Search

Stygiopontius senokuchiae is a hydrothermal-vent-specific copepod species (Dirivultidae, Siphonostomatoida) and abundantly distributes near vent orifices. This species is thought to have an early juvenile planktic stage, and previous studies using bulk tissue stable-isotope and radioisotope data suggest that adults ingest chemoautotrophic microbes growing on vent chimneys. However, ontogenetic changes in their diets have not been investigated. We analysed gut contents of copepodite I to adult stages of S. senokuchiae collected from a hydrothermal-vent chimney at the Izu-Ogasawara Arc to check for the presence of bacterial cells in oral tubes and guts. We compared these results with an unidentified co-occurring calanoid and a species of Ectinosoma (Harpacticoida) and to other Siphonostomatoid copepods, namely Hatschekia labracis (Hatschekiidae), which was attached to a scarbreast tuskfin (Choerodon azurio, Labridae), and Asterocheres sp. 1 (Asterocheridae), which was attached to a spirastrellid sponge. Carbon isotope ratios of S. senokuchiae at different stages and Ectinosoma were measured to complement the nutritional insights obtained from gut-content analysis. Our results clearly showed that most S. senokuchiae individuals possessed bacteria in the oral tube or gut regardless of growth stage, whereas the coexisting Ectinosoma and calanoid did not. The carbon isotopic compositions confirmed that S. senokuchiae gains its nutrition from chemoautotrophic bacteria that use the rTCA carbon-fixation pathway. Comparisons with other Siphonostomatoida copepods suggest that Dirivultidae are specifically adapted to feed on bacteria at hydrothermal-vent chimneys, allowing their high dominance and evolutionary success in these habitats.

Dirivultid copepods (Siphonostomatoida), one of the most successful meiobenthic organisms found at deep-sea hydrothermal vents, have been the focus of most previous ecological studies among meiofauna in these habitats. The ecology of Harpacticoida, a major benthic copepod group in typical deep-sea floor, however, is not well understood in terms of variations in community structure and controlling factors at venting sites. The spatial heterogeneities in benthic harpacticoid composition and their association with environmental parameters were investigated at hydrothermal vent chimney structures in the calderas of three neighbouring sea knolls (Bayonnaise Knoll, Myojin Knoll and Myojin-sho Caldera) in the western North Pacific. While a previous study had reported the distribution of dirivultids was strongly associated with spatial differences in stable carbon isotopic signatures (δ13C) of organic matter in the detritus on active chimneys in the field, multivariate analyses detected no significant corelation between the parameter and harpacticoid composition in this study. Instead, high associations of the harpacticoid composition with differences in water depth and total organic carbon (TOC) concentration were detected. Ectinosomatidae dominated at vent sites with lower TOC values in the shallowest Bayonnaise Knoll, while they were less prevalent at deeper vent fields in the other knolls, where Miraciidae was the most abundant family. This study indicated the availability of vent chemoautotrophic carbon is not a primary factor controlling the composition of harpacticoids even in the habitats on the hydrothermal vents, but instead by the food amount, regardless of its resources (including marine snow from the sea surface), in the study area.

Habitat heterogeneity is one of the major factors shaping community structure and diversity of many fauna. The present study aimed to reveal the influence of habitat heterogeneity on the nematode community structures and diversity in a seasonally hypoxic bay (Omura Bay of Nagasaki in Western Kyushu, Japan). The severity of hypoxia varies typically along north–south axis of the bay, which is intensified southwardly. Nematode abundance and diversity were highest in the northern site than the other sites, and nematode communities were clustered into three groups by sampling site. There were significant differences in composition (Two-way ANOSIM, Rho = 0.726, p

Stygiopontius copepods (Dirivultidae, Siphonostomatoida, Crustacea) are among the most successful meiobenthic organisms at deep-sea hydrothermal vents. Most of their ecology is not yet known, including the spatiotemporal differences in their sex ratios and their controlling factors. We investigated spatial variation in the sex ratio of adult Stygiopontius senokuchiae and its association with environmental parameters, including food quality at hydrothermal vent chimney structures in the calderas of three neighboring sea knolls (Bayonnaise Knoll, Myojin Knoll, and Myojin-sho Caldera) in the western North Pacific Ocean. Their sex ratio was significantly biased to males from 1:1, which was different from some of the other Stygiopontius species. The ratios did not show a significant correlation with the density of total adults. While previous studies have shown that the abundance of S. senokuchiae is positively associated with δ13C values of detritus on active chimneys, multivariate analyses in this study did not detect any significant association between their sex ratio and any investigated parameters including δ13C. These findings suggest that neither population density nor chemoautotrophic food availability drives the sexual difference in the spatial distribution around vents among the adults of the species.

