Content uploaded by Moriaki Yasuhara
Author content
All content in this area was uploaded by Moriaki Yasuhara
Content may be subject to copyright.
A preview of the PDF is not available
Recent Ostracoda from the Northeastern part of Osaka Bay, Southwestern Japan
Abstract and Figures
Ostracodes from Osaka Bay, southwestern Japan, were quantitatively studied in order to examine the relationships between environmental factors and species distribution. At least 109 species belonging to 43 genera were identified from 31 surface sediment samples. On the basis of Q-mode cluster analysis, four biotopes (A, B, C and D) are recognized. Characteristic species of each biotope are as follows: A (central bay at 17.2-37.2 m water depth), Trachyleberis scabrocuneata, Cytheropteron miurense, Kobayashiina donghaiensis, Schizocythere kishinouyei and Krithe japonica; B (inner part of the bay at 16.3-21. 1 m water depth), Bicornucythere bisanensis (form A), Bicornucythere bisanensis (form M), Cytheromorpha acupunctata, Loxoconcha viva and Nipponocythere bicarinata; C (innermost part of the bay at 9.2-15.9 m water depth), B. bisanensis (form A), Loxoconcha tosaensis and Spinileberis quadriaculeata; D (inner part of the bay at 12.6 m water depth), C. acupunctata. These results suggest that species distribution is controlled by bathymetry. High abundance and high species diversities are found in the westernmost area. In contrast, low abundance and low species diversities are observed in the eastern area, where water masses with low dissolved oxygen content spread, especially in the summer. It is shown that the tidal current and the dissolved oxygen contents strongly influence abundance and species diversity of ostracodes in Osaka Bay.
Figures - uploaded by Moriaki Yasuhara
Author content
All figure content in this area was uploaded by Moriaki Yasuhara
Content may be subject to copyright.
... 1-5 sensu Irizuki (1994); Semicytherura sp. sensu Irizuki et al. (2005); Semicytherura sp. 1 sensu Yasuhara and Irizuki (2001); Semicytherura sp. 2 sensu Yamada et al. (2002); Semicytherura sp. 1-2 sensu ; Semicytherura sp. ...
... 11, fig. 8 in Yasuhara and Irizuki 2001). This variation in number of anteroventral denticles indicates that the specimens of Yasuhara and Irizuki (2001) belong to S. kazahana (Fig. 4H), unless they present juvenile and adult of separate species. ...
... 8 in Yasuhara and Irizuki 2001). This variation in number of anteroventral denticles indicates that the specimens of Yasuhara and Irizuki (2001) belong to S. kazahana (Fig. 4H), unless they present juvenile and adult of separate species. However, although the geographical distribution of S. kazahana and S. sasameyuki overlaps, their microhabitats differ with the former being a phytal species typical of rocky shore environments, and the latter living on the silty sands of inner bays (Yamada et al. 2005). ...
The genus Semicytherura Wagner, 1957 has nearly 300 species, is common in shallow and marginal marine habitats, and has a worldwide distribution. It is divided into several species groups, of which the Semicytherura henryhowei Hanai & Ikeya, 1977 group is one of the most frequently recorded in temperate Asia. A previous study indicated that many of its members are actually species complexes, and that several morphotypes could be distinguished by carapace shape and ornamentation. We review these complexes and conclude that the henryhowei group currently contains 29 species, nine of which are undescribed. We also provide an illustrated guide and a key to species, based on newly standardized carapace ridge terminology. This enabled us to describe one new species from the extant (i.e., present-day) sediments in Jeju Island, Korea, S. kiosti sp. nov. We also found one juvenile valve of S. kazahana Yamada, Tsukagoshi & Ikeya, 2005, the first official illustrated record of this species from Korean waters. Our revised spatial and temporal distributions of fossil and extant records from this group provide new insights into trans-Arctic interchange of ostracod fauna from the Late Miocene onwards.
