Megafauna of the UKSRL exploration contract area and eastern Clarion-Clipperton Zone in the Pacific Ocean: Annelida, Arthropoda, Bryozoa, Chordata, Ctenophora, Mollusca

Abstract Background There is growing interest in mining polymetallic nodules from the abyssal Clarion-Clipperton Zone (CCZ) in the tropical Pacific Ocean. Despite having been the focus of environmental studies for decades, the benthic megafauna of the CCZ remain poorly known. To predict and manage the environmental impacts of mining in the CCZ, baseline knowledge of the megafauna is essential. The ABYSSLINE Project has conducted benthic biological baseline surveys in the UK Seabed Resources Ltd polymetallic-nodule exploration contract area (UK-1). Prior to ABYSSLINE research cruises in 2013 and 2015, no biological studies had been done in this area of the eastern CCZ. New information Using a Remotely Operated Vehicle and Autonomous Underwater Vehicle (as well as several other pieces of equipment), the megafauna within the UK Seabed Resources Ltd exploration contract area (UK-1) and at a site ~250 km east of the UK-1 area were surveyed, allowing us to make the first estimates of megafaunal morphospecies richness from the imagery collected. Here, we present an atlas of the abyssal annelid, arthropod, bryozoan, chordate, ctenophore and molluscan megafauna observed and collected during the ABYSSLINE cruises to the UK-1 polymetallic-nodule exploration contract area in the CCZ. There appear to be at least 55 distinct morphospecies (8 Annelida, 12 Arthropoda, 4 Bryozoa, 22 Chordata, 5 Ctenophora, and 4 Mollusca) identified mostly by morphology but also using molecular barcoding for a limited number of animals that were collected. This atlas will aid the synthesis of megafaunal presence/absence data collected by contractors, scientists and other stakeholders undertaking work in the CCZ, ultimately helping to decipher the biogeography of the megafauna in this threatened habitat.