Universal classification schemes have been proposed for the vertical zonation of sessile benthos on intertidal rocky shores; however, generalized patterns among deep-sea meiofauna of different regions remain rare. We conducted a preliminary, family-level study of deep-sea harpacticoid diversity and community composition off Tosa Bay, Shikoku island, Japan, along a transect on a landward continental slope of the Nankai Trough. We then compared these data to those of other subduction zones along the Japanese island arc, including the Kuril, Japan, and Ryukyu trenches. Common bathymetric trends in harpacticoid composition were observed among these subduction zones from upper bathyal to abyssal depths (400–5800 m). Ectinosomatidae was the most abundant family within this depth interval; however, its frequency decreased with increasing depth, whereas the abundance of Pseudotachidiidae was highest at abyssal depths. Harpacticoid composition differed significantly between landward slopes of the Ryukyu Trench and those of the Japan and Kuril trenches. However, harpacticoid composition on the landward slope of the Nankai Trough, between the Ryukyu and Japan trenches, was similar to that around the neighboring subduction zones. These findings imply that patterns of deep-sea harpacticoid community composition are similar along the continental slope of the Japanese island arc, and that the Izu–Ogasawara arc, which subdivides the subduction zones, has not functioned as a physical barrier to harpacticoid expansion between zones.

Copepods in the family Dirivultidae are one of the most successful meiofauna in deep-sea hydrothermal vent fields and are abundant near venting fluid. Although vents are spatially limited ocean habitats, they are distributed widely in the Atlantic, Pacific, and Indian Oceans. However, knowledge of dirivultid biogeography and phylogeography remains limited, especially in the northwestern Pacific. Here, we obtained partial mitochondrial COI gene sequences of three dirivultids from the northwestern Pacific—Stygiopontius senokuchiae and an unidentified Chasmatopontius species from vent fields in the Izu–Bonin Arc and Stygiopontius senckenbergi associated with the squat lobster Shinkaia crosnieri in the Okinawa Trough—and analyzed them in comparison with existing data. The among-species sequence diversity exceeded 80 out of 560 bp (14% or 0.166 in Kimura 2-parameter distance), whereas the within-species diversity was less than 10 bp (2% or 0.018 in Kimura 2-parameter distance), with no genetic saturation. Each species formed a monophyletic clade and the genetic region targeted is deemed reliable for identifying species and populations for these copepods. Among the three genera targeted, only Chasmatopontius formed a monophyletic cluster, while Aphotopontius and Stygiopontius did not. Species delimitation analyses suggested the existence of cryptic species in Chasmatopontius. Subdivision among local populations was observed in Aphotopontius, but not in Stygiopontius in the same distribution, implying potential differences in dispersal ability among different genera of dirivultids. Further sampling is required, to fill the spatial gaps to elucidate the biogeography and evolution of dirivultids in the global deep ocean.

Hypoxia in bottom environments of coastal marine ecosystems is a serious problem adversely affecting both benthic life and local fisheries. In this study, we monitored abundance, composition, and feeding types of nematode communities under pre-, mid-, and post-hypoxic conditions in Omura Bay, Nagasaki, Japan, for three consecutive years (2013-2015). The bay is almost completely enclosed, and experiences hypoxia at the bottom every summer. A positive correlation was found between dissolved oxygen (DO) concentration and nematode abundance over the entire sampling period (p3 mg L?1). These findings will help us to understand how global trends of ocean deoxygenation could shape the meiobenthic community and alter benthic ecosystem functioning in coastal areas., Plankton and Benthos Research, 13(2), pp.59-65; 2018

Intertidal/littoral zones are the areas closest to the coast in marine ecosystems. They include a variety of habitats, such as rocky shores, sandy beaches, soft sediments, salt marshes, and mangroves. The intertidal zone, which is located between the extreme low water of spring tide (ELWS) and the extreme high water of spring tide (EHWS) lines, is exposed to the air at low tide and is underwater at high tide. Accordingly, the organisms that inhabit this zone have adapted to extremely harsh environments. Water is available regularly with the tides but varies from fresh to highly saline. The temperature also fluctuates from quite hot during a summer day to near freezing in colder climates during low tide when the area is exposed to air. Such environmental gradients cause apparent zonation, which creates a vertical distribution pattern of organisms.

While concern over anthropogenic marine litter around coastlines is increasing worldwide, information on this litter in trenches on the seafloor is very sparse. We investigated the amount of marine litter on the deep-sea bottom around the Ryukyu Islands in the Northwest Pacific, based on trawl samples. The density of litter observed in the axis of the Ryukyu Trench (7100 m) and in the basin of the Okinawa Trough ranged from 1.2 × 103 to 7.1 × 103 items km−2, or 7.5–121.4 kg km−2, which was significantly higher than that observed on the adjacent shallower continental slopes or abyssal plain (0.1 × 103 to 0.6 × 103 items km−2; 0.03–9.2 kg km−2). This suggests that trenches and troughs function as “depocenters” for anthropogenic litter because of their deeper and enclosed topographies.

