The tidal flat at the mouth of the Obitsu River is one of the best areas for ecological research around Tokyo Bay. Annual variation in the biomass, collected 1 m grid units, showed that the seagrass Zostera japonica which grows at the middle intertidal zone along the river route, has a great influence on the ecosystem. During the summer it forms zostera beds, providing a good habitat for fauna, while during the winter most of the grass disappears except for the roots. The benthonic diatom fractions also increase during the summer. Analysis of the gut contents showed that Batillaria cumingii contained Cocconeis scutellum, a benthonic diatom which lives on Zostera japonica, while the gastropod Umbonium moniliferum and the bivalves Tapes japonica, Mactra veneriformis, Mactra chinensis, Dosinia Japonica and Solen strictus, on the sandy bottom of the lower tidal zone, contained a wider composition of benthonic diatoms. Stable carbon isotopic analysis showed that the primary producers in the Obitsu River tidal flat consisted of benthonic diatoms (-19 to-16‰), seagrasses (-11 to -14‰) and salt marsh plants (-26.5 to -24.5‰). ¹³C ratios for the Batillaria cumingii from the zostera beds were about 1.5‰ higher than for benthonic diatom fractions from the same location. ¹³C ratios for the other primary or higher consumers are between the ratios of the benthonic diatom fractions and the zostera, suggesting that they have a mixed diet.

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Important parameters to estimate the degree of post-mortem modification of molluscan death assemblages are fragmentation rate of shells (Fr), coexistence index of left and right valves (Cv), and shell size-frequency distributions of dominant species. Fr and Cv values are also useful to infer the loss of information from the living population. Modified information obtained from an empty shell assemblage results from empty shell production from a living population. Post-mortem dispersion of empty shells can be typically considered by a “left-right-phenomenon”. Cv value represents the quantity of informational portential concerning the numbers of left and right valves. Hydrodynamic effect continuously modifies the empty size-frequency distribution of the death assemblage. Nevertheless, transported empty shell assemblages preserve the nature of primary size-frequency distributions in some degrees. However, size selective utilization of empty shells by hermit crab strongly modifies the primary size-frequency distribution of empty shells. There are various primary size-frequency distributions of empty shells, however bimodal type is most dominant type. Small number of medium-sized shells results from the few input of empty shells by rapid shell growth. Two peaks in the primary size-frequency distribution refrect repeated input of empty shells in juvenile and adult stages. It is able to consider that empty shells of juveniles and adults have been accumulated independently at different spots. Both spots are located near the central distributional area of the living population. Variously modified size-frequency distribution patterns of empty shells may have resulted from the differences in areal distribution and amounts of juveniles and adults. Mass mortality by catastrophic event (census) often inputs large amounts of empty shells into the death assemblage, and modifies the size-frequency histogram of empty shells. We can detect the presence of seasonal mass mortality by analysing the modes of size-frequency histograms of death and fossil shell assemblages carefully. In the case of the fossil shell assemblage from the Holocene deposits in Fukuoka City, anoxial death event in the summer stratification period of Paphia (Neotapes) undulata from comparison with the death assemblages in the present Hakata Bay. Changes of the size-ranges of juveniles and adults toward upward sequence were interpreted to refrect positional changes of spots of empty shell production in relation with sea-level changes during the Jomon transgression period.

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Recent progress in study on rhythms of micro-growth striation formation (consisting of alternating growth line and growth increment) in bivalves is reviewed herein. Subtidal bivalves have various average frequency of the striation formation. Periodicities such as daily and 1/2 daily ones are known. Involvement of biological clock in keeping these rhythms are doubtful. Rather, it seems that the periodic fluctuation of environmental factors induces these rhythms. Non-periodic formation seems to be more common. Intertidal bivalves form one growth line at each low tide, provided they are exposed. Growth lines of tidal exposure origin are arranged into specific patterns reflecting the tidal type of bivalve's habitats and their relative height in intertidal zone. These patterns have been successfully used in recognizing intertidal bivalves among fossil material or material from archaeological sites.

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This review article gave an outline of the summary account on the structures of feeding and digestive organs, on the intracellular and extracellular digestion, and on the digestive rythms in the intertidal and subtidal lamellibranch molluscs. The crystalline style is dissolved as the tide ebbs and rapidly reformed when the tide returns. The processes of normal or holding, absorption and digestion, disintegration, and of reconstitution of the digestive tubules are organized in distinct phases throughout the digestive diverticula. In some genera, tidal cycle related twelve hours or twenty four hours cycles are shown in feeding and digesting processes. The cycles seemed to have adaptive significance to habitat condition and mode of life of each species.

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Hermatypic corals exhibt ecological patterns which have been called community structures. Of those, the patterns appeared in zonation, distribution of diversity or coverage and the ecological succession are discussed. Studies on the ecological structures in the Indo-Pacific (in-cluding the Ryukyu Islands) and Atlantic coral reefs are reviewed in order to find out the general community structures. Quantitative models on the origin of the structure are also introduced in some cases. Especially, a dynamical model is proposed for the ecological sucession on the basis of the dynamics of species diversity.

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Geographic and vertical distributions of neritic chitons are reviewed from Hokkaido to Kyushu. The species collected from these areas can be divided geographically into two groups. The former group, distributed in Hokkaido and Sanriku regions, is assigned to cold water species-group. The latter, distributed in western coast of Hokkaido and all other areas from Honshu to Kyushu, is assigned to warm water species-group. Therefore, the distributions of the two species groups overlap along the western coast of Hokkaido and in Sanriku region. Each group is further divisible into two subgroups respectively. Vertical distribution of the species are examined and three subdivisions of intertidal zone plus two deeper divisions are recognized.

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A new approach to taphonomic and paleoecologic reconstruction of benthic animal fossils, is proposed, based on the comparison of the orientation of living and fossil bivalves. This method enables us to reconstruct level of disturbance within the sediment profile of the ancient substrate. Preserved and disappeared paleoecologic information in the fossil assemblage can thus be distinguished. This method was applied to one of the common shallow-sea bivalve fossil assemblages, the shell bed of the late Pleistocene Kiyokawa Formation, Boso Peninsula, central Japan. The result reveals repeated erosion of sea-floor and resultant reworking of the shallow-burrowing bivalves. The fossil assemblage from the shell bed can be understood as a mixture of autochthonous deep burrowers and reworked, but indigenous assemblage of shallow burrowers.

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The ecology of nerineid gastropods from the Koike limestone, Fukushima Prefecture, northeast Japan, is discussed according to sedimentary facies and mode of occurrence. The Koike limestone is made up of six non-stratigraphic sedimentary facies : -1) calcareous sandstone, 2) oolite, 3) bioclast, 4) oolite-bioclast, 5) coral, and 6) lime mud facies.
The mode of occurrence of nerineids is divided into two types, Type A and B, based on shell size and accumulation geometry. Type A yields sporadic, large, well preserved individuals from a limited number of horizons. One or a few nerineid species occur mainly in the oolite and oolitebioclast facies together with large bivalves. This suggests that Type A nerineids may be autochthonous. Conversely, Type B (Nerinea bed) is represented by accumulations of small-sized nerineids with other organic debris. Nerineid shells in Type B are generally orientated parallel and are probably allocthonous.

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The Mizunami Group is well-known as a Miocene sequence which yields well-preserved fossils in abundance. The fossils of the group have been studied well in viewpoints of paleoecology and paleoenvironmentology. Some examples of the studies are introduced and a forthcoming plan of studies including reexamination of stratigraphy, studies for trace fossils and taphonomy, and synthesis of various paleoecological data is presented.

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