... However, highly refractory OM content in oxygen-poor sediments, as found in mud flats and mangroves, can depress both diversity and density since both variables may vary from high (Sharma et al. 2021) to very low (Cai et al. 2020). In contrast to nematodes, most other meiofauna taxa, including copepods and ostracods, are often absent or occur in very low abundance in hypoxic and anoxic sediments (Yasuhara and Irizuki 2001;Yasuhara and Yamazaki 2005;Yasuhara et al. 2012a;Yasuhara 2019). Crustaceans are much more sensitive to oxygen depletion and will decrease in density and diversity much more quickly along an oxygen gradient than nematodes (Elmgren 1975; see Chaps. ...
... Among them, S. bradii, Cytheropteron nodosoalatum, H. fascis, Rabilimis mirabilis, and R. septentrionalis are considered to be circumpolar species (Cronin & Ikeya, 1987;J. Liu et al., 2018;Stepanova et al., 2003;Zhao et al., 2014); Acanthocythereis mutsuensis is considered to be a cold-water species (Zhao & Wang, 1990); Amphileberis nipponica, Aurila cymba, Bicornucythere bisanensis, Cytheromorpha acupunctata, Cytheropteron donghaiense, Pistocythereis bradyformis, Spinileberis quadriaculeata, and Trachyleberis niitsumai are regarded as temperate species (Brandão et al., 2013;Hou & Gou, 2007;Yasuhara & Irizuki, 2001;Yasuhara & Seto, 2006); and Albileberis sinensis, Neomonoceratina delicata, Neosinocythere elongata, and Sinocytheridea impressa are treated as subtropical based on their modern distributions (Cao, 1998;Hong et al., 2017Hong et al., , 2019Hou & Gou, 2007;Irizuki et al., 2009;Niiyama et al., 2019;Yajima et al., 1986;Figure S3 in Supporting Information S1). The autoecologies of the dominant species are summarized in Table S4 in Supporting Information S1. ...
The East Asian winter monsoon (EAWM) is associated with northerly surface winds affecting crop and livestock productivity and social and economic activities across East Asia. However, the relationship between EAWM dynamics and marine biota remains poorly understood. In this study, we analyzed fossil ostracods from sediment cores collected in the northwestern Pacific Ocean to detect the appearance/disappearance of circumpolar species during the late Quaternary. We compared our ostracod records, including dating results, with ice‐rafted debris records from the Sea of Japan, and identified two southward migration events of Arctic ostracods at 120–100 ka (Marine Isotope Stage [MIS] 5d stadial) and 30–15 ka (MIS 3 and 2, peak Last Glacial), which corresponded to a strengthened EAWM system. Our results indicate that an intensified EAWM influenced the Yellow Sea Bottom Cold Water formation, lowered the winter water temperatures, and allowed Arctic ostracods to migrate southward into the Yellow Sea.
... This biofacies includes samples 02,03,04,13,14,15,16,17,23,27,28,30,31,37,41 Bicornucythere bisanensis s.l. is regarded as a temperate species and widely distributed in brackish water bodies along the northern coast of Vietnam, the coasts of China and Japan (e.g., Okubo, 1980;Zhao et al., 1985;Zhou et al., 1996;Yasuhara and Irizuki, 2001;Irizuki et al., 2003;Yasuhara and Seto, 2006;Irizuki et al., 2009;Tanaka et al., 2009;Hong et al., 2017;Hong et al., 2019;Cheung et al., 2019). This species also occurs commonly in inner bay environments with water depths from 7 to 80 m (Wang et al., 1980;Abe, 1988;Wang et al., 1988;Zhao and Wang, 1988). ...
