If shrimp with heads and shells are not available, use 1/2 lb (225g) shrimp without heads but with shells and substitute other seafood ingredients for the shrimp heads in making the seafood stock.
You can substitute pieces of fish for any or all of the seafood in this recipe. Be sure to use margarine instead of butter, because margarine is oilier and seems to conduct more heat. The extra heat, plus the additional oil, develops the gumbo file to a more desirable taste, texture and color. Upon reaching a temperature above 140' (60'C), however, the oil separates out and rises to the surface. Some people prefer to skim the oil off before serving.
If the gumbo is made in advance, do not add the seafood. When ready to serve, bring the gumbo to a rapid boil, lower the heat to a simmer, and add the seafood. Immediately cover the pot, turn off the heat, and let the pot stand covered 6 to 10 minutes.