The bioturbation rate of a tube-building maldanid polychaete, Eulymene sp., collected in Koajiro Bay, was measured by a video observation system. The length and volume of fecal pellets, and defecation interval were measured from more than 80h of video records taken for 17 specimens. Thus, the defecation rate was estimated as 782mm³ day^-1 worm^-1 for each individual. From the results of observations and the density of the worms in the field, the bioturbation rates in situ were estimated. At a station in Koajiro Bay, the amount of sediment reworked by a population of this species was estimated as 291.84cm³m^-2 day^-1 in August, 1992. This would amount to a cover about 0.292 mm thick on the bottom sediment. The bioturbation rate of this species was middle level in Japanese littoral waters.

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Interspecif is competition for space among sessile invertebrates was studied on panels exposed in four different seasons during a year in Tomioka Bay, south Japan. Development of the sessile community on the shaded faces of panels was monitored for 15 months. Competitive relationships between these species were strongly influenced by the timing of panel initiation. Hierarchical competitive relationships occurred on panels which were suspended in October 1991 (autumn), April 1992 (spring), and July 1992 (summer) and monitored until December 1992. On the other hand, network competitive relationship occurred on panels suspended in January 1992 (winter) and monitored until December 1992. After 5 months of panels suspension, a hierarchical relationship was observed on panels exposed in all seasons except winter, when a definite dominance among the organisms was not detected. The hierarchical position of a given species changed on panels initiated in different as well as the same seasons but observed after different time intervals. In general, colonial ascidians were dominant on all panels suspended except those initiated in January 1992 (winter). The patterns of colonization and species diversity on panels initiated in different seasons were different. Major factors responsible for these kinds of competitive relationships are timing of panel initiation, duration of panel submersion, pattern of initial colonization on panel surfaces, and seawater temperature.

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Seasonal migration and activity patterns of Nodilittorina radiata (Eydoux & Souleyet) (Gastropoda: Littorinidae) were investigated by observations of marked snails and transect sampling on a concrete breakwater at Kattoshi, Hakodate Bay, northern Japan. Snails were classified into two size groups, large (≥5mm in shell height, mature) and small (≤4mm, immature) . Marked snails of both groups started descending into the intertidal zone in early summer, and they ascended into the supratidal zone in autumn. The locomotory activity of both groups was high during the summer and low during the winter. For both size groups, the proportion of snails in the intertidal zone was high during the summer and low during the winter. Since the seasonal migration and activity patterns were similar for both large (mature) and small (immature) snails, animals evidently undertake seasonal vertical migrations not only for reproduction, but also for other reasons such as food and refuge from environmental stress.

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To determine the favorable sediment environments for juveniles of the brackishwater bivalve Corbicula japonica, the relationship between the presence of juveniles and limiting factors of the sediment environment that are negatively correlated with juvenile density was investigated in Lake Abashiri, one of the northernmost areas of this bivalve's distribution in Japan. Correlations between juvenile density and sediment factors were estimated by Spearman's rank correlation coefficient. Logit models were made between the presence or absence of juveniles, as the dependent variable, and limiting sediment factors, as independent variables. The models were compared by the Akaike Information Criterion (AIC) . The values at probability 50% of the logit models (V50s) and the standard errors of the univariate logit models were calculated for each sediment factor. We defined favorable sites as those where any of these values are less than the V50s, which seem to provide good criteria to distinguish between favorable and unfavorable sediment conditions for juveniles. The V50s for limiting factors were: organic carbon 1.10 %, organic nitrogen 0.15%, organic carbon / nitrogen ratio 7.43, ignition loss 4.45%, water content 40.97%, and silt-clay (particle diameter<0.063mm) plus very fine sand (0.063-0.125mm) 36.88%. A significant logit model could not be made for total sulfide. These criteria may provide important information for the management of C. japonica resources.

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Genetic differentiation among five populations of the Japanese freshwater crab Geothelphusa dehaani, each comprising one of three different body color types, dark brown (RE), dark purplish brown (DA), and grayish blue (BL), was examined by means of electrophoretic analysis of 16 enzyme loci. Allelic replacement was observed between the BL-type population and populations of the other two types (RE and DA) at three loci (α Gpd, Gpi, and Sod-1), but not between the RE-and DA-type populations. The average of Nei's genetic distance (D) was 0.035 between the RE and DA populations (range of 0.013-0.061), 0.251 between the RE and BL populations (0.244-0.260), and 0.296 between the DA and BL populations. These results suggest that the BL type is only distantly related to the other two color types (RE and DA).

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