A new species of dirivultid copepod (Siphonostomatoida) is described from hydrothermal vents in a volcanic seamount in Izu-Bonin Arc, western part of North Pacific Ocean. The copepod was collected during the research cruise NT13-09 using the R/V Natsushima with the ROV Hyper-Dolphin in April 2013. The type series of the new species was collected from the populations of Paralvinella spp. (Annelida: Alvinellidae) on an active vent chimney at the depth of 795 m. Stygiopontius senokuchiae n. sp. is most closely related to S. teres Humes, 1996 but clearly distinguished from the latter species by the possession of the following characters: the basis of leg 1 with an attenuated inner process; the genital double somite with a conical process lateral to the genital opening; and caudal rami without distal process. The findings of the copepod in the present study represents the first record of nominal species of the Dirivultidae from Japanese waters and a record of the shallowest depth of the genus. A key to species of the genus Stygiopontius from Western Pacific is provided.

We compared meiofaunal assemblages obtained from the landward slope of the Japan Trench off the Sanriku region of Honshu Island before the 2011 off the Pacific coast of Tohoku Earthquake, 4.5 months and 1.5 years after the earthquake. Sediment samples were collected after the earthquake along two transects. Meiofaunal density after the earthquake did not differ from that before the earthquake. However, meiofaunal composition after the earthquake was different from that before the earthquake, and the composition dispersion after the earthquake was lower than that before the earthquake. These results suggested that the turbidity current may have affected meiofaunal composition and reduced its variability. These changes were probably caused by the seismic motion and displacement of the substratum after the earthquake, rather than sediment redeposition induced by the earthquake. To assess the effect of the earthquake more thoroughly, we compared meiofaunal assemblage in the Sanriku region before and after the earthquake with that in the Kuril Trench (comparable productivity to the Sanriku region) and Ryukyu Trench (lower productivity). Differences in meiofaunal composition before and after the earthquake within the Sanriku region were lower than those between the Sanriku and Ryukyu regions. These results suggested that deep-sea meiofaunal assemblages are influenced by large-scale disturbances, but changes are within the range of variation generated by surface productivity levels.

The Yatsushiro Sea is an inland sea in the southwestern part of Kyushu Island, covering an area of 1,200 km2, and is partly connected to the Ariake Sea to the north and the east China Sea to the south. The innermost part of the sea is semi-enclosed by the Shiranui reclaimed land, which was created in 1967. Coastal fishery operators in the region have suggested that fish stocks have declined due to the accumulation of ‘hedoro’ (sludge) on the sea floor, which increased after the land was reclaimed. This project analyzed spatiotemporal changes in the community structure of the meiofauna, which represent ideal organisms for environment assays and in the environmental factors in sediments at four fixed stations around the inner part of the northern Yatsushiro Sea over 3 years (May 2010–February 2013). The results confirm the site of local deposits in the narrow sub tidal section off the Shiranui reclaimed land, where rich organic matter has accumulated and highly sulfidic conditions above the standard criterion value for fisheries were present, along with clear and significant decreases in meiofaunal abundance during the summer. Multivariate analyses, however, did not reveal any strong association between the meiofaunal composition and any environmental factor in sediments, suggesting the organic and sulfidic conditions were not driving the temporal changes in meiofaunal structure in the area. Instead, the meiofaunal changes could be directly related to the low continuous decline in dissolved oxygen and salinity in the water on the sea floor in summer.

The present study investigated spatiotemporal variations in meiofaunal abundance and composition at high taxonomic levels, and their associations with certain measures of food availability around hydrothermal vents on chimney structures and in adjacent non-vent fields in the calderas of three neighboring seamounts (Bayonnaise Knoll, Myojin Knoll, and Myojin-sho Caldera), in the Izu-Ogasawara Arc, western North Pacific Ocean. Total meiofaunal abundance in seafloor sediment at the bases of vent chimneys appeared to be greater than that in other non-vent habitats outside or inside the calderas of all seamounts, which was partly explained by temporal variations at the bases of the chimneys. There was no significant difference in the mean meiofaunal abundances among those habitats. A typical deep-sea meiofaunal composition (nematodes as the most abundant taxon, harpacticoid copepods as the second) was observed in the seafloor sediments in the non-vent fields, and even in the sediments at chimney bases. This was significantly different from the meiofaunal composition observed on the surfaces of vent chimneys, where copepods and their nauplii were most abundant. This spatial difference was significantly correlated with a difference in stable carbon isotope ratios (δ13C) of organic matter in sediment, suggesting that the availability of chemosynthetic food controls the spatial differences in meiofaunal composition around these hydrothermal vents, even at a high taxonomic level.

Meiofaunal assemblages inhabiting the landward slope of the Japan Trench (water depth, 120–5600 m) were examined 4.5 months and 1.5 years after the 2011 Tohoku Earthquake off Sanriku, Miyagi Prefecture, northeast Japan. Two key parameters were compared before (24–30 years) and after (4.5 months in 2011 and 1.5 years in 2012) the earthquake: (a) the bathymetric pattern and (b) the vertical distribution of meiofauna in the sediments. Differences in meiofaunal densities and associated bathymetric patterns were not detected before and after the earthquake. However, the vertical profiles of meiofauna in the sediments differed at some stations. The highest meiofaunal densities occurred in the subsurface layers in 2011 at some stations, with these subsurface peaks being no longer present in 2012. At these stations, the assemblage structure at the higher taxon level differed between 2011 and 2012, with copepod density increasing in 2012. Therefore, copepod abundance appeared to decrease because of the effect of earthquake-related events (e.g. rapid sedimentation induced by the turbidity current). These changes in meiofaunal vertical profiles and assemblage structures detected after the earthquake were probably caused by an increase in organic matter content in the topmost layers. These results indicate that major disturbances to deep-sea sediments mainly influenced the vertical distribution and assemblage structure of meiofauna after the 2011 Tohoku Earthquake, whereas meiofaunal densities remained similar or quickly recovered within 4.5 months of the event.

