 - 4506 Bathyonus caudalis (Garman, 1899) 3039 - 3039 Spectrunculus grandis (Günther, 1877) 1980*† - 4332 2578 - 2578 Psychrolutidae Psychrolutes microporos Nelson, 1995 1473 - 1527 Liparidae Notoliparis kermadecensis (Nielsen, 1964) 5879 - 7669† Mariana snailfish 6198 - 8078† Ethereal snailfish 8007 - 8143 Zoarcidae Gen et spp. 3039 - 4989*† 4074 - 4100*† 3424 - 6162† 5044 - 6142 61

63 Figure 6: Fish species recorded attending the Abyssal-lander and Hadal-lander in the 64 Kermadec (Kerm), South Fiji Basin (SFB), New Hebrides Trench (NHeb) and Mariana Trench 65 (MAR); 1) Neomyxine caesiovitta (Kerm, 997 m), 2) Hydrolagus cf. affinis (Kerm, 1527 m), 3) 66 Centroscymnus cf. coelolepis (Kerm, 1971 m), 4) Etmopterus cf. baxteri (Kerm, 1473 m), 5) 67 Centroscymnus owstonii (Kerm, 1527 m), 6) Amblyraja hyperborea (Kerm, 1527 m), 7) 68 Bathyraja cf. richardsoni (NHeb, 2578 m), 8) Aldrovandia affinis (SFB, 4078 m), 9) 69 Halosauropsis macrochir (Kerm, 1473 m), 10) Bassanago bulbiceps (Kerm, 997 m), 11) 70 Diastobranchus capensis (Kerm, 1527 m), 12) Simenchelys parasitica (Kerm, 1527 m), 13) 71 Synaphobranchus brevidorsalis (NHeb, 2078 m), 14) Alepocephalid spp. (NHeb, 2578 m), 15) 72 Bathygadus sp. (Kerm, 1554 m), 16) Coelorinchus sp. (Kerm, 997 m), 17) Coryphaenoides 73 armatus (Kerm, 3655 m), 18) Coryphaenoides ferrieri (Kerm, 1527 m), 19) Coryphaenoides cf. 74 filicauda (Kerm, 4194 m), 20) Coryphaenoides lecointei (Kerm, 4953 m), 21) Coryphaenoides 75 leptolepis (Kerm, 3665 m), 22) Coryphaenoides longifilis (NHeb, 2087 m), 23) Coryphaenoides 76 rudis (NHeb, 2087 m), 24) Coryphaenoides yaquinae (Kerm, 3940 m), 25) Trachyrincus 77 longirostris (Kerm, 997 m), 26) Macrourid 1 (Kerm, 1527 m), 27) Macrourid 2 (Kerm, 4138 m), 78 28) Macrourid 3 (Kerm, 1527 m), 29) Macrourid 4 (Kerm, 5281 m), 30) Macrourid 5 (Kerm, 79 4953 m), 31) Antimora rostrata (Kerm, 1554 m), 32) Lepidion microcephalus (Kerm, 997 m), 80 33) Mora moro (Kerm, 997 m), 34) Echiodon cryomargarites (Kerm, 1473 m), 35) Barathrites 81 iris (Kerm, 5254 m), 36) ?. Bassogigas sp. (SFB, 4100 m), 37) Bassozetus cf. glutinosus (NHeb, 82 2578 m), 38) Bassozetus cf. compressus (Mar, 4506 m), 39) Bathyonus caudalis (Kerm, 3039 83 m), 40) Spectrunculus grandis (Kerm, 3039 m), 41) Psychrolutes microporos (Kerm, 1527 m), 84 42) Notoliparis kermadecensis (Kerm, 5879 m), 43) Mariana snailfish (Mar, 6198 m), 44) 85 Ethereal snailfish (Mar, 8078 m).

87 The number of fish species declined significantly with increasing depth in the Kermadec (R2 =

2 88 0.732, F1,30 = 81.755, p < 0.001) and Mariana (R = 0.391, F1,23 = 14.771, p < 0.001) trenches.

2 89 The New Hebrides Trench approached significance (R = 0.541, F1,5 = 5.88, p = 0.059) but was 90 potentially restricted by smaller sample size (Figure 7).

91 92 Figure 7: The number of fish species observed by the Abyssal-lander and Hadal-lander lander 93 systems at different depths in each of the study areas. A linear model with 95% confidence 94 interval has been fitted.

96 3.3. Community structure

97 Prior to analysis, potential influences from the experimental design were explored. A RELATE 98 test indicated that the duration of the deployment did not have a significant effect on the 99 MaxN (ρ = 0.048, p = 0.232) or %Ob (ρ = 0.008, p = 0.388) datasets. ANOSIM did not detect a 100 significant effect of lander type in the %Ob dataset (R = 0.006, p = 0.365).

101 Significantly distinct fish community groups are presented in greater detail in supplementary 102 Table S3. In the Kermadec Trench three significant fish community groups in the MaxN dataset 103 (Figure 8a) and five in the %Ob dataset Figure 8b were detected. Within the MaxN dataset a 104 bathyal fish community from 997 - 1971 m depth was detected. Over 70% of the bathyal 105 group’s within group similarity was due to the Synaphobranchid eels Diastobranchus capensis 106 and Simenchelys parasitica. There was an abyssal fish community from 3039 – 4185 m where 107 Coryphaenoides armatus, C. yaquinae, Spectrunculus grandis and the Large Synaphobranchid 108 accounted for more than 90% of the within group similarity. Thirdly, an Abyssal Hadal 109 Transition Zone (AHTZ) fish community occurred from 4707 – 6068 m where Bassozetus spp.,

110 C. yaquinae and Large Synaphobranchid accounted for over 98% of the within group 111 similarity. The %Ob dataset generally agreed in these groups, differing in one deployment at 112 5295 m. The same species accounted for the majority of the within group similarity with the 113 exception of the Large Synaphobranchid. These eels have short staying times at the bait and 114 as such are likely to be underrepresented in both datasets, but more so in the %Ob dataset. 115 A greater depth resolution in the %Ob dataset improved the community boundaries; bathyal 116 997-2503 m, abyssal 3039-4692, AHTZ 4707-6068 m. In addition, the %Ob dataset included 117 data from the deeper-capable Hadal-lander and so included two deeper groups: (1) A 118 shallower hadal community (including 5295 m deployment that fell within the AHTZ 119 community in the MaxN dataset) from 5295–6191 m with Bassozetus spp. accounting for 120 100% of the within group similarity; (2) A deeper hadal group 6750–7669 m with the endemic 121 snailfish Notoliparis kermadecensis accounting for 100% of the within group similarity.

122 123 Figure 8: MDS plot of the Kermadec Trench deployments in the a) MaxN and b) %Ob 124 datasets. Resemblance has been overlaid at 20% (solid line) and 40% (dashed line). 125 Significant groups (SIMPROF, p<0.05) are Bathyal (circle), Abyssal (triangle), AHTZ (square), 126 shallow hadal (cross) and deep hadal (asterisk).

127

128 The South Fiji Basin possessed no detectable community structure in either dataset. 129 Bassozetus spp., Zoarcidae Gen et spp. and Barathrites iris were responsible for more than 130 85% of the similarity between deployments in both datasets. The MaxN dataset also detected 131 the Large Synaphobranchid eel as contributing 15% to similarity between deployments.

132 The New Hebrides Trench possessed no detectable community structure. Both datasets 133 suggested that the shallowest deployment at 2087 m had greater dissimilarity from the other 134 deployments (mean dissimilarity of 2078 m from other deployments in the New Hebrides

135 Trench: 72%, mean between all other deployments 43% in the MaxN dataset) but this was 136 not significant. This deployment is distinct as it contains the macrourids Coryphaenoides rudis 137 and C. longifilis, which were the only macrourids detected in the New Hebrides Trench. Both 138 datasets identified Bassozetus spp. as the OTU responsible for over 75% of the similarity 139 between deployments in the New Hebrides Trench and was observed in every deployment. 140 The MaxN dataset additionally identified the Large Synaphobranchid (17% similarity) while 141 the %Ob dataset identified Zoarcidae Gen et spp. (9% similarity).

