megafaunal composition of cold-water corals and other deep ...€¦ · galaxea, journal of coral...

Galaxea, Journal of Coral Reef Studies 19: 19-30（2017）

Abstract Recent developments in deep-sea surveys have revealed the widespread distribution of cold-water corals over the deep-sea floor of the world ocean. There are no reports, however, concerning the taxonomic composition of cold-water corals and other benthic megafauna in the southern Emperor Seamounts area of the North Pacific Ocean. We analyzed benthic samples collected from a research vessel during scientific surveys and by scientific observers onboard commercial fishing vessels to examine the faunal composition of cold-water corals and other megabenthos in the southern Emperor Seamounts area. Seventy-eight genera of cold-water corals were identified. Gorgonians (Alcyonacea with solid axis) occurred at high frequencies with wide vertical distribution ranges, and appeared to be the major components of habitat-forming cold-water corals in the area. Scleractinia occurred at frequencies similar to those of gorgonians, but over lim-ited depth ranges. Among other benthic megafauna, Crus-tacea and Echinodermata occurred at high frequencies.

The results demonstrates that the regional characteristics of deep-sea benthic megafauna in the southern Emperor Seamounts area is more similar to that near the Hawaiian Islands than those reported from Aleutian, other Alaskan, Californian and Japanese waters.

Keywords benthic megafauna, cold-water corals, Emperor Seamounts, habitat-forming species, vulnerable marine ecosystems

Introduction

Recent developments in deep-sea surveys that use acoustic and submersible devices have revealed the widespread distribution of cold-water corals on the deep-sea floor throughout the world ocean (Heifetz et al. 2005; Lundsten et al. 2009; McClain et al. 2010; Mol et al. 2002; Reed et al. 2006; Roberts et al. 2006). Reports from

Megafaunal composition of cold-water corals and other deep-sea benthos in the southern Emperor

Seamounts area, North Pacific Ocean

Mai MIYAMOTO1*, Masashi KIYOTA1, Takeshi HAYASHIBARA2, Masanori NONAKA3, Yukimitsu IMAHARA4, and Hiroyuki TACHIKAWA5

1 Oceanic Ecosystem Group, National Research Institute of Far Seas Fisheries, Japan Fisheries Research and Education Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa, 236-8648 Japan

2 Coastal Resource and Ecosystem Group, Seikai National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 148 Fukaiota, Ishigaki, Okinawa, 907-0451 Japan

3 Okinawa Churaumi Aquarium, Okinawa Churashima Foundation, 424 Ishikawa Motobu-Cho, Okinawa 905-0206, Japan4 Wakayama Laboratory, Biological Institute on Kuroshio, 300-11 Kire, Wakayama 640-0351 Japan5 Coastal Branch of Natural History Museum and Institute, Chiba, 123 Yoshio, Katsuura, Chiba 299-5242, Japan

* Corresponding author: Mai MiyamotoE-mail: [email protected]

Communicated by Michio Hidaka (Biology Editor)

Original paper

Miyamoto et al.: Cold-water coral fauna in the Emperor Seamounts20

Alaska and Florida in the USA, the northern Atlantic Ocean, and many other areas have shown that some species of cold-water corals build complex structures and produce unique demersal communities in the same way as do shallow-water coral reefs (Heifetz 2002; Heifetz et al. 2005; Lundsten et al. 2009; McClain et al. 2010; Mol et al. 2002; Morris et al. 2013; Reed et al. 2006; Roberts et al. 2006; Stone et al. 2015). Such frameworks of cold-water corals provide habitats or spawning and nursery grounds for fish and many other animals and thereby support diverse ecosystems (Baillon et al. 2012; Roberts et al. 2006). Because of their slow growth, long lifespans, and slow recovery from physical damage, cold-water corals attract attention as important components of vul-nerable marine ecosystems (VMEs) in the context of marine biodiversity conservation and management (Roberts et al. 2009).

