The mouths of two rivers in Japan, the Ano and the Shitomo, are adjacent to each other but show a marked difference in their bivalve assemblages on the tidal flats. From the results of a four-year field investigation on the densities of larvae and recruitants of three dominant bivalves (Nuttallia olivacea, Ruditapes philippinarum, Musculista senhousia), we sought to see whether pre- or post-settlement processes are the important factors determining the features of the bivalve assemblages in each river. They were found to be determined basically by post-settlement processes. However, in some years, they were determined after larval recruitment (>1.0mm in shell length), and in other years immediately after the larvae had settled on the tidal flats or after the bivalves had reached a certain size but before larval recruitment.
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