142 The Mariana Trench shows no community structure in the MaxN dataset. However, the larger 143 and bathymetrically broader %Ob dataset detected two significant fish communities (Figure 144 9): An AHTZ community from 4506-6189 m with Bassozetus spp. and Coryphaenoides 145 yaquinae accounting for more than 99% of the similarity between deployments; and a deeper 146 hadal community from 6831-8143 m with the undescribed ‘Mariana snailfish’ accounting for 147 98.93% of the similarity between deployments. The deepest deployment contains only the 148 OTU designated ‘ethereal snailfish’ (see Linley et al., 2016) which was also detected at 8007 149 m alongside the Mariana snailfish. The deepest deployment therefore possesses greater 150 dissimilarity yet is statistically part of the hadal group.

151 152 Figure 9: MDS plot of the Mariana Trench deployments in the %Ob dataset. Resemblance 153 has been overlaid at 20% (solid line) and 40% (dashed line). Significant groups are AHTZ 154 (square) and Hadal (cross).

155

156 Despite the wide geographic range there are similarities evident in the fish communities. 157 Cluster analyses on all deployments detected similar groups and revealed structure in the 158 New Hebrides Trench and South Fiji Basin (Figure 10). Four significant fish community groups 159 were detected in the MaxN and five in the %Ob dataset. The MaxN dataset contained two

160 significant bathyal groups; a shallower (997–1527 m) cluster of deployments in the Kermadec 161 Trench region (the only location to include such shallow deployments) with Diastobranchus 162 capensis and Simechelys parasitica accounting for 82% of the within group similarity; and a 163 deeper (1971–2087 m) bathyal group of Kermadec and New Hebrides Trench deployments 164 containing only the macrourids Coryphaenoides rudis and C. longifilis and the elasmobranchs 165 Amblyraja hyperborea, Bathyraja cf. richardsoni and Hydrolagus cf. affinis. Other clusters 166 were an abyssal group found only in the Kermadec Trench (3039–4158 m) with 96% of the 167 within group similarity due to Coryphaenoides armatus, C. yaquinae, Spectrunculus grandis 168 and Large Synaphobranchid; an AHTZ group containing deployments from all locations, 169 covering a wide bathymetric range (2578-6898 m), as it contained all New Hebrides Trench 170 deployments but the shallowest. Bassozetus spp., Large Synaphobranchid and 171 Coryphaenoides yaquinae accounted for 96% of the AHTZ within group similarity.

172 The %Ob dataset combined the MaxN shallow and deep bathyal deployments as a single 173 bathyal group (997-2503 m) with Diastobranchus capensis, Simenchelys parasitica and 174 Antimora rostrata accounting for 77% of the within group similarity. Both datasets agreed on 175 a Kermadec Trench abyssal group (3039–4692 m) with 96% of the within group similarity due 176 to Coryphaenoides armatus, C. yaquinae, and Spectrunculus grandis. As observed previously 177 Large Synaphobranchid was not detected in this dataset. Both datasets also agreed on the 178 AHTZ group. The deeper range of the %Ob dataset identified two groups of endemic hadal 179 snailfish; one in the Kermadec Trench (6750–7669 m) containing only Notoliparis 180 Kermadecensis; and a second in the Mariana Trench (6831–8143 m) with the ‘Mariana 181 snailfish’ accounting for 99% of the within group similarity.

182 183 Figure 10: MDS plot of all study deployments in the A) MaxN and B) %Ob datasets. 184 Deployments are labelled with their depth. Similarity boundaries have been overlaid at 20%

185 and annotated. Study locations are Kermadec Trench (triangle), South Fiji Basin (square), 186 Hew Hebrides Trench (circle) and Mariana Trench (cross).

187

188 The prominent abyssal macrourids Coryphaenoides armatus and C. yaquinae were not 189 observed in either the New Hebrides Trench or South Fiji Basin. This resulted in the abyssal 190 group, detected in the adjoining Kermadec Trench, being absent from these locations. In the 191 absence of this group, the AHTZ group appeared to occupy a wider bathymetric range, 192 extending shallower (Figure 11). An endemic hadal snailfish is also notably lacking from the 193 New Hebrides Trench.

194

195 196 Figure 11: Stacked plots of Abyssal-lander MaxN of the fish species that defined the abyssal 197 and AHTZ groups; Large Synaphobranchid (Large Syn.), Coryphaenoides armatus (C. 198 armatus), Coryphaenoides yaquinae (C. yaquinae), Spectrunculus grandis (S. grandis) and 199 Bassozetus spp. (Bass. spp.). Dashed lines indicate deployment depths.

200

201 3.4. Environmental drivers

202 DistLM was used to explore the potential environmental drivers of community structure. All 203 environmental data are available in Table S4. The hadal snailfish OTU were combined for this 204 analysis as their endemism is likely due to their isolation as stenobathic hadal fish, and they 205 are functionally very similar (see Linley et al., 2016). Marginal tests of the MaxN dataset

206 indicated that neither current speed (Psudo-F28 = 0.849, p = 0.488) or salinity (Psudo-F28 = 207 1.162, p = 0.319) had a significant effect on community structure at the observed levels. The 208 analysis could therefore be performed on the larger %Ob dataset which lacked these data. 209 Marginal tests detected significant relationships between community structure and depth

210 (Psudo-F64 = 12.579, p = 0.001), temperature (Psudo-F64 = 7.635, p = 0.001), bottom POC

211 (Psudo-F64 = 7.327, p = 0.001), PPYr (Psudo-F64 = 5.205, p = 0.001), PPMo (Psudo-F64 = 5.949,

212 p = 0.001), but not ‘quarter of day’ (Psudo-F64 = 1.123, p = 0.347) or the sediment classification

213 (Psudo-F64 = 0.868, p = 0.508). Both AIC and BIC based model selection measures agreed that 214 the best environmental predictors for the observed biological pattern were depth, 215 temperature and PPYr (Figure 12). These predictors could explain 36% of the biological 216 variability.

217

218 219 Figure 12: dbRDA of the final DistLM model. Vectors have been added for the explanatory 220 environmental variables. Points represent individual deployments; Kermadec Trench (triangle), 221 South Fiji Basin (square), Hew Hebrides Trench (circle) and Mariana Trench (cross). Similarity 222 boundaries have been overlaid at 20% (solid line) and 40% (dashed line) and annotated with the 223 groups they represent. How well each axis represents the model (fitted) and the actual 224 biological data (total) are displayed on the axis.

225

226 3.5. Feeding observations

227 The Hadal-lander video recorded fish behaviour at the bait. Some species were observed to 228 feed exclusively on the bait (number of observed feeding events in brackets): Simenchelys 229 parasitica (135), Antimora rostrata (10), Coryphaenoides armatus (454), Barathrites iris (16), 230 Spectrunculus grandis (8), Centroscymnus cf. coelolepis (9), Etmopterus cf. baxteri (3) and the 231 Large Synaphobranchid (2). However, some bait-attending species were also observed 232 preying on scavenging invertebrates (mainly amphipods and on occasion, decapods). Species

233 observed to most often feed on the bait but occasionally prey upon other scavengers were: 234 Coryphaenoides yaquinae (89% necrophagy, n = 369) and D. capensis (97% necrophagy, n = 235 145). Species that were observed to prey almost exclusively on other scavengers: Bassozetus 236 spp. (98% predation, n = 290), which were observed specifically targeting large amphipods 237 and decapods (supplementary video 1), and Zoarcid Gen et spp. (97.2% predation, n = 106). 238 All of the hadal snailfish were seen to target amphipods feeding at the bait: Notoliparis 239 kermadecensis (96% predation, n = 531), the Mariana snailfish (98%, n = 753) and the ethereal 240 snailfish (100% predation, n = 8). Ingesting bait while suction feeding amphipods appeared to 241 be accidental in the snailfish and bait material was often ejected from the mouth following 242 what would have been counted as a necrophagy feeding event.

243 The species that defined the fish community groups had distinct feeding strategies: the 244 bathyal group and the Kermadec abyssal group were almost exclusively necrophagous, the 245 AHTZ group comprised predatory and more generalist feeders, while the hadal snailfishes 246 were predators.

247 While abyssal macrourids were absent from the New Hebrides Trench and South Fiji Basin, 248 necrophagy was still observed by the cusk-eels Barathrites iris and Spectrunculus grandis 249 respectively. The most prominent scavenger in the New Hebrides Trench and South Fiji Basin 250 however was the shrimp Cerataspis monstruosus Gray, 1828 (previously recorded as 251 Plesiopenaeus armatus (Spence Bate, 1881); see Bracken-Grissom et al., 2012). While only 252 one or two simultaneous individuals had been observed in the other trenches up to nine 253 occurred in the New Hebrides Trench.