Introduction
The Clarion-Clipperton Zone (CCZ) is an abyssal region of the tropical eastern Pacific Ocean where deep-sea mining may take place in the near future (Ramirez-Llodra et al. 2011, Wedding et al. 2013; Fig. 1). High-grade polymetallic nodules, which could provide a commercial source of copper, cobalt, nickel, and manganese (among other metals), are abundant in this six million km region that lies in Areas Beyond National Jurisdiction (ABNJ), and thus falls under the legal mandate of the International Seabed Authority (ISA) (Wedding et al. 2013). Thus far, 16 exploration leases (each up to 75,000 km in area) have been granted by the ISA in the CCZ, with those for exploitation expected to soon follow (https://www.isa.org.jm/).
The ABYSSLINE (ABYSSal BaseLINE) Project was designed to undertake benthic biological baseline studies in accordance with ISA environmental guidelines within the UK Seabed Resources Ltd (UKSRL) exploration contract area (UK-1) ). The UK-1 exploration contract area is one of the easternmost contract areas in the CCZ and encompasses ~58,000 km of seafloor (Fig. 1). The ABYSSLINE Project was led by scientists from the University of Hawai'i at Mānoa (USA), and included scientists from Hawai'i Pacific University (USA), the Natural History Museum, London (UK), the National 2 2 2 Oceanography Centre, Southampton (UK), Senckenberg Gesellschaft für Naturforschung (Germany), Uni Research (Norway), and the International Research Institute of Stavanger (Norway). The ABYSSLINE Project aimed to evaluate baseline conditions of community structure and biodiversity for megafauna, macrofauna, meiofauna and microbes within the UK-1 contract area and across the CCZ , Shulse et al. 2016. No faunal studies had been undertaken in the UK-1 contract area prior to licensing by the ISA in 2013 and the commencement of the ABYSSLINE Project. Locations of megafaunal surveys during the ABYSSLINE cruises, AB01 and AB02, in the Clarion-Clipperton Zone. (a) The location of the UK Seabed Resources Ltd exploration contract area (UK-1) in the eastern Pacific Ocean. (b) The locations of the 30x30-km survey areas, UK-1 Stratum A and UK-1 Stratum B, in relation to the UK-1 exploration contract area and the AB01 ROV dive site, EPIRB, which was approximately 250 km east of the UK-1 contract area. (c) The locations of ROV dives within UK-1 Stratum A, indicated by purple tracklines labelled with the dive number (e.g. RV01). Stations where imagery was collected with a baited camera (CA01) and samples collected with a baited trap (TR04, TR05, TR06) are also indicated. (d) The locations of AUV dives within UK-1 Stratum B, indicated by purple tracklines labelled with the dive number (e.g. AV01). Stations where imagery was collected with a baited camera (CA02, CA06, CA07, CA08, CA09) and samples collected with a baited trap (TR08) are also indicated. All maps were created by Seafloor Investigations Ltd for the ABYSSLINE Project using ArcGIS software (https://www.arcgis.com/features/). It is expected that nodule mining will drastically alter this unique deep-sea habitat with recovery expected to be slow , Jones et al. 2017, Oebius et al. 2001, Ramirez-Llodra et al. 2011, Vanreusel et al. 2016) and yet, despite increases in technology and the number of expeditions to the area, very little is known about the ecology and biogeography of the fauna inhabiting the region , Bluhm and Gebruk 1999, Foell and Pawson 1986, Jones et al. 2017, Martinez-Arbizu et al. 2013, Pawson 1983, Pawson and Foell 1986, Roux 2004, Roux and Pawson 1999, Shulse et al. 2016, Tilot 2006, Vanreusel et al. 2016, Vecchione 2016, Wang and Lu 2002. The megafauna constitute an important component of the biodiversity in the abyssal deep sea and play a significant role in deep-sea ecosystem function , Smith et al. 2008, Vanreusel et al. 2016.  could not locate a single megafauna record for the UK-1 exploration contract area in OBIS or elsewhere. This is likely the result of a lack of sampling, complete taxonomic identification of specimens and/or ensuring data are publicly available, especially as an abundant and diverse megafauna is already known from the CCZ from photographic and video surveys , Foell and Pawson 1986, Martinez-Arbizu et al. 2013, Pawson 1983, Pawson and Foell 1986, Tilot 2006, Vanreusel et al. 2016, Vecchione 2016, Wang and Lu 2002. To predict and manage the environmental impacts of mining in the CCZ and within the UK-1 exploration contract area, baseline knowledge of the megafauna is essential; in addition, it will allow for a future complete taxonomic and biogeographic synthesis of the fauna of the CCZ (Wedding et al. 2015).
Here, we present the second section (Annelida, Arthropoda, Bryozoa, Chordata, Ctenophora, and Mollusca) of an anticipated four-section image atlas of benthic megafauna that inhabit the UK-1 exploration contract area based on ROV and AUV surveys and samples collected during two cruises of the ABYSSLINE project. The first in this series (Echinodermata) has already been published ). These two sections will be supported by Cnidaria and Porifera sections in the near future. This atlas was crucial during the ABYSSLINE quantitative megafaunal analyses ) and we hope that it will help standardize putative morphospecies and be useful to other scientists and stakeholders undertaking research in the CCZ in the future.

