Bring a large pot of salted water to a boil. Cook the spaghetti according to the package directions. Reserve 1 cup of the cooking water, then drain the spaghetti in a colander.
While the pasta cooks, heat the olive oil in a large skillet over medium-high heat. Add the capers and garlic and cook, stirring, until the garlic is golden, about 1 minute. Add the bruschetta topping, red pepper flakes and a pinch of black pepper and cook, stirring, until hot, about 4 minutes.
Add the drained spaghetti to the skillet along with the tuna, parsley and about 1/2 cup of the reserved cooking water. Add more cooking water to loosen if desired and serve.