254 Feeding events at the bait (corrected for the number of fish present and duration observed) 255 suggested that with increasing depth the frequency of predatory feeding events increased 256 while the frequency of scavenging events was reduced (Figure 13). A significant inverse

257 relationship occurred between depth and the frequency of scavenging events (F1,47 = 19.303,

258 p < 0.001) with no detectable difference between the Kermadec and Mariana trenches (F1,47 259 = 0.442, p = 0.503). No relationship was detected between predatory feeding events with

260 depth (F1,42 = 1.034, p = 0.315) or trench (F1,42 = 0.224, p = 0.137).

261 262 Figure 13: Feeding events (necrophagy or predation) per fish per minute in relation to depth 263 in the Kermadec (triangle) and Mariana (cross) trenches. The significant relationship between 264 necrophagy and depth is plotted with 95% confidence intervals.

265

266 3.6. Summary

267 The observed trends between the identified environmental predictors and the feeding 268 character of each significant fish group is summarised in Figure 14.

269 270 Figure 14: Summary of the environmental conditions influencing fish community structure 271 within the significant fish groups. Depth (m), temperature (°C), estimated surface primary 272 production for the previous year (PPYr; mg C/m2/day), and the proportion of necrophagous 273 (black) and predatory (grey) feeding events observed within those groups.

274

275 4. Discussion

276 Generally, there was agreement between the MaxN and %Ob datasets. Both datasets agreed 277 upon the major fish community groups across all deployments: 1) a varied group of bathyal 278 species < 3000 m depth, 2) a macrourid dominated abyssal group of predominantly 279 scavenging fish, 3) a Bassozetus spp. dominated AHTZ group of predatory and generalist 280 feeders, particularly, in %Ob data, 4) a hadal group comprising of endemic snailfish.

281 4.1. Macrourid or ophidiid dominated communities

282 Abyssal macrourids were prominent components of Kermadec and Mariana trench fish fauna 283 but were absent from the New Hebrides Trench and South Fiji Basin. Macrourids are well 284 represented at bathyal depths around New Caledonia (Clark and Roberts, 2008; Zintzen et al., 285 2011) adjacent the New Hebrides Trench and were observed at bathyal deployments (Table 286 1). The macrourids Coryphaenoides armatus and C. yaquinae readily gather at baited landers 287 (Armstrong et al., 1991; Collins et al., 1999; Cousins et al., 2013b; Henriques et al., 2002; Isaacs 288 and Schwartzlose, 1975; King et al., 2006; Priede and Bagley, 2001). It is highly unlikely that 289 Coryphaenoides armatus and C. yaquinae were present in the New Hebrides Trench and 290 South Fiji Basin and failed to be detected using baited landers. In their absence an ophidiid- 291 based community appears to dominate. Examples from the literature exist of such 292 communities. For example, Anderson et al. (1985) reported an ophidiid dominated 293 community (mainly unidentified Bassozetus spp.) in the Caribbean Sea. The only reported 294 macrourid was restricted to ~3500 m depth. A similar community was reported in the basins 295 east of the Bahamas (Sulak, 1982). Baited lander deployments by Fleury and Drazen (2013) in 296 the Sargasso Sea observed Coryphaenoides armatus, but it was rare and one of the last fish 297 species to arrive. Their deployments were also dominated by a Bassozetus spp., closely 298 resembling those discussed herein, that was never observed to feed on the bait. Janßen et al. 299 (2000) observed similar bait attending behaviour in the ophidiid Holcomycteronus 300 aequatorius. Yeh and Drazen (2009) found a dominance of ophidiids in the low productivity 301 North Pacific subtropical gyre. Janßen et al. (2000) and Christiansen and Martin (2000) 302 reported a surprising lack of macrourid fishes (a single individual was photographed) and an 303 ophidiid based community in the Arabian Sea. Macrourids are only found at bathyal depths 304 in the Arabian Sea (Christiansen and Martin, 2000; Janßen et al., 2000; Shcherbachev and 305 Iwamoto, 1995; Witte, 1999). In the Northern Atlantic Merrett (1992) observed a shift from 306 macrourid to ophidiid community from north to south along the productivity gradient. The 307 majority of the authors cited above suggested surface productivity was the driver of these 308 observed differences in the benthic community.

309 The Hadal-lander video data indicated that Bassozetus spp. were discouraged from bait- 310 attending by the voracious feeding of the scavenging macrourids Coryphaenoides armatus 311 and C. yaquinae, which dispersed the scavenging amphipods on which Bassozetus spp. were

312 feeding. Competition may be more direct when these macrourids act as predators. Sulak 313 (1982) suggested that low-energy ophidiid based communities occurred when competition 314 from more energetically expensive macrourids was eased. In the Kermadec Trench this 315 boundary is bathymetric and the Bassozetus spp. occurs deeper than the macrourids. In the 316 New Hebrides Trench and South Fiji Basin, more energetically-expensive macrourids are 317 excluded, allowing the low-energy ophidiid to dominate and extend shallower than in the 318 other trenches studied. This is in keeping with the Species Interactions-Abiotic Stress 319 Hypothesis (SIASH; see Louthan et al. 2006); the deeper limits of these species are probably 320 set by their physiological depth-tolerances (Yancey et al., 2014), of which Bassozetus spp. is 321 greater, whereas the shallower limit of Bassozetus spp. is set by its competitive exclusion by 322 the macrourids and is seen to relax in their absence.

323 The macrourid based scavenging community is often thought of as representative of the 324 world’s abyssal plains (Christiansen and Martin, 2000; Janßen et al., 2000; Jamieson et al., 325 2011). Examples of deviations from this appear in the literature in a range of areas associated 326 with lower surface productivity and warmer latitudes. Sampling of the deep sea has not been 327 uniform, the cost of vessel time has resulted in sampling biased towards areas close to land 328 masses (Ebbe et al., 2010; Jamieson, 2015), enriched through upwelling and terrestrial runoff 329 and subsequent downslope transport (Gove et al., 2016; Henriques et al., 2002; Merrett, 330 1992; Müller and Suess, 1979) where localised, macrourid based, scavenging communities 331 can occur (Henriques et al., 2002; Sulak, 1982). As a result, our idea of the ubiquitous 332 scavenging abyssal fish communities may be greatly skewed and an ophidiid dominated 333 predatory community may be more common globally.

334 4.2. Feeding

335 Bait-attending species that are observed for long periods but never seen to feed on the bait 336 can be said with some confidence to not rely upon necrophagy as part of their natural feeding 337 strategy. The inverse cannot be confidently said and it is likely that the species that were seen 338 to only feed on the bait also feed on other food sources. Community zonation by trophic 339 guilds is potentially an important factor in the deep sea (Cartes and Carrassón, 2004). In the 340 northern Pacific Ocean scavenging fish species, as a proportion of total fish diversity, have 341 been found to increase from 100 – 3000 m (Yeh and Drazen, 2011, 2009). Despite a generally 342 decreasing trend being observed in the current study, the peak in necrophagous feeding at

343 abyssal depths does not contradict the result of Yeh and Drazen (2011). The reduction in 344 scavenging feeding events with depth found in the Kermadec and Mariana trenches predicted 345 a loss of necrophagy in fish within the AHTZ. This hypothesis is supported by stomach contents 346 and stable isotope analyses performed on collected trap specimens (Gerringer et al., 2016).

347 Drazen et al. (2008) hypothesised that Coryphaenoides armatus and C. yaquinae bypass the 348 benthic food web, gaining most of their energetic input from nekton carcasses. 349 Coryphaenoides armatus and C. yaquinae are very similar and commonly confused species 350 (Iwamoto and Stein, 1974): C. armatus is the more specialised scavenger while C. yaquinae 351 feeds more on infauna and epifauna (Drazen, 2007; Drazen et al., 2008), agreeing with the 352 feeding observations in the current study. Coryphaenoides yaquinae was seen to span the 353 abyssal and AHTZ groups while C. armatus was found only in the abyssal group, illustrating 354 the proposed shift from scavengers to a more generalist feeding character in these 355 community groups.

