Heat a large pot of water for the pasta. Measure out the 12 ounces spaghetti
Scrub your shellfish to remove seaweed and grit. Discard any that are broken or already open and do not close when you handle them. Note: I generally soak the clams in some salted water for a bit giving them the chance to eject any sand.
Heat a large skillet over medium-high heat and add two tablespoons olive oil. Add the mussels and clams along with 1/4 cup water or wine, cover, and cook for 5-10 minutes until the shells open. This signals that the seafood is cooked. The shells should start to open at around 5 minutes. Remove any that have opened and place them in a large bowl. Re-cover the pan, and continue to cook, checking every couple of minutes, until all the shells have opened. NOTE: If any shells are broken or do not open, throw them away.
When mussels and clams are cool, remove the meat and discard the shells. Note: you may serve the mussels and clams in their shells. It is a beautiful presentation but much messier at the table!
Add the spaghetti to the boiling water. Cook for about 8 minutes.
While the spaghetti cooks, heat another 2 tablespoon oil in a large skillet until shimmering. Add the calamari and shallot (cut sides down), and sauté for about 5 minutes. Add the wine and cherry tomatoes and cook for another 3-5 minutes. Remove and discard the shallot.
Add the partially-cooked spaghetti to the skillet along with the reserved cooking liquid from the mussels and clams and 1/2 cup of the pasta water. Bring to a boil, cooking for a couple of minutes. Add the shrimp and the reserved mussel and clam meats. When the shrimp are just opaque, season with freshly ground pepper.
Divide among 4 heated pasta bowls, sprinkle with parsley and serve.