Kawasaki, Japan
Kawasaki, Japan

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Yoshiki T.M.,Japan Agency for Marine - Earth Science and Technology | Yoshiki T.M.,Japan National Research Institute of Fisheries Science | Chiba S.,Japan Agency for Marine - Earth Science and Technology | Sasaki Y.,Suidosha Co. | And 3 more authors.
Fisheries Oceanography | Year: 2015

Spatial and temporal variation in copepod community structure, abundance, distribution and biodiversity were examined in the western subarctic North Pacific (40-53°N, 144-173°E) during 2001-2013. Continuous Plankton Recorder (CPR) observational data during the summer season (June and July) were analyzed. The latitudinal distribution of warm-water species in June shifted northward after 2011 while no apparent latitudinal shift of cold-water and other species was observed. Species number and the Shannon-Wiener biodiversity index (H′) in June tended to increase in the northern area after 2011. The warm-water species abundance and center latitude of warm-water distribution were positively correlated with sea surface temperature (SST) across sampling locations, whereas no significant correlations with SST were observed for cold-water species or other species. Warm SSTs in June after 2011 appeared to cause the northward shift of warm-water species distribution, which in turn contributed to the higher biodiversity in the northern area. This study demonstrated the rapid response of warm-water species to warm SST variation, whereas cold-water and other species did not exhibit such clear responses. These findings indicate that the response of copepods to environmental changes differs among copepod species, highlighting the importance of investigating lower trophic levels to the species level to evaluate individual species' responses to climate change. © 2015 John Wiley & Sons Ltd.

Itoh H.,Suidosha Co. | Nakata K.,Fisheries Research Agency | Sasaki K.,Nishi 23 | Ichikawa T.,Japan National Research Institute of Fisheries Science | Hidaka K.,Japan National Research Institute of Fisheries Science
Plankton and Benthos Research | Year: 2014

Species composition and vertical distribution of oncaeid copepods, which are potentially important prey for juvenile fish, were investigated in the Kuroshio Extension region, the NW Pacific, in April, August, November 1998 and February 2001. Samples were collected from 8 discrete layers in the epipelagic zone (0–200 m depth) using MOCNESS (0.064 mm mesh) during both day and night. Thirty-five oncaeid species were identified. ‘Oncaea’ (s.l.) zernovi and Spinoncaea ivlevi were numerically the dominant species comprising 20.0–48.2% and 15.2–26.8%, respectively, of adult oncaeid copepods in the epipelagic zone. Cluster analysis on all samples revealed that these were separated into three groups with discrete vertical ranges; the first one appearing in the 0–50 m depth surface layer in April and August and consisting mainly of Oncaea (s. str.), the second one located in the deepest layer and composed mostly of ‘O.’ zernovi and S. ivlevi with some mesopelagic species, and the third one located above the second one and having intermediate species composition. Species-specific vertical distributions indicate that most oncaeid populations shifted downward from August to November, when the thermocline remarkably descended. However, most Oncaea spp. did not show a downward shift with the thermocline, and were positively correlated to appendicularian abundances, suggesting that appendicularian houses, known to be oncaeid habitats and to provide food, were a possible factor affecting their vertical distribution. Niche partitioning, allowing coexistence of congeners, might be explained by differences in body size and distribution layers in Oncaea and by differences in distribution layer in Triconia. © The Plankton Society of Japan.

Chiba S.,Japan Agency for Marine - Earth Science and Technology | Batten S.D.,Sir Alister Hardy Foundation for Ocean Science | Yoshiki T.,Japan Agency for Marine - Earth Science and Technology | Sasaki Y.,Suidosha Co. | And 3 more authors.
Ecology and Evolution | Year: 2015