The Emperor Seamount chain is located from 30°N to 55°N and from 168°E to 178°E, extending from the Northwestern Hawaiian Islands to the Aleutian Trench in the western North Pacific Ocean (Fig. 1). In 1965, precious corals were discovered in this area. At the peak in 1981, more than 100 coral fishing boats from Taiwan and Japan are believed to have harvested approximately 300 metric tons of precious corals from this area; however, many things about this coral fishery remain unclear, including

the target species and the area of operations (Grigg 1993). Since 1969, the southern Emperor Seamounts south of 45°N have been used as fishing grounds of commercial fisheries targeting bottom fish such as North Pacific armorhead (Pentaceros wheeleri) and splendid alfonsino (Beryx splendens; Kiyota et al. 2016). To fulfill the global requirement for ecosystem-based fishery management, it is urgent that relevant management bodies assess and manage bottom fisheries impacts on VMEs considering the regional characteristics of benthic fauna and the history of the fisheries (FAO 2009). For the southern Emperor Seamounts area, however, there is no basic in-formation on benthic fauna, such as the taxonomic compo-sition of cold-water corals and other benthos.

In this study, we analyzed samples of megabenthos collected during scientific surveys and as part of a sci-entific observer program for commercial fisheries to ex-amine the composition of cold-water corals and other benthic megafauna in the southern Emperor Seamounts area. We identified cold-water corals to the lowest taxon possible and analyzed their bathymetric distribution. This study provides the first description of the faunal com po-sition of cold-water corals and other megabenthos in the southern Emperor Seamounts area for which there has been little information.

Materials and Methods

Study area and benthic samplingWe analyzed samples of megabenthos that were col-

lected from a research vessel during scientific surveys, and by participants in the scientific observer program covering the commercial bottom fisheries in the southern Emperor Seamounts area. Scientific observers have been onboard all Japanese commercial bottom trawl and gillnet fishing vessels (at most six bottom trawlers and one bottom gillnet vessel) operating in the area since June 2009. The fishing operations were conducted mainly on flat tops by trawlers and at upper slopes by the gillnet vessel. The scientific observer samples analyzed in this study were collected from the fishing operations at the following seamounts during the fishing season (January to October), 2009-2014: south bank of Suiko (44.5-45°N,

Fig. 1　Map of the Emperor Seamount chain. Benthos samples were collected from the south bank of Suiko, Yomei, Nintoku, Northern Koko, Koko, Kinmei, Yuryaku, Kammu, Colahan and C-H seamounts.

Miyamoto et al.: Cold-water coral fauna in the Emperor Seamounts 21

170°E, 969-1190 m deep); Yomei (42-42.5°N, 170.5°E, 1038-1220 m deep), Nintoku (41°N, 170.5°E, 979-1050 m deep); Northern Koko (36.5-37°N, 171.5°E, 840-1250 m deep); Koko (34.5-35°N, 171-172°E, 275-1070 m deep); Kinmei (33.5-34°N, 171-172°E, 575-1340 m deep), Yuryaku (32.5°N, 172°E, 391-1225 m deep), Kammu (32°N, 172-173°E, 349-1300 m deep); and Colahan (31°N, 175-176°E, 8221365 m deep). Megabenthos sam ples were collected from 116 trawl hauls (275-651 m deep) and 111 gillnet sets (378-1365 m deep; Table 1). The sam pling depths were determined by depth at the end of trawl hauls recorded by the net monitoring system (SCANMAR, Simrad Co. Ltd., Vancouver, Canada) or by the seafloor depths at the beginning of gillnet retrievals measured by the echo sounder (JFW-820, Japan Radio Co. Ltd., Tokyo, Japan). The sampled benthos specimens were cryo-pre-served onboard and were brought back to the laboratory for later examination.

