Noriyuki Ikeya

Microfaunal Evidence for Elevated Pliocene Temperatures in the Arctic Ocean

The migration of thermophilic marine Ostracoda into the Arctic Ocean during the Pliocene indicates that winter and summer ocean temperatures around Arctic margins were ≥ 0 °C and > 3 °C, respectively, and that ice-free conditions existed for most or all of the Arctic. By at least 3.5–3.0 Ma, probably earlier, the opening of the Bering Strait allowed marine organisms to migrate through the Arctic Ocean, mostly from the Pacific Ocean. Migrant taxa such as Cythere, Hemicythere, and Neomonoceratina are known from Pliocene deposits of Alaska and Canada as well as Neogene deposits of the North Pacific and Atlantic oceans. On the basis of ecological and Zoogeographic information on ostracode species from more than 800 modern “core top” samples for the North Atlantic, North Pacific, and Arctic Oceans, we determined winter and summer temperature tolerances for migrant taxa to be at or above about 0 °C and 3 °C. This suggests ice-free summers, and probably, a perennially ice-free Arctic Ocean in some regions. Elevated water temperatures in the Arctic Ocean between 3.5 and 2.0 Ma is supported by evidence for late Pliocene increased meridional heat transport in the North Atlantic Ocean.

Late Pliocene climate change 3.4-2.3 Ma: paleoceanographic record from the Yabuta Formation, Sea of Japan

Abstract Late Pliocene paleoceanographic changes in the Sea of Japan between 3.4 and2.3 Ma were investigated through study of molluscs, diatoms, and ostracodes from the Yabuta Formation in Toyama Prefecture. The period 3.4−2.7 Ma was characterized by relatively high sea level and cool water benthic faunas. A progressive paleoceanographic shift towards colder oceanic conditions and lower sea level occurred beginning near 2.7 Ma, intenifying about 2.5 Ma, when important changes in ostracode and molluscan faunas occurred. Between 2.7 and 2.3 Ma, eight glacial events can be inferred based on drops in sea level of 50–60 m and increasing proportions of cold, shallow water ostracode species whose modern ecology and zoogeography indicate colder winter water temperatures (3–4°C). The glacial events between 2.5 and 2.3 Ma were the most intense. Preliminary interpretation of the faunal and oceanographic events of the Yabuta Formation suggests that they correspond to Northern Hemispheric cooling also known from North Atlantic deep-sea oxygen isotope, IRD, and planktic foraminiferal records, North Pacific diatom and radiolarian record, and the Chinese loess sequences. The eight glacial events may record a 41,000-yr obliquity cycle which characterized other late Pliocene climate proxy records. Inferred sea level drops near 2.5−2.3 Ma of about 50–60 m provide direct evidence from an ocean margin setting that supports deep sea oxygen isotopic evidence indicating major changes in global ice volume changes.

The Omma-Manganji ostracod fauna (Plio-Pleistocene) of Japan and the zoogeography of circumpolar species

The Omma-Manganji fauna of Japan signifies a time during the late Pliocene and Pleistocene when arctic-subarctic species migrated far south of their present geographic range in response to oceanographic changes. Omma-Manganji deposits exposed on Hokkaido, northern Honshu, and Sado Islands yielded about 224 species of marine Ostracoda. At least 26 are circumpolar species known previously from Arctic seas off the British Isles, eastern North America, Scandinavia and Europe, comprising between 14 and 47% of the ostracod assemblage in eight of ten formations studied. The 26 circumpolar species and 21 other western Pacific cryophilic species are illustrated and their distribution in Japanese deposits is discussed.

Tide versus eutrophication. Impact on ostracods populations structure of Mikawa Bay (Japan)

Abstract In Mikawa Bay where red tides blooms occur during summer, eutrophication is a very well known process. We examine its effects on distribution of recent ostracods populations. Two synthetic parameters are analyzed : abundance and species diversity. We are able to discriminate two main groups of stations : a correspondence and a cluster analysis allow us to constitute three classes containing both species and stations. Typical open marine species are located near the mouth of the bay and the brackish ones in the inner part of the bay. Influence of tidal currents is evident in the deepest part of the bay. Effects of cutrophication process could occur in the inner part of the bay. Nevertheless it does not seem to be the main factor that controls the structure of the ostracodes population.

Taxonomy, morphology and speciation of the Semicytherura henryhowei group (Crustacea, Ostracoda)

The genus Semicytherura belongs to the family Cytheruridae, and was distinguished from Cytherura on the basis of carapace features. Species of Semicytherura from Japan and adjacent seas can be divided into two groups. One is represented by Semicytherura miurensis Hanai, 1957, characterized by a thin, oval carapace covered with fine reticulation. The other is represented by Semicytherura henryhowei Hanai & Ikeya, 1977, characterized by a thick sub-rectangular carapace in lateral view. Semicytherura henryhowei, which is distributed from Hokkaido to Okinawa in Japan, has been regarded as having several morphotypes distinguishable on outline and reticulation of carapace. However, as a result of detailed observations on the copulatory organ, carapace outline and distributional pattern of pore systems, remarkable differences are shown to exist between the two most frequently occurring morphotypes. In order to recognize S. henryhowei sensu stricto, the carapace of the holotype was re-examined. Consequently, neither of the two morphotypes are considered to belong to S. henryhowei due to differences of carapace outline and distribution of pore systems. The two morphotypes are here regarded as independent taxa, described as new: S. kazahanan. sp. and S. sasameyukin. sp. The geographical distributions of the two new species overlap, but their micro-habitats differ from each other; the former lives on calcareous algae on rocky shores, the latter lives on silty sand bottom within the inner bay. A third new species, S. slipperi sp. nov., is also described. In view of their present geographical distributions and fossil records, the origin of this group of species would appear to be the Japanese islands or adjacent areas in and after the Miocene. This group then migrated to the Arctic Ocean and East Pacific Ocean during or before the middle Pliocene.

