Heat the oil in a large pot over medium heat.
Add the fennel, onion, shallots, celery, fennel seeds and salt, sauté until the onion is translucent, about 8-10 minutes.
Add the garlic and red pepper flakes, sauté 2 minutes. Stir in the tomato paste, stir to combine, and cook for 1 minute. Add wine and cook for 2 minutes. Add tomatoes with their juices, fish stock and bay leaf.
Cover and bring to a boil. Reduce the heat to medium-low. Cover and simmer until the flavors blend, about 30 minutes.
Add the corn and the clams to the pot. Cover and cook until the clams just begin to open, about 5 minutes. Add the shrimp and squid. Simmer gently until the shrimp & squid are just cooked through, and the clams are completely open, about 5 minutes more (discard any clams that do not open).
Season the soup, to taste, with more salt, pepper and red pepper flakes.
Ladle the soup into bowls, sprinkle with tarragon leaves and serve with fresh bread or toast.