Butterfly the shrimp: Peel the shrimp, then cut along the curved side with a small sharp knife, about three-quarters of the way through. Devein the shrimp and set aside.
Bring a large pot of salted water to a boil. Heat the olive oil in a large pot or Dutch oven over medium heat. Add the onion and celery and cook until softened, about 4 minutes. Add the wine and cook until reduced, 1 to 2 minutes. Season with 1/2 teaspoon salt and the red pepper flakes. Add the tomatoes and bring to a simmer; cook until slightly thickened, 10 to 12 minutes.
Meanwhile, add the pasta to the boiling water and cook as the label directs for al dente. Reserve 1/2 cup cooking water, then drain.
Add the mussels to the tomato sauce, cover and cook until they just begin to open, 2 to 4 minutes. Add the shrimp, cover and cook until the shrimp is cooked through and the mussels have opened fully, 2 to 4 more minutes. (Discard any unopened mussels.)
Add the pasta to the sauce and stir to coat, stirring in enough of the reserved cooking water to loosen the sauce. Divide among bowls; top with the parsley and celery leaves.