<p>Quantitative samplings of the macro-benthic communities were conducted on the sandy tidal flats, Sashiki Tidal Flats, that face Yatsushiro Bay in Kumamoto Prefecture, to monitor the seasonal dynamics of the macro-benthic communities and the population dynamics of the short-neck clam, Ruditapes philippinarum. This clam is one of the most dominant species of the macro-benthic communities on the sandy tidal flats of the Japanese coast. The larvae produced in the autumn breeding season settled on the tidal flats between November and December. The density of the newly settled juveniles reached about 85,000 ind. m⁻² in January 2015, and 4,760 ind. m⁻² of the juveniles with shell length of 2 to 5 mm remained until April. However, the newly recruited cohort almost collapsed between May and June. The same phenomenon was observed on the tidal flats in the spring of 2014. It was, therefore, one of the main reasons why the adult clams were sparse on the tidal flats throughout the year. Our results suggest that the predation pressure caused by ray Hemitrygon akajei and gastropod Philine argentata was a major mortality factor that collapsed the clam population in spring. In spring, numerous dips were created by the rays on the tidal flats. It is likely that the young spats newly recruited in the autumn breeding season are predated on by these predators by the spring of the next year.</p>