Using the effective diameter D20 used in Creager’s formula, which is most used to calculate hydraulic conductivity, we evaluated the lower limit of particle size in aestivation sites of Japanese sand lance Ammodytes japonicus (DUNCKER & MOHR). We investigated the relationship between the number of Japanese sand lance collected in their aestivation sites and the effective diameter D20 of the bottom sediment. As a result, the number of Japanese sand lance in the sites where the effective diameter D20 is 0.2 mm or less, where the water permeability drops sharply, was significantly reduced. This suggests that the grain size (pore size) must be large enough to ensure breathing during aestivation as a condition for the establishment of aestivation of in the Japanese sand lance. It was also shown that effective diameter D20 of 0.2 mm or greater, where the aestivation sites of Japanese sand lance are established, generally corresponds to 0.3–4 mm or greater in D50 (median diameter). which are the most used indicators of the grain size distribution of the bottom sediment.
