The seasonal abundance and community structure of meiobenthic harpacticoid copepods were investigated in a nursery pond containing sea bream, Pagrus major (TEMMINCK & SCHLEGEL). Twenty-three species were collected. The mean density of total harpacticoid copepods was significantly higher in 1979 (mean±SD: 41.87±37.77 individuals 10cm^-2) than in 1980 (18.35±12.25 individuals 10cm^-2). In 1979, Amonardia normani (BRADY) was most abundant from May to mid-June, and then Longipedia sp. increased and accounted for 80.1-88.5% of the total harpacticoids in August. In 1980 seven species maintained an equilibrium in terms of species composition. There were significant differences in species diversity H' and evenness J' between the two years. These differences might have been caused by two main following factors. First, the harpacticoid copepod community was not continuous due to the draining of the pond water in winter and replenishment with sea water in the spring. Second, in 1980 the density of juvenile sea bream was nearly one order of magnitude higher than in 1979, so their predation impact on meiobenthic copepods was greater in 1980.

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The fundamental body structures of shrimp-shaped and crab-shaped decapod crustaceans were compared and discussed based on larval morphology. Penaeus japonicus was selected as a representative shrimp-shaped form, since it has ancestral characteristics and eight pairs of natatory thoracic appendages. Among the crab-shaped forms, Parthenope (Platylambrus) validus was selected as a representative for tracing the morphological changes of the processes and the knobs on the carapace through the larval and postlarval development. It is suggested that in decapod zoeas the natatory appendages on the thorax show an evolutionary trend towards a reduction in the number of paired appendages. The zoeas of shrimp-shaped decapods bear three to eight pairs of natatory thoracic appendages, while two pairs of natatory thoracopods characterize the zoeas of crab-shaped decapods. The latter are undeveloped in the third to eighth thoracic segments, and the thorax is rather shortened. Consequently, the posterior edge of the carapace does not extend and remains undeveloped during the zoeal stages, and the carapace takes on a helmet-shaped form. The abdomen of the zoea of crab-shaped decapods is also undeveloped. The fundamental body structure of both the shrimp-shaped and crab-shaped decapods is completed during the zoeal stages. The zoea of the crab-shaped form is less developed than that of the shrimpshaped form in the thorax, carapace and abdomen. Therefore, it seems that the Brachura and the Penaeidae differentiated in the early ontogenetic stages.

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A study was conducted to clarify the process of egg development and the variation of egg size in the Japanese mitten crab Eriocheir japonicus. The duration of egg development varied from 15 days (oviposition in October) to 79 days (oviposition in January). Hatching was not synchronized with the lunar rhythm. The mean egg diameter in each batch varied according to developmental stage and season (292 - 434μm). After a small decrease during cleavage, the egg diameter increased during embryonic development. The diameter just before hatching increased from September to April, but decreased from April to July. The minimum mean diameter during development was slightly smaller during the warmer season. The proportional increase in egg diameter (18 - 28%) was not very high at higher temperatures. During the warmer season smaller eggs were oviposited, and these increased proportionately less in size.

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Oyster reefs, composed of dead oyster (Crassostrea gigas) shells, abound on the subtidal mud bottom of Saroma Lagoon, Hokkaido. According to previous descriptions, these reefs might have been formed in the intertidal zone. However, they have also been observed in the subtidal zone in Saroma Lagoon. Thus, the following five hypotheses were proposed and tested; (1) the reefs sank down into the subtidal zone due to diastrophism after reef formation, (2) the sea level rose after reef formation, (3) the reefs formed in the subtidal zone naturally, (4) dead shells accumulated subtidally and were piled up by strong water movements, and (5) the reefs were piled up artificially. The distribution and contours of the reefs were examined with a sonic depth finder and through visual observations by scuba diving. The reefs, mainly located at depths of 8-10m, had a mean diameter of 16 m and a mean height of 1.5m. The right and left valves of many of the oysters were intact, indicating that these shells had grown in situ. Radiocarbon dating of a dead oyster shell revealed an age of 180 years or less. The sea and land levels did not change significantly during the period of reef formation. These results, combined with the history of the oyster fishery within the lagoon, strongly suggest that the reefs formed recently on the subtidal bottom. We suggest that the method of estimating sea level by excavation of fossil oyster reefs should be cross-checked with other methods.

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