The global distribution of zooplankton community structure is known to follow latitudinal temperature gradients: larger species in cooler, higher latitudinal regions. However, interspecific relationships between temperature and size in zooplankton communities have not been fully examined in terms of temporal variation. To re-examine the relationship on a temporal scale and the effects of climate control thereon, we investigated the variation in copepod size structure in the eastern and western subarctic North Pacific in 2000-2011. This report presents the first basin-scale comparison of zooplankton community changes in the North Pacific based on a fully standardized data set obtained from the Continuous Plankton Recorder (CPR) survey. We found an increase in copepod community size (CCS) after 2006-2007 in the both regions because of the increased dominance of large cold-water species. Sea surface temperature varied in an east-west dipole manner, showing the typical Pacific Decadal Oscillation pattern: cooling in the east and warming in the west after 2006-2007. The observed positive correlation between CCS and sea surface temperature in the western North Pacific was inconsistent with the conventional interspecific temperature-size relationship. We explained this discrepancy by the geographical shift of the upper boundary of the thermal niche, the 9°C isotherm, of large cold-water species. In the eastern North Pacific, the boundary stretched northeast, to cover a large part of the sampling area after 2006-2007. In contrast, in the western North Pacific, the isotherm location hardly changed and the sampling area remained within its thermal niche throughout the study period, despite the warming that occurred. Our study suggests that while a climate-induced basin-scale cool-warm cycle can alter copepod community size and might subsequently impact the functions of the marine ecosystem in the North Pacific, the interspecific temperature-size relationship is not invariant and that understanding region-specific processes linking climate and ecosystem is indispensable. © 2015 The Authors.

Miyamoto H.,University of Tokyo | Miyamoto H.,Tohoku National Fisheries Research Institute | Kotori M.,7 48 1 Kurokawa Cho | Itoh H.,Toho University | And 2 more authors.
Journal of Plankton Research | Year: 2014

We analyzed the species composition of pelagic chaetognaths in the Indo-Pacific region as reported in published papers to demonstrate geographic variation in their species diversity and its relationships with environmental variables. The resulting latitudinal pattern in species richness (SR) was asymmetric to the equator, with a peak at 35°N in the Kuroshio area and a gradual decrease to a minimum at ∼40°S. Shannon's diversity index (H′) showed a similar pattern, except that it plateaued between 20°N and 35°S. Regression analyses and structural equation models (SEMs) showed that temperature was the most important explanatory variable, both for SR and H′. The SEM also demonstrated that chlorophyll a concentration and mixed-layer depth were the second most effective variables for SR and H′, respectively. On the basis of these results, models were constructed to estimate geographical variation in SR and H′. The estimated SR was high in coastal waters in tropical to subtropical areas, while H′ was high at midlatitudes in the open ocean. The present data highlight the importance of the coastal waters of Southeast Asia and the Indian Ocean, but also indicate that a gap still exists in this area, which necessitates further data mining as well as basic research. © 2014 The Author 2014.

Itoh H.,Toho University | Itoh H.,Suidosha Co. | Aoki N.,Chiyoda Corporation
Bulletin of the Plankton Society of Japan | Year: 2010

The seasonal occurrence of planktonic copepods was investigated by vertical hauls of a plankton net (mouth diameter: 36cm; mesh opening: 100 μm) and observations of resting eggs in bottom mud at a fixed station in the innermost part of Tokyo Bay during 1990-92. Fourteen species of Calanoida, 16 species of Cyclopoida and five species of Harpacticoida occurred in the study period. Number of species increased from summer to autumn by inflow of species living outside of the bay. The total abundance of copepodids ranged between 76.3 × 10 2-53.3 × 10 4 inds. m -3 with a peak in July in each year. The most dominant species, Oithona davisae, comprised more than 90% of the adult copepods in summer and fewer than 80% in autumnspring with reduction of its population and expansion of other copepod's populations. The seasonal mean-abundance was high in winter for Acartia omorii, Centropages abdominalis and Danielssenia typica, in summer for Oithona davisae and Oncaea waldemari and in autumn for Paracalanus parvus s.l, Parvocalanus crassirostris, Oithona simplex and Microsetella norvegica. The resting eggs of A. omorii, C. abdominalis, Centropages tenuiremis and Labidocera rotunda were found in the bottom mud from the study site, suggesting that these four calanoids maintain their population as resting-eggs in the seasons when their planktonic populations disappear from the water column, as known in other inlet waters of Japan. The occurrence of epibenthic copepods and resting eggs suggest that the shallow waters where oxygen depletion is relatively weak as in the study site, may play a vital role in sustaining the copepod fauna in Tokyo Bay. The mean abundance of O. davisae in July in the present study was more than five times as much as that obtained in the inner part of the bay in 1948. An estimation on the long-term fluctuation of abundance of this species based on the observed data of temperature, salinity and transparency (converted to chlorophyll-a) since 1950 and a multiple regression formula for the relationships between the copepods' abundance and these environmental factors obtained in the present study period shows that abundance of O. davisae reached more than twice the present level in the 1960s and decreased rapidly in the 1980s. © The Plankton Society of Japan 2010.