Materials and methods
The UKSRL exploration contract area (UK-1) is located in the eastern CCZ in the Pacific Ocean (Fig. 1). There were two ABYSSLINE research cruises to the UK-1 exploration contract area: the AB01 or MV1313 cruise on the R/V Melville from 3 to 27 October 2013, and the AB02 or TN319 cruise on the R/V Thompson from 12 February to 25 March 2015. The AB01 cruise focused on a 30x30-km stratum (UK-1 Stratum A) centered at 13°49′ N, 116°36′ W in the northern portion of the UK-1 contract area (Fig. 1). During the AB01 cruise, multibeam bathymetric surveys indicated an abyssal seafloor characterized by ridges and valleys running from NNW to SSE at 3900-4400 m. The commercial Remotely Operated Vehicle (ROV) Remora III, operated by Phoenix International Holdings, performed video surveys and sample collections at four randomly-located sites within UK-1 Stratum A in the UK-1 contract area. Additionally, surveys were done ~250 km to the east of the UK-1 contract area, at a site called "EPIRB" centered at 13°40′ N, 114°24′ W ( Fig.  1). Work at the EPIRB site was dictated by an emergency response to an Emergency Position Indicating Radio Beacon (EPIRB) distress signal and, although unplanned, provided a useful broader context for our study.
The ROV was equipped with two manipulators, four ROS QLEDIII lights, one 1Cam Alpha Component high-definition downward-looking "science" video camera (1080p video and 24.1 megapixel stills) and one standard-definition forward-looking "pilot" video camera. During surveys, the vehicle had substantial difficulty maintaining constant altitude, direction and velocity over the seabed, thereby limiting both the usable imagery and specimens collected.
The AB02 cruise focused on a 30x30-km stratum (UK-1 Stratum B) centered at 12°28' N, 116°36' W in the central portion of the UK-1 exploration contract area (Fig. 1). During the AB02 cruise, multibeam bathymetric surveys indicated an abyssal seafloor dominated by numerous high-relief volcanic seamounts between 3500-4300 m (Fig. 1). The Autonomous Underwater Vehicle (AUV) REMUS 6000, operated by Woods Hole Oceanographic Institution, performed image surveys at five randomly-located sites within UK-1 Stratum B (Fig. 1). The AUV was equipped with four ROS QLEDIII lights, and one Prosilica GT3400 high-definition downward-looking still camera (9 megapixel stills).
During both AB01 and AB02, two baited camera systems were also used to collect video of scavenging arthropods and fishes at random locations in both the UK-1 Stratum A and UK-1 Stratum B ).

Sample collection
The ROV was the primary tool used to collect specimens on the AB01 cruise, however due to significant difficulties, few megafauna were successfully sampled . Additionally, scavenging arthropods and fishes were captured in a baited trap during both AB01 and AB02 . Once the respective sampling equipment was on deck, megafauna were quickly transferred to containers of chilled seawater, photographed, and a tissue subsample taken for DNA analyses. DNA samples were preserved in 80% ethanol and the remainder of the animal was preserved in buffered 4% formalin-seawater solution or 95% ethanol, depending on the taxon. On board, all collected specimens were also imaged, with the resulting images included in this manuscript. After the cruise, morphological samples were sent to taxonomic experts for identification and all specimens sequenced for a range of DNA markers at the Natural History Museum, London, with tissue samples subsequently archived and made openly available for future taxonomic work . All collected specimens were used for taxonomic identifications including ground-truthing identifications based on images.

