Place shrimp shells in medium saucepan. Add 1 3/4 cups water and a generous pinch of salt.
Bring to boil over medium-high heat. Cover, reduce heat to medium-low and simmer until reduced to 1 cup liquid, about 15 minutes.
Strain into measuring cup, pressing on shells to extract as much liquid as possible.
Meanwhile, heat 3 tablespoons oil in heavy large skillet over medium-high heat. Add shrimp and sauté until opaque, about 2 minutes.
Using slotted spoon, transfer to medium bowl. Reduce heat to low. Add remaining 3 tablespoons oil to same skillet.
Add garlic and sauté until soft, about 1 minute. Stir in leeks, oregano, and lemon peel. Cover and cook until leeks are soft, about 3 minutes.
Uncover; add wine, lemon juice, and shrimp-shell liquid. Increase heat to high and boil until reduced by half, about 2 minutes. Remove from heat.
Cook pasta in large pot of boiling salted water until tender but still firm to bite, stirring occasionally.
Drain, reserving 1 cup cooking liquid. Add pasta and shrimp to skillet with sauce and toss to coat. Add enough reserved cooking liquid to moisten if necessary.
Season to taste with salt and pepper. Transfer to large bowl. Garnish with parsley.