The scientific surveys were conducted from 2009 to 2014 by using R/V Kaiyo-maru (93 m, 2942 GT) of the Fisheries Agency of Japan. During these scientific sur-veys, megabenthos were sampled from 170 hauls using a sea-urchin beam trawl (mouth widgh 1.5 m wide, inner net mesh size 5×5 mm) and a large (mouth width 1.0 m, mesh 5×5 mm) or a small (mouth width 0.5 m, mesh 5 ×5 mm) dredge. During sampling, vessel speed was set at approximately 1 knot (max 1.5 knots); the towing duration was set at 5-10 min for the sea-urchin beam trawl or 2-10 min for the dredges after first contact of the sampling gear with the sea floor. The beam trawl was deployed 116

times mainly on the bedrock or sandy flat tops of sea-mounts, and large and small dredges were deployed 54 times as supplements on sloped and rough bottomed areas (Table 1). The seamounts surveyed were Northern Koko (550-1115 m), Koko (229-1299 m), Yuryaku (460-882 m), Kammu (283-1317 m), Colahan (256-981 m) and C-H (30.5°N, 177.5-178°E, 336-1121 m). The sampling depths were determined by a depth-temperature sensor (COMPACT-TD for deep-sea, JFE Advantech Co, Ltd, Hyogo, Japan) attached to the beam trawl or by the sea floor depths at the end of dredge tows observed by the ship-mounted precision depth recorder (Deep-sea preci-sion depth recorder, NEC Corporation, Tokyo, Japan). The benthos samples were examined preliminary onboard and preserved in 70% ethanol for more detailed later examination on shore.

Sample analysisCold-water coral specimens were identified to the

lowest possible taxon such as species or genus based on external morphology and by microscopic observation of sclerites for Alcyonacea. As surveys of deep-sea animals such as cold-water corals are time-consuming and expen-sive and there are not many experts in classification of cold-water corals, information for some taxa is limited. In the North Pacific, information is relatively abundant for the Alcyonacea (especially the Scleraxonia, Holaxonia and Calcaxonia) and the Scleractinia; in contrast, infor-mation is very limited for the Antipatharia. Because this study aimed to clarify the regional characteristics of the

Table 1　Number of sampling hauls by each gear type and number of taxa collected per seamount.


benthic megafauna in the southern Emperor Seamounts area, cold-water corals were identified to the genus level for most samples, and detailed taxonomic examination such as more complicated microscopic observations of alcyonacean sclerites was deferred as a future task. How-ever, specimens of the Coralliidae were subject to detailed taxonomic examination because of their importance in the past targeted fishery (Grigg 1993). The Scleraxonia, Holaxonia and Calcaxonia, which constituted the order Gorgonacea in the previous classification system, were treated separately from the rest of Alcyonacea and grouped as “gorgonians” in this study, because gorgonians (Al-cyonacea with solid axis) and other Alcyonacea (soft corals) have different growth-forms and are treated as separate VME indicator taxa by many fishery management bodies. Hereafter, we designate Scleraxonia, Holaxonia and Calcaxonia as “gorgonians” and Alcyonacea exclud-ing these three suborders as “Alcyonacea (excluding gor-gonians)”.

The bathymetric range and occurrence frequency of gorgonians, Antipatharia and Scleractinia, which were found in relatively large numbers in the samples, were analyzed in 50 m depth intervals. In the bathymetric range analysis, colonial and habitat-forming Scleractinia were analyzed by species, but other cold-water corals were aggregated at the family level.

To determine the benthic composition and the per-centage of cold-water corals in the total benthos biomass, we analyzed the entire catch collected by scientific surveys. The megabenthos samples other than cold-water corals were classified to the family or higher taxonomic levels, such as order or phylum. Cold-water corals were aggregated to the order or suborder levels. The occurrence and total wet-weight of these taxa in the catch was measured, and the occurrence frequency was calculated as the number of hauls in which the focal taxa appeared divided by the total number of hauls.

Most of the specimens examined in this study are stored in the National Research Institute of Far Seas Fisheries, Yokohama, Japan. Some cold-water coral specimens are temporarily retained for further taxonomic examination at the Natural History Museum and Institute, Chiba and the Okinawa Churaumi Aquarium.

