Bring a large pot of well-salted water to a boil over high heat.

Add the pasta and boil until al dente.

While pasta cooks, heat olive oil in a large skillet on medium-low heat.

Add the garlic and hot pepper flakes and cook until garlic is fragrant and sizzling.

Add tuna and shred it into fine flakes with a fork.

Season with salt.

Keep warm over low heat.

Just before the pasta is ready, set aside 1 cup of boiling water.

Drain pasta and return it to the warm pot set over moderate heat.

Depending on the size of your pot and skillet, either add the spinach and the tuna mixture to the pasta in the pasta pot, or add the drained pasta to the skillet with the tuna mixture and add the arugula.

Toss vigorously with tongs, moistening with some of the reserved pasta water.

:- The arugula will wilt in the heat of the pasta.

:- Divide among warm bowls and serve immediately.