Megafaunal image surveys and analyes
All imagery from both "pilot" and "science" cameras on the ROV (covering roughly 8,000 m ) collected during AB01 was used during the creation of this atlas . All imagery from the AUV (27,178 images covering roughly 500,000 m ) collected during AB02 was also used, although the majority of these images (>20,000 images) were at too high an altitude (>6 m) for megafauna to be resolved and identified. All video from both cameras on the ROV, as well as from the AUV, were viewed multiple times and frames archived of each identifiable megafaunal morphotype or morphospecies.
The criteria used for selection of megafaunal morphospecies during AB01 was that individuals were >2 cm in maximum dimension and that there was sufficient detail to identify them to a putative "species-level" morphotype . The AUV imagery collected during AB02 was lower resolution forcing the criteria to be modified to megafauna >5 cm only being included. Morphospecies that could not be identified to species but appeared morphologically distinct were assigned a unique informal species name (e.g. Polychaeta morphospecies 1). These were identified by taxonomic experts or by using the "Atlas of Abyssal Megafauna Morphotypes of the Clarion-Clipperton Fracture Zone" created for the ISA (http://ccfzatlas.com/) (Foell and Pawson 1986, Martinez-Arbizu et al. 2013, Tilot 2006. Morphospecies from this study that matched morphotypes listed in the "Atlas of Abyssal Megafauna Morphotypes of the Clarion-Clipperton Fracture Zone (Martinez-Arbizu et al. 2013)" have had a section titled 'Nomenclature' added to their data, which includes their identification from the online atlas. This is in an effort to provide coherence between these CCZ atlases. For morphospecies that were morphologically similar to a well-defined species name, we use the open nomenclature expression "cf.", although a precautionary approach was taken.
Notes: Seen on seafloor. No additional distinguishing features to place it beyond class Polychaeta.  cf. Polychaeta morphospecies in situ on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Image attribution: DJ    cf. Polynoidae morphospecies in situ swimming above the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Image attribution: Woods Hole Oceanographic Institution.  cf. Sabellidae morphospecies 1 observed in the UK-1 exploration contract area and eastern CCZ. Images (a-b) correspond with the relevant data above.  cf. Sabellidae morphospecies 2 attached to a polymetallic nodule on the seafloor in the eastern CCZ. Image corresponds with the data above. Scale bar is 10 cm.  cf. Serpulidae morphospecies 1 attached to a polymetallic nodule on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm.  cf. Serpulidae morphospecies 2 attached to a polymetallic nodule on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above.  cf. Acrocirridae morphospecies in situ swimming above the seafloor in the eastern CCZ. Image corresponds with the data above. Scale bar is 10 cm.   cf. Eurythenes morphospecies 1 in situ feeding on bait on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Image attribution: A Leitner and J Drazen, University of Hawai'i.  Eurythenes magellanicus after collection via baited trap from the UK-1 exploration contract area. The carapace length of this specimen was 10 cm. Images (a-c) correspond with the data in (a) above.   A pair (likely male and female) of cf. Podoceridae morphospecies in situ on a sponge stalk in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Image attribution: DJ Amon & CR Smith, University of Hawai'i.

Notes:
The very large dorsal spines on the posterior part of the body are diagnostic. Often found on stalks in pairs.  Hemipenaeus cf. spinidorsalis observed in the UK-1 exploration contract area. Images (a-e) correspond with the relevant data above.   Cerataspis cf. monstrosus in situ feeding on bait on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Image attribution: A Leitner and J Drazen, University of Hawai'i.

Family Glyphocrangonidae Smith, 1884
Genus   cf. Glyphocrangon morphospecies in situ on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution.  cf. Munidopsis morphospecies with several Pachycara cf. nazca in situ on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Image attribution: A Leitner and J Drazen, University of Hawai'i.

Notes:
Pale color, no pigment in eyes, short but strong pincers, three pair of walking legs (pereopods) on each side (not four, as would be seen in a true crab, Brachyura). Mouthparts do not cover oral field (as they usually do in true crabs). To identify the species, one would need a dorsal view.  cf. Bathystylodactylus morphospecies in situ on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution.  cf. Munnopsidae morphospecies observed in the UK-1 exploration contract area. Images (a-b) correspond with the data in (a) above, whereas image (c) corresponds with the data in (b) above.  cf. Smithsonius morphospecies observed in the UK-1 exploration contract area and eastern CCZ. Images (a-d) correspond with the relevant data above.        cf. Phlebobranchia morphospecies 1 attached to a polymetallic nodule on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Notes: Attached to polymetallic nodules by a short, thick stalk of transparent tunic.

