Species richness and endemism
The species richness (i.e. number of species) of chemosynthetic-based communities was highly variable amongst the 42 research sites, ranging from two to 38 species per site (Fig. 2). The vent fields at Minami-Ensei (38 species), Iheya (33 species), Iheya North (35 species), Izena (26 species) and Hatoma (24 species) in the Okinawa Trough and the methane seep at Off Hatsushima Island (hereafter OHI, 29 species) in Sagami Bay contained double or more numbers of species compared with the other sites. Molluscs were the most species-rich phylum at all sites, comprising 68.5 ± 18.0% of the total species on average, followed by annelids (17.0 ± 18.0%) and arthropods (14.8 ± 21.5%). Bivalves were the most species-rich class, constituting 65.9 ± 34.7% of the molluscs, followed by gastropods (29.2 ± 31.5%) and polyplacophores (4.1 ± 10.6%). Annelids consisted of single class of polychaetes. Arthropods consisted of Malacostraca (81.5 ± 15.9%) and Maxillopods (18.5 ± 15.9%). Arthropods were common in the hydrothermal vents, but their distribution in seeps was restricted to Sagami Knoll, OHI and Okinoyama in Sagami Bay. The distribution of arthropods was greater in the vents of the Izu–Bonin Arc (except Sumisu), accounting for 33.0–66.7% (overall = 47.5 ± 20.5%) of the observed species. Regarding the accumulative number of species by region, the Okinawa Trough (82 species) had the most, followed by the Nankai Trough (35 species), Sagami Bay (29 species), Izu–Bonin Arc (27 species), Ryukyu Trench (18 species) and Kuril–Japan Trench (14 species).
Figure 2. Summary of species richness (i.e. the number of species) at 42 vent and seep sites in the Japanese archipelago.
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The occurrences of endemic species were high. In total, 19 of the 42 chemosynthetic sites contained endemic species that were only recorded at a single site (Table 1); many endemic species were found at these 19 sites including the methane-seep sites at Sanriku Escarpment in the Kuril–Japan Trench (5 species), OHI in Sagami Bay (5 species) and Kuroshima in the Ryukyu Trench (5 species) as well as the vent sites at Minami-Ensei (18 species) and Iheya North (4 species) in the Okinawa Trough. Regarding the accumulated number of endemic species by region, 56 endemic species exclusively comprised 68.3% of the total number of species in the Okinawa Trough, followed by the Nankai Trough (20 species, 57.1%), Sagami Bay (13 species, 44.8%), Izu–Bonin Arc (15 species, 44.8%), Kuril–Japan Trench (13 species, 92.9%) and Ryukyu Trench (5 species, 27.8%).
Although different regions had different numbers of sampling sites, the results regarding the numbers of species and endemic species standardized by the number of sites indicated the Okinawa Trough still had most species (13.7 species and 9.3 endemic species) followed by Sagami Bay (9.7 species and 4.3 endemic species), the Ryukyu Trench (9.0 species and 2.5 endemic species), the Kuril–Japan Trench (4.7 species and 4.3 endemic species), the Izu–Bonin Arc (3.0 species and 1.7 endemic species) and the Nankai Trough (1.8 species and 1.1 endemic species).
There were significant correlations between species richness and depth (Spearman rank-correlation coefficient (n = 42), rs = −0.39, P = 0.011) or water temperature (rs = 0.48, P = 0.0022), but no significant correlations were found between species richness and latitude (rs = −0.28, P = 0.068) or longitude (rs = −0.17, P = 0.25). The species richness of the faunal communities tended to be lower at the sites > 2000 m deep, while the species richness were highly variable amongst the sites < 2000 m deep (see Figure S1).
