In a large pot, bring 4 quarts of salted water to a rolling boil. Add the small pasta and cook until al dente, about 8-10 minutes. Drain and rinse under cold water.

If using fresh shrimp, boil in a smaller pot of salted water for 2-3 minutes until pink. Drain, cool, and chop into bite-sized pieces.

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

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

In a large mixing bowl, combine cooled pasta, shrimp, imitation crab, and diced vegetables. Pour the dressing over the mixture and toss gently.

Cover and chill in the refrigerator for at least 1 hour to allow flavors to meld.

Before serving, stir gently and adjust seasoning. Garnish with chopped parsley and serve over lettuce if desired.