A malacostracan crustacean is a type of shellfish that includes lobsters, crayfish, and shrimp. If you're looking for synonyms for this mouthful of a term, you might try "decapod," which refers specifically to crustaceans with ten legs, including the malacostracans. "Shrimplike," "crayfish-like," or "lobster-like" are also options if you want to be more descriptive in your language. Another possibility is to use the specific name of the type of malacostracan you're referring to, such as "lobster" or "prawn," instead of the all-encompassing term. Whatever word you choose, just make sure it accurately conveys the type of shellfish you're talking about.