On Japanese coastal tidal flats, short-necked clam (Ruditapes philippinarum) is one of the most dominant species. This bivalve shell is said to have high ability of the filtration for suspended organic particles as a suspension feeder. High purification ability can be expected at the area of high apparent-filtration-rate by high population density of the clam. So, the apparent-filtration-rate of the short-necked clam in a tidal flat was estimated.
　Banzu Tidal Flat in the Tokyo Bay was selected for theobservation of the density distribution of macrobenthos. Short-necked clam was most common and dominant here. Population density for each size fraction was observed in the intertidal zone. Density is low in winter. In June, Larger individuals among 2-3cm shell length are dominant with relatively lower density at the lower (deeper) station, while small shells under 1cm shell length are dominant with higher density at the higher (shallower) station.
　Filtration rate of a individual shell per unit hour was observed by the filtration experiments for suspended particles. Filtration rate is changed by the water temperature and the shell size. Direct measurement of the filtration velocity was also tried by using the Laser type anemometer. Above the opening of the filtration cylinder of a shell, temporal change and spatial distribution of the current was monitored. Estimated filtration rate from the observed velocity is a little smaller than ,but similar to, the filtration rate from the particles filtration experiments.
　Integrated filtration rate for a certain size-fraction group can be calculated using a) population density for this size-fraction and b) filtration rate per an individual for the same size-fraction. Sum of the filtration rate over all size-fraction gives apparent-filtration-rate per unit area at the station in the tidal flat. Due to the low density and low temperature, areal filtration rate is 1/4～1/6 lower in February than that in June. In June, stations near M.W.L. and L.W.L. show high apparent-filtration-rate per unit area. Maximum rate reaches to 2～3m3/m2/d in June. It is understood that the maintenance of good environment condition for M.W. L. and L.W.L. (including the prevention from anaerobic sediment or anoxic Aoshio attacks) is more important for the enhancement of the biological filtration activity in inter tidal flats.