Pour enough olive oil into a large, deep, heavy frying pan to reach a depth of 3 inches. Heat the oil over medium heat until a deep-fry thermometer registers 350 degrees F.
While the oil is heating, put the buttermilk and the bread crumbs in separate shallow bowls. Working in batches, dip clams and oysters in buttermilk to coat completely. Allow the excess buttermilk to drip back in to the bowl. Dredge clams and oysters in the bread crumbs. Place the clams and oysters on a baking sheet, and continue.
When the oil is hot, fry the clams and oysters in batches, about 3 minutes per batch. Using a slotted spoon, transfer the fried seafood to paper towels to drain.
Immediately sprinkle the fried seafood with freshly squeezed lemon and sea salt. Serve with a bowl of warmed spicy marinara or arrabiata sauce for dipping.