Laizhou Bay is located in the Bohai Sea, eastern China. Since the Late Pleistocene, Laizhou Bay has gone through frequent transgressive and regressive activities and both marine and non-marine sediments have been deposited. In order to further understand the palaeoenvironmental changes since the Late Pleistocene, a core (DK06) was drilled on the coastal plain of Laizhou Bay. From this core, we obtained abundant well-preserved ostracod specimens. After environmental analysis and cluster analysis of these ostracods, four biofacies have been recognised: Biofacies Ⅰ (shallow marine facies), Biofacies Ⅱ (estuarine facies), Biofacies Ⅲ (brackish water facies), Biofacies Ⅳ (freshwater facies). The studied ostracod assemblages suggest that the palaeoenvironment of Laizhou Bay went through four transgressions including three well-known marine transgressions during MIS 1, MIS 3 and probably MIS 5 and three regressions since the late Pleistocene, indicating substantial impact of glacial-interglacial sea level changes on the Bohai Sea coastal areas and their biota.
... Focusstacked optical microscopic images were obtained with a VHX-2000 digital microscope (Keyence Ltd) at the CMCR. We examined the images and observed the specimens under a binocular microscope to identify ostracode taxa by referring to the literature (Ishizaki, 1968;Yasuhara and Irizuki, 2001;Irizuki et al., 2008) (Figure S1). We counted the number of valves and carapaces in each taxon in each sample. ...
The Philippine Sea plate subducts beneath the Eurasia plate at the Nankai Trough, northwestern Pacific, causing crustal deformation, mega‐thrust earthquakes, and tsunami events. Shikoku Island, 150 km northwest of the trough, experiences both coseismic and interseismic deformation. Coastal sediments potentially record vertical crustal movements as relative sea level (RSL) changes. We studied sedimentary facies and microfossil ostracodes in core SKM from southwestern Shikoku Island for evidence of middle Holocene tsunami events and deformation. The core sediments included nine event layers corresponding to storm or tsunami events. Using modern analog techniques, we estimated RSLs from the ostracode assemblages of core SKM and 13 other cores from Shikoku Island and the surrounding region. Then, we subtracted RSL changes due to glacio‐hydro isostatic adjustment from the estimated RSLs to estimate vertical tectonic movement rates in these cores between 8.6 and 4.7 ka. The inferred RSL changes suggest that the Sukumo site has experienced both uplift and subsidence since 8.6 ka. Before 6.6 ka, rates of tectonic crustal movement were higher than the modern‐day rate, and its spatial distribution also differed. After 6.6 ka, tectonic crustal movement showed a similar spatial pattern and occurred at rates close to the modern day interseismic rate. The spatial pattern and rates of tectonic crustal movement could be caused by changes in rupture areas between the Eurasia and the Philippine Sea plates beneath the Shikoku Island and in stress condition of the asthenosphere. Some of the vertical displacements can be explained by the movements of local active faults.
... 22 in Fig. 1) (Abe 1988) and Osaka Bay (Loc. 10 in Fig. 1) (Yasuhara & Irizuki 2001). In the present study, we compared the morphology of the male copulatory organs of various specimens of the genus Bicornucythere, which is widely distributed across the inner bays of Japan, to identify a new species and report patterns of morphological variation. ...
This study is designed to evaluate the male copulatory organs and carapace size of the inner bay benthic ostracod Bicornucythere bisanensis. The male copulatory organs demonstrate noticeable intraspecific variation in the distal lobe, especially the length of the tip, which shows a gradual variation; whereas the thickness of the distal lobe is conservative within the species. The population from central Japan (Aburatsubo Cove) showed remarkable morphological variations, with four copulatory organ types, each type defined by a combination of four shapes of the right and left distal lobes (Shape R, r, L, and l). While the variety of carapace morphotypes have been previously reported, our findings suggest that one morphotype from western Japan (Misumi-cho) should be treated as a distinct species. Thus, we described it as Bicornucythere misumiensis sp. nov.
... Ostracoda distribution depends on environmental factors such as substrates, water temperature, water depth, salinity, and hydrochemical gradients (Smith, 1993;Yasuhara and Irizuki, 2001;Rumes et al., 2016). Ecologically, silty sand and sand are the most favorable substrates for Ostracod growth (Sridhar et al., 2002). ...
