Preheat the oven to 350 degrees. Melt the butter in a skillet over medium high heat and add the onions and bell peppers. Sauté for 5 minutes or until the onions are tender. Remove from the heat and stir in the crabmeat, breadcrumbs, mayonnaise, ½ teaspoon salt and ¼ teaspoon pepper.
Sprinkle the shrimp with ¾ teaspoon salt and ¼ teaspoon pepper. Mound some of the crab meat mixture in each shrimp and press gently to close. Arrange the shrimp stuffing side up on a baking sheet. Bake for 12 minutes or until the shrimp turn pink and the stuffing is light brown.