To clarify the life history of the red-spotted swimming crab Portunus (Portunus) sanguinolentus in Osaka Bay, many crabs were collected from drifting seaweed and in coastal and offshore areas. In addition, the carapace widths of fishermen-caught crabs were measured, catches by fishermen were recorded, and rearing experiments were carried out. The crabs were seen only from June to January in Osaka Bay. The life history of most crabs is reconstructed as follows. Juvenile crabs arrive from the Kii Channel after attaching themselves to drifting seaweed. They grow rapidly and their average carapace widths (including lateral spines) in winter reach about 140 mm for males and 130 mm for females. Ovigerous females appear from August to January, but these scarcely contribute to total reproduction. When the water temperature falls to below ca. 12°C, all crabs die.These characteristics suggest that the introduction of crabs into Osaka Bay occurs through non-effective dispersal.

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The seasonal fluctuation of the meiobenthos was studied at a subtidal coarse sand area in Otsuchi Bay in July and November, 1993, and January and March, 1994. The highest density of meiobenthos was recorded in July (1.210⁷ ind. m^-2) and the lowest in January (5.810⁶ ind. m^-2). Biomass was highest in March (1.510¹ g m^-2) and lowest in January (5.1 g m^-2). For nematodes the highest density was observed in March (1.210⁶ ind. m^-2) and the lowest in January (1.710⁵ ind. m^-2), whereas biomass was maximal in July (0.49 g m^-2) and minimal in January (0.06 g m^-2). Meiobenthos aggregated in the surface layer of the sediment and decreased with depth in July and November; however, it was concentrated in the 23 cm layer in January and in the 12 cm layer in March. A few species of nematodes predominated in each season and the dominant species and their feeding types were different from season to season. In particular, a non-selective deposit feeder, Theristus sp., was extremely abundant in March, soon after the spring phytoplankton bloom. This species may have an r-strategic life history allowing it to respond quickly to the increased food supply.

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The fan worm polychaete, Fabricinuda bikinii (Hartman, 1954), has been previously reported from Bikini Atoll (type locality) in the western Pacific, and Aldabra Atoll in the western Indian Ocean. The species is reported here from Thalassia seagrass beds at Yakata, Okinawa Island. A description of the species is provided based on observations using both light and scanning electron microscopy. Methyl green staining patterns are described and compared to what has been previously reported for this species.

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Monodonta labio (Linné) is a common species on intertidal rocky shores along the Pacific coast of the Boso Peninsula, Honshu, Japan. The study area for this research consisted of bed-rock and cobble-field habitats. Seasonal changes in size distribution in these two habitats, including juveniles, were investigated from April, 1989 to April, 1991. These results showed that small snails concentrated in the cobble-fields, and larger individuals on the bed-rock. To determine the cause of this difference in size distribution, mark-recapture experiments were conducted from April, 1990 to April, 1991. Juvenile snails were shown to recruit only in the cobble-fields, and larger snails more than about 5 mm to migrate from the cobble-fields to bed-rock, mainly during late autumn to winter. This growth-related migration may be an adaptive strategy for utilizing a rocky shore with a variety of microhabitats.

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