Calcareous microfauna of Ostracoda and their diversity from continental slope sediments of Gulf of Mannar, highlighting the bathymetric and ecological variations, are examined in this chapter. Ostracoda are immensely useful and economically viable proxy for delineating ecological as well as bathymetric fluctuations. Two short core sediment samples MC-2 and MC-60 were collected at a water depth of 1,235 and 1,887 m from the study area, and the core-wise distribution of dominant Ostracoda species and their ecological implications were thoroughly investigated. A total of 33 species belonging to 23 genera in MC-2 sample and 10 species belonging to 8 genera in MC-60 sample were taxonomically identified and recorded. Of these, Ostracoda genera Krithe, Bradleya, and Acanthocythereis are dominantly present in the core sample MC-2, and the genus Krithe is abundantly present in the core sample MC-60. Ostracod fauna are found to accommodate themselves in the silty substrate in the Gulf. The relative rate of sedimentation in the Gulf has been detected from the carapaces and open-valve ratio of Ostracoda. Species diversity indices are used to determine the variations in the ecological and bathymetric conditions of Ostracoda obtained from core sediments in the Gulf of Mannar.
Keywords: Ostracoda, Gulf of Mannar, Bathymetry, Ecology, Bay of Bengal
... For abundant species (i.e., Horn), we observed a distinct "Victoria Harbour + Eastern Buffer" biofacies (Figs. 6 and 7). These Victoria Harbour and "Victoria Harbour + Eastern Buffer" biofacies are characterized by sand dwellers (Cytheropteron miurense, Pistocythereis bradyformis, Loxoconcha epeterseni) and phytal taxa (Aurila, Neonesidea, Xestoleberis) ( Fig. 9; Table 2) (Yasuhara and Irizuki, 2001;Yasuhara et al., 2002). So, Mud or intermediate-level productivity may play a role not only in diversity as discussed above but also in faunal composition. ...
Hong Kong is one of the most urbanized coastal cities in the world. Yet, despite extensive anthropogenic impacts, adjacent marine environments harbour tremendous biodiversity. We investigated how the diversity, taxonomic composition, and biogeography of meiobenthic ostracods in Hong Kong's coastal waters vary in response to natural and anthropogenic factors. Our regression models indicated that metal pollution and mud content were the main factors structuring meiofaunal diversity, with eutrophication also playing a role. The highest diversity was observed in the Victoria Harbour region at the center of Hong Kong's urbanized seascape, and the lowest diversities were observed in Mirs Bay, Port Shelter, and Tolo Harbour. Ostracod diversity and biogeography patterns are congruent with published studies of other soft-sediment fauna, which also identified a diversity peak in Hong Kong's urban center and a vast southern water biofacies characterized by muddy and turbid conditions. These results do not apply to organisms that prefer oligotrophic conditions, such as hard corals. For those taxa, eutrophic waters in western Hong Kong are generally not habitable and higher diversities are observed in less productive, eastern waters. Our findings indicate that meiofaunal ostracods can be used as a bioindicator for the diversity of soft sediment benthos, more broadly.
... Ostracods were picked from the coarse fraction. We identified ostracod taxa by referring to Ishizaki (1968), Yasuhara and Irizuki (2001), and Irizuki et al. (2008). We counted the number of valves and valve fragments of each taxon. ...
