Heat the butter in a large saucepan over medium heat.
Add the leeks, fennel, ½ teaspoon salt, and ¼ teaspoon pepper. Cook, stirring occasionally, until the vegetables are tender, 4 to 5 minutes. Stir in the flour.
Add the clam juice, milk, and potatoes and bring to a boil.
Reduce heat and simmer, stirring occasionally, for 12 minutes.
Stir in the shrimp and corn and cook until the potatoes are tender and the shrimp and corn are heated through, 3 to 5 minutes.
Stir in the parsley and lemon juice. Serve with the bread (if using).