The name "gumbo" comes from the Bantu word for okra, a key ingredient in this Creole stew.
Shell and devein shrimp, leaving tail sections intact; reserve shells for stock. Break sausage into 1/2-inch pieces; in a skillet, cook over medium heat until all the fat has been rendered, about 8 minutes. Mix in onions, green peppers, okra, jalapeno, and file Cook until onions have softened, about 10 minutes. Add salt, pepper, and thyme. Reduce heat to low; cook 5 minutes more. Set aside.
In a stockpot, melt dark roux over medium heat; heat shrimp stock in a medium saucepan over high heat. Gradually pour hot stock over roux, whisking to combine well. Cook over medium-low heat until mixture thickens, 6 to 8 minutes. Reduce heat to low, and add reserved sausage-and-vegetable mixture and tomatoes. Stir gumbo well, and let cook for 1 hour.
If using whole crabs, cut them in half lengthwise, and add to gumbo. Cook 2 to 3 minutes. Add shrimp, and cook 3 to 4 minutes, or until opaque. Add crabmeat and parsley, and cook 2 to 4 minutes. Season to taste. Serve hot over dirty rice.
Gumbo file (powdered sassafras leaves) helps thicken the gumbo; it is available in supermarkets.