West Coast Manila clams are small, tender clams that cook very quickly. They work well in this simple low-calorie soup, made by stirring miso into water and adding a little of the clam cooking broth. Too much broth will overpower the miso, so be prudent.
When cooking the clams, most of them will open as the water begins to boil. Quickly remove them from the saucepan to prevent overcooking, which results in tough clams. Throw away any that don’t open. (The reverse holds true for clams before cooking--if the shell is already open and doesn’t close when tapped, throw the clam away. It’s probably dead and could be contaminated.)
You’ll find the clams at fish markets. And pick up dashi, the instant Japanese soup base, at Japanese and Asian markets, as well as some well-stocked supermarkets.