A crayfish is no more a fish than a catfish is a cat; the original name of this edible crustacean was crevice (no relation to the crevice that means gap), which was adopted from French in the fifteenth century; this French name, in turn, was a borrowing of the Old German name for crab, krebiz. After entering English, crevice remained the standard spelling of the crablike creature’s name until the fifteenth century when its associations with the sea caused people to corrupt the pronunciation and spelling of the word, first to crefish and then to crayfish.
These two words are used almost interchangeably to refer to two different types of shellfish, but strictly speaking, only the first one is entitled to the name. The name crawfish is used in the southern United States.