Bring a large pot of water to a boil. Cook pasta according to package directions. Drain.
Meanwhile, heat oil in a large skillet over medium heat. Add onion and cook until starting to soften, about 2 minutes. Add garlic and crushed red pepper and cook, stirring, until fragrant, about 1 minute. Add clams and their juice, wine, lemon zest and lemon juice. Bring to a simmer. Adjust heat to maintain a simmer and cook, stirring occasionally, until reduced by half, 6 to 8 minutes.
Stir in butter and salt. Add the hot pasta and toss to coat. Add arugula and toss until slightly wilted, about 1 minute. Serve with Parmesan, if desired.
Using whole-wheat gives you double the fiber of regular pasta for a more filling meal. Only 5% of the U.S. population gets the daily recommended 25 to 30 grams. This simple swap is an easy way to inch closer to that goal.