In large pot of boiling salted water, cook fettuccine until tender but firm, about 3 minutes.
Meanwhile, in large serving bowl, break tuna into chunks; add red peppers, garlic, pepper and salt. In small bowl, gradually whisk oil into lemon juice; pour half over tuna mixture and mix well.
Reserving 1/2 cup cooking liquid, drain fettuccine; place on tuna mixture. Pour remaining oil mixture over top; top with arugula and toss, adding reserved cooking liquid as desired to moisten.