356 Scavenging behaviour was still observed in the New Hebrides Trench and South Fiji Basin by 357 ophidiid rather than macrourid scavengers. Barathrites iris has been observed feeding at 358 mackerel baits in previous studies (Fleury and Drazen, 2013; Henriques et al., 2002; Janßen et 359 al., 2000) as has Spectrunculus grandis (Cousins et al., 2013b; Henriques et al., 2002; Janßen 360 et al., 2000). These two species were present in the South Fiji Basin and New Hebrides Trench 361 respectively and may represent lower energy/opportunist scavengers. Janßen et al. (2000) 362 suggested flexibility in Barathrites iris and Pachycara spp. (Zoarcidae), as they were observed 363 at baited lander deployments in fewer images at more productive sites where alternative food 364 sources are presumed to reduce the need to scavenge. Facultative scavengers may be a 365 feature of intermediately productive areas or areas with high seasonality such as the Arabian 366 Sea (Janßen et al., 2000). Zoarcids were observed in large numbers (up to 23 individuals) in 367 the New Hebrides Trench while maximum numbers were much lower in the Kermadec and 368 Mariana Trenches (5 and 1 respectively). Pachycara species have been observed to take up 369 long-term residence around large carcasses, often beyond its potential as a food source 370 (Henriques et al., 2002; Jones et al., 1998).

371 The loss of scavenging fishes at the boundary of the hadal zone emphasises the zone’s distinct 372 faunal community (Wolff, 1970) and coincides with ecological boundaries in other faunal 373 groups (Jamieson, 2011; Lacey et al., 2016). Scavenging at hadal depths appears dominated

374 by the amphipods of the Superfamily Lysianassodea (Blankenship-Williams and Levin, 2009; 375 Lacey et al., 2016). Bassozetus spp. appeared to target large amphipods and decapods when 376 it was observed feeding, and the hadal snailfishes also appear to bait-attend to prey on the 377 gathered invertebrate scavengers (necrophagivorey). Unlike fish, scavenging amphipods can 378 be detritivores (Blankenship and Levin, 2007). It would appear that Bassozetus spp. and the 379 other members of the AHTZ and hadal predatory groups rely on the benthic food-web to 380 consolidate surface-derived material through detritivorous and necrophagous invertebrates.

381 4.3. Environmental drivers

382 Depth, surface productivity and temperature were identified as environmental predictors 383 that best explained the fish community structure.

384 Depth is unsurprisingly revealed as the strongest factor affecting fish community composition 385 in trench systems and the surrounding abyssal plains. In addition to depth there appears to 386 be a subtle interaction between surface productivity and temperature that determines 387 whether the abyssal scavenging macrourids are present. A linkage of surface productivity with 388 numbers of animals attracted to baited landers has been observed since Isaacs and 389 Schwartzlose (1975), early pioneers of the method. However, proxies for nutrient input such 390 as particle flux and surface productivity may not represent the resources available to a specific 391 species or benthic fauna as a whole (Carney, 2005). Also, the identified environmental factors 392 may not be the environmental force driving the biological pattern but correlated to it. With 393 this caveat in mind, two hypotheses, based on the available data, are proposed below related 394 to fish energy budget and different water masses within the study areas.

395 4.4. Energy budget

396 Depth and temperature affect a wide range of physiological processes, and are thought to be 397 important to deep-sea organism distribution, with surface productivity acting as a proxy for 398 food availability (Watling et al., 2013). Differences between adjacent areas are more likely to 399 be influenced by energetic input than temperature (Carney, 2005; Watling et al., 2013). The 400 Mariana Trench is the least productive area in this study but it is also colder than the New 401 Hebrides Trench. Coryphaenoides yaquinae were present in the Mariana Trench and it is 402 anticipated that future shallower studies around the Mariana Trench will likely reveal that C. 403 armatus are also present. This species is known from 4100-4220 m in the Japan Trench (Endo

404 and Okamura, 1992). The Peru-Chile Trench is relatively warm (1.8 to 2.25°C; Fujii et al., 2013) 405 and highly eutrophic due to upwelling, the Humbolt current system, and its proximity to land 406 mass (Daneri et al., 2000). Baited lander deployments in the Peru-Chile Trench (Jamieson et 407 al., 2012) observed both Coryphaenoides yaquinae and C. armatus, supporting the idea an 408 interplay between temperature and surface productivity that may explain the absence of 409 these species in the New Hebrides and South Fiji Basin.

410 Low surface productivity (leading to fewer food falls) coupled with higher temperatures 411 (accelerating metabolism) may be prohibitive to scavengers; especially those that expend 412 energy searching for odour plumes such as Coryphaenoides armatus (Armstrong et al., 1991; 413 Drazen, 2008, 2002; Priede et al., 1990). High temperature but high productivity (e.g. Peru- 414 Chile Trench), and low temperature and low productivity (e.g. Mariana Trench) still allow this 415 lifestyle.

416 4.5. Water mass

417 An alternative hypothesis is that the differences between the Kermadec Trench and the South 418 Fiji Basin and New Hebrides Trench are due to different water masses. Boundaries between 419 water masses are often suggested as a driver of zonation but the precise mechanism is not 420 fully understood (Carney, 2005). The most recent biogeographical assessment of the study 421 areas by Watling et al. (2013) considers the trenches beyond 6500 m distinct, but their 422 surrounding abyssal depths a single province agreeing with previous outlines from Beliaev 423 (1989) and Vinogradova (1997). Despite the difference in environmental conditions between 424 the Kermadec and New Hebrides trenches being relatively slight, a faunal divide in this region 425 has been detected in the bathyal ophiuroid (O’Hara et al., 2011) and fish communities (Clark 426 et al., 2003; Zintzen et al., 2011). This ecotone appears to take place at the Tasman Front that 427 marks the path of the East Australian Current (Ridgway and Dunn, 2003). Temperature may 428 have appeared to be a strong environmental driver only because it differentiated the sides of 429 this boundary. Differences in productivity, standing crop and phytoplankton size distribution 430 across a front will have effects up the food chain (Baird et al., 2008). On the Kermadec Trench 431 side of the front more nitrogen crosses the thermocline and is available to phytoplankton 432 (Ellwood et al., 2013), a longer residence time also allows phytoplankton growth to deplete 433 nutrients and develop not only a higher and distinct phytoplankton standing stock, but also 434 larger zooplankton size classes (Baird et al., 2008; Ellwood et al., 2013) and distinct pelagic

435 fish and crustacean fauna (Griffiths and Wadley, 1986). Griffiths and Wadley (1986) found 436 that pelagic fishes from the New Hebrides Trench (northern) side of the front were able to 437 enter the Kermadec (southern) side but the front appeared to act as a barrier to northward 438 movement, as appears the case in the current study. The front is unlikely to directly restrict 439 fishes at abyssal depths and deeper but the link, particularly in scavengers, to the shallower 440 food-webs may drive the mirroring of this pattern at greater depths.

441 While the differences between the Kermadec and Mariana trenches are less pronounced than 442 the differences between them and the New Hebrides Trench, there is still variation. The 443 Mariana Trench was unusual in lacking a large synaphobranchid eel at the depths sampled. 444 Coryphaenoides yaquinae is also more prominent in the Mariana Trench community than it 445 was in the Kermadec Trench.

446 4.6. Limitations

447 It is disappointing that the Bassozetus species that features so prominently in this study could 448 not be positively identified. The revision of this genus by Nielsen and Merrett (2000), was 449 based on preserved specimens. These fish have loose, fluid filled, skin around the head and 450 snout (Machida and Tachibana, 1986; Nielsen et al., 1999; Nielsen and Merrett, 2000) which 451 becomes ‘deflated’ during capture and preservation. This can make correlation between 452 vouchers and in situ images challenging. Fresh or in situ individuals often have longer snout 453 measurements and a differing head profile than that of preserved specimens (Berbel-Filho et 454 al., 2013; Machida and Tachibana, 1986).

455 The inclusion of temperature as an environmental predictor sits well with the proposed 456 energy budget hypothesis and records from other locations, but it is also linked closely to 457 depth (although the relationship is complex due to adiabatic heating). The very strong match 458 of depth as a predictor could explain the weaker explanatory power provided by temperature. 459 However, it is not possible now to resolve if the role of temperature is its direct physiological 460 effect on fish distribution or if it is an indirect proxy for other effects of water masses.

461 The current hypothesis is based on the relative oligotrophic designation of the Mariana 462 Trench as interpreted from surface primary production. However, trenches may accumulate 463 organic matter through downslope funnelling of detritus (George and Higgins, 1979; Ichino et

464 al., 2015; Itou et al., 2000) or chemosynthetic communities (Fujikura et al., 1999). Sediment 465 respirometry in the trench shows high metabolic rates (Glud et al., 2013).

