Changes of microbial population were studied in seawater and in seawater containing sand; both were inoculated with Vibrio parahaemolyticus. The viable cell number of V. parahaemolyticus decreased in all samples of the natural seawater and of the seawater containing natural sea sand. Flagellates and ciliates were observed in these samples. And amoebae, bdellovibrio and myxo bacteria, plaque-forming microorganisms, increased in number in these samples following a decline of V. parahaemolyticus.

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... ut the effectiveness of predation and parasitism decreased in the samples containing sand. The results obtained in this study indicate that the decline of V. parahaemolyticus can be attributed to indigenous predators and parasites in the sea.