The clam Ruditapes philippinarum (Adams) is one of Japan's most important fisheries resources; recently, however, the anuual catch of this clam has declined to less than one quarter of its previous maximum. In order to implement more effective management of this resource, studies on early life stages, particularly the recruitment process that regulates subsequent population dynamics, are essential. To date, the recruitment dynamics of R. Philippinarum have been poorly understood due to a lack of basic information resulting from difficulties in identifying the larvae. To remedy this lack, we studied the recruitment process of R. philippinarum from April, 1998, to March, 1999, in Mikawa Bay, central Japan, using a new monoclonal antibody method. Our study revealed that R. philippinarum produces larvae from April to November in Mikawa Bay, in two discrete periods. An early spawning period occurred from April to July, with the peak abundance of larvae moving from the north-west to the eastern part of the bay. A later spawning period was observed in a relatively limited season from August to November. The distribution of larvae was basically controlled by the current system of the bay, although the larvae tended to become dispersed as they matured. The new monoclonal antibody identification method, applied for the first time in a study of a natural population, was found to be useful for studying R. philippinarum. It also proved to be an effective procedure for rapidly processing a large number of samples.

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The growth rate of the bivalve Nuttallia olivacea in the field was estimated by the marking-recapture and caging methods in a tidal flat at the mouth of the Natori River, Miyagi Prefecture. Individuals with a shell length of 9-20 mm were marked, returned to the tidal flat, and recaptured monthly for measurement of their size and weight. Furthermore, nylon cages filled with sand, into which bivalves 9-20 mm in shell length had been put, were buried in the flat. The size and weight of these bivalves were similarly measured. No significant difference between the two methods was detected by T-analysis of the estimated growth rate values. However, the recapture rate of marked individuals in the marking-recapture trials became lower 3 months after initial release, so that precise estimation became difficult with time. Daily increments in shell length and weight as well as the rate of growth were largest in summer (August to September), i.e., 110μm/day, 16.0 mg/day, and 1.63%/day respectively, and smallest in winter (February to April), i.e., 4.7μm/day, 2.9 mg /day and 0.1%/day, respectively. Water temperature varied from 5°C in February to 27°C in August. The annual growth in terms of shell length and weight was 13.49 (±2.44) mm and 3.14 (±0.89) g from June, 1995, to June, 1996, and 12.14 (±2.67) mm and 2.84 (±0.67) g from September, 1995, to September, 1996.

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The bottom sediments and benthic foraminiferal assemblages in Kagoshima Bay, southern Kyushu, Japan, were quantitatively analyzed from sedimentological and paleoecological viewpoints. The submarine topography and the distribution of bottom sediments in the bay's mouth and central areas suggest that the predominant currents move northwards along the coast of the Osumi Peninsula and southwards along the coast of the Satsuma Peninsula, with a postulated counterclockwise gyre in the central area. Longshore currents flowing southwards along the Satsuma Peninsula are diverted by an extensive area of reclaimed land at Kiire Town, and this influences the distribution of the bottom sediments offshore. The vertical distribution of grain size in sediment cores taken at two stations off the reclaimed land supports the idea mentioned above. Bottom sediments off Ibusuki City were swept away during the period from 1938 to 1972. This erosion seems to have been caused mainly by the excavation of iron sand and the construction of piers. Prior to the installation of further engineering works in nearshore areas, detailed hydrological and sedimentological investigations are needed in order to appraise their likely impact on the surrounding sea bottom and the benthic commuinity.

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Tidal flats in Okinawa are now facing various environmental crises. Large-scale reclamation in the southern central region of Okinawa, where many tidal flats used to exist, has continued since the 1970s. If reclamation keeps up its present pace, almost all of the tidal flats of the southern central Okinawa will disappear in the near future. Tidal flats in northern Okinawa have been greatly affected by clayey silt inflows caused by development on land. One consequence has been simplification of the tidal flat environment and a loss of benthic species diversity. Also, at Manko tidal flat, designated as a wetland of international importance under the Ramsar Convention on Wetlands, rapid sedimentation of clayey silt has led to decreases in benthic animals and plants as well as water birds. Most of the shoreline of Okinawa has been converted or is in the process of being converted to artificial breakwaters, and many forms of life restricted to sandy beach, salt-marsh and coastal wood environments exist on the extreme margin of survival. We have only scarce knowledge of the present state of most organisms living in tidal flats of Okinawa. At the same time, the status of tidal flat environments on Okinawa is worse than it has ever been.

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The present status of the molluscs of the Suô-nada Sea coast is reported, and conservation of the species diversity there is discussed. This place is exceptional in modern Japan due to its extremely high biodiversity. Few people appreciate the rich biota in these tidal-flat ecosystems, and major destruction there has already begun. For the conservation of the biota of the Suô-nada Sea, it is essential to recognize the relationships between species and their microhabitats, because the habitats of most tidal-flat species are very restricted in space. Here I discuss some major problems in the conduct of environmental assessment studies in Japan, as illustrated by research into the expected impact on the molluscan fauna of the construction of the Kaminoseki Nuclear Power Plant on Nagashima Island. These problems include misidentifications, underestimation of faunal or ecological richness, failure of artificial plantings or mitigation, misleading treatment of the concept of conservation, and emphasis on protection of only a few species useful for human beings. I also discuss the role of amateur researchers in the conservation of tidal flats. Amateurs sometimes can provide new and precise information on the status of species, which is important when there have been recent rapid changes in environmental conditions. On the other hand, amateurs must understand that many species are endangered today and should not be overhunted only for private purposes.

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[in Japanese]

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