Bring a large pot of generously salted water to a boil.
Add the spaghetti and cook until al dente, about 10 minutes.
While the pasta cooks, combine the tuna, olive oil, lemon zest, lemon juice, kosher salt, red pepper flakes, and capers in a large serving bowl.
Drain the cooked pasta, reserving a small amount of pasta water if needed.
Add the hot pasta to the bowl and toss well until everything is evenly coated. This simple step allows the flavors to blend beautifully in your Mediterranean Tuna Pasta.
Fold in the arugula and toss until it begins to wilt.
Taste and adjust the seasoning with additional salt or red pepper flakes if desired.
Garnish with chopped parsley or dill and freshly ground black pepper before serving.