In contrast to specific large benthic invertebrates in chemosynthetic ecosystems such as hydrothermal vents, meiofaunal communities in such habitats have been reported to have strong taxonomic overlap with meiofauna in the adjacent “normal” environments. However, meiofauna have only recently been included in studies of those environments and detailed information on these communities is still rare. This is especially true in the Northwest Pacific Ocean, even though there are many seamounts with active vents in the calderas of the region. Nematode community composition at the genus level in sediments from a hydrothermal vent field in the caldera of Myojin Knoll (32°06′N, 139°52′E, depth 1,300 m), a seamount on the Izu-Ogasawara Arc, Japan, was investigated for the first time and was compared with adjacent non-vent areas inside and outside the caldera. Multivariate analyses showed that the composition of nematodes in the hydrothermal field was significantly different from that in the non-hydrothermal fields around the caldera. However, the common genera, such as Oxystomina, Pareudesmoscolex, Desmoscolex, and Microlaimus were found in two, or all three vent fields while their rank contributions differed among the three fields. When the data from Myojin Knoll were compared with those from other deep-sea vent environments in different regions (e.g., North Fiji Basin, East Pacific Rise, Mid-Atlantic Ridge), the nematode composition in the vent field of the Myojin caldera was more similar to that of the non-vent fields around the caldera than the composition in vent fields of other regions. These data from the Northwest Pacific Ocean also suggest the absence of long-range transport systems and local adaptations for meiofauna in hydrothermal vent fields.

We investigated the genus diversity and community composition of harpacticoid copepods and their relationship with environmental factors around the Kuril Trench (490–7090 m). Harpacticoid densities did not decrease with water depth and were highest at 1000 m water depth. Diversity values based on the number of genera, Shannon diversity and the expected number of genera (rarefaction) indicated unimodal patterns with water depth, with peaks at intermediate depth; genus evenness increased with water depth and slightly decreased at hadal depths. This result suggested that the general relationship between water depth and diversity described for macrofauna and megafauna could be extended to meiofauna across all depth ranges. However, the regulating factor that affects harpacticoid diversity could not be identified. The community composition of harpacticoids gradually changed with water depth (from bathyal to hadal depths). In addition, comparison of assemblages between the trench slope, trench floor and abyssal plain suggested that the community found at hadal depth was largely different from those found on the trench slope and abyssal plain. Multivariate analyses suggested that water depth, or certain factors associated with water depth, affects harpacticoid assemblages around the Kuril Trench.

Monitoring sediment microbial community metabolism and structure is instrumental to understanding biogeochemical processes in and ecological impacts on bottom environments. The aim of this study was to determine potential community respiration and to reveal community dynamics of the microorganisms in the dead zone sediments of Omura Bay, Japan. The bay is highly enclosed and develops severe hypoxia in the central regions every summer. We collected sediment core samples from the center of the bay during hypoxia, estimated sediment oxygen consumption by using an adapted in vivo electron transport system activity (in vivo ETSA) assay, enumerated abundance of bacteria, and analyzed bacterial community structure by automated ribosomal intergenic spacer analysis. Higher ETSA and bacterial diversity were found in upper sediments (within 3 cm depth) from the center than the fringe of the bay. Sediment bacterial community structure of the bay center was distinct from that of the fringe. From these results, upper sediment in the dead zone of Omura Bay was characterized by (1) greater community respiration and (2) greater diversity of bacterial components compared with the non-hypoxic sediment of the bay fringe. These characteristics have important implications for understanding the interaction between microbial communities and the development of hypoxia in Omura Bay.

In tidal flat sediments, larger invertebrates often modify the sedimentary environment in a manner that causes smaller animals, such as meiofauna, to become more abundant. In other words, macrofauna can “promote” meiofauna. In summer (August) and autumn (November/early December) 2007, we investigated centimetre-scale horizontal and vertical profiles of total metazoan meiofaunal abundance and community structure around burrows of Uca lactea (De Haan, 1835) (fiddler crabs) and Scopimera globosa (De Haan, 1835) (sand bubbler crabs) on an intertidal sandflat of the Amakusa Islands, western Japan. Meiofaunal community structure changed significantly with sediment depth around burrows of U. lactea in both seasons, whereas vertical differences were less clear for S. globosa. Although no evidence was found that total meiofaunal abundance or the abundances of certain taxonomic groups were positively influenced by burrows of S. globosa, the abundance of nematodes in subsurface sediment around burrows of U. lactea was significantly higher than in controls in autumn. High concentrations of fresh organic matter had not accumulated in the sediment around burrows of either species of crab. Uca lactea, however, made its burrows in significantly finer sediment than did S. globosa. One possibility is that differences in meiofaunal distribution in sediment around the burrows of the two crabs were the result of habitat and behavioural differences between the species.