It is difficult to directly compare the chemical concentrations (i.e. methane and hydrogen sulphide) obtained in the seeps and vents because of differences in sample collection methods. For example, the values in seeps are from sediment pore water, while those in vents are from vent fluid. Regardless, if comparisons are to be made, methane concentrations were higher at the sites in the Okinawa Trough and Sagami Bay, while hydrogen sulphide concentrations were higher at Izena in the Okinawa Trough and Ryuyo in the Nankai Trough (Fig. 3a,b). Although the number of studies was limited (~11 sites), the species richness was significantly correlated with methane concentration (Spearman rank-correlation coefficient (n = 11); rs = 0.781, significant level of rs = 0.618) (Fig. 3a). Conversely, species richness was not significantly correlated with hydrogen sulphide concentration (n = 8, rs = 0.0714, significant level of rs = 0.738) (Fig. 3b).
Figure 3. Correlation between species richness and fluid chemical factors: (a) methane and (b) hydrogen sulphide. Sites in the Izu–Ogaswara Trench and Okinawa Trough are hydrothermal vents (closed symbols), while those in the Kuril–Japan Trench, Nankai Trough, Ryukyu Trench and Sagami Bay are methane seeps (open symbols). Linear fitting conducted by straight-line function (y = ax + b, where x is chemical concentration and y is species richness) was performed using the kaleidagraph© synergy Software package. Chemical data are from pore water at seeps and from fluid at vents: Off Hatsushima Is. (Masuzawa et al., 1992, 1995; Tsunogai et al., 1996), Ryuyo (Tsunogai et al., 2002), Tenryu III (Tsunogai et al., 2002), Atsumi II (Toki et al., 2007), Off Kumano 2500–2900 m site (Toki et al., 2004), Suiyo (Sakai et al., 1994; Tsunogai et al., 1994; Kishida et al., 2004), Minami-Ensei (Chiba et al., 1993; Sakai et al., 1994), Iheya (Gamo et al., 1991; Ishibashi et al., 1995), Iheya North (Sakai et al., 1990), Yonaguni IV (Konno et al., 2006).
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Classification of faunal community
The assemblage compositions of the chemosynthetic-based communities were clustered into eight groups and two outliers by the SIMPROF test (P < 0.05, Fig. 4a). These groups were basically congruent with geographically different regions as follows: Group 1, all sites in the Kuril–Japan Trench; Group 2, nine sites in the Nankai Trough; Group 3, five sites in the Nankai Trough and one site from the Ryukyu Trench; Group 4, five sites in the Nankai Trough, one site from the Ryukyu Trench and one site from the Izu–Bonin Arc; Group 5, all sites (except Sagami Knoll) in Sagami Bay; Group 6, all sites (except Minami-Ensei) in the Okinawa Trough; and Groups 7 and 8, each four site in the Izu–Bonin Arc.
Figure 4. The similarity of macro- and megafaunal community structure in different geological regions: (a) dendrogram of the group average cluster analysis and (b) non-metric MDS plots. Solid branches of the dendrogram indicate significant faunal groups in which the SIMPROF test (P < 0.05) suggested the structure is not random. The results of clusters detected by SIMPROF test are imposed as solid lines in the MDS plots.
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The two-dimensional MDS ordination of the assemblage compositions of the 42 chemosynthetic sites showed a stress value of 0.05 (Fig. 4b), indicating the ordination was acceptable with an excellent representation (Clarke & Warwick, 1994). The MDS ordination plots were consistent with the cluster analysis. All eight groups detected by the multivariate analyses were also consistent with geographically different regions and significantly different (ANOSIM test, Global R = 0.834, P < 0.001). The Nankai Trough was divided into three groups (Groups 2, 3 and 4) basically with respect to depth and horizontal location. Group 2 comprised relatively deeper sites with depths from 2500 to 4800 m, while Group 3 comprised sites with depths from 1400 to 2360 m (1940–2360 m for sites from Nankai Trough). Group 4 comprised the north-eastern sites of the Nankai Trough with depths from 270 to 1200 m.
