Bring a large pot of generously salted water to a boil.
Cook the spaghetti 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 spaghetti, reserving a small amount of pasta water if needed.
Add the hot pasta to the bowl and toss well until evenly coated. This Mediterranean Tuna Pasta develops its flavor as the warm pasta absorbs the lemony olive oil.
Fold in the arugula and continue tossing until the leaves are just wilted.
Taste and adjust seasoning with additional salt or red pepper flakes if desired.
Garnish with chopped parsley or fresh dill and freshly ground black pepper before serving.