Chordates of the UKSRL exploration contract area (UK-1) and the eastern Clarion-Clipperton Zone
Expansive incurrent siphon appears as a smiling mouth on the side of the animal. Large incurrent (atrial) siphon on top appears as a transparent region when open. Nearly circular in shape when viewed from above.   cf. Actinopterygii morphospecies observed swimming above the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution.  Figure 32.
Histiobranchus cf. bathybius observed in the UK-1 exploration contract area. Image (a) corresponds with the data in (a) above, whereas images (b-d) correspond with the data in (b) above.      cf. Coryphaenoides morphospecies observed in the UK-1 exploration contract area. There may be two species, Coryphaenoides armatus and Coryphaenoides yaquinae, in the in situ images, however specimens are required for identification. Images (a-b) correspond with the data in (a) above, whereas image (c-f) correspond with the data in (b-e) above. Notes: Includes both species C. armatus and C. yaquinae. Both species have been recovered from the CCZ but are not readily distinguishable in photographs. Prominent snout and small chin barbel. First dorsal fin prominent and separated from the second dorsal fin which tapers to tip of whip tail. First pelvic fin ray elongated. Body color ranges from white to dark gray often with darker areas around fin insertions and eyes. In the "Atlas of Abyssal Megafauna Morphotypes of the Clarion-Clipperton Fracture Zone" created for the ISA (http://ccfzatlas.com/), this morphospecies is listed as "Halosaur morphotype".  cf. Halosauridae morphospecies observed in the UK-1 exploration contract area. Images (a-b) correspond with the relevant data above.

Materials
a: cf. Halosauridae morphospecies swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. b: cf. Halosauridae morphospecies swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution.
Distinguishable by swimming pattern: majority of slender body nearly completely straight with propulsion driven by flexion of terminal ¼ to ½ of body.

Order Ophidiiformes
Family Ophidiidae Rafinesque, 1810 cf. Ophidiidae morphospecies 1 Nomenclature: In the "Atlas of Abyssal Megafauna Morphotypes of the Clarion-Clipperton Fracture Zone" created for the ISA (http://ccfzatlas.com/), this morphospecies is listed as "Ophidiidae morphotype 1".  cf. Ophidiidae morphospecies 2 in situ on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above.  cf. Ophidiidae morphospecies 5 observed in the UK-1 exploration contract area. Images (a-b) correspond with the relevant data above.

Materials
a: cf. Ophidiidae morphospecies 5 swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. b: cf. Ophidiidae morphospecies 5 swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution.

Materials
Notes: Large round, bulbous head with a tadpole-like body morphology. Reduced eyes. A variety of colormorphs exist ranging from beige to dark brown. May have lighter spots. Single dorsal contiguous with anal fin. Pelvic fins reduced to one ray each and below head. Distinguished from other ophidiids by the large rounded head. Bassozetus cf. nasus observed in the UK-1 exploration contract area. Images (a-c) correspond with the data in (a) above, whereas images (d-e) correspond with the data in (b-c) respectively.  Figure 44.
Bathyonus cf. caudalis observed in the UK-1 exploration contract area and eastern CCZ. Images (a-b) correspond with the data in (a) above, whereas image (c) corresponds with the data in (b) above.  Pachycara cf. nazca observed in the UK-1 exploration contract area and eastern CCZ. Image (a) corresponds with the data in (a) above, whereas images (b-c) correspond with the data in (b), and images (d-f) correspond with the data in (c-e) above.   cf. Mertensiidae morphospecies observed swimming above the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution.

Order Lobata Eschscholtz, 1825
cf. Lobata morphospecies 1 Nomenclature: In the "Atlas of Abyssal Megafauna Morphotypes of the Clarion-Clipperton Fracture Zone" created for the ISA (http://ccfzatlas.com/), this morphospecies is listed as "Unknown Ctenophora morphotype 1".   cf. Lobata morphospecies 1 observed swimming above the seafloor in the UK-1 exploration contract area. Image corresponds with the data above.   cf. Lobata morphospecies 2 observed swimming above the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Image attribution: Woods Hole Oceanographic Institution.  cf. Lobata morphospecies 3 observed in the UK-1 exploration contract area. Images (a-d) correspond with the relevant data above.