The BIO-ENV procedure (Table 2) showed that depth was the only abiotic variable that best grouped sites in a manner consistent with the faunal patterns (ρ = 0.551, P < 0.01). Furthermore, the best 2-variable combinations were depth and latitude (ρ = 0.546), and depth and water temperature (ρ = 0.463).
Table 2. Results of BIO-ENV analysis to select the 10 best subsets of environmental variables
|No. of variables||Correlation||Selections|
|3||0.387||DEP, TEMP, LAT|
|4||0.353||DEP, TEMP, LAT, LON|
|3||0.349||DEP, LAT, LON|
|3||0.345||DEP, TEMP, LON|
The complementarity analysis selected 24 sites to conserve all 155 species inhabiting the 42 sites in this study (Marxan score, 318–1000; Table 1). Of the 24 selected sites, 19 were determined to be irreplaceable sites (i.e. they were selected 1000/1000 times) and the remaining five were flexible sites. The 19 irreplaceable sites corresponded to the 19 sites where endemic species were only found at a single site. Thus, the sites selected on the basis of the principle of complementarity are strongly affected by the presence of endemic species (Table 1). Excluding three of the five flexible sites would still allow for the conservation of all 155 species populations (Table 3). Therefore, the complementarity analysis for all 42 sites concludes at least 21 sites (11 seeps and 10 vents, 50.0% of all sites) including 19 irreplaceable sites, and two flexible sites are required to conserve all 155 species.
Table 3. Flexible site combinations that can be excluded from the priority area for conservation determined by complementarity analysis for all sites pooled
|Combination no.||Site (reference no.)|
|1||Tenryu III (13), Kumano IV (17), Kumano VI (18)|
|2||Tenryu III (13), Kumano VI (18), Off Kumano 2500–2900 m site (20)|
|3||Ternyu III (13), Kumano IV (17), Off Kumano 2500–2900 m site (20)|
|4||Kumano IV (17), Off Kumano 2500–2900 m site (20), Off Muroto 3200–3700 m site (23)|
|5||Kumano IV (17), Kumano VI (18), Off Muroto 3200–3700 m site (23)|
|6||Kumano VI (18), Off Kumano 2500–2900 m site (20), Off Muroto 3200–3700 m site (23)|
The results of the complementarity analysis targeted for each geographically distinct region detected by the multivariate analysis selected 32 sites for conservation (Marxan score, 485–1000; Table 1). The 32 sites selected contained an additional nine irreplaceable sites compared with the previous sites (i.e. the Tokai, Tenryu II, Atsumi II, Zenisu, Off Kumano 2100 m site and Off Kumano 2500–2900 m site in the Nankai Trough, Off Kikaijima in the Ryukyu Trench and Suiyo and Nikko in the Izu–Bonin Arc), giving 28 irreplaceable sites and four flexible sites. Of the four flexible sites, two sites were in Group 2 (the Tenryu III and Off Muroto 3200–3700 m site in the Nankai Trough) while the other two sites were in Group 3 (Kumano IV and Kumano VI in the Nankai Trough), and one site from each group must be protected to conserve all species. Therefore, the complementarity analysis by geographically distinct groups indicated 30 sites (28 irreplaceable + two flexible sites, 71.4% of all sites), comprising 17 seeps and 13 vents, must be protected to conserve at least one population of all 155 species. Although we conducted complementarity analysis for each geographically groups clustered by SIMPROF test, Off Kikaijima and Kuroshima (Ryukyu Trench) in Groups 3 and 4, respectively (Fig. 4a,b), where Nanaki Trough sites dominated, were placed in the Ryukyu Trench for the analysis because these two sites are far from those in the Nankai Trough (784–1768 km). Similarly, Sumisu in Group 4 was included in Izu-Bonin Arc (Group 7). Sagami Knoll, an outlier by the SIMPROF test (Fig. 4a), was also included and analysed in Sagami Bay because the site contained no endemic species compared with the other sites in Sagami Bay (i.e. OHI and Okinoyama, Table 1).