To examine the specific features of trench communities, spatial changes in the assemblages of harpacticoids were investigated at the family level around the Ryukyu Trench (the Ryukyu region) and Kuril Trench (the Kuril region). In the Ryukyu region, there were high average dissimilarities in the harpacticoid assemblages among the trench, trench slope and abyssal plain, indicating that the assemblage structures differ substantially between these topographic settings at the family level. Conversely, in the Kuril region, the average dissimilarities in harpacticoid assemblages between the trench and the trench slope and between the trench and the abyssal plain were lower than that between the slope and the abyssal plain. This result suggests that the hadal assemblage is a transition zone between the slope and the abyssal plain in this region. In addition, the analyses indicate that the composition of harpacticoid assemblages is influenced by the quantity of organic matter in the Ryukyu region, while sediment properties play a key role in the Kuril region. Comparisons of the assemblages between the two regions, however, revealed that the average dissimilarities between the trenches and between the abyssal plains were higher than that between the adjacent slopes. This result suggests that interchange among regions is difficult for deep-sea benthic animals, including harpacticoid copepods, likely due to the presence of physical barriers around trenches.

The abundance and biomass of metazoan meiofauna and their relationships with environmental factors [chloroplastic pigment equivalents (CPE) and sediment characteristics] were studied quantitatively around and within the Kuril Trench (560–7090 m) and the Ryukyu Trench (1290–7150 m), which are located in eutrophic and oligotrophic regions, respectively, of the western North Pacific. Faunal abundance and biomass, as well as the CPE content of sediments, were considerably higher in the Kuril region than in the Ryukyu region. In both cases, CPE tended to decrease with water depth, but relatively high values were found in the deepest areas, suggesting that organic matter has accumulated in both trenches. Meiofaunal abundance and biomass were lower than expected from sediment CPE values at hadal stations below 6000 m. Differences in the density and biomass of meiofauna between these two trenches appeared to reflect differences in overall ocean productivity above them. When the analysis was restricted to each region, however, no association was found between the abundance and biomass of meiofauna and food availability. Furthermore, the factors regulating the bathymetric patterns in these meiofaunal parameters appeared to differ between the two trenches.

We investigated the sex differences in the spatiotemporal distribution, body length, and gut contents of adult Neocervinia itoi, the most abundant cerviniid harpacticoid in Sagami Bay, Japan. Two other cerviniid species, Cerviniopsis sp. A and Cervinia bradyi, were also examined for comparison. The density of N. itoi tended to be higher at stations in or below sea bights with steep slopes in the bay, although there was no correlation between their distribution and chloroplastic pigment equivalent (CPE) in the sediment. In general, the sex ratio was strongly biased toward females in this species; the overall mean percentage of males was approximately 23% of adults. The sex ratio seemed to fluctuate temporally, even at the same site, although this fluctuation was not statistically significant. Comparisons of body size and sex ratio between the three species suggest that sampling bias was not the only cause for the observed skewed sex ratio. Distinct sex differences were observed in gut contents of N. itoi. Detritus-like gut contents were clearly observed in the urosomes of almost all females, whereas few males had gut contents. This result suggests that males do not forage during the adult stage. This non-feeding habit of males may result in a shorter lifespan than that of females and may be one cause of the skewed sex ratio. It remains unknown, however, whether this is a general trend among cerviniid species.

We examined the trophic levels of deep-sea benthic foraminifera and metazoans based on stable carbon and nitrogen isotopic compositions of soft tissue to evaluate the role of foraminifera in deep-sea benthic ecosystems. Living benthic foraminifera and metazoans were collected from 2 bathyal sites in Sagami Bay, Japan (water depths 750 and 1430 m) on 3 occasions (April 2004, November 2004 and August 2005). Both carbon and nitrogen isotopic compositions significantly dif- fered among the analysed foraminiferal species. At the deeper site δ 15 N of the benthic foraminifera ranged from 6.7 to 11.0‰ (typically 7 to 10‰) with considerable interspecies variations. This implies that most benthic foraminifera utilize primarily surface sediments (4.5‰) or particulate organic mat- ter (6.4‰) as their food sources. Many metazoan meiobenthic organisms, in particular polychaetes of meiofaunal size, some harpacticoid copepods, and kinorhynchs (examined only at the shallower site), exhibited δ 15 N heavier than foraminifera, suggesting that they occupy higher trophic levels than benthic foraminifera. Macro- and megabenthos (spatangoids, ophiuroids and Dentalioida) exhibited δ 15 N of 10 to 14‰, suggesting they belong in trophic levels 1 to 2 steps higher than metazoan meiobenthos and benthic foraminifera. Similar isotopic trends were observed at the shallower site. Combining the isotopic evidences and the observations on gut contents of some metazoan meio- benthos, together with previous experimental results, the benthic foraminifera in the bathyal Sagami Bay are considered a bridge in the energy flow from phytodetritus and sediments to metazoans.

