They use that foot to anchor themselves to get secure. he was likely searching for something to grab onto and hold himself in place. My clams like to turn themselves around occassionally, I figure they'll find the place they like.
While a lot of people like to put clams on the sandbed, clams do prefer to be on a rock or something they can hold onto for security. I have mine in the sandbed, but I've placed some small rocks below them so they can hold on, and they seem to be happy.
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