466 Acknowledgments

467 This work was funded by the TOTAL Foundation (France) through the projects ‘Multi- 468 disciplinary investigations of the deepest scavengers on Earth’ (2010-2012) and ‘Trench 469 Connection’ (2013-2015) awarded to A.J.J. The 2014 field work was funded by the HADES-K 470 and HADES-M projects supported by the National Science Foundation (NSF) and the Schmidt 471 Ocean Institute (SOI) respectively. We thank the crew and company of the RV Kaharoa 472 (KAH1109, KAH1202, KAH1301 and KAH1310), and NIWA Vessels Management, New Zealand. 473 We also thank the crew and company of the RV Thomas G. Thompson (TN309; HADES-K) and 474 the RV Falkor (FK141109; HADES-M). T.D.L. and A.J.J. are supported by the Marine Alliance 475 for Science and Technology for Scotland (MASTS) pooling initiative, whose support is 476 gratefully acknowledged. P.J.M. and M.R.C. participated in the study through the New 477 Zealand Foundation for Research, Science and Technology (now Ministry for Business, 478 Innovation and Education) funded project ‘Impact of Resource Use on Vulnerable Deep-Sea 479 Communities’ (CO1X0906). J.C.D and M.E.G. were supported by NSF (OCE-1130712) and 480 M.E.G was also funded by the NSF Graduate Research Fellowships Program, both of whose 481 support is gratefully acknowledged. ALS contribution was supported (in part) by the NZ 482 Ministry of Research and Innovation through Te Papa subcontract within NIWA’s Biodiversity 483 and Biosecurity Science Programme (previously NIWA’s FoRST contract C01X0502). Thanks 484 are extended to Dr Kenneth J. Sulak of the U.S. Geological Survey and Dr Jørgen Nielsen of the 485 Natural History Museum of Denmark for discussion on the identification of the 486 synaphobranchid eels and Bassozetus spp. respectively. Thanks also to Matteo Ichino of the 487 National Oceanographic Centre who assisted with the benthic POC estimates and Heather 488 Stewart of the British Geological Survey who categorised the sediment types seen in the 489 lander images.

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815

816 Supplementary material 817

818 Table S1: Baited lander deployment details, listed by region and increasing depth. ‘Region’ 819 refers to the study region; Kermadec Trench (Kerm), New Hebrides Trench (NHeb), South 820 Fiji Basin (SFB) and Mariana Trench (Mar). ‘Cruise’ is the expedition where the data was 821 taken. ‘Equip’ is which lander was used; Abyssal-lander (AL) or the Hadal-lander (HL). 822 ‘Deployment’ is the deployment number during that cruise. ‘Date’ is shown in the format 823 dd/mm/yyyy. Latitude and longitude are shown in decimal degrees. Depth is in metres as 824 calculated from the system’s on-board pressure sensor. Duration is the number of minutes 825 recorded at the seabed. * indicates deployments that did not include fish.

Depth Duration Region Cruise Equip Dep # Date Lat Long (m) (min) Kerm KAH1301 AL 18 28/01/2013 -38.914 178.505 997 480 Kerm KAH1301 AL 20 28/01/2013 -39.004 178.570 1473 676 Kerm HADES-K AL 01 11/04/2014 -37.202 178.864 1527 1227 Kerm HADES-K HL 01 12/04/2014 -37.213 178.857 1554 1444 Kerm KAH1301 AL 03 21/01/2013 -33.993 -179.933 1971 1241 Kerm KAH1301 HL 05 21/01/2013 -34.002 -179.821 2503 396 Kerm KAH1301 AL 06 22/01/2013 -33.999 -179.501 3039 1146 Kerm KAH1301 HL 08 22/01/2013 -34.000 -179.206 3534 77 Kerm HADES-K AL 14 14/05/2014 -31.855 -176.818 3655 715 Kerm KAH1301 AL 09 23/01/2013 -34.001 -179.059 3940 994 Kerm HADES-K HL 02 13/04/2014 -37.741 179.836 3975 2055 Kerm HADES-K AL 02 13/04/2014 -37.728 179.820 4061 1498 Kerm HADES-K AL 03 15/04/2014 -37.681 179.859 4138 1140 Kerm HADES-K AL 06 20/04/2014 -37.001 -179.883 4158 781 Kerm HADES-K HL 06 20/04/2014 -37.002 -179.877 4194 737 Kerm HADES-K HL 03 15/04/2014 -37.679 179.873 4332 1010 Kerm KAH1301 HL 11 23/01/2015 -34.000 -178.953 4519 551 Kerm HADES-K HL 08 23/04/2014 -37.223 -179.770 4692 1459 Kerm HADES-K AL 08 23/04/2014 -37.220 -179.774 4707 991 Kerm HADES-K AL 04 16/04/2014 -37.180 -179.748 4953 1599 Kerm HADES-K AL 05 19/04/2014 -37.166 -179.744 5046 703 Kerm HADES-K HL 04 16/04/2014 -37.179 -179.731 5075 1959 Kerm KAH1301 AL 12 24/01/2013 -34.001 -178.782 5092 1042 Kerm HADES-K HL 05 19/04/2014 -37.181 -179.748 5135 722 Kerm HADES-K AL 13 14/05/2014 -31.810 -176.906 5254 972 Kerm HADES-K HL 07 22/04/2014 -37.040 -179.686 5281 763 Kerm HADES-K AL 07 22/04/2014 -37.039 -179.688 5295 684 Kerm KAH1301 AL 15 25/01/2013 -34.000 -178.507 5460 1057 Kerm KAH1301 HL 14 24/01/2013 -34.000 -178.656 5601 73 * Kerm HADES-K AL 12 12/05/2014 -31.861 -177.727 5646 626 Kerm HADES-K AL 10 02/05/2014 -35.898 -178.947 5879 1217

Kerm HADES-K AL 11 11/05/2014 -31.902 -177.595 6037 1039 Kerm HADES-K AL 09 01/05/2014 -35.899 -178.953 6068 1191 Kerm HADES-K HL 09 01/05/2014 -35.914 -178.959 6191 1111 Kerm KAH1301 HL 17 25/01/2013 -34.000 -178.439 6552 567 * Kerm HADES-K HL 12 05/05/2014 -35.854 -178.914 6750 1030 Kerm HADES-K HL 10 02/05/2014 -35.833 -178.869 7243 1172 Kerm HADES-K HL 11 03/05/2014 -35.671 -178.876 7669 712 Kerm HADES-K HL 13 06/05/2014 -34.371 -178.190 7905 1141 * SFB KAH1310 AL 05 10/11/2013 -24.972 171.033 4074 1097 SFB KAH1310 AL 38 29/11/2013 -27.732 174.229 4078 573 SFB KAH1310 AL 01 09/11/2013 -25.144 171.048 4100 910 SFB KAH1310 HL 06 10/11/2013 -24.971 171.106 4101 969 NHeb KAH1310 AL 30 23/11/2013 -21.263 168.197 2087 925 NHeb KAH1310 AL 36 26/11/2013 -21.202 168.219 2578 651 NHeb KAH1310 AL 22 20/11/2013 -21.104 168.143 3424 477 NHeb KAH1310 AL 09 16/11/2013 -21.024 168.268 4123 1131 NHeb KAH1310 HL 10 16/11/2013 -21.023 168.335 4148 964 NHeb KAH1310 AL 28 22/11/2013 -20.950 168.490 4835 183 * NHeb KAH1310 AL 13 17/11/2013 -20.872 168.517 5192 266 NHeb KAH1310 AL 25 22/11/2013 -20.854 168.537 5344 472 NHeb KAH1310 AL 21 19/11/2013 -20.838 168.561 6056 653 NHeb KAH1310 AL 17 18/11/2013 -20.768 168.559 6162 619 NHeb KAH1310 AL 35 25/11/2013 -20.716 168.556 6397 560 NHeb KAH1310 AL 32 25/11/2013 -20.646 168.608 6898 927 Mar HADES-M AL 02 11/11/2014 12.726 144.726 4506 578 Mar HADES-M HL 02 11/11/2014 12.722 144.710 4703 667 Mar HADES-M AL 01 10/11/2014 12.744 144.647 4902 785 Mar HADES-M HL 17 30/11/2014 11.423 144.433 4998 768 Mar HADES-M HL 16 29/11/2014 11.418 144.412 5044 831 Mar HADES-M HL 24 07/12/2014 12.684 144.652 5641 933 Mar HADES-M AL 05 26/11/2014 11.603 144.862 6008 518 Mar HADES-M HL 14 26/11/2014 11.591 144.847 6010 607 Mar HADES-M HL 03 12/11/2014 12.521 144.747 6089 672 Mar HADES-M HL 04 13/11/2014 12.521 144.744 6097 651 Mar HADES-M AL 04 13/11/2014 12.519 144.740 6130 646 Mar HADES-M HL 15 27/11/2014 11.607 144.833 6142 2114 Mar HADES-M AL 03 12/11/2014 12.524 144.733 6198 658 Mar HADES-M HL 05 14/11/2014 12.626 144.711 6831 838 Mar HADES-M HL 06 15/11/2014 12.627 144.761 6931 763 Mar HADES-M HL 12 24/11/2014 11.810 144.977 7012 676 Mar HADES-M HL 07 16/11/2014 12.411 144.893 7415 2000 Mar HADES-M HL 08 18/11/2014 12.409 144.895 7440 626 Mar HADES-M HL 13 25/11/2014 11.878 144.977 7485 650 Mar HADES-M HL 23 06/12/2014 12.333 144.671 7716 724 Mar HADES-M HL 09 20/11/2014 12.305 144.687 7941 951 Mar HADES-M HL 18 01/12/2014 11.920 144.923 8004 666