Results

Composition of cold-water coralsWe examined 213 cold-water coral specimens from the

commercial trawl, 211 from the commercial gillnet, and 1174 from the scientific surveys. The specimens included 34 Stylasterina, 2 Pennatulacea, 69 Alcyonacea (excluding gorgonians), 998 gorgonians, 125 Antipatharia and 370 Scleractinia. In total 29 families and 78 distinct genera of coral were identified. Many genera were considered to include multiple species as indicated by “spp.” in Table 2. One family (Stylasteridae) and two genera were identified in the Stylasterina (hydrocorals), Hydrozoa (Table 2). The identified taxa in the Octocorallia included one genus in the family Pennatulidae (sea pens) and one unknown genus of the order Pennatulacea, three genera and one unknown genus in the family Clavulariidae of the suborder Stolonifera, and two genera in the family Alcyoniidae and, one genus in each of the families Nephtheidae, Nidaliidae and Paralcyoniidae of the Alcyoniina, Al-cyonacea (excluding gorgonians). The specimens iden-tified as scleraxonian gorgonians included six taxa in three families (Anthothelidae, Paragorgiidae, and Coral-liidae), 14 genera in four families of the Holaxonia, and 15 genera in three families of the Calcaxonia (Table 2). Among the gorgonians sampled, more genera were in the Plexauridae (11) of the Holaxonia and the Primnoidae (12) of the Calcaxonia than in other families. The genus Acanthogorgia was the most abundant of the gorgonians, accounting for 119 of the 998 (11.9%) gorgonian speci-mens. Several gorgonian specimens belonged to the family Coralliidae, known as precious corals, including Hemicorallium abyssale (Bayer 1956) and Hemicorallium laauense (Bayer 1956). In the Antipatharia (black corals), Hexo corallia, nine genera in four families were identified. The genus Antipathes was the most abundant of the Anti-patharia (25 out of 86 specimens). The Scleractinia (stony corals) comprised 22 genera in 7 families, of which 17 were identified to species. Most of the scleractinian speci-mens were of small, unattached solitary species (e.g., Letepsammia formosissima (Moseley 1876), Flabellum sp.), however, some colonial framework-building species such as Solenosmilia variabilis Duncan 1873, Desmo-


Table 2　Cold-water coral taxa collected in the southern Emperor Seamounts area.


phyllum pertusum (Linnaeus 1758), Enallopsammia ros-trata (Pourtalès 1867) and Madrepora oculata Linnaeus 1758 were also collected at Northern Koko, Koko, Yuryaku, Kammu, Colahan and C-H seamounts.

The largest number of cold-water corals taxa were collected from Koko seamount (2 stylasterinan genera, 2 pennatulacean genera, 9 alcyonacean genera, 38 gorgonian genera, 6 antipatharian genera and 21 scleractinian genera); Koko is the largest seamount in the Emperor seamount chain and the sampling there was the most intensive (Table 1). Also rich in cold-water coral taxa were Colahan (2, 9, 22, 4, and 12 stylasterinan, alcyo-nacean, gorgonian, antipatharian and scleractinian genera, respectively) and Kammu seamounts (2, 1, 3, 20, 3, and

14 stylasterinan, pennatulacean, alcyonacean, gorgonian, antipatharian and scleractinian genera, respectively; Table 1).

Bathymetric distribution of cold-water coralsGorgonians were distributed over the widest depth

range, from 275 to 1353 m (Fig. 2). Most of the Acan tho-gorgiidae, Plexauridae and Primnoidae occurred at rela-tively shallower depth ranges, down to 500-600 m. In contrast to other gorgonian families, Chrysogorgiidae oc-curred over a relatively wider range, between 250 and over 1300 m. Antipatharia also were found over wide depth ranges but mainly in zones shallower than 500 m. The family Schizopathidae showed the widest vertical

Table 2　Continued.


distribution among the Antipatharia, reaching depths of 1299 m. Among Scleractinia, Solenosmilia variabilis, Enal lopsammia rostrata and Madrepora oculata, were principally distributed deeper than 550 m. E. rostrata was confirmed over the widest depth range among the three

species, from 306 to 1245 m. Desmophyllum pertusum was distributed shallower than 700 m; this species is known as a framework-builder in many other regions, but only fragments have been collected from the southern Emperor Seamounts. Most of the other solitary and small

Fig. 2　Bathymetric distribution (50 m bins) of cold-water corals in the southern Emperor Seamounts area. Shading in the cells indicates the frequency of occurrence of the taxa in the samples. Arrows indicate the total depth range of occurrence.


species of the Scleractinia were found at relatively shal-lower depths. Among the solitary Scleractinia, the family Caryophylliidae showed the widest vertical distribution (250-1149 m).