Material
a: cf. Lobata morphospecies 3 swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. b: cf. Lobata morphospecies 3 swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. c: cf. Lobata morphospecies 3 swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. d: cf. Lobata morphospecies 3 swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution.
Notes: Dark red pigmented body flattened in tentacular plane. With no long filamentous tentacles. Long comb rows.  Lampocteis cf. cruentiventer observed swimming above the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Image attribution: Woods Hole Oceanographic Institution. Notes: Infaunal, two-valves that meet at line; view in Fig. 53 a, c virtually into expanded siphon which is extended; Fig. 53b shows closed valves. Valve length enigmatically large (ca. 3-4 cm) for these depths, essentially 50% larger than expected.
Notes: Octopod with large paired fins emerging from lateral mantle, secondary web between the arms and primary web, and two rows of long cirri lateral to single row of arm suckers. Head about the same width as mantle. Mantle notably elongate, vague translucence. Distal arms often coiled aborally, exposing the secondary web. cf. Cirroteuthidae morphospecies observed in the UK-1 exploration contract area. Images (a-d) correspond with the relevant data above.
a: cf. Cirroteuthidae morphospecies swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. b: cf. Cirroteuthidae morphospecies swimming above the seafloor. Image attribution: Woods Hole Oceanographic Institution. c: cf. Cirroteuthidae morphospecies swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. d: cf. Cirroteuthidae morphospecies swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. cf. Grimpoteuthis morphospecies observed in the UK-1 exploration contract area. Images correspond with the data above.
a: cf. Grimpoteuthis morphospecies swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. b: cf. Grimpoteuthis morphospecies swimming above the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. c: cf. Grimpoteuthis morphospecies in situ on the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution. d: cf. Grimpoteuthis morphospecies in situ on the seafloor. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution.
Notes: Octopod with moderate-sized paired fins emerging from lateral mantle, simple interbrachial web, often two-toned with light mantle, darker arms/web, and two rows of cirri lateral to arm suckers. Arms often held nearly straight. Often seen near or on the seafloor. Notes: Finless, benthic octopod with two rows of arm suckers, head narrower than mantle, arms coiled aborally when still, extended into sediment when walking. Swimming is propelled by mantle jets, not by fins. Ventrum often darker than dorsum, but not necessarily.