Mar HADES-M HL 10 21/11/2014 11.920 144.923 8007 1199 Mar HADES-M HL 11 23/11/2014 11.925 144.915 8078 576 Mar HADES-M HL 22 05/12/2014 12.289 144.669 8143 806 Mar HADES-M HL 20 03/12/2014 12.160 144.671 8964 770 * Mar HADES-M HL 19 02/12/2014 12.043 144.882 9059 620 * Mar HADES-M HL 21 04/12/2014 12.070 144.602 10545 1185 * 826

827

828 Table S2: Trap deployments. Headings are as in Table S1. ‘Equip’ denotes the equipment 829 used; Large Fish Trap (LFT) and Small Fish Trap (SFT). * indicates deployments that did not 830 include fish.

Region Cruise Equip Dep # Date Lat Long Depth (m)

Kerm KAH1301 LFT 21 28/01/2013 -38.914 178.503 1013 Kerm KAH1301 LFT 19 28/01/2013 -39.003 178.575 1490 Kerm HADES-K LFT 01 11/04/2014 -37.204 178.878 1561 Kerm KAH1301 LFT 02 21/01/2013 -33.967 -179.967 1980 * Kerm KAH1301 LFT 04 21/01/2013 -33.992 -179.891 1980 Kerm KAH1301 LFT 07 22/01/2013 -34.000 -179.339 3268 Kerm HADES-K LFT 19 14/05/2014 -31.850 -176.821 3569 Kerm HADES-K SFT 04 15/05/2014 -31.854 -176.822 3601 Kerm HADES-K LFT 02 13/04/2014 -37.736 179.810 3865 Kerm HADES-K LFT 06 20/04/2014 -37.000 -179.888 4149 Kerm KAH1301 LFT 10 23/01/2013 -34.000 -179.022 4193 Kerm HADES-K LFT 03 15/04/2014 -37.671 179.872 4204 Kerm HADES-K LFT 08 23/04/2014 -37.219 -179.773 4779 * Kerm HADES-K LFT 04 16/04/2014 -37.186 -179.721 4830 Kerm HADES-K LFT 05 20/04/2014 -37.165 -179.734 4989 Kerm HADES-K LFT 07 21/04/2014 -37.166 -179.729 5112 Kerm KAH1301 LFT 13 24/01/2013 -34.000 -178.685 5242 Kerm KAH1301 LFT 16 25/01/2013 -33.982 -178.501 5612 * Kerm HADES-K LFT 18 13/05/2014 -31.915 -176.919 5958 * Kerm HADES-K LFT 09 30/04/2014 -35.904 -178.967 6061 * Kerm HADES-K LFT 17 12/05/2014 -31.890 -177.533 6456 Kerm HADES-K SFT 01 11/05/2014 -32.862 -177.479 7187 Kerm HADES-K LFT 12 04/05/2014 -35.832 -178.874 7200 Kerm HADES-K SFT 03 14/05/2014 -31.830 -177.075 7227 Kerm HADES-K LFT 16 11/05/2014 -31.914 -177.482 7251 Kerm HADES-K LFT 10 02/05/2014 -35.667 -178.885 7392 Kerm HADES-K LFT 11 03/05/2014 -35.672 -178.883 7516 Kerm HADES-K SFT 02 12/05/2014 -31.909 -177.482 7554 Kerm HADES-K LFT 13 06/05/2014 -34.362 -178.187 7948 * Kerm HADES-K LFT 14 07/05/2014 -32.845 -177.662 9198 * Kerm HADES-K LFT 15 08/05/2014 -31.931 -177.301 10005 *

SFB KAH1310 LFT 03 09/11/2013 -25.143 171.069 4100 SFB KAH1310 LFT 04 09/11/2013 -25.144 171.096 4100 SFB KAH1310 SFT 39 29/11/2013 -27.747 174.250 4100 * NHeb KAH1310 SFT 31 23/11/2013 -21.276 168.209 2000 * NHeb KAH1310 SFT 37 26/11/2013 -21.219 168.667 2500 * NHeb KAH1310 SFT 23 20/11/2013 -21.113 168.165 3400 * NHeb KAH1310 LFT 11 16/11/2013 -21.024 168.291 4100 NHeb KAH1310 LFT 12 16/11/2013 -21.023 168.314 4100 * NHeb KAH1310 SFT 29 22/11/2013 -20.935 168.477 4700 NHeb KAH1310 LFT 15 17/11/2013 -20.893 168.528 5180 * NHeb KAH1310 LFT 16 17/11/2013 -20.912 168.538 5180 NHeb KAH1310 SFT 26 21/11/2013 -20.833 168.525 5300 NHeb KAH1310 SFT 34 25/11/2013 -20.759 168.486 5600 * NHeb KAH1310 SFT 33 24/11/2013 -20.795 168.546 6000 * NHeb KAH1310 LFT 19 18/11/2013 -20.804 168.576 6200 * Mar HADES-M LFT 02 11/11/2014 12.736 144.700 4441 Mar HADES-M LFT 17 30/11/2014 11.423 144.461 4994 * Mar HADES-M LFT 16 29/11/2014 11.443 144.460 5072 * Mar HADES-M SFT 17 07/12/2014 12.716 144.640 5156 * Mar HADES-M LFT 01 10/11/2014 12.724 144.657 5231 * Mar HADES-M LFT 21 07/12/2014 12.726 144.665 5255 * Mar HADES-M LFT 14 26/11/2014 11.598 144.878 6034 * Mar HADES-M LFT 15 27/11/2014 11.607 144.854 6068 * Mar HADES-M LFT 04 14/11/2014 12.526 144.736 6081 Mar HADES-M SFT 01 13/11/2014 12.528 144.746 6115 * Mar HADES-M LFT 03 13/11/2014 12.548 144.735 6230 * Mar HADES-M SFT 02 15/11/2014 12.641 144.738 6865 * Mar HADES-M LFT 12 24/11/2014 11.811 144.945 6898 Mar HADES-M LFT 06 15/11/2014 12.627 144.748 6914 Mar HADES-M SFT 09 24/11/2014 11.815 144.986 6949 Mar HADES-M SFT 03 15/11/2014 12.610 144.768 6961 Mar HADES-M LFT 13 25/11/2014 11.826 144.009 6974 Mar HADES-M LFT 05 14/11/2014 12.598 144.779 7062 Mar HADES-M SFT 10 25/11/2014 11.853 144.984 7237 * Mar HADES-M SFT 04 16/11/2014 12.415 144.912 7495 Mar HADES-M LFT 07 16/11/2014 12.423 144.871 7497 Mar HADES-M SFT 05 18/11/2014 12.424 144.871 7502 * Mar HADES-M LFT 08 18/11/2014 12.426 144.912 7509 Mar HADES-M LFT 19 05/12/2014 12.277 144.620 7626 Mar HADES-M LFT 20 06/12/2014 12.350 144.681 7652 Mar HADES-M SFT 16 06/12/2014 12.329 144.685 7744 * Mar HADES-M LFT 10 21/11/2014 11.913 144.951 7841 Mar HADES-M SFT 07 21/11/2014 11.927 144.962 7907 Mar HADES-M LFT 18 01/12/2014 11.906 144.935 7912 * Mar HADES-M LFT 09 20/11/2014 12.303 144.674 7929 Mar HADES-M SFT 06 20/11/2014 12.304 144.680 7949