Composition of benthic megafaunaIn total, 21 classes of 10 phyla of benthic megafauna

were collected during the scientific surveys and classified into 35 taxonomic groups for which frequencies of occur-rence and wet-weights were calculated (Fig. 3). Among all of the benthic taxa identified, Ophiuroidea, Anomura, other Decapoda and Echinoidea occurred at the highest frequencies (i.e. in more than 70% of all hauls). Gorgo-nians, Scleractinia, Asteroidea and Prosobranchia also occurred at high frequencies (in more than 60% of all

hauls). The wet-weight of Echinoidea was highest (63.6 kg in total) with Ophiuroidea second (41.7 kg). Porifera had a high wet-weight (40.6 kg), comparable to Ophiu-roidea, although its frequency of occurrence was relatively low (33.7%). Gorgonians and Scleractinia followed these taxa and had the highest and second highest wet-weights among the cold-water corals (29.7 and 23.1 kg, respec-tively). The frequencies of occurrence and total wet-weights of Alcyonacea (excluding gorgonians), Sty-lasterina and Antipatharia were relatively low. The fre-quency of occurrence and total wet-weight of Pennatulacea were the lowest of all identified cold-water corals.

Several specimens of Zoantharia, Anomura and Ophiu-roidea were confirmed to live on the colonies of gorgonians such as Primnoidae. Other sessile megafauna such as

Fig. 3　Frequency of occurrence (A) and total wet-weights (B) of benthic megafauna collected by scientific surveys in the southern Emperor Seamounts area. Only top 25 groups are shown in this figure. Other groups (e.g. Pennatulacea, Sipuncla or Pycnogoida) were uncommon and are excluded. The Actiniaria, Polychaeta and Crustacea are epibenthos. Black bars indicate cold-water coral taxa. * Alcyonacea (excluding gorgonians)


gorgonians and Stylasterina attached to the dead skeletons of living colonial Scleractinia. Similarly, epibenthos such as commensal Polychaeta and Crustacea also inhabited colonial Scleractinia.

Discussion

Our results demonstrate that gorgonians and Scleractinia contribute substantially to the species richness and abun-dance of benthic megafauna in the southern Emperor Seamounts area. Both coral groups had high frequency of occurrence and wet-weights compared to other cold-water corals and most other benthic taxa. Among corals, the number of gorgonian taxa was highest, with 38 genera in 10 families confirmed. Two species of the family Coral-liidae (Hemicorallium abyssale and H. laauense) were iden tified, and several additional species, including new spe cies were suspected. Most of the specimens of these additional species were identified tentatively as Pleu ro-corallium cf. pusillum (Kishinouye 1904), because of the inadequate taxonomic description and the absence of type specimens for P. pusillum. These results indicate that several species of the Coralliidae inhabit the southern Emperor Seamounts area and might have been harvested by the past coral fishing (Grigg 1993).

Many of the gorgonians collected were in the family Primnoidae, a group known to provide important habitat structure for commercially important fish and other animals (Krieger and Wing 2002; Buhl-Mortensen and Mortensen 2004, 2005; Stone 2006; Stone et al. 2015). The Scleractinia comprised large number of taxa (22 genera in 7 families) but their bathymetric range was slightly narrower than that of the gorgonians. Most of the scleractinians sampled were solitary and small species. Scleractinia are known to form large reef structures in the North Atlantic Ocean and other regions (Mol et al. 2002; Morris et al. 2013; Reed et al. 2006). In this study, some framework-building species of the Scleractinia were identified, such as Solenosmilia variabilis, Desmophyllum pertusum, Enallopsammia rostrata and Madrepora oculata. S. variabilis is generally known to have a cosmo-politan distribution, but it had not previously been re-corded from the Antarctic, North Pacific and Eastern

Pacific Oceans (Cairns 1995). The S. variabilis specimens collected from Koko, Kammu and Colahan seamounts in this study provide the first record of this species in the North Pacific Ocean. The result of a visual seafloor surveys using a remotely operated vehicle (ROV) and drop camera suggested that there might be small reefs of S.variabilis, E rostrate and M. oculata on Colahan sea-mount (Hayashibara and Nishida 2017). But framework-building capacity of D. pertusum has not been confirmed by the seafloor observation in the southern Emperor Sea-mounts area.