To estimate species turnover rates on scales of several tens of km in deep-sea benthic animals, we analyzed spatial and inter-annual changes in species diversity and composition of cerviniids, a typical group of deep-sea harpacticoids, at stations in and around Sagami Bay, central Japan. Associations with environmental factors were also investigated. Generally, bathymetrical patterns in diversity of benthos are unimodal and peak at depths of 2,000–3,000 m. In Sagami Bay, cerviniid diversity did not follow this trend; both species richness and evenness were negatively correlated with water depth. Multivariate analyses [detrended correspondence analysis (DCA) and non-metric multi-dimensional scaling] suggested that temporal changes in species composition of cerviniids are smaller than spatial changes that occur on horizontal scales of several tens of km. Community structure does not change completely on these scales in the bathyal zone around Sagami Bay. DCA also showed that bathymetrical changes in species composition can be regulated by certain factors associated with water depth.

Standing stocks of deep-sea metazoan meiofauna were investigated in the Sulu Sea, one of the most isolated marginal basins with comparably warm bottom-water temperatures (∼10 °C) at depths below 1000 m. A decline in the abundance and biomass of organisms with increasing water depth occurred in the basin, but the abundances at bathyal and abyssal sites in the Sulu Basin appeared to be lower than standard values worldwide when adjusted for water depth. There is no significant correlation between meiofaunal abundance and the concentration of chloroplastic pigment equivalents (CPE) in the sediment, an indicator of the amount of organic matter derived from primary production. The ratios of meiofaunal abundance to CPE concentration at the sampling sites were as small as those observed at comparable depths in the Red Sea. These suggest that the quantity of “food” is not a primary factor limiting the density of organisms in the basin. It is hypothesized that a higher respiratory activity of benthos caused by warm bottom water and a lower food quality could be important factors regulating the metazoan meiofaunal standing stocks in the Sulu Sea.

Benthic foraminiferal food sources were examined in the central part of Sagami Bay, Japan (water depth 1450 m) based on an in situ feeding experiment with 13 C-labeled food materials. In this study, 3 different 13 C-labeled food materials were used: the unicellular marine algae Dunaliella tertiolecta, the marine diatom Chaetoceros sociale, and the marine bacterium Vibrio algi- nolyticus. The first two are representatives of phytodetritus and the third of organic matter produced in the sediments. Each type of food material was injected into a series of in situ culture cores and incubated for up to 21 d. We observed that some benthic foraminiferal species selectively ingested 13 C-labeled algae from the sedimentary organic matter. On the other hand, benthic foraminifera ingested 13 C-labeled bacteria unselectively from sedimentary organic matter. Total benthic forami- nifera assimilated 8.8 mg C m -2 d -1 of sedimentary organic matter without phytodetritus assimilation. Based on the assimilation rates estimated in this experiment, we recognized 3 types of feeding strat- egy among deep-sea benthic foraminifera in Sagami Bay. There are those that ingest (1) fresh phyto- detritus selectively (phytophagous species: Uvigerina akitaensis, Bolivina spissa, Bolivina pacifica); (2) fresh phytodetritus selectively but sedimentary organic matter as well when phytodetritus is absent (seasonal-phytophagous species: Bulimina aculeata, Textularia kattegatensis, Globobulimina affinis); and (3) sedimentary organic matter at random (deposit feeders: Cyclammina cancellata, Chilostomella ovoidea). These different types of carbon utilization should be considered not only for understanding modern ecosystems on the deep-sea floor but also for paleoceanographic reconstruc- tions using the abundance and distribution, or isotopic composition, of benthic foraminifera.

Little is known about temporal changes in the diversity and species composition of deep-sea metazoan meiofauna and their relationships with changes in the food supply. Those changes were studied for benthic copepod assemblages based on 2-year time-series data at a bathyal site in Sagami Bay (1430 m depth), central Japan, where annual fluctuation in the abundance of benthic foraminiferans was previously observed. Species diversity of benthic copepods at the site was as high as, or slightly higher than, that observed at other deep-sea sites, but did not fluctuate temporally through the study period. Multivariate analyses did not reveal any clear seasonal or directional change occurring over the longer term in their species composition, although there was some consistent pattern. These results indicate a lack of, or only weak, seasonality in the diversity and species structure of the deep-sea benthic copepod assemblages, even though the fresh organic food supply fluctuates seasonally. They also suggest that there are differences between copepods and foraminiferans in the response to changes in environmental factors, and that spatial differences in the composition of copepod communities are greater than temporal ones at this deep-sea site.