Miyamoto H.,University of Tokyo | Matsuura H.,Tokai University | Itoh H.,Suidosha Co. | Mizushima T.,Tokai University | Kubota T.,Tokai University
Bulletin of the Plankton Society of Japan | Year: 2012

A review on the fauna of chaetognaths in Sagami and Suruga Bays, located on the Pacific coast of central Japan, was compiled here. Hitherto 30 species have been found in the bays, with 27 species occurring in both bays. The chaetognath fauna is composed of a small number of coastal species and a large number of oceanic species introduced mainly from the Kuroshio Current and North Pacific Intermediate Water. The three vertical zones (epipelagic, upper mesopelagic and lower mesopelagic zone) are recognized, corresponding to the vertical structure of water masses. However, undescribed species have been collected from deep water in recent studies, suggesting the existence of unknown benthopelagic fauna in the bays. The seasonal change of abundance and species composition is remarkable in the epipelagic zone; a coastal species and some oceanic species dominate in spring-summer and autumn-winter, respectively. There are marked differences between previous studies in the months of peak abundance and the magnitude of peaks. For a better understanding of the mechanisms causing such variations, it is necessary to investigate long-term seasonal changes on the basis of observations of the physical and biological environment, as well as study the population dynamics, life history, and interspecific relationships of chaetognaths.

Tachibana A.,Tokyo University of Marine Science and Technology | Ishimaru T.,Tokyo University of Marine Science and Technology | Itoh H.,Suidosha Co. | Yoshida Y.,Yokohama Port and Airport Technology Investigation Office
Journal of Oceanography | Year: 2013

Seasonal change in the meso-sized copepod community structure in the central part of Tokyo Bay was investigated from January 2006 to December 2008. Three seasonal community groups were detected, and seasonal shifts of these communities are explained by life history characteristics of indicator species and seasonal changes in the hydrographical environment. In the winter-spring community, Acartia omorii and Centropages abdominalis dominate because of high growth rates at low temperature. A shift to the early summer community is caused by a diapause of Ce. abdominalis as resting eggs and an increase in the growth and egg-production rate of Pseudodiaptomus marinus at high temperature. A shift to the summer-fall community is caused by a diapause of A. omorii at hypoxic and high temperature conditions and an increase of Temora turbinata, Paracalanus parvus and other oceanic species by an enhancement of the estuary circulation. Then, the community returns to a winter-spring one by the recovery of A. omorii and Ce. abdominalis with low temperatures and oxygenation of bottom water and by the disappearance of oceanic warm-water species at low temperature. Seasonal community shifts occurred almost regularly, but the shift from a winter-spring community to an early summer one occurred 1 month early in 2007 when the water temperature was warmer than in other years. © 2013 The Oceanographic Society of Japan and Springer Japan.