Mar HADES-M SFT 15 05/12/2014 12.271 144.643 7957 * Mar HADES-M SFT 08 23/11/2014 11.930 144.929 7966 Mar HADES-M LFT 11 23/11/2014 11.928 144.924 8028 * Mar HADES-M SFT 11 01/12/2014 11.944 144.933 8223 * Mar HADES-M SFT 13 03/12/2014 12.209 144.635 8847 * Mar HADES-M SFT 12 02/12/2014 12.035 144.883 8942 * Mar HADES-M SFT 14 04/12/2014 12.072 144.593 10250 * 831

832

833

834 Table S3: Similarity percentage analysis (SIMPER) of the significant groups identified by similarity profile analysis (SIMPROF) for each study site 835 and for all deployments pooled. The groups are named as they appear in the text with the depth ranges they occupy. Average similarity of 836 each group is the average Brey-Curtis similarity between all pairs of deployments within the group, Av.Sim is each species contribution to that 837 Average similiarity. Av.MaxN or Av.%Ob is the average MaxN or %Ob of that species in the deployments in that group. Sim/SD is the ratio of 838 the average contribution (Av.Sim) divided by the standard deviation (SD) of those contributions across all pairs of samples making up this 839 average. Contrib% is the percentage of the average similarity that each fish species contributed, Cum.% is the cumulative Contrib% in order of 840 largest to smallest contribution. Fish species are only shown which contribute the first 90% of Cum.%. Large Synaphobranchid is abbreviated to 841 Large Syn.

842

Kermadec Trench MaxN %Ob Bathyal Average similarity: 43.75, Depth range: 997-1971 m Bathyal Average similarity: 38.85, Depth range: 997-2503 m Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% D. capensis 11.76 16.17 2.79 36.96 36.96 D. capensis 21.25 14.69 1.27 37.82 37.82 S. parasitica 16.32 14.93 2.06 34.12 71.08 S. parasitica 19.27 13.39 2.77 34.48 72.3 A. rostrata 1.21 3.70 0.86 8.45 79.53 A. rostrata 6.45 4.61 0.79 11.86 84.16 C. cf. coelolepis 0.56 2.93 0.91 6.70 86.23 H. cf. affinis 1.02 1.91 0.68 4.92 89.08 Macrourid 1 0.25 1.06 0.41 2.42 88.65 C. cf. coelolepis 0.53 1.08 0.76 2.78 91.86 C. owstonii 0.25 1.06 0.41 2.42 91.08

Abyssal Average similarity: 68.02, Depth range: 3039-4158 m Abyssal Average similarity: 61.62, Depth range: 3039-4692 m Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% C. armatus 12.25 30.97 6.71 45.53 45.53 C. armatus 58.06 40.35 4.55 65.49 65.49 S. grandis 5.29 19.16 8.81 28.17 73.69 C. yaquinae 14.75 12.96 1.28 21.03 86.52 C. yaquinae 2.92 11.02 1.33 16.2 89.89 S. grandis 9.99 5.85 0.63 9.49 96.01 Large Syn. 0.45 4.26 0.79 6.27 96.16

AHTZ Average similarity: 59.42, Deph range: 4707-6068 m AHTZ Average similarity: 65.32, Depth range: 4707-6068 m Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% Bassozetus spp. 4.41 29.82 1.75 50.17 50.17 Bassozetus spp. 55.06 53.21 3.85 81.46 81.46 C. yaquinae 1.08 15.14 1.24 25.47 75.64 C. yaquinae 4.88 7.5 0.78 11.47 92.93 Large Syn. 0.79 13.46 1.27 22.65 98.3 Shallow Hadal Average similarity: 28.93, Depth range: 5295-6191 m Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% Bassozetus spp. 0.50 28.93 NA 100 100

Deep Hadal Average similarity: 84.15, Depth range: 6750-7669 m Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% N. kermadecensis 72.25 84.15 6.8 100 100

South Fiji Basin MaxN %Ob Av. similarity: 61.18, Depth range: 4074:4101 m Average similarity: 57.34, Depth range: 4074:4101 m Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% Bassozetus spp. 4.16 31.37 8.28 51.27 51.27 Bassozetus spp. 36.72 29.18 2.79 50.88 50.88 B. iris 0.92 10.74 0.85 17.55 68.82 zoarcid spp. 10.18 16.85 0.88 29.38 80.27 zoarcid spp. 0.86 9.95 0.90 16.26 85.08 B. iris 5.66 7.84 0.88 13.66 93.93 Large Syn. 0.56 9.13 0.91 14.92 100.00

New Hebrides Trench MaxN %Ob Average similarity: 47.77, Depth range: 2087-6898 m Average similarity: 44.20, Depth range: 2578-6896 m Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% Bassozetus spp. 3.46 37.34 2.46 78.15 78.15 Bassozetus spp. 34.93 37.36 1.9 84.53 84.53 Large Syn. 0.52 8.22 0.63 17.2 95.36 zoarcid spp. 5.34 3.79 0.34 8.57 93.11

Mariana Trench MaxN %Ob Average similarity: 62.79, Depth range: 4506-6198 m AHTZ Average similarity: 62.68, Depth range: 4506-6189 m Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% Bassozetus spp. 2.50 44.58 7.41 71.00 71.00 Bassozetus spp. 30.25 40.55 1.97 64.70 64.70 C. yaquinae 0.77 18.21 1.15 29.00 100.00 C. yaquinae 12.46 21.82 1.39 34.82 99.51

Hadal Average similarity: 61.54, Depth range: 6831-8143 m Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% Mariana snailfish 41.73 60.88 1.95 98.93 98.93

843

All MaxN %Ob Shallow Bathyal Average similarity: 51.29, Depth range: 997-1527 Bathyal Average similarity: 31.58, Depth range: 997-2503 m Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% D. capensis 15.92 21.16 7.21 41.26 41.26 D. capensis 15.60 10.5 0.89 33.23 33.23 S. parasitica 24.11 20.73 3.75 40.41 81.67 S. parasitica 14.14 9.57 1.29 30.29 63.52 A. rostrata 0.64 1.98 0.58 3.85 85.53 A. rostrata 5.11 4.21 0.84 13.33 76.85 P. microporos 0.45 1.98 0.58 3.85 89.38 H. cf. affinis 1.30 3.16 0.75 10.00 86.85 Bathygadus sp. 0.45 1.98 0.58 3.85 93.23 C. cf. coelolepis 0.38 0.77 0.60 2.44 89.29 C. longifilis 0.34 0.49 0.22 1.55 90.85 Deep bathyal Average similarity: 47.29, Depth range: 1971-2087 Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% Abyssal Average similarity: 61.62, Depth range: 3039-4692 m C. rudis 1.00 9.46 NA 20.00 20.00 Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% C. longifilis 1.00 9.46 NA 20.00 40.00 C. armatus 58.06 40.35 4.55 65.49 65.49 cf. B. richardsoni 1.00 9.46 NA 20.00 60.00 C. yaquinae 14.75 12.96 1.28 21.03 86.52 H. cf. affinis 1.00 9.46 NA 20.00 80.00 S. grandis 9.99 5.85 0.63 9.49 96.01 A. rostrata 2.25 9.46 NA 20.00 100.00 AHTZ Average similarity: 52.65, Depth range: 2578-6898 m Abyssal Average similarity: 68.02, Depth range: 3039-4158 Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% Bassozetus spp. 39.56 43.17 2.43 82.00 82.00 C. armatus 12.25 30.97 6.71 45.53 45.53 C. yaquinae 3.31 5.54 0.50 10.52 92.53 S. grandis 5.29 19.16 8.81 28.17 73.69 C. yaquinae 2.92 11.02 1.33 16.20 89.89 Mariana snailfish Average similarity: 61.54 Depth range: 6831-8143 m Large Syn. 0.45 4.26 0.79 6.27 96.16 Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% Mariana snailfish 41.73 60.88 1.95 98.93 98.93 AHTZ Average similarity: 50.69, Depth range: 2578-6898 Species Av. MaxN Av.Sim Sim/SD Contrib% Cum.% N. kermadecensis Average similarity: 84.15, Depth range: 6750-7669 m Bassozetus spp. 3.84 36.12 2.45 71.27 71.27 Species Av. %Ob Av.Sim Sim/SD Contrib% Cum.% Large Syn. 0.46 7.94 0.71 15.67 86.93 N. kermadecensis 72.25 84.15 6.80 100.00 100.00 C. yaquinae 0.32 5.23 0.49 10.33 97.26 844

845 Table S4: The environmental predictors used for the DistLM analysis. Site denotes the study site: Kermadec Trench (Kerm), South Fiji Basin 846 (SFB), New Hebrides Trench (NHeb) and Mariana Trench (Mar). The equipment used (Equip): Abyssal-lander (AL) or Hadal-lander (HL). The 847 deployment depth (Depth), salinity (Sal), temperature (Temp) and current speed (Current speed) were recorded by on-board sensors. Quarter 848 of day is the 6 hr block from midnight which the lander arrived at the seabed. Bottom POC is an estimate of the average daily particulate 849 organic carbon (POC) reaching the seabed at the point of the deployment. Estimated average surface primary production if given for the 850 previous year (PPYr) and previous month (PPMo). Seabed type is sediment type category of the visible seabed: bedrock (1), cobbles and 851 pebbles (2), gravelly fine-grained sediment (3), muddy gravel (4), slightly gravelly fine-grained sediment (5), fine-grained sediment (6). The 852 Community Group is the name of the group which the similarity profile analysis (SIMPROF) ascribed the deployment (from the combined %Ob 853 analysis).