Guinotte et al. (2006) noticed a striking relationship between the vertical distribution of cold-water Scleractinia and the depth of the aragonite saturation horizon (ASH). They indicated that the ASH in the North Pacific (50-600 m) is substantially shallower than that in the North Atlantic and other oceans. The bathymetric distribution of framework-building Scleractinia in the southern Emperor Seamounts may be partially affected by the shallow depth of the ASH (approximately 400-800 m deep at present day; Guinotte et al. 2006). For example, in this study, colonial framework-building scleractinian species were distributed at 300-1250 m but in other regions, Sclerac-tinia are known to occur and construct large reefs even in zones deeper than 2000 m (Guinotte et al. 2006; Roberts et al. 2006). The depth of the ASH is likely to be the factor limiting the calcification capacity of Scleractinia in the southern Emperor Seamounts area. In this area, gorgonians occurred over the widest depth range and at the highest frequencies compared to other cold-water corals. The axes of gorgonian corals are composed of protein, calcite and aragonite (Fabricius and Alderslade 2001). Thus, the calcification capacity of gorgonians are thought to be less dependent on ASH than are those of Scleractinia but more dependent on the calcite saturation horizon that is typically deeper than the ASH (Guinotte et al. 2006). Antipatharia generally occurred at low frequency, but some families were distributed down to the depths comparable to gor-gonians. Growth rates of Antipatharia are not dependent on the ASH because their axes are composed of protein (Goldberg et al. 1994).

Cold-water corals provide habitats for other animals (Roberts et al. 2006). Various invertebrates such as Porifera, Crustacea and Ophiuroidea are known to be


associated with cold-water corals in the North Pacific Ocean (Parrish and Baco 2007; Stone and Shotwell 2007, Miyamoto and Kiyota 2017). We confirmed that many specimens of Zoantharia, Anomura and Ophiuroidea in our samples were attached to colonies of gorgonians such as Primnoidae. Similarly, colonial Scleractinia were at-tached or inhabited by other benthic animals in the southern Emperor Seamounts area.

Many environmental factors and biological processes affect the regional species composition and diversity of the benthic fauna of the deep-sea floor and seamounts (Levin et al. 2001; Samadi et al. 2007). For example, in the North Atlantic Ocean, regionally distinctive habitats such as scleractinian reefs and large patches of the Pen-natulacea and Porifera have been documented and iden-tified as VMEs (Henry and Roberts 2007; Kenchington et al. 2014). The benthic megafauna of the southern Emperor Seamounts area is characterized by the dominance of gorgonians and Scleractinia and lower occurrence fre-quencies of Pennatulacea, Stylasterina, and Porifera. In this study, Echinodermata, Crustacea and Prosobranchia were most prevalent, while Echinodermata and Porifera contributed most to the wet weight. Porifera wet weights were relatively large due to their high water contents. Prosobranchia was not reported as a main component of the deep-sea communities in Hawaiian and Alaskan waters (Parrish and Baco 2007; Stone and Shotwell 2007).

Even within the North Pacific Ocean, deep-sea faunas are known to vary regionally. In Alaskan waters, Al-cyonacea (excluding gorgonians), gorgonians, Pennatu-lacea and Stylasteridea are main components of habitat-forming species (Heifetz 2002; Heifetz et al. 2005; Stone and Shotwell 2007). Stone and Shotwell (2007) compared the deep-sea coral ecosystems within the Alaskan region: In the Gulf of Alaska, extensive Pennatulacea groves in the western Gulf of Alaska around the Kodiak Islands are known as important coral features in the region. Gulf of Alaska seamounts share the major taxonomic components of the continental shelf but are characterized by the absence of Stylasterina and a paucity of Pennatulacea. More than 300 islands comprise the Aleutian archipelago, form the boundary between the deep North Pacific Ocean and shallower Bering Sea, and support the most abundant and diverse cold-water coral fauna in Alaska. Stone (2006)

and Stone and Shotwell (2007) reported that Crustacea, Ophiuroidea, Polychaeta and many species of Porifera are associated with cold-water corals in the Aleutian archi-pelago. In the Bering Sea, cold-water corals are patchily distributed on the broad continental shelf and along the shallow continental slopes, but the coral fauna in this region appear to be less diverse. There are no records of colonial or habitat-forming Scleractinia from the Aleutian and Alaskan regions (Heifetz 2002; Stone and Shotwell 2007).