Itoh H.,Suidosha Co. | Nishida S.,University of Tokyo
Journal of Natural History | Year: 2015

The spatiotemporal distributions of the planktonic copepod communities in Tokyo Bay in 1986–1987, when it was in a more eutrophicated condition than at present, were examined using zooplankton samples collected by vertical hauls of Kitahara’s quantitative net (mesh opening, 0.1 mm) from 19 stations in July, August, October, December 1986 and February 1987. The total abundance of copepods ranged from 3 × 104 to 2750 × 104 ind. m−2 with peaks in July and August and a remarkable decrease in October. Oithona davisae dominated most samples with the subdominant species Acartia omorii in the inner and central areas in December and February, and Paracalanus parvus s.l. in the outer area in October, December and February. The copepod communities in the inner and central areas were classified into 2–4 groups in July, August and October, whereas they mostly comprised a single group in December and February. These differences in distribution pattern might be related to seasonal changes in environmental factors such as river discharge, intrusion of high-salinity outer-bay water, and hypoxic water. In comparison with the community in July 1948, the area of dominance of O. davisae had expanded to the whole bay and A. omorii had decreased in the inner area in July 1986, while other copepods such as P. parvus s.l. and Microsetella norvegica had decreased in the whole bay. © 2015 Taylor & Francis.

Itoh H.,Suidosha Co. | Nishioka J.,Hokkaido University | Tsuda A.,University of Tokyo
Progress in Oceanography | Year: 2014

We investigated the community structure of mesozooplankton in the western part of the Sea of Okhotsk in late summer, 2006. We recognized four communities belonging to two assemblages. A coastal assemblage dominated by the arctic planktonic snail Limacina helicina consisted of a gulf community characterized by brackish copepods and a continental shelf community characterized by the hydrozoan medusa Aglantha digitale and the arctic copepod Calanus glacialis. The other assemblage, characterized by the oceanic copepod Neocalanus plumchrus, consisted of a continental slope community characterized by a diverse species composition and a basin community characterized by the oceanic copepod N. cristatus. The continental slope community contained species from the coastal waters and was distributed along the course of the East Sakhalin current. This community may have been assembled by the incorporation of coastal water into the oceanic waters by the strong current. Small coastal copepods such as Oithona similis and Pseudocalanus spp. were the main components in all communities in terms of numbers, but larger copepods such as Neocalanus spp. and Metridia okhotensis were important in terms of weight, especially in the continental slope and basin communities. The population structures of the dominant species suggest that overall biological production is maintained by continuous reproduction or growth (or both) of L. helicina and small coastal copepods after the onset of seasonal dormancy of the large oceanic copepods in late summer. © 2014 Elsevier Ltd.

Itoh H.,Suidosha Co. | Nishida S.,University of Tokyo
Plankton and Benthos Research | Year: 2014

We examined the potential advantage of the association of Hemicyclops species with decapod burrows in their avoidance from fish predators through gut-content analysis of fishes collected from the estuary of the Tama River, Japan, and an experiment applying model burrows. The small gobiid species Pseudogobius masago, collected from an area dominated by burrows of the ocypodid crab Macrophthalmus japonicus, showed a high frequency of Hemicyclops gomsoensis in their guts relative to the other co-occurring fish species. In the experiment, no significant difference was observed in the numbers of H. gomsoensis eaten by the goby between the treatments with and without the model burrows. Another goby, Acanthogobius flavimanus, from an area dominated by burrows of the mud shrimp Upogebia major, showed very low frequencies of H. gomsoensis in their guts, as compared with those in P. masago. They had also ingested H. spinulosus and H. ctenidis, in spite of the overwhelmingly high abundance of H. gomsoensis in the burrows. The number of H. gomsoensis eaten by A. flavimanus was significantly lower in the treatments with the model burrows than in the absence of burrows. Since A. flavimanus was the most dominant demersal fish in the study area, H. gomsoensis seemed to avoid predation from this potentially strong predator by inhabiting the decapod burrows. H. ctenidis and H. spinulosus seemed to avoid predation by trespassers by inhabiting the smaller burrows of polychaetes, but this strategy may be less efficient against large predators that feed on polychaetes. © The Plankton Society of Japan.

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