The Asari （Manila）clam, Ruditapes philippinarum, is one of the most commercially important shellfish in the world. This species is native to coasts of the western Pacific. One population of this clam, on the Nakatsu tidal flats, Japan, has collapsed in recent decades. We relate extreme variations in tidal flat, sediment surface temperature to differences in survival of juvnile Asari clams recruiting into these flats. Most juveniles clams, settled into Nakatsu sediments in autumn rather than spring or early summer. Although there was little seasonal difference in water temperature in this period, spring to early summer sediment surface temperatures were considerly higher than those experienced in autumn. Low tidal sediment surface temperature from late June to mid August indicated over 40℃a critical temperature for survival of newly settled Asari clam. Extremes in sediment surface temperature during low tide in spring to early summer and autumn were caused by differences in day length, timing （daylight） occurrence of the low tide and seasonal variation in spring tidal height. When low tidal ground temperature fell bellow the critical level for survival of settling Asari clam in late September, survivorship was greater, ultimately contributing more individuals to the population than in any other season.