This study examined the effects of bottom disturbance associated with wave action on the burrowing behaviors of the surf clam Mactra chinensis (shell length = 28.5 - 85.0mm) using a vertical circulating water channel. The burrowing speed of the clam tended to increase with shell length and Shields number as an index of bottom disturbance regardless of burrowing posture toward water flow. The relationship between the burrowing speed (V : mm/s), Shell length (L : mm) and Shields number (ψ) was described as V=0.022L+5.989ψ. A large number of the clams apparently burrowed with orienting their lateral toward the flow. The critical conditions when the clams could not burrow into the sand and would be washed out of the sand by bottom disturbance were shown as functions of the shell length and Shields number with ψ =3.087×10-4L＋0.081 and ψ=0.004L＋0.099, respectively. The critical Shields number to wash the clams out of the sand corresponded to that of shallower limit of water depth in the clam habitat.
