Boil salted water in a large pot. Add pasta and cook for 2 minutes. In a separate pan on medium, heat butter. Once melted, sauté diced fennel. Add pasta and 2 ounces pasta water and cook for another 1 to 2 minutes on high heat. Stir in the crab, zest of entire lemon, crème fraiche and chives. Season with salt, pepper and lemon juice.