Discussion
Although many of the morphospecies included here remain taxonomically ambiguous, we provide the first image atlas of annelid, arthropod, bryozoan, chordate, ctenophore and mollusc morphospecies inhabiting the UK-1 exploration contract area and the eastern CCZ. At least 55 distinct morphospecies (8 Annelida, 12 Arthropoda, 4 Bryozoa, 22 Chordata, 5 Ctenophora, and 4 Mollusca) were observed, although this is likely an underestimate given the poor image quality and presence of cryptic species . When the echinoderm megafauna are included, at least 117 morphospecies from seven phyla are documented . This is the second-highest total species richness recorded in the CCZ region for these seven phyla. This is remarkable given that this total is from one exploration contract area (UK-1) and a single dive site east of the contract area, and currently excludes two of the most speciose phyla, Cnidaria and Porifera, which will be treated in separate atlases in the future , Foell and Pawson 1986, Tilot 2006, Wang and Lu 2002. However, we recognise that the comparison of species richness for these seven phyla aross the region is only valid if sampling effort was similar or standardised ).
The numbers of annelid morphospecies in this study are similar to those in the only other CCZ megafauna study that includes annelids (Tilot 2006), although it is not clear whether there is overlap between these morphospecies. However, the number of arthropod morphospecies in Tilot (2006) (> 20) is considerably more than observed during this study and others in the CCZ , Foell and Pawson 1986, Wang and Lu 2002. Interestingly, the observations of Hymenopenaeus nereus in the cf. Muusoctopus morphospecies observed on the seafloor in the UK-1 exploration contract area. Image corresponds with the data above. Scale bar is 10 cm. Image attribution: Woods Hole Oceanographic Institution.
UKSRL exploration area and at the EPIRB site represent a range extension for this species (Hendrickx and Wicksten 2016). Similarly, Eurythenes magellanicus, collected from the UKSRL exploration area and confirmed by molecular and morphological analyses, has been previously recorded only from the Drake Passage and Brazil Basin (d'Acoz and Havermens 2015), representing a significant range extension for this species. Tilot (2006) recorded nearly 30 chordate morphospecies, the highest diversity in the CCZ thus far, though comparable to that recorded here.  observed eight baitattending fish morphospecies from the first scavenging experiments in the CCZ. The quantitative study by  which utilised many of the images from AB01 included in this study reported six fish morphospecies from UK-1 Stratum A and EPIRB only, which included both bait-attending and non-bait-attending. The total number of fish morphospecies from  and     and  were the first to report and confirm the presence of these species in the UKSRL exploration contract area. It should be noted that distinguishing between C. armatus and C. yaquinae in imagery is very difficult and so morphological and molecular analyses of specimens should be used to confirm identification. The presence of Pachycara cf. nazca in the UKSRL exploration contract area also respresents a range extension for this species as it is previously known from the abyssal southeast Pacific Ocean off Peru (Anderson and Bluhm 1996).
The rarer megafaunal phyla in the CCZ appear to be the Bryozoa, Ctenophora and Mollusca but this study has recorded the highest diversities thus far for each of these phyla (four, five and four morphospecies respectively). Only one other megafaunal CCZ study has included bryozoans, with one species recorded (Foell and Pawson 1986). Previous studies identified between one and two ctenophore morphospecies in the CCZ (Martinez-Arbizu et al. 2013, Tilot 2006, whereas as many as five mollusc morphospecies have been observed. Between two and four cephalopod morphospecies (Foell and Pawson 1986, Martinez-Arbizu et al. 2013, Tilot 2006, Vecchione 2016, Wang and Lu 2002 have been recorded from elsewhere in the CCZ, however this may underestimate the cephalopod biodiversity as these highly mobile megafauna are able to evade ROVs and other imaging platforms. Megafaunal bivalves have only been mentioned briefly previously, perhaps because, like bryozoans, they are not observed easily in deep-sea imagery due to their small size , Tilot 2006, Wang and Lu 2002. These morphospecies represent a range of functional traits: the serpulid and sabellid polychaetes and bryozoans are sessile suspension feeders, reliant on the polymetallic nodules as hard substrate, whereas most of the arthropods, chordates, cephalopods and ctenophores, are predatory , Martinez-Arbizu et al. 2013). This differs from the echinoderms recorded in the UKSRL area and at the EPIRB site as most of those morphospecies were deposit feeders of suspension feeders . Approximately one third of the morphospecies in this atlas have been observed in other contract areas in the CCZ , Foell and Pawson 1986, Martinez-Arbizu et al. 2013, Tilot 2006, Vecchione 2016, Wang and Lu 2002, although this may be an overestimate given the presence of cryptic species and the problems identifying megafauna from imagery, as has been experienced during studies in other poorly-explored areas , Bickford et al. 2007, Linse et al. 2007, Vrijenhoek 2009). Information similar to that presented here will likely be crucial to informing the future environmental management of the region.
While this image atlas, as well as the echinoderm atlas , has expanded the knowledge of benthic fauna in the UK-1 exploration contract area and overall CCZ, there is still a need for further high-quality imagery of fauna, and especially physical megafaunal specimens to groundtruth the imaged morphospecies via detailed morphological and molecular analyses. We expect that a number of the morphospecies included in this atlas may be new to science, new records, or poorly known, but this can only be confirmed when specimens are collected and analysed. Molecular analyses are especially important given the presence of cryptic species. The limited collection of voucher specimens in the CCZ continues to severely hamper reliable estimation of species richness and species distributions. Although the taxonomic identification of preserved material is always necessary, we hope that this atlas will aid scientists by showing what these morphospecies look like in situ in their natural surroundings, as well as by providing some ecological information (e.g. feeding modes, preferred habitat etc.). This information will be important in estimating the human impact on this ecosystem. Furthermore, the appearance of morphospecies captured in situ in images can drastically differ from that of collected or preserved material, especially when relatively rudimentary collection equipment (trawls, dredges etc.) are used. As mentioned in , those working in the CCZ must share detailed descriptions of their equipment and methods to facilitate data standardization and statistically-rigorous regional comparisons. It is also important that the ISA-sponsored online atlas continues to be updated with new imagery (such as the images in this atlas), and that the morphospecies are properly identified with the help of taxonomists.