Abstract
Bivalves on subtropical sandy beaches are potentially important food resources but, in general, estimating their abundance has been thought to be difficult due to a high variability in their density. This study quantitatively demonstrates the vertical distribution of Atactodea striata and Latona (=Donax) faba on a sandy beach in Ishigaki Island in southwestern Japan in the summer. Both species occurred over a wide vertical range in the intertidal zone with their peaks around the mean tide level. Associated bivalve fauna consisted of Donacilla picta, Davila plana, and Katelysia hiantina, but all were very rare. The vertical distribution patterns of A. striata and L. faba did not change between the low and high tides due to the small range of migration, which was revealed by the recapture of marked bivalves during 6 hrs and 24 hrs on very fine and calm days in the summer. Size-frequency histograms showed that small individuals (probably juveniles) were predominant in the populations of both species. The low efficiency of sampling at high tide and the intraspecific zonation by size suggested that the sampling should be carried out over the entire range of the intertidal zone at low tide.