Profiles of the radioisotopes 210 Pb and 137 Cs were determined in 15 sediment cores collected from Sagami Bay, Japan. The activities of 210 Pb ex (unsupported) in core top sediments increased with water depth from 25 dpm g −1 on the upper continental slope off the mouth of Tokyo Bay to an average of 283 dpm g −1 at the deep-sea station SB. The high 210 Pb trapping efficiency of settling particles expected from the results of the sediment trap experiment near the SB site suggests that effective 210 Pb enrichment in surface sediments may occur during resuspension and lateral transportation of particles via the benthic nepheloid layer on the continental slope. In several cores, 137 Cs profiles showed an increase, a distinct peak, and then a decrease to an undetectable level downcore. These profiles can be compared with the temporal change of bomb-produced 137 Cs fallout. The mean sedimentation rates estimated by the 210 Pb ex inventory method, rather than using 210 Pb ex profiles, ranged from 0.06 g cm −2 y −1 to 0.14 g cm −2 y −1 . The average value of the rates in SB cores was calculated to be 0.11 g cm −2 y −1 , which was similar to that calculated under the assumption that the age of the 137 Cs peak corresponds to its maximum fallout year in 1963. Although 137 Cs inventories represented one tenth of the anthropogenic fallout of 137 Cs until 1997, they correlated with the increase in 210 Pb ex inventory. This suggests that the scavenging of refractory 137 Cs as well as 210 Pb by settling particles in the water column can lead to the formation of a time marker layer even in deep-sea sediment core, such as at the SB site.

Sex ratios and reproductive activity of benthic copepod assemblages were investigated at the bathyal site (depth 1430 m) in Sagami Bay, central Japan. The ratio of adult females to adult males was approximately 3.5:1, significantly different from 1:1, although this parameter did not show a seasonal pattern. On the other hand, the percentage of ovigerous females among adult females and the ratio of nauplii to total copepods appeared to fluctuate seasonally in 1997 and 1998. Statistical tests, however, could not detect significant difference in either parameter. We discuss the possibility that the reproductive activity of copepods was enhanced by the increased supply of fresh phytodetritus to the sea floor.

Deep-sea benthic ecosystems are mainly sustained by sinking organic materials that are produced in the euphotic zone. “Benthic–pelagic coupling” is the key to understanding both material cycles and benthic ecology in deep-sea environments, in particular in topographically flat open oceanic settings. However, it remains unclear whether “benthic–pelagic coupling” exists in eutrophic deep-sea environments at the ocean margins where areas of undulating and steep bottom topography are partly closely surrounded by land. Land-locked deep-sea settings may be characterized by different particle behaviors both in the water column and in relation to submarine topography. Mechanisms of particle accumulation may be different from those found in open ocean sedimentary systems. An interdisciplinary programme, “Project Sagami”, was carried out to understand seasonal carbon cycling in a eutrophic deep-sea environment (Sagami Bay) with steep bottom topography along the western margin of the Pacific, off central Japan. We collected data from ocean color photographs obtained using a sea observation satellite, surface water samples, hydrographic casts with turbidity sensor, sediment trap moorings and multiple core samplings at a permanent station in the central part of Sagami Bay between 1997 and 1998. Bottom nepheloid layers were also observed in video images recorded at a real-time, sea-floor observatory off Hatsushima in Sagami Bay. Distinct spring blooms were observed during mid-February through May in 1997. Mass flux deposited in sediment traps did not show a distinct spring bloom signal because of the influence of resuspended materials. However, dense clouds of suspended particles were observed only in the spring in the benthic nepheloid layer. This phenomenon corresponds well to the increased deposition of phytodetritus after the spring bloom. A phytodetrital layer started to form on the sediment surface about two weeks after the start of the spring bloom. Chlorophyll-a was detected in the top 2 cm of the sediment only when a phytodetritus layer was present. Protozoan and metazoan meiobenthos increased in density after phytodetritus deposition. Thus, “benthic–pelagic coupling” was certainly observed even in a marginal ocean environment with undulated bottom topography. Seasonal changes in features of the sediment–water interface were also documented.

Temporal variations of sinking particle flux, together with their organic chemical properties, were monitored in the deep basin of Sagami Bay, Japan, using sediment traps with very high time resolutions from March 1997 to August 1998. At a height of 350 m above the bottom (about 1200 m water depth), the averaged total mass flux was more than 1000 mg/m2/day, which is about 10 times higher than those obtained for open ocean regions near Sagami Bay. While large amounts of phytodetritus, derived from phytoplankton blooms in the surface water, were transported downward in spring, the following extraordinary patterns in the temporal variability of sinking particle flux were also observed: (1) A sustained large flux of sinking particles during low productive periods from summer to winter in 1997. (2) An episodic increase of sinking particle flux in June 1998. (3) A difference in the temporal variability of sinking particles between the spring bloom periods of 1997 and 1998. The content of total organic carbon (TOC) and the stable carbon isotopic ratio (δ13C) of TOC demonstrated that the large fluxes observed in (1) and (2) could be attributed to the resuspension of phytodetritus deposited on the sea floor during the spring bloom period, and the abrupt erosion of surface sediment on the continental slope, respectively. The concentration of suspended particles in the deep water column affect the apparent flux of sinking particles. At the same time, sinking particles exported from surface waters during the spring bloom both decrease and increase suspended particle concentration through scavenging and rebound processes, respectively. Finally, the apparent difference in sinking particle flux between 1997 and 1998, (3), could be explained by differences in the extent of the scavenging process, which depend on the flux and quality of exported particles from the surface waters.

