Preheat oven to 180 degrees C. Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Heat oil in a large saucepan over medium heat. Sautee garlic and mushrooms until tender. Pour in Alfredo sauce. Stir in prawns, scallops and crabmeat. Simmer 5 to 10 minutes, or until heated through. In a medium bowl, combine ricotta cheese, egg and pepper.
In a 20x30cm baking dish, layer lasagne, ricotta mixture, Alfredo mixture and grated cheese. Repeat layers until all ingredients are used, ensuring that there is grated cheese for the top.
Bake uncovered in preheated oven for 45 minutes. Cover, and bake 15 minutes.