Current Bottom PPMo Depth Sal Temp Quarter PPYr Seabed Community Site Equip speed (cm POC (mg (mg m2 day- (m) (PSU) (0C) of day (mg m2 day-1) type Group s-1) m2 day-1) 1) Kerm AL 997 34.43 5.181 11.73 2 11.38 861.46 852.86 6 Bathyal Kerm AL 1473 34.49 3.248 7.00 4 7.57 823.30 878.50 6 Bathyal Kerm AL 1527 34.53 2.842 9.45 2 7.64 727.08 452.38 6 Bathyal Kerm HL 1554 2.951 2 7.88 732.73 459.74 6 Bathyal Kerm AL 1971 34.57 2.426 7.08 3 4.36 495.87 493.99 5 Bathyal Kerm HL 2503 2.000 2 3.95 488.59 459.82 5 Bathyal Kerm AL 3039 34.69 1.589 3.52 4 3.10 451.95 435.68 6 Abyssal Kerm HL 3534 1.291 2 2.68 441.76 420.64 6 Abyssal Kerm AL 3655 34.65 1.336 10.10 1 1.72 417.55 290.64 1 Abyssal Kerm AL 3940 34.70 1.067 5.69 4 2.47 440.55 415.58 6 Abyssal Kerm HL 3975 1.165 4 3.59 645.55 501.69 2 Abyssal Kerm AL 4061 34.73 1.206 9.87 4 3.62 647.22 503.12 4 Abyssal Kerm AL 4138 34.65 1.066 5.23 3 3.50 638.87 470.58 1 Abyssal Kerm AL 4158 34.68 1.118 6.78 4 2.98 616.10 435.41 6 Abyssal Kerm HL 4194 1.131 4 2.95 616.71 435.01 6 Abyssal Kerm HL 4332 1.063 3 3.48 636.61 467.09 3 Abyssal Kerm HL 4519 1.054 2 2.29 438.01 534.50 6 Abyssal

Kerm HL 4692 1.051 2 2.91 608.94 450.21 6 Abyssal Kerm AL 4707 34.64 1.038 4.90 4 2.89 608.91 450.43 6 AHTZ Kerm AL 4953 34.67 1.061 2.06 4 2.88 607.73 450.47 1 AHTZ Kerm AL 5046 34.72 1.063 6.21 4 2.86 607.21 450.60 5 AHTZ Kerm HL 5075 1.086 4 2.90 607.20 449.73 6 AHTZ Kerm AL 5092 34.71 1.066 5.84 3 2.13 449.73 437.94 6 AHTZ Kerm HL 5135 1.088 4 2.88 607.76 450.46 6 AHTZ Kerm AL 5254 34.65 1.077 11.36 2 1.70 416.07 286.16 AHTZ Kerm HL 5281 1.104 1 2.76 601.61 453.68 6 AHTZ Kerm AL 5295 34.67 1.081 6.37 4 2.76 602.02 454.43 1 AHTZ Kerm AL 5460 3 2.01 450.57 431.54 6 AHTZ Kerm AL 5646 34.71 1.119 11.32 4 1.73 417.62 287.86 4 AHTZ Kerm AL 5879 34.65 1.142 9.95 3 2.38 498.42 350.51 3 AHTZ Kerm AL 6037 34.64 1.165 4.64 3 1.73 414.49 284.52 3 AHTZ Kerm AL 6068 34.64 1.165 6.17 3 2.39 497.64 350.52 3 AHTZ Kerm HL 6191 1.194 4 2.40 495.23 352.03 6 AHTZ Kerm HL 6750 1.278 3 2.34 502.51 347.24 1 Hadal Kerm HL 7243 1.354 2 2.34 503.20 344.67 4 Hadal Kerm HL 7669 1.425 3 2.29 487.80 342.47 1 Hadal SFB AL 4074 34.65 1.844 5.86 3 1.76 320.62 334.44 5 AHTZ SFB AL 4078 34.64 1.891 14.41 4 1.84 336.02 351.77 3 AHTZ SFB AL 4100 3 1.81 323.73 341.69 5 AHTZ SFB HL 4101 1.859 3 1.77 319.68 334.58 3 AHTZ NHeb AL 2087 34.60 2.171 5.99 3 2.26 271.54 287.81 3 Bathyal NHeb AL 2578 34.60 1.926 11.08 3 1.97 269.40 285.22 3 AHTZ NHeb AL 3424 34.68 1.805 8.44 3 1.88 267.23 287.18 5 AHTZ NHeb AL 4123 34.65 1.844 4.46 3 1.73 266.94 267.47 6 AHTZ NHeb HL 4148 1.860 3 1.69 266.92 262.81 6 AHTZ NHeb AL 5192 34.65 1.971 9.75 4 1.55 262.38 255.67 3 AHTZ

NHeb AL 5344 34.63 1.991 2.04 3 1.54 262.04 257.39 5 AHTZ NHeb AL 6056 34.70 2.094 3.45 4 1.53 261.96 258.58 1 AHTZ NHeb AL 6162 34.67 2.111 1.81 4 1.49 258.80 261.97 5 AHTZ NHeb AL 6397 4 1.46 256.68 262.26 6 AHTZ NHeb AL 6898 4 1.51 256.70 258.83 6 AHTZ Mar AL 4506 34.64 1.468 10.67 4 0.58 122.76 97.78 1 AHTZ Mar HL 4703 1.493 4 0.58 123.09 97.86 4 AHTZ Mar AL 4902 34.62 1.498 3.23 4 0.65 124.50 99.64 4 AHTZ Mar HL 4998 1.468 3 0.59 119.98 114.46 1 AHTZ Mar HL 5044 1.478 3 0.58 120.13 114.04 4 AHTZ Mar HL 5641 1.554 3 0.59 123.84 96.42 3 AHTZ Mar AL 6008 34.69 1.592 3.21 4 0.56 121.96 104.33 6 AHTZ Mar HL 6010 1.600 4 0.57 122.06 103.84 3 AHTZ Mar HL 6089 1.609 4 0.54 120.48 96.04 1 AHTZ Mar HL 6097 1.612 4 0.53 120.52 96.03 5 AHTZ Mar AL 6130 34.65 1.610 7.64 4 0.53 120.56 96.04 3 AHTZ Mar HL 6142 1.618 3 0.56 121.98 103.68 3 AHTZ Mar AL 6198 34.69 1.620 6.25 4 0.53 120.69 95.99 3 AHTZ Mar HL 6831 1.725 4 0.58 122.17 95.14 6 Hadal Mar HL 6931 1.740 4 0.57 121.07 94.90 6 Hadal Mar HL 7012 1.748 4 0.57 120.10 101.61 3 Hadal Mar HL 7415 1.813 4 0.57 120.14 95.76 1 Hadal Mar HL 7440 1.817 4 0.57 120.13 95.68 1 Hadal Mar HL 7485 1.827 4 0.55 119.67 101.87 3 Hadal Mar HL 7716 1.865 4 0.53 121.63 96.24 4 Hadal Mar HL 7941 1.900 3 0.53 121.69 96.00 6 Hadal Mar HL 8004 1.915 4 0.54 119.95 104.61 5 Hadal Mar HL 8078 1.926 4 0.55 120.05 105.02 1 Hadal Mar HL 8143 1.937 4 0.53 121.96 95.74 1 Hadal

854