The seasonal carbon cycle was studied in the bathyal environment of Sagami Bay, Japan, to determine whether “benthic–pelagic coupling” takes place in this eutrophic marginal oceanic setting. Both Japanese sea color observation satellite (ADEOS) photography and sediment trap moorings have been used since 1996 for monitoring sea surface primary production. Video records at a real time deep-sea floor observatory off Hatsushima Island in Sagami Bay were also used to monitor the deposition of phytodetritus on the sea floor. At this location, a spring bloom starts in mid-February and ends in mid-April. About 2 weeks after the start of the spring bloom, phytodetrital material is deposited on the sea floor. Video records clearly show that phytodetritus deposition has taken place in the spring of every year since 1994, even though the exact timing is different from year to year. The population size of benthic foraminifera is highly correlated to this phytodetritus deposition. The phytodetritus triggers rapid, opportunistic reproduction of the shallow infaunal taxa, Bolivina pacifica, Stainforthia apertura and Textularia kattegatensis. Shallow infaunal species mainly occur in the phytodetrital layer or just below this layer during the spring. This indicates that such opportunistic species are key indicators of phytodetrital deposition. The deep infaunal taxa Globobulimina affinis and Chilostomella ovoidea show less pronounced seasonal fluctuations in population size, but nevertheless exhibit some response to phytodetrital deposition. Thus the seasonal flux of organic matter is the most important determinant of population size, microhabitats and reproduction of benthic foraminifera in Sagami Bay.

Temporal changes in the vertical distribution of metazoan meiofaunal groups were examined at a bathyal site (depth 1450 m) in Sagami Bay from December 1996 to August 1998. Among major meiofaunal groups, the vertical distributions of copepods (adults and copepodites), and kinorhynchs changed seasonally and they concentrated in the shallower part in the sediment profile when CPE concentrations were higher. Seasonal changes in the vertical distributions of ovigerous females and nauplii of copepods were not related to CPE concentrations, but related to that of total copepods. On the other hand, nematodes, ostracods, and polychaetes did not show seasonal migrations. These results suggest that there is a difference in the response to the sedimentation of organic matter between metazoan meiofaunal taxa.

An interdisciplinary research project was carried out to understand seasonal carbon cycling in a deep-sea ecosystem in Sagami Bay, central Japan. Temporal-changes in the chloroplastic pigment (CPE) concentration in the sediment, as well as the abundance, the biomass and the metabolic activity of benthic organisms were studied. CPE was detectable throughout the year, and its amount increased in the spring when a fluffy layer was observed on the surface of the sediment. Significant seasonal fluctuation of bacterial abundance was found, but the range was small, the maximal value being only 1.6 times larger than the minimal one. Metabolic activity did not show significant temporal difference. The abundance and biomass of metazoan meiofauna seemed to fluctuate seasonally, but ANOVA did not confirm it statistically. Our results suggest that if enough organic matter is supplied constantly, the deep-sea benthic community will be stable even though seasonality of organic matter flux associated with the spring bloom exists., Un programme de recherche interdisciplinaire a été consacré au cycle saisonnier du carbone dans un écosystème profond de la baie de Sagami, au centre du Japon. Les variations temporelles de la concentration du pigment chloroplastique (CPE) dans le sédiment ont été étudiées, ainsi que l'abondance, la biomasse et l'activité métabolique des organismes benthiques. La concentration CPE est détectable pendant toute l'année, avec une augmentation au printemps lorsqu'une couche pelucheuse est observée à la surface du sédiment. L'abondance bactérienne présente une fluctuation saisonnière significative mais de faible amplitude, avec un rapport de 1,6 entre les valeurs maximale et minimale. L'activité métabolique ne varie pas de manière significative avec le temps. L'abondance et la biomasse de la méiofaune de métazoaires semblent fluctuer avec les saisons, sans confirmation statistique Anova. Les résultats montrent que la communauté benthique profonde reste stable si l'apport de matière organique est suffisant et continu, même avec une variabilité saisonnière associée à la floraison printanière.

A new aegisthid copepod, Cerviniopsis reducta sp. nov. is described from the deep sea in Sagami Bay, Japan. The newspecies has superficial resemblance to C. minutiseta Ito, 1983 in the armature formula of swimming legs. However theydiffer from each other in the shape of setae of the swimming legs, the distal margin of operculum, length of caudal rami,and the location of setae on P5 exopod. Also, the male of Neocervinia itoi Lee & Yoo, 1998 is described on the basis ofsamples collected from around the type locality in Sagami Bay, Japan. Sexual dimorphism of N. itoi male can be observedin the fused rostrum, atrophied mouthparts, P5, and P6. The sixth leg is symmetrical and both gonopores are presumablyactive, based on the presence of two spermatophores internally in the genital segment. This paper reports for the first timeon the sexually dimorphic characters in the genus Neocervinia Huys, Mobjerg & Kristensen, 1997, reinstating its generic status with the newly revealed male characters.