Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 10 minutes. Drain, stir in butter, and set aside.
Combine marinara sauce, spinach, onion, mushrooms, tomato paste, garlic, cayenne, parsley, sea salt, and black pepper in a saucepan. Bring to a simmer, stirring occasionally, while you prepare the cheese mixture.
Combine lump crab, ricotta cheese, mozzarella cheese, 1 cup Parmesan-Romano cheese, eggs, and clam juice together in a large mixing bowl. Blend thoroughly together.
Spoon a light layer of sauce into the bottom of the prepared pan. Add a layer of spaghetti, followed by a layer of cheese mixture. Repeat until ingredients have all been added, making sure sauce makes up the top layer. Sprinkle with 1/4 cup Parmesan cheese.
Bake, uncovered, in the preheated oven until crab is pink and cheese is melted, about 35 minutes. Remove from the oven and let rest for 10 minutes. Serve.