Carapace formation of the podocopid ostracode Semicytherura species (Crustacea: Ostracoda)

Details of ostracode carapace structures were examined by SEM and TEM. The podocopine ostracode Semicytherura kazahana has major ridges on the carapace surface and develops its prismatic layer inside the adult carapace. Electron microscopy at the final molt reveals that the major ridges arise from the highly dense formation of pits within the underlying swollen epidermis, and that disappearance of the epidermis in the presumptive area of the prismatic layer occurs after the calcification of the outer lamella cuticle, and just before synthesis of the membranous layer. These facts suggest that the formation of the carapace in Semicytherura takes place via a more complex process than that of the other podocopid ostracodes.

The Interspecific Relations between Three Close Species of the Genus Cythere O. F. Miüler, 1785

The genus Cythere was first proposed by O. F. Muller in 1785, with the European C. lutea as the type species. Almost a century after this proposal, the genus had expanded to contain some 1,000 species. Hanai (1959) researched Cythere , but restricted it, however, to species that are closely related to C. lutea , the type species of this genus. This communication deals with the interspecific relationships between C. uranipponica Hanai, 1959 and C. nishinipponica Okubo, 1976. These, both fossil and Recent, are distributed around the Japanese Archipelago and are very similar to each other. The two species were compared in detail, particularly with respect to morphological elements such as the size, outline, ridges, pits, distribution patterns of the normal pore and radial pore canals, muscle scars, hingement, chitinous appendages, and ecological elements as well as their geographical and stratigraphical distribution. The study has revealed that the morphology of the carapace is very similar in the two species, and that it is difficult to distinguish between them on this basis. In particular, the distribution pattern of the sieve-type normal pore canals, including both their number and position, which has been considered to be a criterion unique to each species, is not necessarily a very stable character in either of the two species studied, and the difference between them was found to be very small. The soft parts were also studied, but no significant difference was observed between the chitinous appendages, antennules, antennae, and legs. However, a striking difference was found in the copulatory organs of the males of the two forms. The species reported by Schornikov (1974) as C. uranipponica was obviously different from C. uranipponica in the distribution pattern of the sieve-type normal pore canals and the morphology of the copulatory organs of the male, and therefore we propose it here as a separate species C. schornikovi n. sp.

Preface: The phylogeny, fossil record and ecological diversity of ostracod crustaceans

After more than two centuries of research, more than 65,000 living and fossil ostracod species have been described and studied, yet much remains to be learned about this ancient, widespread and diverse group of bivalved arthropods. Their higher classification and phylogeny are subjects of vigorous debate, as is their position in the broader picture of crustacean phylogeny. At the same time, major advances in our understanding of ostracod lineages and their relationships are resulting from the application of innovative approaches and techniques. This preface provides a contextual overview of the 15 contributions to this volume, which resulted from the 14th International Symposium on Ostracoda (ISO2001) held in 2001at Shizuoka, Japan. As such it provides a cross-section of topics at the forefront of research on the evolution and diversity of Ostracoda, and indicates directions for future work.

Morphological Variations of Cytheromorpha acupunctata (Brady) in Continuous Populations at Hamana-ko Bay, Japan

Continuous monthly populations of Cytheromorpha acupunctata (Brady, 1880) at a fixed station in Hamana-ko Bay, Pacific coast of central Japan, were examined to understand the morphological features of this species. From their population structures, reproduction is considered to take place throughout the year. Precocious sexual dimorphism is observed clearly in the A-1 stage and possibly in A-2, as suggested by biometrical analysis. The extent of reticulation on the carapace is found on a wide and continuous scale in different forms. Differences in size are also observed, in association with differences in ornamentation: well-developed reticulations tend to occur in small carapaces, less-ornamented carapaces are apt to be large. The morphological changes occur seasonally, i. e. , samples collected in summer contain more individuals with well-developed reticulation and a small carapace, whereas the reverse can be noticed in specimens collected in winter. Variation is presumably influenced by environmental factors, such as water temperature, along with genetic factors.

Taxonomy and Microhabitats of the Genus Spinileberis (Ostracoda, Crustacea) from Japan

Abstract. The taxonomy of three Recent Spinileberis species (S. quadriaculeata, S. furuyaensis, S. pulchra) and one new fossil species, S. lubrica sp. nov. from Japan was studied, and the phylogenetic relationship between the four species was also considered. The relationship with S. lubrica sp. nov., which is an extinct species, was analysed by comparing its carapace morphology with those of the three living species. On the basis of the observation of the cross-section of four Spinileberis shells, the crystallographic structure of the valve is similar between S. pulchra and S. furuyaensis, and S. quadriaculeata and S. lubrica sp. nov., respectively. On the other hand, the relative thickness of the valve is variable among the four Spinileberis species. The same number and similar distributional pattern of normal pores between S. lubrica sp. nov. and S. quadriaculeata suggest that S. lubrica sp. nov. evolved from the common ancestor of S. quadriaculeata and S. lubrica sp. nov. A consideration of the characteristics of the new fossil species, together with the three recent ones, leads us to believe that the species of the inner bay of Japan, S. lubrica sp. nov., is likely to have adapted to very fine sand or silt with relatively high-salinity water, in contrast to the other three Spinileberis species. The complete male soft parts of S. furuyaensis are also herein described for the first time.