In a frying pan large enough to hold the cooked pasta and sauce, heat 3 tablespoons of the oil over high heat. Add the shrimp and a pinch of salt. Sauté just until the shrimp are pink, about 1 minute. Using a slotted spoon, transfer shrimp to a plate. Add the squid and a pinch of salt to the pan and sauté until opaque, about 1 minute. Transfer to the plate.

Add the remaining 3 tablespoons oil, the garlic, the parsley, and the pepper flakes to the pan and cook over medium heat until the garlic is golden, about 1 minute. Add the clams (discard any that do not close to the touch) and tomatoes, cover, and cook over low heat, stirring occasionally, until the clams open, about 5 minutes. Discard any that failed to open. Add the shrip and squid and remove from the heat.

Meanwhile, bring a large pot three-fourths full of water to a boil. Add the pasta and salt and cook until al dente. Drain, reserving a ladleful of the water.

Add the pasta to the sauce and stir and toss over medium heat, adding some of the cooking water if too dry. Transfer to a warmed serving bowl and serve.