Tsunami deposits in Kyushu Island, southwestern Japan, have been attributed to the 7.3 ka Kikai caldera eruption, but their origin has not been confirmed. We analyzed a 83-cm-thick Holocene event deposit in the SKM core, obtained from incised valley fill in the coastal lowlands near Sukumo Bay, southwestern Shikoku Island. We confirmed that the event deposit contains K-Ah volcanic ash from the 7.3 ka eruption. The base of the event deposit erodes the underlying inner-bay mud, and the deposit contains material from outside the local terrestrial and marine environment, including angular quartz porphyry from a small inland exposure, oyster shell debris, and a coral fragment. Benthic foraminifers and ostracods in the deposit indicate various habitats, some of which are outside Sukumo Bay. The sand matrix contains low-silica volcanic glass from the late stage of the Kikai caldera eruption. We also documented the same glass in an event deposit in the MIK1 core, from the incised Oyodo River valley in the Miyazaki Plain on southeastern Kyushu. These two 7.3 ka tsunami deposits join other documented examples that are widely distributed in southwestern Japan including the Bungo Channel and Beppu Bay in eastern Kyushu, Tachibana Bay in western Kyushu, and Zasa Pond on the Kii Peninsula as well as around the caldera itself. The tsunami deposits near the caldera have been divided into older and younger 7.3 ka tsunami deposits, the younger ones matching the set of widespread deposits. We attribute the younger 7.3 ka tsunami deposits to a large tsunami generated by a great interplate earthquake in the northern part of the Ryukyu Trench and (or) the western Nankai Trough just after the late stage of the Kikai caldera eruption and the older 7.3 ka tsunami deposits to a small tsunami generated by an interplate earthquake or Kikai caldera eruption.
Scientists studying the biodiversity and biogeography of meiofauna encounter many uncertainties regarding the causes and consequences of natural and anthropogenic-driven changes in biodiversity patterns they observe worldwide. Recently developed novel analytical and computational technologies are facilitating more systematic and integrated approaches to the study of meiofauna biodiversity. In this chapter, we reflect on the state of the art in biodiversity and biogeography research with a focus on the most abundant and diverse meiofauna taxa including nematodes and copepods. Other occasionally abundant meiofauna taxa such as carbonate-shelled crustacean ostracods and protist foraminiferans, which are present in the fossil record, allow meiobenthologists to understand the links between shifts in biodiversity and major historical events in the marine environment. Sample-size dependency and the lack of standardization across benthic surveys currently hamper the integration of disparate meiofauna studies into wider research of seafloor biodiversity and biogeography. We discuss habitat-specific meiofauna biodiversity patterns that are observed at different scales and identify the main drivers of such patterns. Important factors include physical characteristics of the seafloor, biogeochemical processes, ecosystem productivity, geographical location, but also the interactions of meiofauna with other ecosystem components including their prey, their predators, competitors, and habitat facilitators. We discuss the importance of meiobenthic biodiversity for ecosystem functioning and touch on the biogeography of dominant meiofauna taxa by looking at what we know about the importance of endemism versus cosmopolitanism, the growing insights in population genetics and cryptic speciation, the phylogenic processes underpinning them, and critical gaps in our knowledge. We conclude by identifying some dynamic areas of research and inquiry for future generations of meiobenthologists studying the biodiversity and biogeography of meiofauna.
Gamagyang Bay is located on the southern border of the Korean Peninsula. It is a geo-graphically half-closed shallow embayment with a ria-type coastal topography, which provides a variety of environments for ostracod habitation. Among the ostracod species, Pistocythereis bradyi, P. bradyformis and two members of the Keijella bisanensis species group are dominant. Although all these species crawl on the muddy bottom surface in a similar way, their main distribution ranges are limited to either the inner or the outer half of the bay. Each of the four species was recognised as having two distinct morphs within the species. Each morph has its own pattern of geographical distribution which is probably controlled by environmental factors or by the origin and history of migration of the species into Gamagyang Bay. Variation in P. bradyi, P. bradyformis and K. bisanensis occurs in the features of the mural elements, the continuity of the ventral ridge and the H/L ratio of the carapace, respectively.
Twenty-three species distributed among 17 genera of ostracodes were discriminated from the bottom sediments of the Nakanoumi Estuary, Shimane Prefecture (Table 1), and among them four species are described as new to science. No ostracode specimen was found from Shinjiko Lake. To analyse the fauna calculations were made by the Jaccard coefficient for similarity of the ostracode species association (R-matrix), and the simple matching coefficient for station similarity (Q-matrix). The clustering was carried out using the weighted variable group method with simple arithmetic average. The result from the clustering method was compared with that of the principal factor analysis based upon Q-matrix. Using Motomura's Law log Y+aX=b, the relation of the recognized biotopes to the physical conditions and ostracode distribution are discussed.