Bring a large pot of salted water to a rolling boil. Add the small pasta shapes and cook according to package directions until al dente, about 8-10 minutes. Drain and rinse under cold water.

If using fresh shrimp, boil water and add the shrimp, cooking for 2-3 minutes until pink and opaque. Drain and cool, then chop into bite-sized pieces. If using pre-cooked shrimp, chop them.

Chop the celery, red bell pepper, and red onion into small pieces. Finely chop the fresh parsley for garnish.

In a medium bowl, whisk together mayonnaise, sour cream, Dijon mustard, lemon juice, Old Bay seasoning, salt, and pepper until smooth.

In a large bowl, combine the cooled pasta, shrimp, imitation crab, chopped vegetables, and dressing. Toss gently to coat the ingredients evenly.

Cover and chill in the refrigerator for at least 1 hour, or overnight for better flavor.

Before serving, sprinkle with freshly chopped parsley and serve on a bed of lettuce if desired.