Wash mussels and clams under cold running water and scrub with a stiff brush. Remove mussels' beards by sharply pulling. Place shellfish in a large bowl, cover with a wet towel, and reserve in refrigerator.
Core and seed bell peppers. Cut into fine julienne, about 2 inches long. Cut onion, zucchini, and yellow squash into similar julienne. Cut sausage or prosciutto into 2-inch lengths and then into fine julienne. Combine the shellfish, julienne vegetables, sausage, tomato, olives, fish stock, port, and pepper in a large stockpot or Dutch oven. Cover and cook over high heat until shells open, about 8 to l0 minutes. Taste before adjusting seasonings since shellfish and bottled clam juice are quite salty. Ladle into soup bowls, sprinkle with chopped parsley, and serve immediately.