Step 1Scrub the clams with a stiff brush under running water to remove all grit. Do not use any open clams or mussels or any with damaged or cracked shells.
Step 2To prepare the mussels, remove any stringy "beard" on the outside of the shells and rinse and scrub each shell with a brush under running water to remove grit.
Step 3Rinse the periwinkles under running water in a strainer to remove grit, then rinse each shell to remove any clinging dirt.
Step 4Pour the water into a large saucepan. Cut or break the dried seaweed into four pieces. Add it to the water and let stand 30 minutes.
Step 5Bring the seaweed broth to boil over high heat. Remove the seaweed pieces just before the water begins to boil; discard. When the broth comes to a full boil, stir in the bonito shavings. Cook 30 seconds, then remove from the heat and let it stand, without stirring, until the flakes settle to the bottom, about 10 minutes.
Step 6Strain the broth through a cheesecloth-lined strainer. Gather together the ends of the cheesecloth and squeeze out the remaining liquid. Pour the broth back into the saucepan. Place the miso in a small bowl and stir a little broth into it until smooth. Add the thinned miso to the broth. Set aside.
Step 7Pour 1 cup water into bottom of a
Step 85-quart pot. Add the clams, mussels and periwinkles. Cover the pot and heat to simmering over medium-high heat. Reduce heat and simmer 5 to 8 minutes, remove the cover and check to see whether they have all opened. Discard any that haven't.
Step 9Heat the miso broth just to a simmer. Divide the shellfish into serving bowls. Strain the cooking liquid through a cheesecloth-lined strainer into the miso broth.
Step 10Spoon the broth over the shellfish in the bowls. Thinly slice two of the green onions diagonally and cut thin strips from the green parts of the remaining two green onions and use to garnish each bowl. Serve with small picks for removing the periwinkles from their shells.