Lundsten et al. (2009) reported from ROV video observation that Cnidaria, Porifera and Echinodermata were the dominant megabenthic taxa on three seamounts off California. Among Cnidaria, gorgonians were the most frequently observed cold-water corals, and Anti-patharia increased in deeper zones. However, the Cali-fornian seamounts differed from the southern Emperor Seamounts in higher occurrences of the Alcyonacea (excluding gorgonians) and scarcity of the Scleractinia.

Limited information is available for the composition of benthic megafauna in northwestern Pacific Ocean around Japan. Tow camera and submersible surveys from Hok-kaido through Ryukyu Island revealed the differences in the dominant taxa by location. Brachiopoda was dominant on Shiribeshi Seamount off the west coast of Hokkaido (observed at 115-620 m deep; Fujikura et al. 1991), while Ophiuroidea was dominant in the western Tsugaru Strait (569-580 m deep; Wakutsubo and Koganezaki 1987). On the Ogasawara Ridge (210-400 m) and Kerama Bank (185-310 m), several cold-water corals were confirmed by sea-floor observation surveys from ROVs and sub-mersibles. However, major components of cold-water corals in these areas, Stylaster sp. as Stylasterina, Plu-mulariidae as gorgonian and Parantipathes sp. as Anti-patharia (Fujioka 1995; Okamura 1989), differed from those in the southern Emperor Seamounts area. In Sagami Bay, 260 species of octocorals were recorded, including 144 gorgonians, 80 Alcyonacea and 36 Pennatulacea of which Alcyonacea and Pennatulacea occurred most frequently (Matsumoto et al. 2007).

The composition of the deep-sea benthic megafauna in the southern Emperor Seamounts area appears to be most similar to that of the Hawaiian Islands. In the Hawaiian Islands, gorgonians, Antipatharia, zoantharina gold corals


(Kulamanamana haumeaae Sinniger, Ocaña and Baco, 2013), and Scleractinia are considered the main compo-nents of the cold-water coral fauna whereas Stylaserina and Pennatulacea occur low frequently (Parrish and Baco 2007). Many commensal invertebrates such as Zoantharia, sea anemones, Crustacea, Polychaeta and Ophiuroidea are known to be associated with these cold-water corals (Parrish and Baco 2007).

This study demonstrated 78 unique coral taxa at a depth range between 275 and 1353 m and along a broad region of the southern Emperor Seamount chain. Gorgonians were the most common and diverse cold-water coral taxa; they provide structural habitats and attachment substrate for other benthic animals. The Scleractinia, including framework-building species occurred at frequencies sim-ilar to those of gorgonians, but they were only found over limited depth ranges. Within the North Pacific, the deep-sea benthic megafauna on the southern Emperor Sea-mounts is more similar to that around the Hawaiian Is-lands than in the Aleutian Islands, other Alaskan waters, the Californian seamounts and the seamounts and conti-nental slopes off Japan. Continued collection and detailed taxonomic examination of cold-water coral specimens from the southern Emperor Seamounts region would provide for species-level comparisons of the deep-sea coral megafauna with different regions.

Acknowledgments

This study was conducted as part of the “Project on Evaluation of the Status of the Sea Floor Environment of Fishing Grounds in the High Seas”, Fisheries Agency of Japan. We are grateful to the captain, crew, and researchers of R/V Kaiyo-maru for their cooperation in scientific surveys. We are also grateful to the two re viewers, Dr. Robert Stone and Dr. Amy Baco-Taylor for their valuable comments and suggestions.

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Received: 26 August 2016Accepted: 20 April 2017

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